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Table of contents

1. History of the development of histology. Development of histology in Russia
2. Research methods in histology. Preparation of a histological specimen
3. Introduction to the course of histology
4. Morphology and functions of the cytoplasm and cell organelles
5. Morphology and functions of the nucleus. cell reproduction
6. General Embryology
7. Human Embryology
8. General principles of tissue organization
9. Epithelial tissue
10. Blood and lymph
11. hematopoiesis
12. Immunocytopoiesis and participation of immune cells in immune responses
13. Connective tissues. Connective tissue proper
14. Connective tissues. Skeletal connective tissues
15. Muscle tissues. Skeletal muscle tissue
16. Muscle tissue. Cardiac and smooth muscle tissue
17. nervous tissue
18. Nervous system
19. The cardiovascular system
20. Endocrine system
21. Digestive system
22. Respiratory system
23. Leather and its derivatives
24. Excretory system
25. Sexual system
26. Female reproductive system
27. Organ of vision
28. Organs of taste and smell
29. The structure of the organ of hearing and balance
30. Organs of hematopoiesis and immunological protection

SECTION I. GENERAL HISTOLOGY

Topic 1. HISTORY OF THE DEVELOPMENT OF HISTOLOGY. DEVELOPMENT OF HISTOLOGY IN RUSSIA

In the history of the development of histology, three main periods can be distinguished: pre-microscopic, microscopic and modern.

The pre-microscopic period (from the beginning of the 1665th century BC to XNUMX) is associated with the names of Aristotle, Galen, Vesalius and other great scientists of that time. This period of development of histology is characterized by attempts to isolate heterogeneous tissues in animals and humans using methods of anatomical preparation.

Microscopic period - 1665 - 1950 The beginning of this period is associated with the name of the English physicist R. Hooke, who invented the microscope and used it for the systematic study of various, including biological, objects. He published the results of his research in the book "Monograph". R. Hooke first introduced the term "cell". Subsequently, there was a continuous improvement of microscopes and their ever wider use for the study of biological tissues and organs. Particular attention was paid to the structure of the cell. Among the outstanding scientists of that time, one can single out M. Malpighi, A. Leeuwenhoek, N. Gru.

J. Purkinje described the presence of the cytoplasm and nucleus in animal cells, and somewhat later R. Brown discovered the nucleus in plant cells. The botanist M. Schleiden was engaged in the study of the origin of cells - cytokinesis. As a result of his research, T. Schwann formulated the cell theory:

1) all plant and animal organisms are made up of cells;

2) all cells develop according to the general principle - from cytoblastoma;

3) each cell has an independent vital activity, and the vital activity of an organism is the sum of the activity of cells.

R. Virchow in 1858 clarified that the development of cells is carried out by dividing the original cell. The theory developed by T. Schwann is still relevant today.

Modern provisions of cell theory:

1) a cell is the smallest unit of a living thing;

2) the cells of animal organisms are similar in structure;

3) cell reproduction occurs by dividing the original cell;

4) multicellular organisms are complex associations of cells and their derivatives, combined into systems of tissues and organs and interconnected by cellular, humoral and nervous mechanisms of regulation.

Further improvement of microscopes made it possible to identify smaller structures in cells:

1) plate complex (K. Golgi - 1897);

2) mitochondria (E van Benda - 1897);

3) centrioles (T. Boveri - 1895);

4) endoplasmic reticulum (K. Porter - 1945);

5) lysosomes (K. Duve - 1949).

The mechanisms of division of plant (ID Chistyakov, 1874) and animal cells (P.I. Peremezhko, 1978) were described.

The modern stage in the development of histology began in 1950, when the electron microscope was first used to study biological objects. However, the modern stage of development of histology is characterized by the introduction of not only electron microscopy, but also other methods: cyto- and histochemistry, historadiography, etc. In this case, a complex of various methods is usually used, which make it possible not only to compile a qualitative idea of ​​the structures under study, but also to obtain subtle quantitative characteristics. At present, various morphometric methods are especially widely used, including automated processing of the information received using a personal computer.

Histology in Russia was developed by scientists from the medical faculties of Russian universities, where strong histological schools were formed:

1) Moscow school (A. I. Babukhin, I. F. Ognev). The main area of ​​activity is the histogenesis of muscle and nervous tissue, histophysiological approaches to the study of the sense organs, especially the organ of vision;

2) St. Petersburg Histological School at the Medical-Surgical Academy (K. E. Baer - embryologist, N. M. Yakubovich, M. D. Lavdovsky - neurohistologist and A. A. Maksimov - author of the unitary theory of hematopoiesis);

3) St. Petersburg Histological School at the University (F. V. Ovsyannikov - research of the sense organs, A. S. Dogel - neurohistologist, etc.);

4) Kyiv histological school (P. I. Peremezhko studied cell division, development of organs);

5) Kazan histological school - K. A. Arshtein, A. S. Dogel, A. E. Smirnov, T. A. Timofeev, B. I. Lavrentiev. This school developed the neurohistological direction.

The most prominent scientists in the field of histology in Russia were A. A. Zavarzin and N. G. Khlopin, who studied the patterns of tissue development in phylogenesis.

Topic 2. RESEARCH METHODS IN HISTOLOGY. PREPARATION OF HISTOLOGICAL PREPARATION

The main research method in histology is microscopy - the study of histological preparations under a microscope. Recently, microscopy has been combined with other methods - histochemistry and historadiography. For microscopy, various designs of microscopes are used, which allow studying various parameters of histological preparations.

The following types of microscopy are distinguished:

1) light microscopy (the most common type of microscopy, while the resolution of the microscope is 0,2 microns);

2) ultraviolet microscopy (resolution of the microscope is 0,1 microns);

3) luminescent microscopy (used to determine certain chemical structures in the histological specimen under study);

4) phase contrast microscopy (used to detect and study certain structures in unstained histological preparations);

5) polarizing microscopy (used mainly to study fibrous structures);

6) dark field microscopy is used to study living objects;

7) incident light microscopy (designed to study thick objects);

8) electron microscopy (the most modern type of microscopy with a resolution of 0,1 - 0,7 nm). There are two types of electron microscopy - transmission (transmission) and scanning (or solution) microscopy, which displays surface ultrastructures.

Histological and cytochemical methods are used to determine the composition of chemicals and their amount in certain structures. The principle of the method lies in the chemical reaction between the reagent and the substrate contained in the test substance. In this case, the resulting reaction by-products can be detected using light or luminescent microscopy.

The method of histoautoradiography makes it possible to reveal the composition of chemicals in the structures under study and the intensity of the exchange by the inclusion of radioactive isotopes. This method is most often used in animal experiments.

The interferonometry method makes it possible to determine the dry mass of a substance in living or fixed objects.

The cell culture method is the cultivation of cells in test tubes or in special capsules in the body and the subsequent examination of living cells under a microscope.

The method of vital staining is the introduction of a dye (trepan blue) into the blood or into the abdominal cavity of the animal, which during the life of the animal is captured by certain cells - macrophages, and after the slaughter of the animal and the preparation of the drug, cells containing the dye are determined and counted.

Immunomorphological methods allow using preliminary immune reactions (based on antigen-antibody interaction) to determine the subpopulation of lymphocytes, the degree of foreignness of cells, to carry out histological typing of tissues and organs, i.e., to determine their histocompatibility for further transplantation.

The differential centrifugation method is the study of individual organelles or even their fragments isolated from a cell. To do this, a piece of the organ under study is rubbed, filled with saline, and then dispersed in a centrifuge at various speeds (from 2 to 150 thousand per 1 min). As a result of centrifugation, fractions of interest are obtained, which are then studied by various methods.

Methods of morphometry - quantitative methods. They allow you to determine the size and volume of the nucleus - karyometry, cells - cytometry, organelles - electronic morphometry, as well as determine the number of cells of various populations and subpopulations. These methods are widely used in scientific research.

Various experimental methods - food and water load, physical methods (UHF, microwave, lasers, magnets). They are used to study the reaction of structures of interest to a particular impact and are combined with the methods of morphometry, cyto- and histochemistry. These methods are also used in scientific research.

Thus, the main and most common method of study in histology is microscopy. Preparation of a histological preparation includes the following steps.

1. Taking material - a piece of tissue or organ. When taking material, the following rules must be observed:

1) sampling should be carried out as soon as possible after the death or slaughter of the animal, if possible from a living object, in order to preserve the structure of the studied cells as best as possible;

2) the sampling of the material should be carried out with a sharp instrument so as not to injure the tissues;

3) the thickness of the piece should not exceed 5 mm so that the fixing solution can penetrate the entire depth of the tissue;

4) it is necessary to mark the piece, indicating the name of the body, the number of the animal or the name of the person, the date of sampling.

2. Fixing the material. This stage is carried out in order to stop the metabolic processes in the cell and save it from decay. To do this, a piece of tissue taken for examination is immersed in a fixing solution. The solution can be simple (alcohol or formalin) and complex (Carnoy's solution, Zinker's fixative). The fixative causes protein denaturation and keeps the cell structure in a state close to life. Fixation can also be carried out by freezing - cooling with liquid nitrogen or a jet of carbon dioxide.

3. Pouring tissue pieces into sealing media (paraffin, resins) - or freezing. This stage is necessary in order to subsequently make a thin section of the tissue under study.

4. Preparation of sections on a microtome or ultramicrotome using special knives. After that, sections for light microscopy are glued to glass slides, and for electron microscopy, they are mounted on special grids.

5. Staining of sections or their contrasting (for electron microscopy). Before staining the sections, it is necessary to remove the sealing medium - to perform deparaffing. With the help of coloring, the contrast of the studied structures is achieved. Dyes can be divided into basic, acidic and neutral. The most widely used basic dyes (hematoxylin) and acidic (eosin). Complex dyes are also often used.

6. Section clearing in xylene and toluene. They are encapsulated in resins (balm and polystyrene) and covered with a coverslip.

After these procedures, the drug can be examined under a light microscope. Light microscope sections placed under glass can be stored for a long time and reused. For electron microscopy, each section is used only 1 time, while it is photographed, and the study of tissue structures is carried out according to the electron diffraction pattern.

If the tissue has a liquid consistency (for example, blood, bone marrow), then the preparation is made in the form of a smear on a glass slide, which is then also fixed, stained and studied.

From brittle parenchymal organs, preparations are made in the form of an organ imprint, this organ is fractured, then a glass slide is applied to the fracture site, on which free cells are glued. After that, the drug is fixed and studied.

From some organs (for example, the mesentery, pia mater) or from loose fibrous connective tissue, film preparations are made by stretching or crushing between two glasses, followed by fixation and pouring into resins.

Topic 3. INTRODUCTION TO THE COURSE OF HISTOLOGY

Histology is the science of the structure, development and vital activity of the tissues of living organisms. Consequently, histology studies one of the levels of organization of living matter - tissue.

There are the following levels of organization of living matter:

1) cellular;

2) fabric;

3) structural and functional units of the organ;

4) organ;

5) systemic;

6) organismic;

7) population and other levels.

Histology is considered as a discipline that includes four main sections:

1) cytology, which studies the structure of the cell;

2) embryology, which studies the formation of cells and tissues during fetal development;

3) general histology - studies the structure, functional, cellular elements of various tissues;

4) private (or macroscopic) histology, which studies the structures of certain organs and their systems.

Thus, there are several sections in histology that study certain levels of organization of living matter, starting from the cellular and ending with the organ and system that make up the body.

Histology refers to the morphological sciences. Unlike anatomy, which studies the structure of organs at the macroscopic level, histology studies the structure of organs and tissues at the microscopic and electron microscopic levels. At the same time, the approach to the study of various elements is made taking into account the function they perform. This method of studying the structures of living matter is called histophysiological, and histology is often referred to as histophysiology. When studying living matter at the cellular, tissue and organ levels, not only the shape, size and location of the structures of interest are considered, but the chemical composition of the substances that form these structures is determined by the methods of cyto- and histochemistry. The studied structures are also considered taking into account their development both in the prenatal period and during the initial ontogenesis. It is with this that the need to include embryology in histology is connected.

The main object of histology in the system of medical education is the body of a healthy person, and therefore this academic discipline is referred to as human histology.

The main task of histology as an academic subject is the presentation of knowledge about the microscopic and ultramicroscopic (electron-microscopic) structure of cells, tissues of organs and systems of a healthy person in close connection with their development and functions. This is necessary for further study of human physiology, pathological anatomy, pathological physiology and pharmacology. Knowledge of these disciplines shapes clinical thinking.

The task of histology as a science is to elucidate the patterns of structure of various tissues and organs in order to understand the physiological processes occurring in them and the possibility of controlling these processes.

Topic 4. MORPHOLOGY AND FUNCTIONS OF THE CYTOPLASMA AND CELL ORGANELLES

Cytology is the science of the structure, development and vital activity of cells. Consequently, cytology studies the regularities of the structural and functional organization of the first (cellular) level of organization of living matter. A cell is the smallest unit of living matter that has independent vital activity and the ability to reproduce itself. Subcellular formations (nucleus, mitochondria and other organelles), although they are living structures, do not have independent vital activity.

A cell is an ordered, structured system of biopolymers limited by an active membrane, forming a nucleus and cytoplasm, participating in a single set of metabolic and energy processes that maintain and reproduce the entire system as a whole.

A cell is a living system consisting of a cytoplasm and a nucleus and is the basis of the structure, development and life of all animal organisms.

The main components of the cell:

1) core;

2) cytoplasm.

According to the ratio of the nucleus and cytoplasm (nuclear-cytoplasmic ratio), cells are divided into:

1) cells of the nuclear type (the volume of the nucleus prevails over the volume of the cytoplasm);

2) cells of the cytoplasmic type (the cytoplasm prevails over the nucleus).

In shape, the cells are round (blood cells), flat, cubic or prismatic (cells of different epithelium), spindle-shaped (smooth muscle cells), process (nerve cells), etc. Most cells contain one nucleus, but one cell can have 2, 3 and more nuclei (multinuclear cells). In the body there are structures (symplasts, syncytium) containing several tens or even hundreds of nuclei. However, these structures are formed either as a result of the fusion of individual cells (symplasts) or as a result of incomplete cell division (syncytium). The morphology of these structures will be considered in the study of tissues.

Structural components of the cytoplasm of an animal cell:

1) plasmolemma (cytolemma);

2) hyaloplasm;

3) organelles;

4) inclusions.

The plasma membrane surrounding the cytoplasm is often considered as one of the organelles of the cytoplasm.

Plasmolemma (cytolemma)

The plasmalemma is the shell of an animal cell that delimits its internal environment and ensures the interaction of the cell with the extracellular environment.

Plasma membrane functions:

1) delimiting (barrier);

2) receptor;

3) antigenic;

4) transport;

5) formation of intercellular contacts.

The chemical composition of plasma membrane substances: proteins, lipids, carbohydrates.

The structure of the plasmalemma:

1) a double layer of lipid molecules, which forms the basis of the plasmolemma, in which protein molecules are sometimes included;

2) supramembrane layer;

3) submembrane layer found in some cells.

Each lipid molecule has two parts:

1) hydrophilic head;

2) hydrophobic tails.

The hydrophobic tails of lipid molecules bind to each other and form a lipid layer. Hydrophilic heads are in contact with the external and internal environment.

Protein molecules are built into the bilipid layer of the membrane locally and do not form a continuous layer. According to the function performed, plasma membrane proteins are divided into:

1) structural;

2) transport;

3) receptor proteins;

4) enzyme proteins;

5) antigenic determinants.

Proteins and hydrophilic lipid heads located on the outer surface of the plasmalemma are usually associated with chains of carbohydrates and form complex polymeric molecules. It is these macromolecules that make up the epimembrane layer - the glycocalyx. A significant part of the surface glycoproteins and glycolipids normally performs receptor functions: it perceives hormones and other biologically active substances. Such cellular receptors transmit perceived signals to intracellular enzyme systems, enhancing or inhibiting metabolism, and thereby affect cell function.

There are the following methods of transport of substances:

1) a method of diffusion of substances (ions, some low molecular weight substances) through the plasmalemma without energy consumption;

2) active transport of substances (amino acids, nucleotides, etc.) with the help of carrier proteins with energy consumption;

3) vesicular transport (produced by means of vesicles (vesicles)). It is divided into endocytosis - the transport of substances into the cell, exocytosis - the transport of substances from the cell.

In turn, endocytosis is divided into:

1) phagocytosis - capture and movement into the cell;

2) pinocytosis - the transfer of water and small molecules.

The process of phagocytosis is divided into several phases:

1) adhesion (sticking) of the object to the cytolemma of the phagocytic cell;

2) the absorption of the object by first forming a deepening of the invagination, and then moving it into the hyaloplasm.

In those tissues in which cells or their processes are tightly adjacent to each other (epithelial, smooth muscle, etc.), connections are formed between the plasma membranes of contacting cells - intercellular contacts.

Types of intercellular contacts:

1) simple contact - 15 - 20 nm (communication is carried out due to the contact of glycocalyx macromolecules). Simple contacts occupy the most extensive areas of adjoining cells. With the help of simple contacts, a weak bond is carried out - adhesion, which does not prevent the transport of substances into the intercellular spaces. A variation of a simple contact is a lock-type contact, when the plasmolemms of neighboring cells, together with sections of the cytoplasm, seem to bulge into each other, which results in an increase in the area of ​​\uXNUMXb\uXNUMXbcontacting surfaces and a stronger mechanical bond;

2) desmosomal contact - 0,5 µm. Desmosomal junctions (or adhesion patches) are small areas of interaction between cells. Each such site has a three-layer structure and consists of two semi-desmosomes - electron-dense sections located in the cytoplasm at the points of cell contact, and an accumulation of electron-dense material in the intermembrane space - 15 - 20 nm. The number of desmosomal contacts in one cell can reach 2000. The functional role of desmosomes is to provide mechanical contact between cells;

3) tight contact. This contact is also called end plates. They are localized in organs (stomach, intestines), in which the epithelium delimits the aggressive contents of these organs, for example, gastric juice containing hydrochloric acid. Tight junctions are located only between the apical parts of the cells, covering each cell along the entire perimeter. There are no intermembrane spaces in these areas, and bilipid membranes of neighboring cells merge into a single bilipid membrane. In adjacent areas of the cytoplasm of adjoining cells, an accumulation of electron-dense material is noted. The functional role of tight junctions is a strong mechanical connection of cells, an obstacle to the transport of substances through intercellular spaces;

4) gap-like contact (or nexuses) - 0,5 - 3 microns (both membranes are pierced in the transverse direction by protein molecules (or connexons) containing hydrophilic channels through which the exchange of ions and micromolecules of neighboring cells is carried out, which ensures their functional connection) . These contacts are limited areas of contacts of neighboring cells. An example of gap-like junctions (nexuses) are the contacts of cardiomyocytes, while through them there is a distribution of biopotentials and a friendly contraction of the cardiac muscles;

5) synaptic contact (or synapse) - specific contacts between nerve cells (interneuronal synapses) or between nerve and muscle cells (myoneural synapses). The functional role of synapses is the transmission of a nerve impulse or a wave of excitation (inhibition) from one cell to another or from a nerve cell to a muscle cell.

Hyaloplasm

Hyaloplasm (or cytoplasm matrix) makes up the internal environment of the cell. It consists of water and various biopolymers (proteins, nucleic acids, polysaccharides, lipids), of which the main part is proteins of various chemical and functional specificities. The hyaloplasm also contains amino acids, monosugars, nucleotides and other low molecular weight substances.

Biopolymers form a colloidal medium with water, which, depending on the conditions, can be dense (in the form of a gel) or more liquid (in the form of a sol), both in the entire cytoplasm and in its individual sections. In the hyaloplasm, various organelles and inclusions are localized and interact with each other and with the environment of the hyaloplasm. Moreover, their location is most often specific to certain cell types. Through the bilipid membrane, the hyaloplasm interacts with the extracellular environment. Consequently, hyaloplasm is a dynamic environment and plays an important role in the functioning of individual organelles and the vital activity of cells as a whole.

Organelles

Organelles are permanent structural elements of the cytoplasm of a cell that have a specific structure and perform certain functions.

Organelle classification:

1) common organelles inherent in all cells and providing various aspects of the cell's vital activity;

2) special organelles that are present in the cytoplasm of only certain cells and perform specific functions of these cells.

In turn, common organelles are divided into membranous and non-membrane.

Special organelles are divided into:

1) cytoplasmic (myofibrils, neurofibrils, tonofibrils);

2) cell surface organelles (cilia, flagella).

Membrane organelles include:

1) mitochondria;

2) endoplasmic reticulum;

3) lamellar complex;

4) lysosomes;

5) peroxisomes.

Non-membrane organelles include:

1) ribosomes;

2) cell center;

3) microtubules;

4) microfibrils;

5) microfilaments.

The principle of the structure of membrane organelles

Membrane organelles are closed and isolated areas (compartments) in the hyaloplasm, having their own internal structure. Their wall consists of a bilipid membrane and proteins like a plasmalemma. However, bilipid membranes of organelles have specific features: the thickness of bilipid membranes of organelles is less than that of plasmolemms (7 nm versus 10 nm), membrane membranes differ in the number and content of proteins built into them.

However, despite the differences, the membranes of organelles have the same structural principle, therefore they have the ability to interact with each other, integrate, merge, disconnect, lace up.

The general principle of the structure of organelle membranes can be explained by the fact that they are all formed in the endoplasmic reticulum, and then their functional rearrangement occurs in the Golgi complex.

Mitochondria

Mitochondria are the most isolated structural elements of the cytoplasm of the cell, which have a largely independent vital activity.

There is an opinion that in the past mitochondria were independent living organisms, after which they penetrated into the cytoplasm of cells, where they lead a saprophytic existence. Proof of this may be the presence of a genetic apparatus (mitochondrial DNA) and a synthetic apparatus (mitochondrial ribosomes) in mitochondria.

The shape of mitochondria can be oval, round, elongated, and even branched, but oval-elongated prevails. The mitochondrial wall is formed by two bilipid membranes separated by a space of 10–20 nm. At the same time, the outer membrane covers the entire mitochondrion in the form of a bag along the periphery and delimits it from the hyaloplasm. The inner membrane delimits the internal environment of the mitochondria, while it forms folds inside the mitochondria - cristae. The internal environment of the mitochondria (mitochondrial matrix) has a fine-grained structure and contains granules (mitochondrial DNA and ribosomes).

The function of mitochondria is the production of energy in the form of ATP.

The source of energy in mitochondria is pyruvic acid (pyruvate), which is formed from proteins, fats and carbohydrates in the hyaloplasm. Pyruvate oxidation occurs in the mitochondrial matrix, and on the mitochondrial cristae, electron transfer, ADP phosphorylation, and ATP formation take place. The ATP produced in the mitochondria is the only form of energy that is used by the cell to carry out various processes.

Endoplasmic reticulum

The endoplasmic reticulum (ER) in different cells can be presented in the form of flattened cisterns, tubules, or individual vesicles. The wall consists of a bilipid membrane.

There are two types of EPS:

1) granular (granular, or rough);

2) non-granular (or smooth). On the outer surface of the membranes of the granular ER contains attached ribosomes.

In the cytoplasm during electron microscopic examination, two types of EPS can be detected, however, one of them predominates, which determines the functional specificity of the cell. These two varieties of EPS are not independent and isolated forms, since a more detailed study can reveal the transition of one variety to another.

Functions of granular EPS:

1) synthesis of proteins intended for removal from the cell (for export);

2) separation (segregation) of the synthesized product from the hyaloplasm;

3) condensation and modification of the synthesized protein;

4) transport of the synthesized products to the tanks of the lamellar complex;

5) synthesis of lipid membrane components.

Functions of smooth EPS:

1) participation in the synthesis of glycogen;

2) lipid synthesis;

3) detoxification function (neutralization of toxic substances by combining them with other substances).

Golgi lamellar complex

The lamellar complex is called the transport apparatus of the cell.

The lamellar Golgi complex (mesh apparatus) is represented by an accumulation of flattened cisterns and small vesicles bounded by a bilipid membrane. The lamellar complex is subdivided into subunits - dictyosomes. Each dictyosome is a stack of flattened cisterns, along the periphery of which small vesicles are localized. At the same time, in each flattened tank, the peripheral part is somewhat expanded, and the central part is narrowed. There are two poles in the dictyosome: the cispole (directed by the base towards the nucleus) and the transpole (directed towards the cytolemma). It has been established that transport vacuoles approaching the cispole carry products synthesized in EPS to the Golgi complex. Vesicles are laced from the transpole, carrying the secret to the plasmalemma for its release from the cell. Some of the small vesicles filled with enzyme proteins remain in the cytoplasm and are called lysosomes.

Function of the lamellar complex:

1) transport (removes the products synthesized in it from the cell);

2) condensation and modification of substances synthesized in granular EPS;

3) formation of lysosomes (together with granular ER);

4) participation in carbohydrate metabolism;

5) synthesis of molecules that form the glycocalyx of the cytolemma;

6) synthesis, accumulation, excretion of mucins (mucus);

7) modification of membranes synthesized in EPS and their transformation into plasmalemma membranes.

Lysosomes

Lysosomes - the smallest organelles of the cytoplasm, are bodies bounded by a bilipid membrane and containing an electron-dense matrix consisting of a set of hydrolytic enzyme proteins (more than thirty types of hydrolases) capable of splitting any polymeric compounds (proteins, fats, carbohydrates), their complexes into monomeric fragments.

The function of lysosomes is to ensure intracellular digestion, i.e., the breakdown of both exogenous and endogenous biopolymer substances.

Lysosome classification:

1) primary lysosomes - electron dense bodies;

2) secondary lysosomes - phagolysosomes, including autophagolysosomes;

3) tertiary lysosomes or residual bodies.

True lysosomes are called small electron-dense bodies that form in a lamellar complex. The digestive function of lysosomes begins only after fusion with a phagosome (a phagocytosed substance surrounded by a bilipid membrane) and the formation of a phagolysosome, in which phagocytosed material and lysosomal enzymes are mixed. After this, the splitting of the biopolymer compounds of the phagocytosed material into monomers - amino acids, sugars - begins. These molecules freely penetrate through the membrane of the phagolysosome into the hyaloplasm and are then utilized by the cell - they go to generate energy or build new intracellular macromolecular compounds.

Some compounds cannot be cleaved by lysosome enzymes and are therefore excreted unchanged from the cell by exocytosis (the reverse process of phagocytosis). Substances of a lipid nature are practically not broken down by enzymes, but accumulate and compact in the phagolysosome. These formations were called tertiary lysosomes (or residual bodies).

In the process of phagocytosis and exocytosis, membranes are recirculated in the cell: during phagocytosis, part of the plasmolemma is laced off and forms a phagosome shell; during exocytosis, this shell is again built into the plasmolemma.

Damaged, altered or obsolete cell organelles are utilized by it by the mechanism of intracellular phagocytosis with the help of lysosomes. Initially, these organelles are surrounded by a bilipid membrane, and a vacuole, an autophagosome, is formed. Then one or more lysosomes merge with it, and an autophagolysosome is formed, in which the hydrolytic cleavage of biopolymer substances is carried out, as in the phagolysosome.

Lysosomes are found in all cells, but in unequal numbers. Specialized cells - macrophages - contain a large number of primary and secondary lysosomes in the cytoplasm. They perform a protective function in tissues, absorb a significant number of exogenous substances - bacteria, viruses, other foreign agents and decay products of their own tissues.

Peroxisomes

Peroxisomes are microbodies of the cytoplasm (0,1 - 1,5 μm), similar in structure to lysosomes, but differ from them in that their matrix contains crystal-like structures, and among enzyme proteins there is catalase, which destroys hydrogen peroxide formed during oxidation amino acids.

Ribosomes

Ribosomes are the apparatus for the synthesis of protein and polypeptide molecules.

According to localization, they are divided into:

1) free, (located in the hyaloplasm);

2) non-free (or attached), - which are associated with EPS membranes.

Each ribosome consists of small and large subunits. Each subunit of the ribosome consists of ribosomal RNA and protein - ribonucleoprotein. Subunits are formed in the nucleolus, and assembly into a single ribosome is carried out in the cytoplasm. For protein synthesis, individual ribosomes with the help of matrix (information) RNA are combined into chains of ribosomes - polysomes. Free and attached ribosomes, in addition to differences in their localization, are characterized by a certain functional specificity: free ribosomes synthesize proteins.

Cell Center

Cell center - cytocenter, centrosome. In a nondividing cell, the cell center consists of two main structural components:

1) diplosomes;

2) centrosphere.

The diplosome consists of two centrioles (maternal and daughter) located at right angles to each other. Each centriole consists of microtubules forming a hollow cylinder, 0,2 µm in diameter and 0,3–0,5 µm long. Microtubules are combined into triplets (three tubes each), forming a total of nine triplets. The centrosphere is a structureless section of the hyaloplasm around the diplosome, from which microtubules extend radially (like a radiant sphere).

Functions of the cytocenter:

1) the formation of a fission spindle in the prophase of mitosis;

2) participation in the formation of microtubules of the cell scaffold;

3) playing the role of basic bodies of cilia in the ciliated epithelial cells of the centriole.

The position of centrioles in some epithelial cells determines their polar differentiation.

Microtubules

Microtubules - hollow cylinders (outer diameter - 24 mm, inner - 15 mm), are independent organelles, forming a cytoskeleton. They can also be part of other organelles - centrioles, cilia, flagella. The wall of microtubules consists of the globular protein tubulin, which is formed by separate rounded formations of a globule with a diameter of 5 nm. Globules can be in the hyaloplasm in a free state or connect with each other, resulting in the formation of microtubules. They can then again disintegrate into globules. Thus, spindle microtubules are formed and then disintegrate in different phases of mitosis. However, in the composition of centrioles, cilia and flagella, microtubules are stable formations. Most of the microtubules are involved in the formation of the intracellular scaffold, which maintains the shape of the cell, determining a certain position of the organelles in the cytoplasm, and also predetermines the direction of intracellular movements. Tubulin proteins do not have the ability to contract, therefore, microtubules do not contract. In the composition of cilia and flagella, microtubules interact with each other, they slide relative to each other, which ensures the movement of these organelles.

microfibrils

Microfibrils (intermediate filaments) are thin, non-branching filaments.

Basically, microfibrils are localized in the cortical (submembrane) layer of the cytoplasm. They consist of a protein that has a certain structure in cells of various classes (in epithelial cells it is a keratin protein, in muscle cells it is desmin).

The functional role of microfibrils is to participate, along with microtubules, in the formation of the cell scaffold, performing a supporting function.

Microtubules can combine into bundles and form tonofibrils, which are considered as independent organelles and perform a supporting function.

Microfilaments

Microfilaments are even thinner filamentous structures (5 - 7 nm), consisting of contractile proteins (actin, myosin, tropomyosin).

Microfilaments are localized mainly in the cortical layer of the cytoplasm.

Together, microfilaments make up the contractile apparatus of the cell, which provides various types of movements: the movement of organelles, the flow of hyaloplasm, the change in the cell surface, the formation of pseudopodia, and the movement of the cell.

The accumulation of microfilaments in muscle fibers forms special organelles of muscle tissue - myofibrils.

Inclusions

Inclusions are non-permanent structural components of the cytoplasm. Classification of inclusions:

1) trophic;

2) secretory;

3) excretory;

4) pigment.

During the life of cells, random inclusions can accumulate - medication, particles of various substances.

Trophic inclusions - lecithin in eggs, glycogen or lipids in various cells.

Secretory inclusions are secretory granules in secreting cells (eg, zymogenic granules in pancreatic acinar cells, secretory granules in various endocrine cells).

Excretory inclusions are substances that need to be removed from the cell (for example, granules of uric acid in the epithelium of the renal tubules).

Pigment inclusions - melanin, hemoglobin, lipofuscin, bilirubin. These inclusions give the cell that contains them a certain color: melanin stains the cell black or brown, hemoglobin yellow-red, bilirubin yellow. Pigment cells are found only in certain types of cells: melanin - in melanocytes, hemoglobin - in erythrocytes. Lipofuscin, unlike the other pigments mentioned, can be found in many cell types. The presence of lipofuscin in cells (especially in a significant amount) indicates aging and functional inferiority.

Topic 5. MORPHOLOGY AND FUNCTIONS OF THE NUCLEUS. CELL REPRODUCTION

The human body contains only eukaryotic (nuclear) cell types. Nuclear-free structures (erythrocytes, platelets, horny scales) are secondary formations, since they are formed from nuclear cells as a result of their specific differentiation.

Most cells contain a single nucleus, only rarely are binucleated and multinucleated cells. The shape of the nucleus is most often rounded (spherical) or oval. In granular leukocytes, the nucleus is subdivided into segments. The nucleus is usually localized in the center of the cell, but in the cells of the epithelial tissue it can be shifted to the basal pole.

Structural elements of the nucleus are clearly expressed only in a certain period of the cell cycle - in interphase. During cell division (mitosis or meiosis), pronounced changes in cell structures occur: some disappear, others are significantly transformed.

Structural elements of the core

The structural elements of the nucleus listed below are well expressed only in the interphase:

1) chromatin;

2) nucleolus;

3) karyoplasm;

4) karyolemma.

Chromatin is a dye-receptive substance (chromos), hence its name. Chromatin consists of chromatin fibrils 20–25 km thick, which can be loosely or compactly located in the nucleus.

On this basis, euchromatin can be distinguished - loose (or decondensed) chromatin, weakly stained with basic dyes, and heterochromatin - compact (or condensed) chromatin, well stained with basic dyes.

In preparing the cell for division in the nucleus, chromatin fibrils spiralize and chromatin is converted into chromosomes. After division in the nuclei of daughter cells, despiralization of chromatin fibrils occurs, and the chromosomes are again converted into chromatin. Thus, chromatin and chromosomes are different states of the same substance.

According to the chemical structure, chromatin consists of:

1) deoxyribonucleic acid (DNA) - 40%;

2) proteins - about 60%;

3) ribonucleic acid (RNA) - 1%.

Nuclear proteins are presented in two forms:

1) alkaline (histone) proteins - 80 - 85%;

2) acidic proteins - 15 - 20%.

Histone proteins are associated with DNA and form a deoxynucleoprotein, which is a chromatin fibrils, clearly visible under electron microscopy. In certain areas of chromatin fibrils, transcription from DNA to various RNA is carried out, with the help of which the synthesis of protein molecules subsequently takes place. Transcription processes in the nucleus are carried out only on free chromosomal fibrils, i.e., on euchromatin. In condensed chromatin, these processes are not carried out, therefore, heterochromatin is called inactive chromatin.

The ratio of euchromatin and heterochromatin is an indicator of the synthetic activity of the cell. DNA replication occurs on chromatin fibrils in the S-period of interphase. These processes can also occur in heterochromatin, but much longer.

The nucleolus is a spherical formation (1 - 5 microns in diameter), which perceives basic dyes well and is located among the chromatin. One nucleus can contain from 1 to 4 or even more nucleoli. In young and frequently dividing cells, the size of the nucleoli and their number are increased. The nucleolus is not an independent structure. It is formed only in interphase, in certain regions of some chromosomes - nucleolar organizers, which contain genes encoding a ribosomal RNA molecule. In the region of the nucleolar analyzer, transcription from DNA is carried out. In the nucleolus, ribosomal RNA combines with protein and the formation of a subunit of the ribosome.

Microscopically in the nucleolus distinguish:

1) fibrillar component (localized in the central part of the nucleolus and is a thread of ribonucleoprotein (RNP));

2) granular component (located in the peripheral part of the nucleolus and is an accumulation of ribosome subunits).

In the prophase of mitosis, when the spiralization of chromatin fibrils and the formation of chromosomes occur, the processes of RNA transcription and synthesis of the ribosome subunit cease, and the nucleolus disappears. At the end of mitosis, decondensation of chromosomes occurs in the nuclei of newly formed cells, and a nucleolus appears.

Karyoplasm (nucleoplasm or nuclear juice) consists of water, proteins and protein complexes (nucleoproteins, glycoproteins), amino acids, nucleotides, sugars. Under a light microscope, the karyoplasm is structureless, however, with electron microscopy, small granules (15 nm) consisting of ribonucleoproteins can be found in it. Karyoplasmic proteins are mainly enzyme proteins, including glycolysis enzymes that break down carbohydrates with the formation of ATP.

Non-histone proteins (acidic) form a structural network in the nucleus (nuclear protein matrix), which, together with the nuclear envelope, takes part in creating the internal environment.

With the participation of karyoplasm, the metabolism in the nucleus, the interaction of the nucleus and cytoplasm are carried out.

The karyolemma is a nuclear envelope that separates the contents of the nucleus from the cytoplasm (barrier function), while at the same time ensuring a regulated metabolism between the nucleus and the cytoplasm. The nuclear envelope is involved in the fixation of chromatin.

The karyolemma consists of two bilipid membranes, the outer and inner nuclear membranes, separated by a perinuclear space 20–100 nm wide. The karyolemma has pores 80–90 nm in diameter. In the pore region, the outer and inner nuclear membranes pass into each other, and the perinuclear space is closed. The lumen of the pore is closed by a special structural formation - the pore complex, which consists of fibrillar and granular components. The granular component is represented by protein granules 25 nm in diameter, arranged along the edge of the pore in 3 rows. Fibrils depart from each granule and unite in a central granule located in the center of the pore. The pore complex plays the role of a diaphragm that regulates its permeability. The pore size is stable for a given cell type, but the number of pores may change during cell differentiation. There are no pores in the nuclei of spermatozoa. Attached ribosomes can be localized on the outer surface of the nuclear membrane. In addition, the outer nuclear membrane may continue into the EPS channels.

Functions of somatic cell nuclei:

1) storage of genetic information encoded in DNA molecules;

2) repair (restoration) of DNA molecules after their damage with the help of special reparative enzymes;

3) reduplication (doubling) of DNA in the synthetic period of interphase;

4) transfer of genetic information to daughter cells during mitosis;

5) implementation of the genetic information encoded in DNA for the synthesis of protein and non-protein molecules: the formation of an apparatus for protein synthesis (information, ribosomal and transfer RNA).

Functions of germ cell nuclei:

1) storage of genetic information;

2) the transfer of genetic information during the fusion of female and male germ cells.

Cellular (life) cycle

The cell (or life) cycle of a cell is the time of existence of a cell from division to the next division or from division to death. The cell cycle is different for different cell types.

In the body of mammals and humans, the following types of cells are distinguished, localized in different tissues and organs:

1) frequently dividing cells (poorly differentiated cells of the intestinal epithelium, basal cells);

2) rarely dividing cells (liver cells - hepatocytes);

3) non-dividing cells (nerve cells of the central nervous system, melanocytes, etc.).

The life cycle of these cell types is different.

The life cycle of frequently dividing cells is the time of their existence from the beginning of division to the next division. The life cycle of such cells is often called the mitotic cycle.

This cell cycle is divided into two main periods:

1) mitosis (or division period);

2) interphase (cell life span between two divisions).

There are two main methods of reproduction (reproduction) of cells.

1. Mitosis (karyokenesis) - indirect cell division, inherent mainly in somatic cells.

2. Meiosis (reduction division) is characteristic only for germ cells.

There are also descriptions of the third method of cell division - amitosis (or direct division), which is carried out by constriction of the nucleus and cytoplasm with the formation of two daughter cells or one binuclear one. However, it is currently believed that amitosis is characteristic of old and degenerating cells and is a reflection of cell pathology.

These two methods of cell division are divided into phases or periods.

Mitosis is divided into four phases:

1) prophase;

2) metaphase;

3) anaphase;

4) telophase.

Prophase is characterized by morphological changes in the nucleus and cytoplasm.

The following transformations take place in the kernel:

1) condensation of chromatin and the formation of chromosomes consisting of two chromatids;

2) disappearance of the nucleolus;

3) disintegration of the karyolemma into individual vesicles.

The following changes occur in the cytoplasm:

1) reduplication (doubling) of centrioles and their divergence to opposite poles of the cell;

2) formation of a fission spindle from microtubules;

3) reduction of granular ER and also a decrease in the number of free and attached ribosomes.

In metaphase, the following happens:

1) the formation of a metaphase plate (or parent star);

2) incomplete separation of sister chromatids from each other.

Anaphase is characterized by:

1) complete divergence of chromatids and the formation of two equivalent dipole sets of chromosomes;

2) divergence of chromosome sets to the poles of the mitotic spindle and divergence of the poles themselves.

Telophase is characterized by:

1) decondensation of chromosomes of each chromosome set;

2) formation of the nuclear membrane from the bubbles;

3) cytotomy, (constriction of a binuclear cell into two daughter independent cells);

4) the appearance of nucleoli in daughter cells.

Interphase is divided into three periods:

1) I - J1 (or presynthetic period);

2) II - S (or synthetic);

3) III - J2 (or postsynthetic period).

In the presynthetic period, the following processes occur in the cell:

1) enhanced formation of the synthetic apparatus of the cell - an increase in the number of ribosomes and various types of RNA (transport, informational, ribosomal);

2) increased protein synthesis necessary for cell growth;

3) preparation of the cell for the synthetic period - the synthesis of enzymes necessary for the formation of new DNA molecules.

The synthetic period is characterized by doubling (reduplication) of DNA, which leads to a doubling of the ploidy of diploid nuclei and is a prerequisite for subsequent mitotic cell division.

The postsynthetic period is characterized by increased synthesis of messenger RNA and all cellular proteins, especially tubulins, necessary for the formation of the fission spindle.

The cells of some tissues (for example, hepatocytes), upon exiting mitosis, enter the so-called J0 period, during which they perform their numerous functions for a number of years without entering the synthetic period. Only under certain circumstances (when a part of the liver is damaged or removed) do they enter the normal cell cycle (or synthetic period), synthesizing DNA, and then mitotically divide. The life cycle of such rarely dividing cells can be represented as follows:

1) mitosis;

2) J1-period;

3) J0-period;

4) S-period;

5) J2-period.

Most of the cells of the nervous tissue, especially the neurons of the central nervous system, do not further divide after leaving mitosis in the embryonic period.

The life cycle of such cells consists of the following periods:

1) mitosis - I period;

2) growth - II period;

3) long-term functioning - III period;

4) aging - IV period;

5) death - V period.

Over a long life cycle, such cells constantly regenerate according to the intracellular type: protein and lipid molecules that make up various cellular structures are gradually replaced by new ones, i.e., cells are gradually renewed. During the life cycle, various, primarily lipid inclusions accumulate in the cytoplasm of nondividing cells, in particular lipofuscin, which is currently considered as an aging pigment.

Meiosis - a method of cell division, in which there is a decrease in the number of chromosomes in daughter cells by 2 times, is characteristic of germ cells. In this method of division, there is no DNA reduplication.

In addition to mitosis and meiosis, endoreproduction is also released, which does not lead to an increase in the number of cells, but contributes to an increase in the number of working structures and an increase in the functional ability of the cell.

This method is characterized by the fact that after mitosis, the cells first enter the J1- and then the S-period. However, such cells, after DNA duplication, do not enter the J2 period and then mitosis. As a result, the amount of DNA becomes doubled - the cell becomes polyploid. Polyploid cells can re-enter the S-period, as a result of which they increase their ploidy.

In polyploid cells, the size of the nucleus and cytoplasm increases, the cells become hypertrophied. Some polyploid cells enter mitosis after DNA replication, but it does not end with cytotomy, since such cells become binuclear.

Thus, during endoreproduction, there is no increase in the number of cells, but the amount of DNA and organelles increases, and, consequently, the functional ability of a polyploid cell.

Not all cells have the ability to endoreproduce. Endoreproduction is most characteristic for liver cells, especially with increasing age (for example, in old age, 80% of human hepatocytes are polyploid), as well as for acinar cells of the pancreas and bladder epithelium.

Cell response to external influence

This cell morphology is not stable and constant. When the body is exposed to various adverse environmental factors, various changes occur in the structure of the cell. Depending on the impact factors, the change in cellular structures occurs differently in the cells of different organs and tissues. At the same time, changes in cellular structures can be adaptive and reversible or maladaptive, irreversible (pathological). It is not always possible to determine the boundary between reversible and irreversible changes, since adaptive ones can turn into maladaptive ones with further action of the environmental factor.

Changes in the nucleus under the influence of environmental factors:

1) swelling of the nucleus and its displacement to the cell periphery;

2) expansion of the perinuclear space;

3) the formation of invaginations of the karyolemma (the invagination of individual sections of its membrane into the nucleus);

4) chromatin condensation;

5) pycnosis (wrinkling of the nucleus and compaction (coagulation of chromatin));

6) karyorrhexis (disintegration of the nucleus into fragments);

7) karyolysis (dissolution of the nucleus).

Changes in the cytoplasm:

1) thickening and then swelling of mitochondria;

2) degranulation of granular ER (desquamation of ribosomes and fragmentation of tubules into separate vacuoles);

3) expansion of cisterns and disintegration of the lamellar Golgi complex into vacuoles;

4) swelling of lysosomes and activation of their hydrolases;

5) increase in the number of autophagosomes;

6) disintegration of the fission spindle and the development of pathological mitosis during mitosis.

Changes in the cytoplasm may be due to:

1) structural changes in the plasmalemma, which leads to an increase in its permeability and hydration of the hyaloplasm;

2) metabolic disorders, which leads to a decrease in the content of ATP;

3) a decrease in splitting or an increase in the synthesis of inclusions (glycogen, lipids) and their excessive accumulation.

After the elimination of adverse environmental factors, adaptive changes in structures disappear and the cell morphology is completely restored. With the development of non-adaptive changes, even after the elimination of the action of adverse environmental factors, the changes continue to grow, and the cell dies.

Topic 6. GENERAL EMBRYOLOGY

Definition and components of embryology

Embryology is the science of the patterns of development of animal organisms from the moment of fertilization to birth (or hatching on eggs). Consequently, embryology studies the intrauterine period of development of an organism, that is, a part of ontogeny.

Ontogeny - the development of an organism from fertilization to death, is divided into two periods:

1) embryonic (embryogenesis);

2) postembryonic (postnatal).

The development of any organism is preceded by progenesis.

Progenesis includes:

1) gametogenesis - the formation of germ cells (spermatogenesis and ovogenesis);

2) fertilization.

Oocyte classification

The cytoplasm of most eggs contains inclusions - lecithin and yolk, the content and distribution of which differ significantly in different living organisms.

According to the content of lecithin, we can distinguish:

1) alecitary eggs (yellowless). This group includes helminth eggs;

2) oligolecytic (small yolk). Characteristic of the lancelet ovum;

3) polylecytic (multi-yolk). Inherent in the eggs of some birds and fish.

According to the distribution of lecithin in the cytoplasm, they distinguish:

1) isolecytic eggs. Lecithin is distributed evenly in the cytoplasm, which is typical for oligolecytic eggs;

2) telolecytic. The yolk is concentrated at one of the poles of the egg. Among telolecytic eggs, moderately telolecytic (characteristic of amphibians), sharply telolecytic (occur in fish and birds) and centrolecytic (their yolk is localized in the center, which is typical for insects) are distinguished.

A prerequisite for ontogenesis is the interaction of male and female germ cells, while fertilization occurs - the process of fusion of female and male germ cells (syngamy), as a result of which a zygote is formed.

Fertilization can be external (in fish and amphibians), while male and female germ cells go into the external environment, where they merge, and internal - (in birds and mammals), while spermatozoa enter the genital tract of the female body, into in which fertilization takes place.

Internal fertilization, unlike external, is a complex multi-phase process. After fertilization, a zygote is formed, the development of which continues with external fertilization in water, in birds - in an egg, and in mammals and humans - in the mother's body (womb).

Embryogenesis periods

Embryogenesis, according to the nature of the processes occurring in the embryo, is divided into three periods:

1) crushing period;

2) the period of gastrulation;

3) the period of histogenesis (formation of tissues), organogenesis (formation of organs), systemogenesis (formation of functional systems of the body).

Splitting up. The lifespan of a new organism in the form of a single cell (zygote) lasts in different animals from several minutes to several hours and even days, and then fragmentation begins. Cleavage is the process of mitotic division of the zygote into daughter cells (blastomeres). Cleavage differs from normal mitotic division in the following ways:

1) blastomeres do not reach the original size of the zygote;

2) blastomeres do not diverge, although they are independent cells.

There are the following types of crushing:

1) complete, incomplete;

2) uniform, uneven;

3) synchronous, asynchronous.

The eggs and the zygotes formed after their fertilization, containing a small amount of lecithin (oligolecithal), evenly distributed in the cytoplasm (isolecithal), are completely divided into two daughter cells (blastomeres) of equal size, which then simultaneously (synchronously) divide again into blastomeres. This type of crushing is complete, uniform and synchronous.

Oocytes and zygotes containing a moderate amount of yolk are also completely crushed, but the resulting blastomeres are of different sizes and are not crushed simultaneously - crushing is complete, uneven, asynchronous.

As a result of crushing, an accumulation of blastomeres is first formed, and the embryo in this form is called a morula. Then, fluid accumulates between the blastomeres, which pushes the blastomeres to the periphery, and a cavity filled with fluid is formed in the center. At this stage of development, the embryo is called a blastula.

Blastula consists of:

1) blastoderm - shells of blastomeres;

2) blastocele - a cavity filled with fluid.

The human blastula is a blastocyst. After the formation of the blastula, the second stage of embryogenesis begins - gastrulation.

Gastrulation is the process of formation of germ layers, which are formed through the reproduction and movement of cells. The process of gastrulation in different animals proceeds differently. There are the following types of gastrulation:

1) delamination (splitting of the accumulation of blastomeres into plates);

2) immigration (movement of cells into the developing embryo);

3) invagination (invagination of a layer of cells into the embryo);

4) epiboly (fouling of slowly dividing blastomeres with rapidly dividing ones with the formation of an outer layer of cells).

As a result of gastrulation, three germ layers are formed in the embryo of any animal species:

1) ectoderm (outer germ layer);

2) endoderm (inner germ layer);

3) mesoderm (middle germ layer).

Each germ layer is a separate layer of cells. Between the sheets, there are initially slit-like spaces, into which process cells soon migrate, forming together the germinal mesenchyme (some authors consider it as the fourth germinal layer).

The germinal mesenchyme is formed by the eviction of cells from all three germ layers, mainly from the mesoderm. The embryo, consisting of three germ layers and mesenchyme, is called the gastrula. The process of gastrulation in the embryos of different animals differs significantly both in terms of methods and time. The germ layers and mesenchyme formed after gastrulation contain presumptive (presumptive) tissue rudiments. After this, the third stage of embryogenesis begins - histo- and organogenesis.

Histo- and organogenesis (or differentiation of germ layers) is a process of transformation of tissue rudiments into tissues and organs, and then the formation of functional systems of the body.

Histo- and organogenesis is based on the following processes: mitotic division (proliferation), induction, determination, growth, migration and differentiation of cells. As a result of these processes, axial rudiments of organ complexes (notochord, neural tube, intestinal tube, mesodermal complexes) are first formed. At the same time, various tissues are gradually formed, and from the combination of tissues, anatomical organs are laid down and develop, uniting into functional systems - digestive, respiratory, reproductive, etc. At the initial stage of histo- and organogenesis, the embryo is called the embryo, which later turns into a fetus.

At present, it has not been finally established how from one cell (zygote), and later from identical germ layers, cells completely different in morphology and function are formed, and from them - tissues (epithelial tissues, horny scales, nerve cells and glial cells). Presumably, genetic mechanisms play a leading role in these transformations.

The concept of the genetic basis of histo- and organogenesis

After the egg is fertilized by the sperm, a zygote is formed. It contains genetic material, consisting of maternal and paternal genes, which are then transferred during division to daughter cells. The sum of all the genes of the zygote and the cells formed from it constitutes the genome that is characteristic only for this type of organism, and the features of the combination of maternal and paternal genes in a given individual constitute its genotype. Consequently, any cell that is formed from a zygote contains genetic material of the same quantity and quality, i.e., the same genome and genotype (the only exceptions are germ cells, they contain half the genome set).

In the process of gastrulation and after the formation of germ layers, cells located in different sheets or in different parts of the same germ layer influence each other. This influence is called induction. Induction is carried out by isolating chemicals (proteins), but there are also physical methods of induction. Induction affects primarily the cell genome. As a result of induction, some genes of the cellular genome are blocked, i.e., they become inoperative, transcription of various RNA molecules is not performed from them, therefore, protein synthesis is not carried out either. As a result of induction, some genes are blocked, while others are free - working. The sum of the free genes of a given cell is called its epigen. The very process of epigenome formation, i.e., the interaction of induction and genome, is called determination. After the formation of the epigenome, the cell becomes determined, i.e. programmed to develop in a certain direction.

The sum of cells located in a certain area of ​​the germ layer and having the same epigenome is the presumptive rudiments of a certain tissue, since all these cells will differentiate in the same direction and become part of this tissue.

The process of cell determination in different parts of the germ layers occurs at different times and can proceed in several stages. The formed epigenome is stable and after mitotic division is transferred to daughter cells.

After cell determination, i.e., after the final formation of the epigenome, differentiation begins - the process of morphological, biochemical and functional specialization of cells.

This process is provided by transcription from active genes determined by RNA, and then the synthesis of certain proteins and non-protein substances is carried out, which determine the morphological, biochemical and functional specialization of cells. Some cells (for example, fibroblasts) form an intercellular substance.

Thus, the formation of cells with different structure and functions from cells containing the same genome and genotype can be explained by the process of induction and the formation of cells with different epigenomes, which then differentiate into cells of different populations.

Extra-embryonic (provisional) organs

Part of the blastomeres and cells after crushing the zygote goes to the formation of organs that contribute to the development of the embryo and fetus. Such organs are called extra-embryonic.

After birth, some extra-embryonic organs are rejected, while others in the last stages of embryogenesis undergo reverse development or are rebuilt. Different animals develop an unequal number of provisional organs that differ in structure and function.

Mammals, including humans, develop four extra-embryonic organs:

1) chorion;

2) amnion;

3) yolk sac;

4) allantois.

The chorion (or villous membrane) performs protective and trophic functions. Part of the chorion (villous chorion) is introduced into the mucous membrane of the uterus and is part of the placenta, which is sometimes considered as an independent organ.

Amnion (or water shell) is formed only in terrestrial animals. Amnion cells produce amniotic fluid (amniotic fluid), in which the embryo develops, and then the fetus.

After the baby is born, the chorionic and amniotic membranes are shed.

The yolk sac develops to the greatest extent in embryos formed from polylecithal cells, and therefore contains a lot of yolk, hence its name. The yolk tag performs the following functions:

1) trophic (due to the trophic inclusion (yolk), the embryo is nourished, especially developing in the egg, at later stages of development, the yolk circle of blood circulation is formed to deliver trophic material to the embryo);

2) hematopoietic (in the wall of the yolk sac (in the mesenchyme) the first blood cells are formed, which then migrate to the hematopoietic organs of the embryo);

3) gonoblastic (primary germ cells (gonoblasts) are formed in the wall of the yolk sac (in the endoderm), which then migrate to the anlage of the sex glands of the embryo).

Allantois - blind protrusion of the caudal end of the intestinal tube, surrounded by extra-embryonic mesenchyme. In animals developing in the egg, the allantois reaches a great development and acts as a reservoir for the metabolic products of the embryo (mainly urea). That is why allantois is often called the urinary sac.

In mammals, there is no need for the accumulation of metabolic products, since they enter the mother's body through the uteroplacental bloodstream and are excreted by her excretory organs. Therefore, in such animals and humans, allantois is poorly developed and performs other functions: umbilical vessels develop in its wall, which branch out in the placenta and due to which the placental circulation is formed.

Topic 7. HUMAN EMBRYOLOGY

Progenesis

Consideration of patterns of embryogenesis begins with progenesis. Progenesis - gametogenesis (spermatogenesis and ovogenesis) and fertilization.

Spermatogenesis is carried out in the convoluted tubules of the testes and is divided into four periods:

1) breeding period - I;

2) growth period - II;

3) ripening period - III;

4) period of formation - IV.

The process of spermatogenesis will be considered in detail when studying the male reproductive system. The human spermatozoon consists of two main parts: the head and the tail.

The head contains:

1) nucleus (with a haploid set of chromosomes);

2) case;

3) acrosome;

4) a thin layer of cytoplasm surrounded by a cytolemma.

The tail of the spermatozoon is divided into:

1) liaison department;

2) intermediate department;

3) main department;

4) terminal department.

The main functions of the spermatozoon are the storage and transfer of genetic information to the eggs during their fertilization. The fertilizing ability of spermatozoa in the female genital tract lasts up to 2 days.

Ovogenesis is carried out in the ovaries and is divided into three periods:

1) the period of reproduction (in embryogenesis and during the 1st year of postembryonic development);

2) a period of growth (small and large);

3) maturation period.

The egg cell consists of a nucleus with a haploid set of chromosomes and a pronounced cytoplasm, which contains all organelles, with the exception of the cytocenter.

Shells of the egg:

1) primary (plasmolemma);

2) secondary - shiny shell;

3) tertiary - radiant crown (layer of follicular cells).

Fertilization in humans is internal - in the distal part of the fallopian tube.

It is divided into three phases:

1) remote interaction;

2) contact interaction;

3) penetration and fusion of pronuclei (syncaryon phase).

Three mechanisms underlie the remote interaction:

1) rheotaxis - the movement of spermatozoa against the flow of fluid in the uterus and fallopian tube;

2) chemotaxis - directed movement of spermatozoa to the egg, which releases specific substances - gynogamones;

3) canacitation - activation of spermatozoa by gynogamones and the hormone progesterone.

After 1,5 - 2 hours, the spermatozoa reach the distal part of the fallopian tube and come into contact with the egg.

The main point of the contact interaction is the acrosomal reaction - the release of enzymes (trypsin and hyaluronic acid) from sperm acrosomes. These enzymes provide:

1) separation of the follicular cells of the radiant crown from the egg;

2) gradual but incomplete destruction of the zona pellucida.

When one of the spermatozoa reaches the plasmolemma of the egg, a small protrusion forms in this place - the fertilization tubercle. After that, the penetration phase begins. In the region of the tubercle of the plasmolemma, the egg and sperm merge, and part of the sperm (the head, connecting and intermediate sections) is in the cytoplasm of the egg. The plasmolemma of the sperm is integrated into the plasmolemma of the egg. After this, a cortical reaction begins - the release of cortical granules from the egg by the type of exocytosis, which merge between the plasma membrane of the egg and the remains of the zona pellucida, harden and form a fertilization membrane that prevents other spermatozoa from penetrating the egg. Thus, in mammals and humans, monospermy is ensured.

The main event of the penetration phase is the introduction into the cytoplasm of the egg of the genetic material of the spermatozoa, as well as the cytocenter. This is followed by swelling of the male and female pronuclei, their convergence, and then fusion - the synacryon. Simultaneously in the cytoplasm, the movements of the contents of the cytoplasm and the isolation (segregation) of its individual sections begin. This is how presumptive (presumptive) rudiments of future tissues are formed - the stage of tissue differentiation passes.

Conditions necessary for the fertilization of an egg:

1) the content of at least 150 million spermatozoa in the ejaculate, with a concentration of at least 1 million in 60 ml;

2) patency of the female genital tract;

3) normal anatomical position of the uterus;

4) normal body temperature;

5) alkaline environment in the female genital tract.

From the moment of the fusion of the pronuclei, a zygote is formed - a new unicellular organism. The lifetime of the zygote organism is 24-30 hours. From this period, ontogenesis begins and its first stage is embryogenesis.

Embryogenesis

Human embryogenesis is divided (in accordance with the processes occurring in it) into:

1) crushing period;

2) the period of gastrulation;

3) the period of histo- and organogenesis.

In obstetrics, embryogenesis is divided into other periods:

1) initial period - 1st week;

2) the embryonic period (or the period of the embryo) - 2 - 8 weeks;

3) the fetal period - from the 9th week until the end of embryogenesis.

I. The period of crushing. Crushing in humans is complete, uneven, asynchronous. Blastomeres are of unequal size and are divided into two types: dark large and light small. Large blastomeres are less frequently divided, located about the center and constitute the embryoblast. Small blastomeres are more often crushed, located on the periphery of the embryoblast, and subsequently form a trophoblast.

The first cleavage begins approximately 30 hours after fertilization. The plane of the first division passes through the region of the guide bodies. Since the yolk is evenly distributed in the zygote, it is extremely difficult to isolate the animal and vegetative poles. The region of separation of the directional bodies is usually called the animal pole. After the first crushing, two blastomeres are formed, somewhat different in size.

Second crush. The formation of the second mitotic spindle in each of the resulting blastomeres occurs shortly after the end of the first division, the plane of the second division runs perpendicular to the plane of the first crushing. In this case, the conceptus passes into the stage of 4 blastomeres. However, cleavage in humans is asynchronous, so a 3-cell conceptus can be observed for some time. At stage 4 of the blastomeres, all major types of RNA are synthesized.

Third crush. At this stage, the asynchrony of cleavage is manifested to a greater extent, as a result, a conceptus is formed with a different number of blastomeres, while it can be conditionally divided into 8 blastomeres. Prior to this, the blastomeres are located loosely, but soon the concept becomes denser, the contact surface of the blastomeres increases, and the volume of the intercellular space decreases. As a result, convergence and compaction are observed, which is an extremely important condition for the formation of tight and slit-like contacts between blastomeres. Before formation, uvomorulin, a cell adhesion protein, begins to integrate into the plasma membrane of blastomeres. In blastomeres of early conceptus, uvomorulin is evenly distributed in the cell membrane. Later, accumulations (clusters) of uvomorulin molecules form in the area of ​​intercellular contacts.

On the 3rd - 4th day, a morula is formed, consisting of dark and light blastomeres, and from the 4th day begins the accumulation of fluid between the blastomeres and the formation of a blastula, which is called a blastocyst.

The developed blastocyst consists of the following structural formations:

1) embryoblasts;

2) trophoblasts;

3) blastocele filled with liquid.

Cleavage of the zygote (formation of the morula and blastocyst) is carried out in the process of slow movement of the embryo through the fallopian tube to the body of the uterus.

On the 5th day, the blastocyst enters the uterine cavity and is in it in a free state, and from the 7th day, the blastocyst implants into the uterine mucosa (endometrium). This process is divided into two phases:

1) the phase of adhesion - adhesion to the epithelium;

2) the phase of invasion - penetration into the endometrium.

The whole process of implantation occurs on the 7th - 8th day and lasts for 40 hours.

The introduction of the embryo is carried out by destroying the epithelium of the uterine mucosa, and then the connective tissue and walls of the endometrial vessels with proteolytic enzymes, which are secreted by the blastocyst trophoblast. In the process of implantation, the histiotrophic type of nutrition of the embryo changes to hemotrophic.

On the 8th day, the embryo is completely immersed in its own plate of the uterine mucosa. At the same time, the defect in the epithelium of the area of ​​implementation of the embryo overgrows, and the embryo is surrounded on all sides by gaps (or cavities) filled with maternal blood pouring out of the destroyed vessels of the endometrium. In the process of embryo implantation, changes occur both in the trophoblast and in the embryoblast, where gastrulation occurs.

II. Gastrulation in humans is divided into two phases. The first headlight of gastrulation occurs on the 7th - 8th day (in the process of implantation) and is carried out by the method of delamination (an epiblast, hypoblast is formed).

The second phase of gastrulation occurs from the 14th to the 17th day. Its mechanism will be discussed later.

In the period between I and II phases of gastrulation, i.e., from the 9th to the 14th day, an extraembryonic mesenchyme and three extraembryonic organs are formed - the chorion, amnion, yolk sac.

Development, structure and functions of the chorion. In the process of implantation of the blastocyst, its trophoblast, as it penetrates, from a single layer becomes a two-layer one and consists of a cytotrophoblast and a sympathotrophoblast. Sympathotrophoblast is a structure in which a single cytoplasm contains a large number of nuclei and cell organelles. It is formed by the fusion of cells pushed out of the cytotrophoblast. Thus, the embryoblast, in which the first phase of gastrulation occurs, is surrounded by an extra-embryonic membrane, consisting of cyto- and symplastotrophoblast.

In the process of implantation, cells are evicted from the embryoblast into the cavity of the blastocyst, forming an extra-embryonic mesenchyme, which grows from the inside to the cytotrophoblast.

After that, the trophoblast becomes three-layered - it consists of a symplastotrophoblast, a cytotrophoblast and a parental leaf of the extraembryonic mesenchyme and is called the chorion (or villous membrane). Over the entire surface of the chorion, villi are located, which initially consist of cyto- and symplastotrophoblast and are called primary. Then the extra-embryonic mesenchyme grows into them from the inside, and they become secondary. However, gradually, on most of the chorion, the villi are reduced and are preserved only in that part of the chorion that is directed to the basal layer of the endometrium. At the same time, the villi grow, vessels grow into them, and they become tertiary.

During the development of the chorion, two periods are distinguished:

1) the formation of a smooth chorion;

2) the formation of the villous chorion.

The placenta is subsequently formed from the villous chorion.

Chorionic functions:

1) protective;

2) trophic, gas exchange, excretory and others, in which chorine takes part, being an integral part of the placenta and which the placenta performs.

Development, structure and functions of the amnion. The extraembryonic mesenchyme, filling the cavity of the blastocyst, leaves free small areas of the blastocoel adjacent to the epiblast and hypoblast. These areas make up the mesenchymal anlage of the amniotic vesicle and yolk sac.

The amnion wall consists of:

1) extra-embryonic ectoderm;

2) extra-embryonic mesenchyme (visceral layer).

The functions of the amnion are the formation of amniotic fluid and a protective function.

Development, structure and functions of the yolk sac. The cells that make up the extraembryonic (or yolk) endoderm are evicted from the hypoblast, and, growing from the inside of the mesenchymal anlage of the yolk sac, form the wall of the yolk sac along with it. The wall of the yolk sac is composed of:

1) extra-embryonic (yolk) endoderm;

2) extra-embryonic mesenchyme.

Functions of the yolk sac:

1) hematopoiesis (formation of blood stem cells);

2) the formation of sex stem cells (gonoblasts);

3) trophic (in birds and fish).

Development, structure and functions of allantois. Part of the germinal endoderm of the hypoblast in the form of a finger-like protrusion grows into the mesenchyme of the amniotic stalk and forms the allantois. The allantois wall consists of:

1) germinal endoderm;

2) extra-embryonic mesenchyme.

Functional role of allantois:

1) in birds, the allantois cavity reaches a significant development and urea accumulates in it, therefore it is called the urinary sac;

2) a person does not need to accumulate urea, therefore the allantois cavity is very small and completely overgrown by the end of the 2nd month.

However, blood vessels develop in the mesenchyme of the allantois, which connect with the vessels of the body of the embryo at their proximal ends (these vessels appear in the mesenchyme of the body of the embryo later than in the allantois). With their distal ends, the allantois vessels grow into the secondary villi of the villous part of the chorion and turn them into tertiary ones. From the 3rd to the 8th week of intrauterine development, due to these processes, the placental circle of blood circulation is formed. The amniotic leg, together with the vessels, is pulled out and turns into the umbilical cord, and the vessels (two arteries and a vein) are called umbilical vessels.

The mesenchyme of the umbilical cord is transformed into a mucous connective tissue. The umbilical cord also contains the remains of allantois and the yolk stalk. The function of allantois is to contribute to the performance of the functions of the placenta.

At the end of the second stage of gastrulation, the embryo is called gastrula and consists of three germ layers - ectoderm, mesoderm and endoderm and four extraembryonic organs - chorion, amnion, yolk sac and allantois.

Simultaneously with the development of the second phase of gastrulation, the germinal mesenchyme is formed by cell migration from all three germ layers.

On the 2nd - 3rd week, i.e., during the second phase of gastrulation and immediately after it, the rudiments of axial organs are laid:

1) chords;

2) neural tube;

3) intestinal tube.

The structure and functions of the placenta

The placenta is a formation that provides a link between the fetus and the mother's body.

The placenta consists of the maternal part (basal part of the decidua) and the fetal part (villous chorion - a derivative of the trophoblast and extraembryonic mesoderm).

Functions of the placenta:

1) the exchange between the organisms of the mother and fetus of metabolite gases, electrolytes. The exchange is carried out using passive transport, facilitated diffusion and active transport. Sufficiently freely, steroid hormones can pass into the body of the fetus from the mother;

2) the transport of maternal antibodies, which is carried out with the help of receptor-mediated endocytosis and provides passive immunity to the fetus. This function is very important, since after birth the fetus has passive immunity to many infections (measles, rubella, diphtheria, tetanus, etc.), which the mother either had or was vaccinated against. The duration of passive immunity after birth is 6-8 months;

3) endocrine function. The placenta is an endocrine organ. It synthesizes hormones and biologically active substances that play a very important role in the normal physiological course of pregnancy and fetal development. These substances include progesterone, human chorionic somatomammotropin, fibroblast growth factor, transferrin, prolactin, and relaxin. Corticoliberins determine the term of childbirth;

4) detoxification. The placenta helps detoxify some drugs;

5) placental barrier. The placental barrier includes syncytiotrophoblast, cytotrophoblast, basement membrane of the trophoblast, connective tissue of the villi, basement membrane in the wall of the fetal capillary, endothelium of the fetal capillary. The hematoplacental barrier prevents the contact of the blood of the mother and the fetus, which is very important for protecting the fetus from the influence of the mother's immune system.

The structural and functional unit of the formed placenta is cotyledon. It is formed by the stem villus and its branches containing the vessels of the fetus. By the 140th day of pregnancy, about 10-12 large, 40-50 small and up to 150 rudimentary cotyledons have been formed in the placenta. By the 4th month of pregnancy, the formation of the main structures of the placenta ends. The lacunae of a fully formed placenta contain about 150 ml of maternal blood, which is completely exchanged within 3-4 minutes. The total surface of the villi is about 15 m², which ensures a normal level of metabolism between the organisms of the mother and fetus.

The structure and functions of the decidua

The decidua is formed throughout the endometrium, but first of all it is formed in the area of ​​implantation. By the end of the 2nd week of intrauterine development, the endometrium is completely replaced by the decidua, in which the basal, capsular, and parietal parts can be distinguished.

The decidua surrounding the chorion contains the basal and capsular parts.

Other sections of the decidua are lined with parietal part. Spongy and compact zones are distinguished in the decidua.

The basal part of the decidua is part of the placenta. It separates the ovum from the myometrium. In the spongy layer there are many glands that persist until the 6th month of pregnancy.

By the 18th day of pregnancy, the capsular part completely closes over the implanted fetal egg and separates it from the uterine cavity. As the fetus grows, the capsular part protrudes into the uterine cavity and fuses with the parietal part by the 16th week of intrauterine development. In full-term pregnancy, the capsular part is well preserved and is distinguishable only in the lower pole of the fetal egg - above the internal uterine os. The capsular part does not contain surface epithelium.

The parietal part until the 15th week of pregnancy thickens due to the compact and spongy zones. In the spongy zone of the parietal part of the decidua, the glands develop until the 8th week of pregnancy. By the time the parietal and capsular parts merge, the number of glands gradually decreases, they become indistinguishable.

At the end of full-term pregnancy, the parietal part of the decidua is represented by several layers of decidua cells. From the 12th week of pregnancy, the surface epithelium of the parietal part disappears.

Loose connective tissue cells around the vessels of the compact zone are sharply enlarged. These are young decidual cells, which are similar in structure to fibroblasts. As differentiation proceeds, the size of decidual cells increases, they acquire a rounded shape, their nuclei become light, and the cells are more closely adjacent to each other. By the 4th - 6th week of pregnancy, large light decidual cells predominate. Some decidual cells are of bone marrow origin: apparently, they are involved in the immune response.

The function of decidual cells is the production of prolactin and prostaglandins.

III. mesoderm differentiation. In each mesodermal plate, it differentiates into three parts:

1) dorsal part (somites);

2) intermediate part (segmental legs, or nephrotomes);

3) ventral part (splanchiotoma).

The dorsal part thickens and is subdivided into separate sections (segments) - somites. In turn, three zones are distinguished in each somite:

1) peripheral zone (dermatome);

2) central zone (myotoma);

3) medial part (sclerotoma).

Trunk folds form on the sides of the embryo, which separate the embryo from extraembryonic organs.

Due to the trunk folds, the intestinal endoderm folds into the primary intestine.

The intermediate part of each mesodermal wing is also segmented (with the exception of the caudal section - nephrogenic tissue) into segmented legs (or nephrotomes, nephrogonotomes).

The ventral part of each mesodermal wing is not segmented. It splits into two sheets, between which there is a cavity - the whole, and is called the "splanchiotoma". Therefore, the splanchiotome consists of:

1) visceral leaf;

2) parental sheet;

3) cavities - coelom.

IV. differentiation of the ectoderm. The outer germ layer differentiates into four parts:

1) neuroectoderm (from it the neural tube and ganglionic plate are kneaded);

2) skin ectoderm (skin epidermis develops);

3) transitional plastic (the epithelium of the esophagus, trachea, bronchi develops);

4) placodes (auditory, lens, etc.).

V. Endoderm differentiation. The inner germ layer is subdivided into:

1) intestinal (or germinal), endoderm;

2) extra-embryonic (or yolk), endoderm.

From the intestinal endoderm develop:

1) epithelium and glands of the stomach and intestines;

2) liver;

3) pancreas.

Organogenesis

The development of the vast majority of organs begins from the 3rd - 4th week, i.e. from the end of the 1st month of the existence of the embryo. Organs are formed as a result of the movement and combination of cells and their derivatives, several tissues (for example, the liver consists of epithelial and connective tissues). At the same time, cells of different tissues have an inductive effect on each other and thus provide directed morphogenesis.

Critical periods in human development

In the process of development of a new organism, there are such periods when the whole organism or its individual cells, organs and their systems are the most sensitive to exogenous and endogenous environmental factors. It is customary to call such periods critical, since it is at this time that changes can occur in them, which in the future will lead to disruption of normal development and the formation of anomalies - violations of the normal anatomical structure of organs without violating their functions, defects - violations of the anatomical structure of organs with violation of their functions. functions, deformities - pronounced anatomical violations of the structure of organs, with a violation of their functions, often incompatible with life.

The critical periods in human development are as follows:

1) gametogenesis (spermato- and ovogenesis);

2) fertilization;

3) implantation (7 - 8 days);

4) placentation and laying of axial complexes (3rd - 8th week);

5) stage of enhanced brain growth (15-20 weeks);

6) formation of the reproductive apparatus and other functional systems (20 - 24 weeks);

7) the birth of a child;

8) neonatal period (up to 1 year);

9) puberty (11 - 16 years).

In embryogenesis, critical periods for certain groups of cells occur when the epigenome is formed and determination is carried out, which determines the further differentiation of cells in a certain direction and the formation of organs and tissues. It is during this period that various chemical and physical influences can lead to a disruption in the formation of the natural epigenome, i.e., to the formation of a new one, which determines cells to develop in a new, unusual direction, leading to the development of anomalies, defects and deformities.

Adverse factors include smoking, alcohol intake, drug addiction, harmful substances contained in the air, drinking water, food, and some medications. Currently, due to the environmental situation, the number of newborns with various above-mentioned deviations is increasing.

Topic 8. GENERAL PRINCIPLES OF TISSUE ORGANIZATION

Tissue is a historically (phylogenetically) established system of cells and non-cellular structures that has a common structure, and sometimes origin, and is specialized in performing certain functions. Tissue is a new (after cells) level of organization of living matter.

Structural components of tissue: cells, cell derivatives, intercellular substance.

Characterization of the structural components of the tissue

Cells are the main, functionally leading components of tissues. Almost all tissues are composed of several types of cells. In addition, cells of each type in tissues can be at different stages of maturity (differentiation). Therefore, in tissue, such concepts as cell population and cell differon are distinguished.

A cell population is a collection of cells of a given type. For example, loose connective tissue (the most common in the body) contains:

1) population of fibroblasts;

2) population of macrophages;

3) population of tissue basophils, etc.

Cellular differon (or histogenetic series) is a collection of cells of a given type (a given population) that are at different stages of differentiation. The initial cells of differon are stem cells, followed by young (blast) cells, maturing cells and mature cells. Distinguish between complete differon or incomplete, depending on whether there are cells of all types of development in the tissues.

However, tissues are not just an accumulation of various cells. Cells in tissues are in a certain relationship, and the function of each of them is aimed at performing the function of the tissue.

Cells in tissues influence each other either directly through gap-like junctions (nexuses) and synapses, or at a distance (remotely) through the release of various biologically active substances.

Cell derivatives:

1) symplasts (fusion of individual cells, for example, muscle fiber);

2) syncytium (several cells interconnected by processes, for example, the spermatogenic epithelium of the convoluted tubules of the testis);

3) postcellular formations (erythrocytes, platelets).

The intercellular substance is also a product of the activity of certain cells. The intercellular substance consists of:

1) an amorphous substance;

2) fibers (collagen, reticular, elastic).

The intercellular substance is not equally expressed in different tissues.

Development of tissues in ontogenesis (embryogenesis) and phylogenesis

In ontogenesis, the following stages of tissue development are distinguished:

1) the stage of orthotopic differentiation. At this stage, the rudiments of future specific tissues are localized first in certain areas of the egg and then in the zygote;

2) stage of blastomeric differentiation. As a result of zygote cleavage, presumptive (supposed) tissue rudiments are localized in different blastomeres of the embryo;

3) the stage of rudimentary differentiation. As a result of gastrulation, presumptive tissue rudiments are localized in certain areas of the germ layers;

4) histogenesis. This is the process of transformation of the rudiments of tissues and tissues as a result of proliferation, growth, induction, determination, migration and differentiation of cells.

There are several theories of tissue development in phylogenesis:

1) the law of parallel series (A. A. Zavarzin). Animal and plant tissues of different species and classes that perform the same functions have a similar structure, that is, they develop in parallel in animals of different phylogenetic classes;

2) the law of divergent evolution (N. G. Khlopin). In phylogeny, there is a divergence of tissue characteristics and the emergence of new tissue varieties within the tissue group, which leads to the complication of animal organisms and the emergence of a variety of tissues.

Fabric classifications

There are several approaches to the classification of tissues. The morphofunctional classification is generally accepted, according to which four tissue groups are distinguished:

1) epithelial tissues;

2) connective tissues (tissues of the internal environment, musculoskeletal tissues);

3) muscle tissue;

4) nervous tissue.

Tissue homeostasis (or maintaining the structural constancy of tissues)

The state of the structural components of tissues and their functional activity are constantly changing under the influence of external factors. First of all, rhythmic fluctuations in the structural and functional state of tissues are noted: biological rhythms (daily, weekly, seasonal, annual). External factors can cause adaptive (adaptive) and maladaptive changes, leading to the disintegration of tissue components. There are regulatory mechanisms (interstitial, intertissue, organismal) that ensure the maintenance of structural homeostasis.

Interstitial regulatory mechanisms are provided, in particular, by the ability of mature cells to secrete biologically active substances (keylons), which inhibit the reproduction of young (stem and blast) cells of the same population. With the death of a significant part of mature cells, the release of chalones decreases, which stimulates proliferative processes and leads to the restoration of the number of cells in this population.

Interstitial regulatory mechanisms are provided by inductive interaction, primarily with the participation of lymphoid tissue (immune system) in maintaining structural homeostasis.

Organismic regulatory factors are provided by the influence of the endocrine and nervous systems.

Under some external influences, the natural determination of young cells can be disrupted, which can lead to the transformation of one tissue type into another. This phenomenon is called "metaplasia" and occurs only within a given tissue group. For example, the replacement of a single-layer prismatic epithelium of the stomach with a single-layer flat.

Tissue regeneration

Regeneration is the restoration of cells, tissues and organs, aimed at maintaining the functional activity of this system. In regeneration, such concepts as the form of regeneration, the level of regeneration, and the method of regeneration are distinguished.

Forms of regeneration:

1) physiological regeneration - restoration of tissue cells after their natural death (for example, hematopoiesis);

2) reparative regeneration - restoration of tissues and organs after their damage (trauma, inflammation, surgical interventions, etc.).

Regeneration levels:

1) cellular (intracellular);

2) tissue;

3) organ.

Regeneration methods:

1) cellular;

2) intracellular;

3) substitution.

Factors regulating regeneration:

1) hormones;

2) mediators;

3) keylons;

4) growth factors, etc.

Tissue Integration

Tissues, being one of the levels of organization of living matter, are part of the structures of a higher level of organization of living matter - the structural and functional units of organs and the composition of organs in which integration (combination) of several tissues occurs.

Integration mechanisms:

1) intertissue (usually inductive) interactions;

2) endocrine influences;

3) nervous influences.

For example, the composition of the heart includes cardiac muscle tissue, connective tissue, epithelial tissue.

Topic 9. EPITHELIAL TISSUES

Characterization of epithelial tissues

They form the outer and inner layers of the body.

Functions of the epithelium:

1) protective (barrier);

2) secretory;

3) excretory;

4) suction.

Structural and functional features of epithelial tissues:

1) arrangement of cells in layers;

2) location of cells on the basement membrane;

3) the predominance of cells over the intercellular substance;

4) polar differentiation of cells (to the basal and apical poles);

5) absence of blood and lymphatic vessels;

6) high ability of cells to regenerate.

Structural components of epithelial tissue:

1) epithelial cells (epithelial cells);

2) basement membrane.

Epitheliocytes are the main structural elements of epithelial tissues.

The basement membrane (about 1 µm thick) consists of:

1) thin collagen fibrils (from collagen protein of the fourth type);

2) an amorphous substance (matrix) consisting of a carbohydrate-protein-lipid complex.

Basement membrane functions:

1) barrier (separation of the epithelium from the connective tissue);

2) trophic (diffusion of nutrients and metabolic products from the underlying connective tissue and back);

3) organizing (attachment of epitheliocytes with the help of hemidesmosomes).

Classification of epithelial tissues

There are the following types of epithelium:

1) integumentary epithelium;

2) glandular epithelium.

Genetic classification of epithelia (according to N. G. Khlopin):

1) epidermal type (develops from the ectoderm);

2) enterodermal type (develops from the endoderm);

3) whole nephrodermal type (develops from the mesoderm);

4) ependymoglial type (develops from neuroectoderm);

5) angiodermal type (or vascular endothelium developing from the mesenchyme).

Topographic classification of the epithelium:

1) skin type (skin epidermis);

2) gastrointestinal;

3) renal;

4) hepatic;

5) respiratory;

6) vascular (vascular endothelium);

7) epithelium of serous cavities (peritoneum, pleura, pericardium).

The glandular epithelium forms most of the body's glands. Consists of glandular cells (glandulocytes) and basement membrane.

Gland classification

By number of cells:

1) unicellular (goblet gland);

2) multicellular (the vast majority of glands).

According to the location of cells in the epithelial layer:

1) endoepithelial (goblet gland);

2) exoepithelial.

By the method of removing the secret from the gland and by structure:

1) exocrine glands (have an excretory duct);

2) endocrine glands (do not have excretory ducts and secrete (hormones) into the blood or lymph).

According to the method of secretion from the glandular cell:

1) merocrine;

2) apocrine;

3) holocrine.

According to the composition of the allocated secret:

1) protein (serous);

2) mucous membranes;

3) mixed (protein-mucous);

4) sebaceous.

By structure:

1) are simple;

2) complex;

3) branched;

4) unbranched.

Phases of the secretory cycle of glandular cells

There are the following phases of the secretory cycle of glandular cells:

1) absorption of the initial products of secretion;

2) synthesis and accumulation of the secret;

3) secretion (according to the merocrine or apocrine type);

4) restoration of the glandular cell.

Topic 10. BLOOD AND LYMPH

Characteristics and composition of blood

Blood is a tissue or one of the types of connective tissues.

The blood system includes the following components:

1) blood and lymph;

2) organs of hematopoiesis and immunopoiesis;

3) blood cells that have moved out of the blood into the connective and epithelial tissues and are able to return (recycle) back into the bloodstream (lymphocytes).

Blood, lymph and loose unformed connective tissue make up the internal environment of the body.

Blood functions:

1) transport. This function of the blood is extremely diverse. Blood carries out the transfer of gases (due to the ability of hemoglobin to bind oxygen and carbon dioxide), various nutrients and biologically active substances;

2) trophic. Nutrients enter the body with food, then are broken down in the gastrointestinal tract to proteins, fats and carbohydrates, absorbed and carried by the blood to various organs and tissues;

3) respiratory. Carried out in the form of transport of oxygen and carbon dioxide. Hemoglobin oxygenated in the lungs (oxyhemoglobin) is delivered by blood through the arteries to all organs and tissues where gas exchange (tissue respiration) occurs, oxygen is consumed for aerobic processes, and carbon dioxide is bound by blood hemoglobin (carboxyhemoglobin) and is delivered to the lungs through the venous blood flow, where it again occurs oxygenation;

4) protective. There are cells and systems in the blood that provide non-specific (complement system, phagocytes, NK cells) and specific (T- and B-systems of immunity) protection;

5) excretory. The blood removes the decay products of macromolecules (urea and creatinine are excreted by the kidneys with urine).

Together, these functions provide homeostasis (the constancy of the internal environment of the body).

Components of blood:

1) cells (shaped elements);

2) liquid intercellular substance (blood plasma).

The ratio of blood parts: plasma - 55 - 60%, formed elements - 40 - 45%.

Blood plasma consists of:

1) water (90 - 93%);

2) substances contained in it (7 - 10%).

Plasma contains proteins, amino acids, nucleotides, glucose, minerals, metabolic products.

Plasma proteins:

1) albumins;

2) globulins (including immunoglobulins);

3) fibrinogen;

4) enzyme proteins, etc.

The function of plasma is the transport of soluble substances.

Due to the fact that the blood contains both true cells (leukocytes) and post-cellular formations (erythrocytes and platelets), in the aggregate it is customary to refer to them collectively as formed elements.

Qualitative and quantitative composition of blood (blood test) - hemogram and leukocyte formula.

Hemogram of an adult:

1) erythrocytes contain:

a) for men - 3,9 - 5,5 x 10¹² in 1 l, or 3,9 - 5,5 million in 1 μl, hemoglobin concentration 130 - 160 g/l;

b) in women - 3,7 - 4,9 x 10¹², hemoglobin - 120 - 150 g / l;

2) platelets - 200 - 300 x 10⁹ in 1 l;

3) leukocytes - 3,8 - 9 x 10⁹ in 1 l.

Structural and functional characteristics of blood cells

Erythrocytes are the predominant population of blood cells. Morphological features:

1) do not contain a nucleus;

2) do not contain most organelles;

3) the cytoplasm is filled with pigment inclusions (hemoglobin).

Form of erythrocytes:

1) biconcave discs - discocytes (80%);

2) the remaining 20% ​​- spherocytes, planocytes, echinocytes, saddle-shaped, bifocal.

The following types of red blood cells can be distinguished by size:

1) normocytes (7,1 - 7,9 microns, concentration of normocytes in peripheral blood - 75%);

2) macrocytes (more than 8 microns in size, the number is 12,5%);

3) microcytes (less than 6 microns in size - 12,5%).

There are two forms of erythrocyte hemoglobin:

1) HbA;

2) HbF.

In an adult, HbA is 98%, HbF is 2%. In newborns, HbA is 20%, HbF is 80%. The life span of erythrocytes is 120 days. Old erythrocytes are destroyed by macrophages, mainly in the spleen, and the iron released from them is used by maturing erythrocytes.

In the peripheral blood, there are immature forms of erythrocytes called reticulocytes (1 - 5% of the total number of erythrocytes).

Red blood cell functions:

1) respiratory (transport of gases: O² and CO²);

2) transport of other substances adsorbed on the surface of the cytolemma (hormones, immunoglobulins, drugs, toxins, etc.).

Platelets (or platelets) are fragments of the cytoplasm of special cells of the red bone marrow (megakaryocytes).

Components of a platelet:

1) hyalomere (the base of the plate, surrounded by the plasmalemma);

2) granulomere (granularity represented by specific granules, as well as fragments of granular EPS, ribosomes, mitochondria, etc.).

Shape - rounded, oval, process.

According to the degree of maturity, platelets are divided into:

1) young;

2) mature;

3) old;

4) degenerative;

5) gigantic.

Life expectancy - 5 - 8 days.

Platelet function - participation in the mechanisms of blood coagulation through:

1) bonding of plates and formation of a blood clot;

2) the destruction of the plates and the release of one of the many factors that contribute to the transformation of globular fibrinogen into filamentous fibrin.

Leukocytes (or white blood cells) are nuclear blood cells that perform a protective function. They are contained in the blood from several hours to several days, and then leave the bloodstream and show their functions mainly in the tissues.

Leukocytes represent a heterogeneous group and are divided into several populations.

Leukocyte formula

Leukocyte formula - the percentage of various forms of leukocytes (to the total number of leukocytes equal to 100%).

Morphological and functional characteristics of granular leukocytes

Neutrophilic leukocytes (or neutrophils) are the largest population of leukocytes (65 - 75%). Morphological features of neutrophils:

1) segmented nucleus;

2) in the cytoplasm, small granules staining in a slightly oxyphilic (pink) color, among which nonspecific granules can be distinguished - varieties of lysosomes, specific granules. Organelles in leukocytes are not developed. The size in the smear is 10 - 12 microns.

According to the degree of maturity, neutrophils are divided into:

1) young (metamyelocytes) - 0 - 0,5%;

2) stab - 3 - 5%;

3) segmented (mature) - 60 - 65%.

An increase in the percentage of young and stab forms of neutrophils is called a shift of the leukocyte formula to the left and is an important diagnostic indicator. A general increase in the number of neutrophils in the blood and the appearance of young forms are observed in various inflammatory processes in the body. Currently, neutrophilic leukocytes can determine the gender of the blood - in women, one of the segments has a perinuclear satellite (or appendage) in the form of a drumstick.

The life expectancy of neutrophils is 8 days, of which 8-12 hours they are in the blood, and then they enter the connective and epithelial tissues, where they perform their main functions.

Functions of neutrophils:

1) phagocytosis of bacteria;

2) phagocytosis of immune complexes ("antigen - antibody");

3) bacteriostatic and bacteriolytic;

4) release of keyons and regulation of leukocyte reproduction.

Eosinophilic leukocytes (or eosinophils). The content is normal - 1 - 5%. Dimensions in smears - 12 - 14 microns.

Morphological features of eosinophils:

1) there is a two-segment core;

2) large oxyphilic (red) granularity is noted in the cytoplasm;

3) other organelles are poorly developed.

Among the granules of eosinophils, nonspecific azurophilic granules are isolated - a type of lysosome containing the enzyme peroxidase and specific granules containing acid phosphatase. Organelles in eosinophils are poorly developed.

According to the degree of maturity, eosinophils are also divided into young, stab and segmented, but the definition of these subpopulations in clinical laboratories is rarely performed.

Methods for neutralizing histamine and serotonin include phagocytosis and adsorption of these biologically active substances on the cytolemma, the release of enzymes that break them down extracellularly, and the release of factors that prevent the release of histamine and serotonin.

Functions of eosinophils - participation in immunological (allergic and anaphylactic) reactions: inhibit (inhibit) allergic reactions by neutralizing histamine and serotonin.

The participation of eosinophils in allergic reactions explains their increased content (up to 20 - 40% or more) in the blood in various allergic diseases (worm infestations, bronchial asthma, cancer, etc.).

The lifespan of eosinophils is 6-8 days, of which the stay in the bloodstream is 3-8 hours.

Basophilic leukocytes (or basophils). This is the smallest population of granular leukocytes (0,5 - 1%), however, there are a huge number of them in the total mass in the body.

The dimensions in the smear are 11 - 12 microns.

Morphology:

1) a large, weakly segmented nucleus;

2) the cytoplasm contains large granules;

3) other organelles are poorly developed.

The functions of basophils are participation in immune (allergic) reactions through the release of granules (degranulation) and the above biologically active substances contained in them, which cause allergic manifestations (tissue edema, blood filling, itching, spasm of smooth muscle tissue, etc.).

Basophils also have the ability to phagocytosis.

Morphological and functional characteristics of non-granular leukocytes

Agranulocytes do not contain granules in the cytoplasm and are subdivided into two completely different cell populations - lymphocytes and monocytes.

Lymphocytes are the cells of the immune system.

Lymphocytes, with the participation of auxiliary cells (macrophages), provide immunity, i.e., protection of the body from genetically alien substances. Lymphocytes are the only blood cells capable of mitotic division under certain conditions. All other leukocytes are terminal differentiated cells. Lymphocytes are a heterogeneous (heterogeneous) population of cells.

By size, lymphocytes are divided into:

1) small (4,5 - 6 microns);

2) medium (7 - 10 microns);

3) large (more than 10 microns).

In the peripheral blood, up to 90% are small lymphocytes and 10-12% are medium. Large lymphocytes are not normally found in peripheral blood. In electron microscopic examination, small lymphocytes can be divided into light and dark.

Small lymphocytes are characterized by:

1) the presence of a large round nucleus, consisting mainly of heterochromatin, especially in small dark lymphocytes;

2) a narrow rim of basophilic cytoplasm, which contains free ribosomes and weakly expressed organelles - the endoplasmic reticulum, single mitochondria and lysosomes.

Medium lymphocytes are characterized by:

1) a larger and loose nucleus, consisting of euchromatin in the center and heterochromatin along the periphery;

2) in the cytoplasm, in comparison with small lymphocytes, the endoplasmic reticulum and the Golgi complex are more developed, there are more mitochondria and lysosomes.

According to the sources of development, lymphocytes are divided into:

1) T-lymphocytes. Their formation and further development is associated with the thymus (thymus gland);

2) B-lymphocytes. Their development in birds is associated with a special organ (the bag of Fabricius), and in mammals and humans, with its analogue that has not yet been precisely established.

In addition to the sources of development, T- and B-lymphocytes differ among themselves and in their functions.

By function:

1) B-lymphocytes and plasma cells formed from them provide humoral immunity, i.e., protection of the body from foreign corpuscular antigens (bacteria, viruses, toxins, proteins, etc.) contained in the blood, lymphatic fluid;

2) T-lymphocytes, which are divided into the following subpopulations according to their functions: killers, helpers, suppressors.

However, this simple classification is outdated, and it is now accepted to classify all lymphocytes by the presence of receptors (CD) on their membrane. In accordance with this, lymphocytes CD3, CD4, CD8, etc. are isolated.

According to life expectancy, lymphocytes are divided into:

1) short-lived (weeks, months) - mainly B-lymphocytes;

2) long-lived (months, years) - mainly T-lymphocytes.

Monocytes are the largest blood cells (18 - 20 microns), having a large bean-shaped or horseshoe-shaped nucleus and a well-defined basophilic cytoplasm, which contains multiple pinocytic vesicles, lysosomes and other common organelles.

According to their function - phagocytes. Monocytes are not fully mature cells. They circulate in the blood for 2-3 days, after which they leave the bloodstream, migrate to different tissues and organs and turn into various forms of macrophages, the phagocytic activity of which is much higher than that of monocytes. Monocytes and macrophages formed from them are combined into a single macrophage system (or mononuclear phagocytic system (MPS)).

Features of the leukocyte formula in children

In newborns in the general blood test of erythrocytes 6 - 7 x 10¹² per liter - physiological erythrocytosis, the amount of hemoglobin reaches 200 g per 1 liter, leukocytes 10 - 30 x 10⁹ in 1 liter - physiological age-related leukocytosis, the number of platelets is the same as in adults - 200 - 300 x 10⁹ in l.

After birth, the number of red blood cells and hemoglobin gradually decreases, first reaching adult levels (5 million in 1 μl), and then physiological anemia develops. The level of red blood cells and hemoglobin reaches adult levels only during puberty. The white blood cell count decreases to 2 - 10 x 15 10 weeks after birth⁹ in 1 liter, and by the period of puberty reaches the values ​​of an adult.

The greatest changes in the leukocyte formula in children are noted in the content of lymphocytes and neutrophils. The remaining indicators do not differ from the values ​​of adults.

At birth, the ratio of neutrophils and lymphocytes is similar to that of adults - 65 - 75% to 20 - 35%. In the first days of a child's life, there is a decrease in the concentration of neutrophils and an increase in the content of lymphocytes, on the 4th - 5th day their number is compared - 45% each (first physiological crossover). Further, physiological lymphocytosis is observed in children - up to 65% and physiological neutropenia - 25%, the lowest neutrophil counts are observed by the end of the second year of life. After that, a gradual increase in the content of neutrophils and a decrease in the concentration of lymphocytes begin, at the age of 4-5 years, a second physiological crossover is observed. By puberty, the ratio of neutrophils and lymphocytes comes to the level of an adult.

The constituent components and functions of lymph

Lymph consists of lymphoplasm and formed elements, mainly lymphocytes (98%), as well as monocytes, neutrophils, and sometimes erythrocytes. Lymphoplasma is formed by the penetration of tissue fluid into the lymphatic capillaries, and then discharged through the lymphatic vessels of various calibers and flows into the venous system. Along the way, lymph passes through the lymph nodes, in which it is cleared of exogenous and endogenous particles, and is also enriched with lymphocytes.

Functions of the lymphatic system:

1) tissue drainage;

2) enrichment with lymphocytes;

3) purification of the lymph from exogenous and endogenous substances.

Topic 11. Bleeding

Hematopoiesis (hemocytopoiesis) is the process of formation of blood cells.

There are two types of hematopoiesis:

1) myeloid;

2) lymphoid.

In turn, myeloid hematopoiesis is divided into:

1) erythrocytopoiesis;

2) granulocytopoiesis;

3) thrombopoiesis;

4) monocytopoiesis.

Lymphoid hematopoiesis is divided into:

1) T-lymphocytopoiesis;

2) B-lymphocytopoiesis.

In addition, hematopoiesis is divided into two periods:

1) embryonic;

2) postembryonic.

The embryonic period leads to the formation of blood as a tissue and therefore represents the histogenesis of blood. Postembryonic hematopoiesis is the process of physiological regeneration of blood as a tissue.

Embryonic period of hematopoiesis

It is carried out in embryogenesis in stages, replacing different organs of hematopoiesis. Accordingly, there are three stages:

1) yolk;

2) hepatothymusolienal;

3) medullothymus-lymphoid.

1. The yolk stage is carried out in the mesenchyme of the yolk sac starting from the 2nd - 3rd week of embryogenesis, from the 4th it decreases and completely stops by the end of the 3rd month.

First, in the yolk sac, as a result of the proliferation of mesenchymal cells, the so-called blood islands are formed, which are focal accumulations of process cells.

The most important moments of the yolk stage are:

1) the formation of blood stem cells;

2) the formation of primary blood vessels.

Somewhat later (on the 3rd week), vessels begin to form in the mesenchyme of the body of the embryo, but they are empty slit-like formations. Pretty soon, the vessels of the yolk sac are connected to the vessels of the body of the embryo, and the yolk circle of blood circulation is established. From the yolk sac through these vessels, stem cells migrate into the body of the embryo and populate the anlage of future hematopoietic organs (primarily the liver), in which hematopoiesis is then carried out.

2. Hepatotimusolienal stage) of hematopoiesis is carried out first in the liver, a little later in the thymus (thymus gland), and then in the spleen. In the liver, mainly myeloid hematopoiesis occurs (only extravascularly) from the 5th week until the end of the 5th month, and then gradually decreases and completely stops by the end of embryogenesis. The thymus is laid down on the 7th - 8th week, and a little later T-lymphocytopoiesis begins in it, which continues until the end of embryogenesis, and then in the postnatal period until its involution (at 25 - 30 years). The spleen is laid on the 4th week, from the 7th - 8th week it is populated with stem cells, and universal hematopoiesis begins in it, i.e. both myelo- and lymphopoiesis. Hematopoiesis is especially active in the spleen from the 5th to the 7th months, and then myeloid hematopoiesis is gradually suppressed, and by the end of embryogenesis (in humans) it completely stops.

3. Medullothymus-lymphoid stage of hematopoiesis. The laying of the red bone marrow begins from the 2nd month, hematopoiesis begins in it from the 4th month, and from the 6th month it is the main organ of myeloid and partially lymphoid hematopoiesis, that is, it is a universal hematopoietic organ. At the same time, lymphoid hematopoiesis is carried out in the thymus, spleen and lymph nodes.

As a result of the successive change of hematopoietic organs and the improvement of the process of hematopoiesis, blood is formed as a tissue, which in newborns has significant differences from the blood of adults.

Post-embryonic period of hematopoiesis

It is carried out in the red bone marrow and lymphoid organs (thymus, spleen, lymph nodes, tonsils, lymphoid follicles).

The essence of the process of hematopoiesis lies in the proliferation and gradual differentiation of stem cells into mature blood cells.

In the scheme of hematopoiesis, two series of hematopoiesis are presented:

1) myeloid;

2) lymphoid.

Each type of hematopoiesis is subdivided into varieties (or series) of hematopoiesis.

Myelopoiesis:

1) erythrocytopoiesis (or erythrocyte series);

2) granulocytopoiesis (or granulocyte series);

3) monocytopoiesis (or monocytic series);

4) thrombocytopenia (or platelet series).

Lymphopoiesis:

1) T-lymphocytopoiesis (or T-lymphocytic series;

2) B-lymphocytopoiesis;

3) plasmacytopoiesis.

In the process of gradual differentiation of stem cells into mature blood cells, intermediate cell types are formed in each row of hematopoiesis, which form classes of cells in the hematopoiesis scheme.

In total, six classes of cells are distinguished in the hematopoietic scheme.

I class - stem cells. By morphology, the cells of this class correspond to a small lymphocyte. These cells are pluripotent, that is, they are able to differentiate into any blood cell. The direction of differentiation depends on the content of formed elements in the blood, as well as on the influence of the microenvironment of stem cells - inductive influences of stromal cells of the bone marrow or other hematopoietic organ. Maintaining the stem cell population is carried out as follows. After mitosis of a stem cell, two are formed: one enters the path of differentiation to a blood cell, and the other takes on the morphology of a small lymphocyte, remains in the bone marrow, and is a stem cell. The division of stem cells occurs very rarely, their interphase is 1–2 years, while 80% of stem cells are at rest and only 20% are in mitosis and subsequent differentiation. Stem cells are also called collin-forming units because each stem cell produces a group (or clone) of cells.

Class II - semi-stem cells. These cells are limitedly pluripotent. There are two groups of cells - the precursors of myelopoiesis and lymphopoiesis. Morphologically similar to a small lymphocyte. Each of these cells gives rise to a clone of the myeloid or lymphoid series. The division occurs every 3-4 weeks. The maintenance of the population is carried out similarly to pluripotent cells: after mitosis, one cell enters further differentiation, and the second remains semi-stem.

Class III - unipotent cells. This class of cells is poetinsensitive - the precursors of their hematopoietic series. In morphology, they also correspond to a small lymphocyte and are capable of differentiation into only one blood cell. The frequency of division of these cells depends on the content of poetin in the blood - a biologically active substance specific for each series of hematopoiesis - erythropoietin, thrombopoietin. After mitosis of cells of this class, one cell enters into further differentiation to a uniform element, and the second maintains a population of cells.

Cells of the first three classes are combined into a class of morphologically unidentifiable cells, since all of them resemble a small lymphocyte in morphology, but their developmental abilities are different.

Class IV - blast cells. Cells of this class differ in morphology from all others. They are large, have a large loose nucleus (euchromatin) with 2-4 nucleoli, the cytoplasm is basophilic due to the large number of free ribosomes. These cells often divide, and all daughter cells enter into further differentiation. Blasts of various hematopoietic lines can be identified by their cytochemical properties.

Class V - maturing cells. This class is characteristic of its hematopoietic series. In this class, there may be several varieties of transitional cells from one (prolymphocyte, promonocyte) to five in the erythrocyte series. Some maturing cells can enter the peripheral circulation in small numbers, such as reticulocytes or stab leukocytes.

VI class - mature shaped elements. These classes include erythrocytes, platelets and segmented granulocytes. Monocytes are not terminally differentiated cells. They then leave the bloodstream and differentiate into the final class, the macrophages. Lymphocytes differentiate into a final class upon encountering antigens, whereby they turn into blasts and divide again.

The set of cells that make up the line of stem cell differentiation into a certain uniform element forms a differon (or histogenetic series). For example, erythrocyte differon is:

1) stem cell (class I);

2) semi-stem cell - the precursor of myelopoiesis (class II);

3) unipotent erythropoietin-sensitive cell (class III);

4) erythroblast (class IV);

5) maturing cell - pronormocyte, basophilic normocyte, polychromatophilic normocyte, oxyphilic normocyte, reticulocyte (class V);

6) erythrocyte (class VI).

In the process of maturation of erythrocytes in class V, the synthesis and accumulation of hemoglobin, the reduction of organelles and the cell nucleus occur. Normally, the replenishment of erythrocytes is carried out due to the division and differentiation of maturing cells - pronormocytes, basophilic and polychromatophilic normocytes. This type of hematopoiesis is called homoplastic. With severe blood loss, the replenishment of erythrocytes is carried out not only by the strengthening of maturing cells, but also by cells of IV, III, II and even class I - a heteroplastic type of hematopoiesis occurs.

Topic 12. IMMUNOCYTOPOIESIS AND THE PARTICIPATION OF IMMUNE CELLS IN IMMUNE REACTIONS

Unlike myelopoiesis, lymphocytopoiesis in the embryonic and postembryonic periods is carried out in stages, replacing different lymphoid organs. As noted earlier, lymphocytopoiesis is divided into:

1) T-lymphocytopoiesis;

2) B-lymphocytopoiesis.

In turn, they are divided into three stages:

1) bone marrow stage;

2) the stage of antigen-independent differentiation, carried out in the central immune organs;

3) the stage of antigen-dependent differentiation, carried out in peripheral lymphoid organs.

T-lymphocytopoiesis

The first stage is carried out in the lymphoid tissue of the red bone marrow, where the following cell classes are formed:

1) stem cells - class I;

2) semi-stem cells precursors of T-lymphocytopoiesis - class II;

3) unipotent T-poietin-sensitive cells, precursors of T-lymphocytopoiesis. These cells migrate into the bloodstream and reach the thymus (thymus) - class III.

The second stage is antigen-independent differentiation, which takes place in the thymus cortex. In this case, further formation of T-lymphocytes occurs. Stromal cells secrete thymosin, under the influence of which the transformation of unipotent cells into T-lymphoblasts occurs. They are class IV cells in T-lymphocytopoiesis. T-lymphoblasts turn into T-prolymphocytes (class V cells), and they turn into T-lymphocytes - class VI.

In the thymus, three subpopulations of T-lymphocytes develop independently from unipotent cells - T-killers, T-helpers, T-suppressors.

The resulting T-lymphocytes acquire different receptors for various antigens in the thymus cortex, while the antigens themselves do not enter the thymus. Protection of the thymus gland from the ingress of foreign antigens is carried out due to the presence of the hematothymic barrier and the absence of afferent vessels in the thymus.

As a result of the second stage, subpopulations of T-lymphocytes are formed, which have different receptors for certain antigens. The thymus also produces T-lymphocytes that have receptors for the antigens of their own tissues, but such cells are immediately destroyed by macrophages.

After the formation of T-lymphocytes, without penetrating into the thymus medulla, they enter the bloodstream and are carried to the peripheral lymphoid organs.

The third stage (antigen-independent differentiation) is carried out in T-dependent zones of peripheral lymphoid organs - lymph nodes and spleen. Here, conditions are created for the meeting of the antigen with the T-lymphocyte (killer, helper or suppressor) that has a receptor for this antigen.

Most often, there is not a direct interaction of a T-lymphocyte with an antigen, but an indirect one - through a macrophage. When a foreign antigen enters the body, it is first phagocytosed by a macrophage (completed phagocytosis), partially cleaved, and the antigenic determinant is brought to the surface of the macrophage, where it is concentrated. Then these determinants are transmitted by macrophages to the corresponding receptors of various subpopulations of T-lymphocytes. Under the influence of a specific antigen, a blastotransformation reaction occurs - the transformation of a T-lymphocyte into a T-lymphoblast. Further differentiation of cells depends on which subpopulation of T-lymphocytes has interacted with the antigen.

T-killer lymphoblast gives the following clones of cells.

1. T-killers (or cytotoxic lymphocytes), which are effector cells that provide cellular immunity. T-killers provide the primary immune response - the body's reaction to the first interaction with the antigen.

In the process of destruction of a foreign antigen by killers, two main mechanisms can be distinguished: contact interaction - the destruction of a section of the cytolemma of the target cell and distant interaction - the release of cytotoxic factors that act on the target cell gradually and for a long time.

2. T-memory cells. These cells, when the body encounters the same antigen again, provide a secondary immune response that is stronger and faster than the primary one.

T-helper lymphoblast produces the following cell clones:

1) T-helpers that secrete the mediator lymphokine, which stimulates humoral immunity. It is an immunopoiesis inducer;

2) T-memory cells.

T-suppressor lymphoblast produces the following cell clones:

1) T-suppressors;

2) T-memory cells.

Thus, during the third stage of T-lymphocytopoiesis, the formation of effector cells of each subpopulation of T-lymphocytes (T-killers, T-helpers and T-suppressors) with a certain function, and T-memory cells that provide a secondary immune response occurs.

In cellular immunity, two mechanisms can be distinguished for the destruction of target cells by killers - contact interaction, in which a section of the cytolemma of the target cell is destroyed and its death, and distant interaction - the release of cytotoxic factors that act on the target cell gradually and cause its death after a certain time .

B-lymphocytopoiesis

In the process of B-lymphocytopoiesis, the following stages can be distinguished.

The first stage is carried out in the red bone marrow, where the following cell classes are formed:

1) stem cells - class I;

2) semi-stem cells, precursors of lymphopoiesis - class II;

3) unipotent B-lymphopoietin-sensitive cells - precursors of B-lymphocytopoiesis - class III.

The second stage - antigen-independent differentiation - in birds is carried out in a special organ - the bursa of Fabricius, in mammals, including humans, such an organ has not been found. Most researchers believe that the second stage (as well as the first) is carried out in the red bone marrow, where B-lymphoblasts, class IV cells, are formed. Then they proliferate into B-prolymphocytes - class V cells and into B-lymphocytes - class VI cells. During the second stage, B-lymphocytes acquire a variety of receptors for antigens. At the same time, it was found that receptors are represented by proteins - immunoglobulins, which are synthesized in the maturing B-lymphocytes themselves, then brought to the surface and integrated into the plasma membrane. The terminal chemical groups of these receptors are different, and this explains the specificity of their perception of certain antigenic determinants of different antigens.

The third stage - antigen-dependent differentiation is carried out in B-dependent zones of peripheral lymphoid organs - in the spleen and lymph nodes. Here, B-lymphocytes meet with antigens, their subsequent activation and transformation into an immunoblast. This happens only with the participation of additional cells - macrophages, T-helpers and T-suppressors. Therefore, for the activation of B-lymphocytes, the cooperation of the following cells is necessary - a B-lymphocyte, a T-helper or a T-suppressor, as well as a humoral antigen - a bacterium, a virus, or a polysaccharide protein. The interaction process proceeds as follows: the antigen-presenting macrophage phagocytizes the antigen and brings the antigenic determinant to the surface of the cell membrane, after which the determinant acts on B-lymphocytes, T-helpers and T-suppressors. Thus, the influence of the antigenic determinant on the B-lymphocyte is not enough for the blastotransformation reaction; it proceeds after the activation of the T-helper and the release of an activating lymphokine by it. After that, the B-lymphocyte turns into an immunoblast. After the proliferation of the immunoblast, clones of cells are formed - plasmocytes - effector cells of humoral immunity, they synthesize and secrete into the blood immunoglobulins - antibodies of various classes and B-memory cells.

Immunoglobulins (antibodies) interact with specific antigens, an antigen-antibody complex is formed, thus neutralizing foreign antigens.

T-helpers play the following function in the implementation of humoral immunity - they contribute to the reaction of blastotransformation, replace the synthesis of non-specific immunoglobulins with specific ones, stimulate the synthesis and release of immunoglobulins by plasma cells.

T-suppressors are activated by the same antigens and secrete lymphokines that inhibit the formation of plasma cells and their synthesis of immunoglobulins up to complete cessation. Thus, the effect of T-killers and T-helpers on the B-lymphocyte regulates the response of humoral immunity.

Topic 13. CONNECTIVE TISSUE. PROPER CONNECTIVE TISSUES

The concept of "connective tissues" (tissues of the internal environment, support-trophic tissues) combines tissues that are not the same in morphology and functions, but have some common properties and develop from a single source - mesenchyme.

Structural and functional features of connective tissues:

1) internal location in the body;

2) the predominance of the intercellular substance over the cells;

3) variety of cellular forms;

4) common source of origin - mesenchyme.

Functions of connective tissues:

1) trophic (metabolic);

2) support;

3) protective (mechanical, non-specific and specific);

4) reparative (plastic), etc.

The most common in the body are fibrous connective tissues and especially loose fibrous unformed tissue, which is part of almost all organs, forming stroma, layers and layers, accompanying blood vessels.

Morphological and functional characteristics of loose fibrous irregular connective tissue

It consists of cells and intercellular substance, which, in turn, consists of fibers (collagen, elastic, reticular) and amorphous substance.

Morphological features that distinguish loose fibrous connective tissue from other types of connective tissues:

1) variety of cell forms (nine cell types);

2) the predominance of amorphous matter in the intercellular substance over the fibers.

Functions of loose fibrous connective tissue:

1) trophic;

2) supporting (forms the stroma of parenchymal organs);

3) protective (nonspecific and specific (participation in immune reactions) protection);

4) depot of water, lipids, vitamins, hormones;

5) reparative (plastic).

Cell types (cell populations) of loose fibrous connective tissue:

1) fibroblasts;

2) macrophages (histiocytes);

3) tissue basophils (mast cells);

4) plasma cells;

5) fat cells (lipocytes);

6) pigment cells;

7) adventitial lashes;

8) pericytes;

9) blood cells - leukocytes (lymphocytes, neutrophils).

Structural and functional characteristics of cell types

Fibroblasts are the predominant cell population of loose fibrous connective tissue. They are heterogeneous in terms of maturity and functional specificity and therefore are divided into the following subpopulations:

1) poorly differentiated cells;

2) differentiated (or mature cells, or fibroblasts proper);

3) old fibroblasts (definitive) - fibrocytes, as well as specialized forms of fibroblasts;

4) myofibroblasts;

5) fibroclasts.

The predominant form is mature fibroblasts, the function of which is to synthesize and release collagen and elastin proteins, as well as glycosaminoglycans, into the intercellular environment.

The structural organization of fibroblasts is characterized by a pronounced development of a synthetic apparatus - a granular endoplasmic reticulum and a transport apparatus - the lamellar Golgi complex. The remaining organelles are poorly developed. In fibrocytes, the granular ER and the lamellar complex are reduced. The cytoplasm of fibroblasts contains microfilaments containing the contractile proteins actin and myosin, but these organelles are especially developed in myofibroblasts, due to which they tighten the young connective tissue during scar formation. Fibroclasts are characterized by the content in the cytoplasm of a large number of lysosomes. These cells are able to secrete lysosomal enzymes into the intercellular environment and, with their help, split collagen or elastic fibers into fragments, and then phagocytize the split fragments intracellularly. Consequently, fibroclasts are characterized by lysis of the intercellular substance, including fibers (for example, during uterine involution after childbirth).

Thus, various forms of fibroclasts form the intercellular substance of the connective tissue (fibroblasts), maintain it in a certain structural and functional state (fibrocytes), and destroy it under certain conditions (fibroclasts). Due to these properties of fibroblasts, the reparative function of the connective tissue is carried out.

Macrophages are cells that perform a protective function, primarily through phagocytosis of large particles.

According to modern data, macrophages are polyfunctional cells. Macrophages are formed from monocytes after they leave the bloodstream. Macrophages are characterized by structural and functional heterogeneity depending on the degree of maturity, area of ​​localization, as well as their activation by antigens or lymphocytes.

The protective function of macrophages manifests itself in various forms:

1) non-specific protection (through phagocytosis of exogenous and endogenous particles and their intracellular digestion);

2) release into the extracellular environment of lysosomal enzymes and other substances;

3) specific (or immunological protection - participation in a variety of immune reactions).

Macrophages are divided into fixed and free. Connective tissue macrophages are motile or wandering and are called histiocytes.

There are macrophages of serous cavities (peritoneal and pleural), alveolar, liver macrophages (Kupffer cells), macrophages of the central nervous system - glial macrophages, osteoclasts.

All types of macrophages are combined into a mononuclear phagocytic system (or macrophage system) of the body.

According to the functional state, macrophages are divided into residual (inactive) and activated. Depending on this, their intracellular structure also differs.

The most characteristic structural feature of macrophages is the presence of a pronounced lysosomal apparatus, i.e., the cytoplasm contains many lysosomes and phagosomes.

A feature of histocytes is the presence on their surface of numerous folds, invaginations and pseudopodia, reflecting the movement of cells or the capture of various particles by them. The plasmolemma of macrophages contains a variety of receptors, with the help of which they recognize various, including antigenic particles, as well as various biologically active substances.

By phagocytizing antigenic substances, macrophages secrete, concentrate, and then carry their active chemical groups - antigenic determinants onto the plasma membrane, and then transfer them to lymphocytes. This function is called antigen presenting. With the help of this function, macrophages trigger antigenic reactions, since it has been established that most antigenic substances are not able to trigger immune reactions on their own, that is, act directly on lymphocyte receptors. In addition, activated macrophages secrete some biologically active substances - monokines, which have a regulatory effect on various aspects of immune responses.

Macrophages are involved in the final stages of immune responses of both humoral and cellular immunity. In humoral immunity, they phagocytize antigen-antibody immune complexes, and in cellular immunity, under the influence of lymphokines, macrophages acquire killer properties and can destroy foreign, including tumor, cells.

Thus, macrophages are not immune cells, but take part in immune reactions.

Macrophages also synthesize and secrete about a hundred different biologically active substances into the intercellular environment. Therefore, macrophages can be classified as secretory cells.

Tissue basophils (mast cells) are true cells of loose fibrous connective tissue.

The function of these cells is to regulate local tissue homeostasis.

This is achieved through the synthesis of tissue basophils and the subsequent release into the intercellular environment of glycosamino-glycans (heparin and chondroitin sulfuric acids), histamine, serotonin and other biologically active substances that affect the cells and intercellular substance of the connective tissue.

These biologically active substances have the greatest influence on the microvasculature, where they cause an increase in the permeability of hemocapillaries, enhance the hydration of the intercellular substance. Mast cell products influence immune responses and the processes of inflammation and allergy.

The sources of mast cell formation have not yet been fully established.

The ultrastructural organization of tissue basophils is characterized by the presence of two types of granules in the cytoplasm:

1) metachromatic granules stained with basic dyes with a color change;

2) orthochromatic granules stained with basic dyes without color change and representing lysosomes.

When tissue basophils are excited, biologically active substances are released from them in the following ways:

1) with the help of the allocation of granules - degranulation;

2) with the help of diffuse release of histamine through the membrane, which increases vascular permeability and causes hydration of the main substance, thereby enhancing the inflammatory response.

Mast cells are involved in immune responses. When some foreign substances enter the body, plasma cells synthesize class E immunoglobulins, which are then adsorbed on the cytolemma of mast cells. When the same antigens enter the body again, “antigen-antibody” immune complexes are formed on the surface of mast cells, which cause a sharp degranulation of tissue basophils, and biologically active substances released in large quantities cause the rapid onset of allergic and anaphylactic reactions.

Plasma cells (plasmocytes) are cells of the immune system (effector cells of humoral immunity).

Plasma cells are formed from B-lymphocytes when exposed to antigenic substances.

Most of them are localized in the organs of the immune system (lymph nodes, spleen, tonsils, follicles), but a significant part of plasma cells is distributed in the connective tissue.

The functions of plasma cells are the synthesis and release of antibodies into the intercellular environment - immunoglobulins, which are divided into five classes.

Plasma cells have a well-developed synthetic and excretory apparatus. Electron diffraction patterns of plasmocytes show that almost the entire cytoplasm is filled with a granular endoplasmic reticulum, except for a small area adjacent to the nucleus and in which the Golgi lamellar complex and the cell center are located. When studying plasmocytes under a light microscope with the usual histological staining - hematoxylin-eosin, they have a round or oval shape, basophilic cytoplasm, an eccentrically located nucleus containing clumps of heterochromatin in the form of triangles (wheel-shaped nucleus). A pale colored area of ​​the cytoplasm is adjacent to the nucleus - a "light courtyard", in which the Golgi complex is localized. The number of plasma cells reflects the intensity of immune responses.

Fat cells (adipocytes) are found in loose connective tissue in different amounts in different parts of the body and in different organs.

Functions of fat cells:

1) depot of energy resources;

2) water depot;

3) depot of fat-soluble vitamins, etc.

Fat cells are located in groups near the vessels of the microvasculature. With a significant accumulation, they form white adipose tissue. Adipocytes have a characteristic morphology: almost the entire cytoplasm is filled with one fat drop, and the organelles and the nucleus are pushed to the periphery. With alcohol fixation and holding the battery of alcohols, the fat dissolves, and the cell takes the form of a signet ring, and the accumulation of fat cells in the histological preparation has a cellular, honeycomb-like appearance. Lipids are detected only after formalin fixation by histochemical methods - sudan and osmium.

Pigment cells (pigmentocytes, melanocytes) - process-shaped cells containing pigment inclusions (melanin) in the cytoplasm. Pigment cells are not true cells of the connective tissue, since, firstly, they are localized not only in the connective tissue, but also in the epithelial tissue, and secondly, they are formed not from mesenchymal cells, but from neural crest neuroblasts.

Adventitial cells are localized in the adventitia of the vessels. They have an elongated and flattened shape. The cytoplasm of these cells is weakly basophilic and contains a small amount of organelles. Some authors consider adventitial cells as independent cellular elements of the connective tissue, others believe that they are a source for the development of fibroblasts, fat and smooth muscle cells.

Pericytes - cells localized in the walls of capillaries - in the splitting of the basement membrane.

Leukocytes - lymphocytes and neutrophils. Normally, the connective tissue necessarily contains various amounts of blood cells - lymphocytes and neutrophils. In inflammatory conditions, their number increases sharply (lymphocytic and leukocyte infiltration).

Intercellular substance of connective tissue

It consists of two structural components:

1) from the main (or amorphous) substance;

2) from fibers.

The main (or amorphous) substance consists of proteins and carbohydrates. Proteins are represented mainly by collagen, as well as albumins and globulins.

Carbohydrates are represented by polymeric forms, mainly glycosaminoglycans (sulfated - chondroitin sulfuric acids, dermatan sulfate, etc.)

Carbohydrate components retain water, depending on the water content, the fabric can be more or less dense.

The amorphous substance ensures the transport of substances from the blood to the cells and vice versa, including transport from the connective tissue to the epithelial.

An amorphous substance is formed primarily due to the activity of fibroblasts - collagens and glycosaminoglycans, as well as due to blood plasma substances - albumins and globulins.

Depending on the concentration of water, the main amorphous substance can be more or less dense, which determines the functional role of this type of tissue.

The fibrous component is represented by collagen, elastic and reticular fibers. In various organs, the ratio of these fibers is not the same: collagen fibers predominate in loose fibrous connective tissue.

Collagen fibers have different thickness (from 1 - 3 to 10 or more microns). They have high strength and low elongation. Each collagen fiber consists of two chemical components:

1) fibrillar protein collagen;

2) carbohydrate component - glycosaminoglycans and proteoglycans.

Both of these components are synthesized by fibroblasts and released into the extracellular environment, where they are assembled and fiber is built.

There are five levels in the structural organization of collagen fibers.

Level I - polypeptide. Collagen is represented by polypeptide chains, consisting of three amino acids - proline, glycine, lysine.

Level II - molecular, represented by a collagen protein molecule 280 nm long, 1,4 nm wide, consisting of three polypeptide chains twisted into a spiral.

Level III - protofibrillar (thickness 10 nm, consists of several longitudinally arranged collagen molecules interconnected by hydrogen bonds).

IV level - microfibrils (thickness from 11 - 12 nm, and more). They consist of 5 - 6 protofibrils connected by lateral bonds.

Level V - fibril (or collagen fiber) thickness 1 - 10 microns, consisting of several microfibrils - depending on the thickness, associated with glycosaminoglycans and proteoglycans. Collagen fibers have a transverse striation due to both the arrangement of amino acids in the polypeptide chain and the arrangement of chains in the collagen molecule. Collagen fibers with the help of carbohydrate components are combined into bundles up to 150 microns thick.

Depending on the order of amino acids in polypeptide chains, on the degree of their hydroxylation, and on the quality of the carbohydrate component, twelve types of collagen protein are distinguished, of which only five types are well studied.

These types of collagen protein are included not only in collagen fibers, but also in the basement membranes of epithelial tissue and blood vessels, cartilage, vitreous body and other formations. With the development of some pathological processes, collagen breaks down and enters it into the blood. In blood plasma, the type of collagen is determined biochemically, and, consequently, the presumable area of ​​its decay and its intensity are also determined.

Elastic fibers are characterized by high elasticity, the ability to stretch and contract, but little strength.

They are thinner than collagen, do not have transverse striation, branch along the way and anastomose with each other, forming an elastic network. The chemical composition of elastic fibers is elastin protein and glycoproteins. Both components are synthesized and secreted by fibroblasts, and in the vascular wall - by smooth muscle cells. The elastin protein differs from the collagen protein both in the composition of amino acids and in their hydroxylation. Structurally, the elastic fiber is organized as follows: the central part of the fiber is represented by an amorphous component of elastin molecules, and the peripheral part is represented by a small fibrillar network. The ratio of amorphous and fibrillar components in elastic fibers can be different. Most fibers are dominated by the amorphous component. When the amorphous and fibrillar components are equal, the fibers are called elaunin. There are also oxytalone elastic fibers, consisting only of the fibrillar component. Elastic fibers are localized primarily in those organs that constantly change their volume - in the lungs, blood vessels.

Reticular fibers are similar in composition to collagen fibers.

Reticular fibers consist of type III collagen and a carbohydrate component. They are thinner than collagen, have a slightly pronounced transverse striation. Branching and anastomosing, they form small-loop networks, hence their name. In reticular fibers, unlike collagen fibers, the carbohydrate component is more pronounced, which is well detected by silver nitrate salts, therefore these fibers are also called argyrophilic. It should be remembered that immature collagen fibers, consisting of precollagen protein, also have argyrophilic properties. According to their physical properties, reticular fibers occupy an intermediate position between collagen and elastic. They are formed due to the activity of reticular cells. They are localized mainly in the hematopoietic organs, making up their stroma.

Dense fibrous connective tissue

It differs from the loose one in the predominance of the fibrous component in the intercellular substance over the amorphous one.

Depending on the nature of the location of the fibers, dense fibrous connective tissue is divided into formed (the fibers of this type of tissue are arranged in an orderly manner, most often parallel to each other) and unformed (the fibers are arranged randomly).

Dense formed connective tissue is presented in the body in the form of tendons, ligaments, fibrous membranes.

Dense fibrous unformed connective tissue forms a mesh layer of the dermis of the skin.

In addition to containing a large number of fibers, dense fibrous connective tissue is characterized by a lack of cellular elements, which are mainly represented by fibrocytes.

Tendon structure

The tendon consists mainly of dense, formed connective tissue, but also contains loose fibrous connective tissue, which forms layers.

On the transverse and longitudinal sections of the tendon, it can be seen that it consists of parallel collagen fibers forming bundles of I, II and III orders.

The bundles of the first order are the thinnest, separated from each other by fibrocytes. The bundles of the second order consist of several bundles of the first order, surrounded on the periphery by a layer of loose fibrous connective tissue that makes up the endotenonium. The bundles of the III order consist of bundles of the II order and are surrounded by more pronounced layers of loose fibrous connective tissue - perithenonium.

The entire tendon is surrounded by epithenonium along the periphery.

In the layers of loose fibrous connective tissue, vessels and nerves pass, providing trophism and innervation of the tendon.

Age features of fibrous connective tissues

In newborns and children, in the fibrous connective tissue, the amorphous substance contains a lot of water bound by glycosaminoglycans. Collagen fibers are thin and consist not only of protein, but also of precollagen. Elastic fibers are well developed. The amorphous and fibrous components of the connective tissue together determine the elasticity and firmness of the skin in children. With increasing age in postnatal ontogenesis, the content of glycosaminoglycans in the amorphous tissue substance decreases, and, accordingly, the water content also decreases. Collagen fibers grow and form thick and coarse bundles. Elastic fibers are largely destroyed. As a result, the skin of the elderly and old people becomes inelastic and flabby.

Connective tissues with special properties

Reticular tissue consists of reticular cells and reticular fibers. This tissue forms the stroma of all hematopoietic organs (with the exception of the thymus) and, in addition to the supporting function, performs other functions: it provides trophism for hematopoietic cells and influences the direction of their differentiation.

Adipose tissue consists of accumulations of fat cells and is divided into two types: white and brown adipose tissue.

White adipose tissue is widely distributed in various parts of the body and in the internal organs, it is unequally expressed in different subjects and throughout ontogenesis. It is a collection of typical fat cells (adipocytes).

Metabolic processes are actively taking place in fat cells.

Functions of white adipose tissue:

1) energy depot (macroergs);

2) water depot;

3) depot of fat-soluble vitamins;

4) mechanical protection of some organs (eyeball, etc.).

Brown adipose tissue is found only in newborns.

It is localized only in certain places: behind the sternum, near the shoulder blades, on the neck, along the spine. Brown adipose tissue consists of an accumulation of brown fat cells, which differ significantly from typical adipocytes both in morphology and in the nature of their metabolism. The cytoplasm of brown fat cells contains a large number of liposomes distributed throughout the cytoplasm.

Oxidative processes in brown fat cells are 20 times more intense than in white ones. The main function of brown adipose tissue is to generate heat.

Mucous connective tissue is found only in the embryonic period in the provisional organs and, above all, in the umbilical cord. It consists mainly of an intercellular substance in which fibroblast-like cells that synthesize mucin (mucus) are localized.

Pigmented connective tissue is a tissue area that contains an accumulation of melanocytes in (the area of ​​the nipples, scrotum, anus, choroid of the eyeball).

Topic 14. CONNECTIVE TISSUE. SKELETAL CONNECTIVE TISSUES

Skeletal connective tissues include cartilaginous and bone tissues that perform supporting, protective and mechanical functions, as well as taking part in the metabolism of minerals in the body. Each of these types of connective tissue has significant morphological and functional differences, and therefore they are considered separately.

cartilage tissues

Cartilaginous tissue consists of cells - chondrocytes and chondroblasts, as well as dense intercellular substance.

Chondroblasts are located singly along the periphery of the cartilaginous tissue. They are elongated flattened cells with basophilic cytoplasm containing a well-developed granular ER and lamellar complex. These cells synthesize the components of the intercellular substance, release them into the intercellular environment, and gradually differentiate into the definitive cells of the cartilage tissue - chondrocytes. Chondroblasts are capable of mitotic division. The perichondrium surrounding the cartilaginous tissue contains inactive, poorly differentiated forms of chondroblasts, which, under certain conditions, differentiate into chondroblasts that synthesize the intercellular substance, and then into chondrocytes.

An amorphous substance contains a significant amount of mineral substances that do not form crystals, water, or dense fibrous tissue. Vessels in the cartilage tissue are normally absent. Depending on the structure of the intercellular substance, cartilage tissues are divided into hyaline, elastic and fibrous cartilage tissue.

In the human body, hyaline cartilage tissue is widespread and is part of the large cartilages of the larynx (thyroid and cricoid), trachea, and cartilage of the ribs.

Elastic cartilage tissue is characterized by the presence of both collagen and elastic fibers in the cellular substance (cartilaginous tissue of the auricle and cartilaginous part of the external auditory canal, cartilage of the external nose, small cartilages of the larynx and middle bronchi).

Fibrous cartilage tissue is characterized by the content of powerful bundles of parallel collagen fibers in the intercellular substance. In this case, chondrocytes are located between the bundles of fibers in the form of chains. According to physical properties, it is characterized by high strength. It is found in the body only in limited places: it forms part of the intervertebral discs (annulus fibrosus), and is also localized at the points of attachment of ligaments and tendons to hyaline cartilage. In these cases, a gradual transition of connective tissue fibrocytes into cartilage chondrocytes is clearly seen.

When studying cartilage tissues, the concepts of "cartilaginous tissue" and "cartilage" should be clearly understood.

Cartilage tissue is a type of connective tissue, the structure of which is superimposed above. Cartilage is an anatomical organ that consists of cartilage and perichondrium. The perichondrium covers the cartilaginous tissue from the outside (with the exception of the cartilaginous tissue of the articular surfaces) and consists of fibrous connective tissue.

There are two layers in the perichondrium:

1) external - fibrous;

2) internal - cellular (or cambial, germ).

In the inner layer, poorly differentiated cells are localized - prechondroblasts and inactive chondroblasts, which, in the process of embryonic and regenerative histogenesis, first turn into chondroblasts, and then into chondrocytes.

The fibrous layer contains a network of blood vessels. Therefore, the perichondrium, as an integral part of the cartilage, performs the following functions:

1) provides trophic avascular cartilaginous tissue;

2) protects cartilage tissue;

3) provides regeneration of cartilaginous tissue in case of its damage.

The trophism of the hyaline cartilage tissue of the articular surfaces is provided by the synovial fluid of the joints, as well as fluid from the vessels of the bone tissue.

The development of cartilage tissue and cartilage (chondrohystogenesis) is carried out from the mesenchyme.

bone tissues

Bone tissue is a type of connective tissue and consists of cells and intercellular substance, which contains a large amount of mineral salts, mainly calcium phosphate. Minerals make up 70% of bone tissue, organic - 30%.

Functions of bone tissue:

1) support;

2) mechanical;

3) protective (mechanical protection);

4) participation in the mineral metabolism of the body (depot of calcium and phosphorus).

Bone cells - osteoblasts, osteocytes, osteoclasts. The main cells in the formed bone tissue are osteocytes. These are process-shaped cells with a large nucleus and weakly expressed cytoplasm (nuclear-type cells). The cell bodies are localized in the bone cavities (lacunae), and the processes - in the bone tubules. Numerous bone tubules, anastomosing with each other, penetrate the bone tissue, communicating with the perivascular space, form the drainage system of the bone tissue. This drainage system contains tissue fluid, through which the exchange of substances is ensured not only between cells and tissue fluid, but also in the intercellular substance.

Osteocytes are definitive forms of cells and do not divide. They are formed from osteoblasts.

Osteoblasts are found only in developing bone tissue. In the formed bone tissue, they are usually contained in an inactive form in the periosteum. In developing bone tissue, osteoblasts surround each bone plate along the periphery, tightly adhering to each other.

The shape of these cells can be cubic, prismatic and angular. The cytoplasm of osteoblasts contains a well-developed endoplasmic reticulum, the Golgi lamellar complex, many mitochondria, which indicates a high synthetic activity of these cells. Osteoblasts synthesize collagen and glycosaminoglycans, which are then released into the extracellular space. Due to these components, an organic matrix of bone tissue is formed.

These cells provide mineralization of the intercellular substance through the release of calcium salts. Gradually releasing the intercellular substance, they seem to be walled up and turn into osteocytes. At the same time, intracellular organelles are significantly reduced, synthetic and secretory activity is reduced, and the functional activity characteristic of osteocytes is preserved. Osteoblasts localized in the cambial layer of the periosteum are in an inactive state; synthetic and transport organelles are poorly developed in them. When these cells are irritated (in case of injuries, bone fractures, etc.), a granular EPS and a lamellar complex rapidly develop in the cytoplasm, active synthesis and release of collagen and glycosaminoglycans, the formation of an organic matrix (bone callus), and then the formation of definitive bone fabrics. In this way, due to the activity of osteoblasts of the periosteum, bones regenerate when they are damaged.

Osteoclasts - bone-destroying cells, are absent in the formed bone tissue, but are contained in the periosteum and in places of destruction and restructuring of bone tissue. Since local processes of bone tissue restructuring are continuously carried out in ontogeny, osteoclasts are also necessarily present in these places. In the process of embryonic osteohistogenesis, these cells play a very important role and are present in large numbers. Osteoclasts have a characteristic morphology: these cells are multinucleated (3-5 or more nuclei), have a rather large size (about 90 microns) and a characteristic shape - oval, but the part of the cell adjacent to the bone tissue has a flat shape. In the flat part, two zones can be distinguished: the central (corrugated part, containing numerous folds and processes), and the peripheral part (transparent) in close contact with the bone tissue. In the cytoplasm of the cell, under the nuclei, there are numerous lysosomes and vacuoles of various sizes.

The functional activity of the osteoclast is manifested as follows: in the central (corrugated) zone of the cell base, carbonic acid and proteolytic enzymes are released from the cytoplasm. The released carbonic acid causes demineralization of bone tissue, and proteolytic enzymes destroy the organic matrix of the intercellular substance. Fragments of collagen fibers are phagocytosed by osteoclasts and destroyed intracellularly. Through these mechanisms, resorption (destruction) of bone tissue occurs, and therefore osteoclasts are usually localized in the depressions of bone tissue. After the destruction of bone tissue due to the activity of osteoblasts, which are evicted from the connective tissue of the vessels, a new bone tissue is built.

The intercellular substance of bone tissue consists of the main (amorphous) substance and fibers, which contain calcium salts. The fibers consist of collagen and are folded into bundles, which can be arranged in parallel (orderly) or randomly, on the basis of which the histological classification of bone tissues is built. The main substance of bone tissue, as well as other types of connective tissues, consists of glycosamino- and proteoglycans.

The bone tissue contains less chondroitin sulfuric acids, but more citric and others, which form complexes with calcium salts. During the development of bone tissue, an organic matrix is ​​first formed - the main substance and collagen fibers, and then calcium salts are deposited in them. They form crystals - hydroxyapatites, which are deposited both in an amorphous substance and in fibers. Providing bone strength, calcium phosphate salts are also both a depot of calcium and phosphorus in the body. Thus, bone tissue takes part in the mineral metabolism of the body.

When studying bone tissue, one should also clearly separate the concepts of "bone tissue" and "bone".

Bone is an organ whose main structural component is bone tissue.

Bone as an organ consists of such elements as:

1) bone tissue;

2) periosteum;

3) bone marrow (red, yellow);

4) vessels and nerves.

The periosteum (periosteum) surrounds the bone tissue along the periphery (with the exception of the articular surfaces) and has a structure similar to the perichondrium.

In the periosteum, the outer fibrous and inner cellular (or cambial) layers are isolated. The inner layer contains osteoblasts and osteoclasts. A vascular network is localized in the periosteum, from which small vessels penetrate into the bone tissue through perforating channels.

Red bone marrow is considered as an independent organ and belongs to the organs of hematopoiesis and immunogenesis.

Bone tissue in the formed bones is mainly represented by a lamellar form, however, in different bones, in different parts of the same bone, it has a different structure. In the flat bones and epiphyses of the tubular bones, the bone plates form crossbars (trabeculae) that make up the cancellous substance of the bone. In the diaphysis of tubular bones, the plates are tightly adjacent to each other and form a compact substance.

All types of bone tissue develop mainly from the mesenchyme.

There are two types of osteogenesis:

1) development directly from the mesenchyme (direct osteohistogenesis);

2) development from the mesenchyme through the cartilage stage (indirect osteohistogenesis).

The structure of the diaphysis of a tubular bone. On the transverse section of the diaphysis of the tubular bone, the following layers are distinguished:

1) periosteum (periosteum);

2) the outer layer of common (or general) plates;

3) a layer of osteons;

4) the inner layer of common (or general) plates;

5) internal fibrous plate (endosteum).

External common plates are located under the periosteum in several layers, without forming a single ring. Osteocytes are located between the plates in the gaps. Perforating channels pass through the outer plates, through which perforating fibers and vessels penetrate from the periosteum into the bone tissue. The perforating vessels provide trophism to the bone tissue, and the perforating fibers firmly connect the periosteum with the bone tissue.

The osteon layer consists of two components: osteons and insertion plates between them. The osteon is the structural unit of the compact substance of the tubular bone. Each osteon consists of 5-20 concentrically layered plates and the osteon channel, in which the vessels (arterioles, capillaries, venules) pass. There are anastomoses between the canals of adjacent osteons. Osteons make up the bulk of the bone tissue of the diaphysis of the tubular bone. They are located longitudinally along the tubular bone, respectively, by force (or gravitational) lines and provide a support function. When the direction of the lines of force changes, as a result of a fracture or curvature of the bones, osteons that do not carry a load are destroyed by osteoclasts. However, osteons are not completely destroyed, and part of the bone plates of the osteon along its length are preserved, and such remaining parts of the osteon are called insertion plates.

During postnatal osteogenesis, there is a constant restructuring of the bone tissue, some osteons are resorbed, others are formed, so there are intercalated plates or remnants of previous osteons between the osteons.

The inner layer of the common plates has a structure similar to the outer one, but it is less pronounced, and in the region of the transition of the diaphysis to the epiphyses, the common plates continue into trabeculae.

Endooste - a thin connective tissue plate lining the cavity of the diaphysis canal. The layers in the endosteum are not clearly expressed, but among the cellular elements there are osteoblasts and osteoclasts.

Classification of bone tissue

There are two types of bone tissue:

1) reticulofibrous (coarse-fibered);

2) lamellar (parallel fibrous).

The classification is based on the nature of the location of collagen fibers. In reticulofibrous bone tissue, bundles of collagen fibers are thick, tortuous, and randomly arranged. In the mineralized intercellular substance, osteocytes are randomly located in the lacunae. Lamellar bone tissue consists of bone plates, in which collagen fibers or their bundles are arranged parallel in each plate, but at right angles to the course of the fibers of neighboring plates. Between the plates in the gaps are osteocytes, while their processes pass through the tubules through the plates.

In the human body, bone tissue is represented almost exclusively by a lamellar form. Reticulofibrous bone tissue occurs only as a stage in the development of some bones (parietal, frontal). In adults, it is located in the area of ​​attachment of the tendons to the bones, as well as in place of the ossified sutures of the skull (sagittal suture, scales of the frontal bone).

Development of bone tissue and bones (osteohistogenesis)

All types of bone tissue develop from one source - from the mesenchyme, but the development of different bones is not the same. There are two types of osteogenesis:

1) development directly from the mesenchyme - direct osteohistogenesis;

2) development from the mesenchyme through the cartilage stage - indirect osteohistogenesis.

With the help of direct osteohistogenesis, a small number of bones develop - the integumentary bones of the skull. At the same time, reticulofibrous bone tissue is first formed, which soon collapses and is replaced by lamellar one.

Direct osteogenesis proceeds in four stages:

1) the stage of formation of skeletal islands in the mesenchyme;

2) the stage of formation of osseoid tissue - an organic matrix;

3) the stage of mineralization (calcification) of osteoid tissue and the formation of reticulofibrous bone tissue;

4) the stage of transformation of reticulofibrous bone tissue into lamellar bone tissue.

Indirect osteogenesis begins from the 2nd month of intrauterine development. First, in the mesenchyme, due to the activity of chondroblasts, a cartilaginous model of the future bone from hyaline cartilage tissue, covered with perichondrium, is laid. Then there is a replacement, first in the diaphysis, and then in the epiphyses of the bone cartilage tissue. Ossification in the diaphysis is carried out in two ways:

1) perichondral;

2) endochondral.

First, in the area of ​​the diaphysis of the cartilaginous anlage of the bone, osteoblasts are evicted from the perichondrium and form reticulofibrous bone tissue, which, in the form of a cuff, covers the cartilaginous tissue along the periphery. As a result, the perichondrium turns into a periosteum. This method of bone formation is called perichondral. After the formation of the bone cuff, the trophism of the deep sections of the hyaline cartilage in the area of ​​the diaphysis is disturbed, as a result of which calcium salts are deposited here - cartilage shoaling. Then, under the inductive influence of calcified cartilage, blood vessels grow into this zone from the periosteum through the holes in the bone cuff, the adventitia of which contains osteoclasts and osteoblasts. Osteoclasts destroy the stagnant cartilage, and around the vessels, due to the activity of osteoblasts, lamellar bone tissue is formed in the form of primary osteons, which are characterized by a wide lumen (channel) in the center and fuzzy boundaries between the plates. This method of bone tissue formation in the depth of cartilage tissue is called endochondral. Simultaneously with endochondral ossification, the coarse-fibered bone cuff is restructured into lamellar bone tissue, which makes up the outer layer of the general plates. As a result of perichondral and endochondral ossification, the cartilaginous tissue in the area of ​​the diaphysis is replaced by bone. In this case, a cavity of the diaphysis is formed, which is first filled with red bone marrow, which is then replaced by white bone marrow.

The epiphyses of tubular bones and spongy bones develop only endochondral. Initially, in the deep parts of the cartilaginous tissue of the epiphysis, shallowing is noted. Then, vessels with osteoclasts and osteoblasts penetrate there, and due to their activity, the cartilage tissue is replaced by lamellar tissue in the form of trabeculae. The peripheral part of the cartilage tissue is preserved in the form of articular cartilage. Between the diaphysis and the epiphysis, cartilage tissue is preserved for a long time - the metaepiphyseal plate, due to the constant reproduction of the cells of which the bone grows in length.

In the metaepiphyseal plate, the following cell zones are distinguished:

1) border zone;

2) zone of columnar cells;

3) zone of vesicular cells.

Approximately by the age of 20, the metaepiphyseal plate is reduced, synostosis of the epiphyses and diaphysis occurs, after which the growth of the bone in length stops. In the process of bone development due to the activity of osteoblasts of the periosteum, bones grow in thickness. Regeneration of bones after their damage and fractures is carried out due to the activity of periosteal osteoblasts. Reorganization of bone tissue is carried out constantly throughout osteogenesis: some osteons or their parts are destroyed, others are formed.

Factors affecting the process of osteohistogenesis and the state of bone tissue

The following factors influence the process of osteohistogenesis on the state of bone tissue.

1. The content of vitamins A, C, D. The lack of these vitamins in food leads to a violation of the synthesis of collagen fibers and to the disintegration of existing ones, which is manifested by fragility and increased fragility of bones. Insufficient formation of vitamin D in the skin leads to a violation of bone tissue calcification and is accompanied by insufficient bone strength and flexibility (for example, with rickets). An excess of vitamin A activates the activity of osteoclasts, which is accompanied by bone resorption.

2. The optimal content of thyroid and parathyroid hormones - calcitonin and parathyroid hormone, which regulate the calcium content in the blood serum. The state of bone tissue is also influenced by the level of sex hormones.

3. Bone curvature leads to the development of a piezoelectric effect - stimulation of osteoclasts and bone resorption.

4. Social factors - food, etc.

5. Environmental factors.

Age-related changes in bone tissue

With increasing age, the ratio of organic and inorganic substances in the bone tissue changes towards an increase in inorganic and a decrease in organic, which is accompanied by an increase in bone fragility. This may explain the significant increase in the incidence of fractures in the elderly.

Topic 15. MUSCLE TISSUES. SKELETAL MUSCLE TISSUE

Almost all types of cells have the property of contractility due to the presence in their cytoplasm of the contractile apparatus, represented by a network of thin microfilaments (5-7 nm), consisting of contractile proteins actin, myosin, tropomyosin. Due to the interaction of these microfilament proteins, contractile processes are carried out and the movement of hyaloplasm, organelles, vacuoles in the cytoplasm, the formation of pseudopodia and plasmolemma invaginations, as well as the processes of phago- and pinocytosis, exocytosis, cell division and movement are ensured. The content of contractile elements (and, consequently, contractile processes) is not equally expressed in different types of cells. Contractile structures are most pronounced in cells whose main function is contraction. Such cells or their derivatives form muscle tissues that provide contractile processes in hollow internal organs and vessels, movement of body parts relative to each other, maintaining posture and moving the body in space. In addition to movement, during contraction, a large amount of heat is released, and therefore, muscle tissues are involved in the thermoregulation of the body.

Muscle tissues are not the same in structure, sources of origin and innervation, and functional features.

Any kind of muscle tissue, in addition to contractile elements (muscle cells and muscle fibers), includes cellular elements and fibers of loose fibrous connective tissue and vessels that provide trophism and transfer the forces of contraction of muscle elements.

Muscle tissue is divided according to its structure into smooth (non-striated) and striated (striated). Each of the two groups, in turn, is divided into species according to sources of origin, structure and functional features.

Smooth muscle tissue, which is part of the internal organs and blood vessels, develops from the mesenchyme. Special muscle tissues of neural origin include smooth muscle cells of the iris, epidermal origin - myoepithelial cells of the salivary, lacrimal, sweat and mammary glands.

Striated muscle tissue is divided into skeletal and cardiac. Both of these varieties develop from the mesoderm, but from its different parts: skeletal - from somite myotomes, cardiac - from visceral sheets of splanchiotomes.

Striated skeletal muscle tissue

As already noted, the structural and functional unit of this tissue is the muscle fiber. It is an elongated cylindrical formation with pointed ends from 1 to 40 mm long (and according to some sources - up to 120 mm), with a diameter of 0,1 mm. The muscle fiber is surrounded by a sheath of sarcolemma, in which two sheets are clearly distinguished under an electron microscope: the inner sheet is a typical plasmalemma, and the outer one is a thin connective tissue plate (basal plate).

The main structural component of the muscle fiber is the myosymplast. Thus, the muscle fiber is a complex formation and consists of the following main structural components:

1) myosymplast;

2) myosatellite cells;

3) basal plate.

The basal plate is formed by thin collagen and reticular fibers, belongs to the supporting apparatus and performs an auxiliary function of transferring contraction forces to the connective tissue elements of the muscle.

Myosatellite cells are growth elements of muscle fibers that play an important role in the processes of physiological and reparative regeneration.

The myosymplast is the main structural component of the muscle fiber, both in terms of volume and functions. It is formed by the fusion of independent undifferentiated muscle cells - myoblasts.

Myosymplast can be considered as an elongated giant multinucleated cell, consisting of a large number of nuclei, cytoplasm (sarcoplasm), plasmolemma, inclusions, general and specialized organelles.

In the myosymplast, there are up to 10 thousand longitudinally elongated light nuclei located on the periphery under the plasmalemma. Fragments of a weakly expressed granular endoplasmic reticulum, a lamellar Golgi complex, and a small number of mitochondria are localized near the nuclei. There are no centrioles in the symplast. The sarcoplasm contains inclusions of glycogen and myoglobin.

A distinctive feature of the myosymplast is also the presence in it:

1) myofibrils;

2) sarcoplasmic reticulum;

3) tubules of the T-system.

Myofibrils - the contractile elements of the myosymplast are localized in the central part of the sarcoplasm of the myosymplast.

They are combined into bundles, between which there are layers of sarcoplasm. A large number of mitochondria (sacrosomes) are localized between the myofibrils. Each myofibril extends longitudinally throughout the entire myosymplast and, with its free ends, is attached to its plasmolemma at the conical ends. The diameter of the myofibril is 0,2 - 0,5 microns.

According to their structure, myofibrils are heterogeneous in length, divided into dark (anisotropic), or A-disks, and light (isotropic), or I-disks. Dark and light discs of all myofibrils are located at the same level and cause the transverse striation of the entire muscle fiber. The disks, in turn, consist of thinner fibers - protofibrils, or myofilaments. Dark discs are made up of myosin, light discs are made up of actin.

In the middle of the I-disk across the actin microfilaments, there is a dark strip - a telophragm (or Z-line), in the middle of the A-disk there is a less pronounced mesophragm (or M-line).

Actin myofilaments in the middle of the I-disk are held together by proteins that make up the Z-line, and with their free ends partially enter the A-disk between thick myofilaments.

In this case, six actin filaments are located around one myosin filament. With a partial contraction of the myofibril, the actin filaments seem to be drawn into the A-disk, and a light zone (or H-strip) is formed in it, bounded by the free ends of the microfilaments. The width of the H-band depends on the degree of contraction of the myofibril.

The section of the myofibril located between the two Z-bands is called the sarcomere and is the structural and functional unit of the myofibril. The sarcomere includes the A-disk and two halves of the I-disk located on either side of it. Therefore, each myofibril is a collection of sarcomeres. It is in the sarcomere that contraction processes take place. It should be noted that the terminal sarcomeres of each myofibril are attached to the myosymplast plasmolemma by actin myofilaments.

Structural elements of a sarcomere in a relaxed state can be expressed by the formula:

Z + 1/2I = 1/2A + b + 1/2A + 1/2I + Z.

The contraction process is carried out by the interaction of actin and myosin filaments with the formation of actomyosin "bridges" between them, through which the actin filaments are drawn into the A-disk and the sarcomere is shortened.

Three conditions are necessary for the development of this process:

1) the presence of energy in the form of ATP;

2) the presence of calcium ions;

3) presence of biopotential.

ATP is produced in sarcosomes (mitochondria), located in large quantities between myofibrils. The fulfillment of the second and third conditions is carried out with the help of special organelles of muscle tissue - the sarcoplasmic reticulum (an analogue of the endoplasmic reticulum of ordinary cells) and the T-tubules system.

The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum and consists of dilated cavities and anastomosing tubules surrounding the myofibrils.

In this case, the sarcoplasmic reticulum is subdivided into fragments surrounding individual sarcomeres. Each fragment consists of two terminal cisterns connected by hollow anastomosing tubules - L-tubules. In this case, the terminal tanks cover the sarcomere in the region of the I-disk, and the tubules - in the region of the A-disk. The terminal cisterns and tubules contain calcium ions, which, upon receipt of a nerve impulse and reaching a wave of depolarization of the membranes of the sarcoplasmic reticulum, leave the cisterns and tubules and are distributed between actin and myosin microfilaments, initiating their interaction.

After the wave of depolarization ceases, calcium ions rush back to the terminal cisterns and tubules.

Thus, the sarcoplasmic reticulum is not only a reservoir for calcium ions, but also plays the role of a calcium pump.

The wave of depolarization is transmitted to the sarcoplasmic reticulum from the nerve ending, first through the plasmalemma, and then through the T-tubules, which are not independent structural elements. They are tubular invaginations of the plasmalemma into the sarcoplasm. Penetrating deep, T-tubules branch and cover each myofibril within one bundle strictly at a certain level, usually at the level of the Z-band or somewhat more medially - in the area of ​​\uXNUMXb\uXNUMXbjunction of actin and myosin filaments. Therefore, each sarcomere is approached and surrounded by two T-tubules. On the sides of each T-tubule are two terminal cisterns of the sarcoplasmic reticulum of neighboring sarcomeres, which, together with the T-tubules, form a triad. Between the wall of the T-tubule and the walls of the terminal cisterns there are contacts through which the depolarization wave is transmitted to the membranes of the cisterns and causes the release of calcium ions from them and the onset of contraction.

Thus, the functional role of T-tubules is to transfer excitation from the plasma membrane to the sarcoplasmic reticulum.

For the interaction of actin and myosin filaments and subsequent contraction, in addition to calcium ions, energy is also needed in the form of ATP, which is produced in sarcosomes, which are located in large numbers between myofibrils.

Under the influence of calcium ions, the ATP-ase activity of myosin is stimulated, which leads to the breakdown of ATP with the formation of ADP and the release of energy. Thanks to the released energy, "bridges" are established between the heads of the myosin protein and certain points on the actin protein, and due to the shortening of these "bridges", actin filaments are pulled between myosin filaments.

Then these bonds break down, using the energy of ATP and the myosin head, new contacts are formed with other points on the actin filament, but located distal to the previous ones. This is how the gradual retraction of actin filaments between myosin filaments and shortening of the sarcomere occurs. The degree of this contraction depends on the concentration of free calcium ions near the myofilaments and on the content of ATP.

When the sarcomere is fully contracted, the actin filaments reach the M-band of the sarcomere. In this case, the H-band and I-disks disappear, and the sarcomere formula can be expressed as follows:

Z + 1/2IA + M + 1/2AI + Z.

With a partial reduction, the sarcomere formula will look like this:

Z + 1/nI + 1/nIA + 1/2H + M + 1/2H + 1/nAI + 1/nI + Z.

Simultaneous and friendly contraction of all sarcomeres of each myofibril leads to contraction of the entire muscle fiber. The extreme sarcomeres of each myofibril are attached by actin myofilaments to the plasmolemma of the myosymplast, which has a folded character at the ends of the muscle fiber. At the same time, at the ends of the muscle fiber, the basal plate does not enter the folds of the plasmalemma. It is perforated by thin collagen and reticular fibers, penetrate deep into the folds of the plasmolemma and attach in those places where the actin filaments of the distal sarcomeres are attached from the inside.

This creates a strong connection between the myosymplast and the fibrous structures of the endomysium. Collagen and reticular fibers of the end sections of muscle fibers, together with the fibrous structures of endomysium and perimysium, together form muscle tendons that attach to certain points of the skeleton or are woven into the reticular layer of the dermis of the skin in the facial area. Due to muscle contraction, parts or the whole body move, as well as a change in the relief of the face.

Not all muscle fibers are the same in their structure. There are two main types of muscle fibers, between which there are intermediate ones that differ primarily in the features of metabolic processes and functional properties and, to a lesser extent, in structural features.

Type I fibers - red muscle fibers, are characterized primarily by a high content of myoglobin in the sarcoplasm (which gives them a red color), a large number of sarcosomes, high activity of the succinate dehydrogenase enzyme in them, and high activity of slow-acting ATPase. These fibers have the ability of slow but prolonged tonic contraction and low fatigue.

Type II fibers - white muscle fibers, characterized by a low content of myoglobin, but a high content of glycogen, high activity of phosphorylase and fast-type ATPase. Functionally, fibers of this type are characterized by the ability of a faster, stronger, but shorter contraction.

Between the two extreme types of muscle fibers are intermediate, characterized by a different combination of these inclusions and different activities of the listed enzymes.

Any muscle contains all types of muscle fibers in their various quantitative ratios. In the muscles that maintain the posture, red muscle fibers predominate, in the muscles that provide movement of the fingers and hands, red and transitional fibers predominate. The nature of the muscle fiber can change depending on the functional load and training. It has been established that the biochemical, structural and functional features of the muscle fiber depend on the innervation.

Cross transplantation of efferent nerve fibers and their endings from red fiber to white (and vice versa) leads to a change in metabolism, as well as structural and functional features in these fibers to the opposite type.

The structure and physiology of the muscle

A muscle as an organ consists of muscle fibers, fibrous connective tissue, blood vessels, and nerves. A muscle is an anatomical formation, the main and functionally leading structural component of which is muscle tissue.

Fibrous connective tissue forms layers in the muscle: endomysium, perimysium, epimysium, and tendons.

Endomysium surrounds each muscle fiber, consists of loose fibrous connective tissue and contains blood and lymphatic vessels, mainly capillaries, through which the trophic fiber is provided.

The perimysium surrounds several muscle fibers collected in bundles.

Epimysium (or fascia) surrounds the entire muscle, contributes to the functioning of the muscle as an organ.

Histogenesis of skeletal striated muscle tissue

From the myotomes of the mesoderm, poorly differentiated cells - myoblasts - are evicted to certain areas of the mesenchyme. In the area of ​​contacts of myoblasts, the cytolemma disappears, and a symplastic formation is formed - a myotube, in which nuclei in the form of a chain are located in the middle, and along the periphery, myofibrils begin to differentiate from myofilaments.

Nerve fibers grow to the myotube, forming motor nerve endings. Under the influence of efferent nerve innervation, the restructuring of the muscle tube into a muscle fiber begins: the nuclei move to the periphery of the symplast to the plasmolemma, and the myofibrils occupy the central part. From the folds of the endoplasmic reticulum, the sarcoplasmic reticulum develops, surrounding each myofibril throughout its entire length. The plasmalemma of the myosymplast forms deep tubular protrusions - T-tubules. Due to the activity of the granular endoplasmic reticulum, first of the myoblasts, and then of the muscular tubes, proteins and polysaccharides are synthesized and secreted using the lamellar complex, from which the basal plate of the muscle fiber is formed.

During the formation of the myotube, and then the differentiation of the muscle fiber, part of the myoblasts is not part of the symplast, but is adjacent to it, located under the basal plate. These cells are called myosatellites and play an important role in the process of physiological and reparative regeneration. It has been established that the laying of striated skeletal muscles occurs only in the embryonic period. In the postnatal period, their further differentiation and hypertrophy are carried out, but the number of muscle fibers does not increase even under conditions of intensive training.

Regeneration of skeletal muscle tissue

In muscle, as in other tissues, two types of regeneration are distinguished: physiological and reparative. Physiological regeneration is manifested in the form of hypertrophy of muscle fibers.

This is expressed in an increase in their thickness and length, an increase in the number of organelles, mainly myofibrils, the number of nuclei, which is manifested by an increase in the functional ability of the muscle fiber. It has been established by radioisotope methods that an increase in the content of nuclei in muscle fibers is achieved by the division of myosatellite cells and the subsequent entry into the myosymplast of daughter cells.

The increase in the number of myofibrils is carried out using the synthesis of actin and myosin proteins by free ribosomes and the subsequent assembly of these proteins into actin and myosin myofilaments in parallel with the corresponding sarcomere filaments. As a result of this, myofibrils thicken first, and then their splitting and the formation of daughter ones. It is possible that new actin and myosin myofilaments are formed not in parallel, but end to end with existing ones, which results in their elongation.

The sarcoplasmic reticulum and T-tubules in a hypertrophied muscle fiber are formed due to the growth of the previous elements. With certain types of muscle training, a predominantly red type of muscle fibers (for stayers in athletics) or a white type can be formed.

Age-related hypertrophy of muscle fibers is intensely manifested with the onset of motor activity of the body (1-2 years), which is primarily due to increased nervous stimulation. In old age, as well as under conditions of slight muscle load, atrophy of special and general organelles, thinning of muscle fibers and a decrease in their performance occurs.

Reparative regeneration develops after damage to muscle fibers.

With this method, regeneration depends on the size of the defect. With significant damage along the muscle fiber, myosatellites in the area of ​​damage and in adjacent areas are disinhibited, proliferate intensively, and then migrate to the area of ​​the defect in the muscle fiber, where they are embedded in chains, forming a microtubule.

The subsequent differentiation of the microtubule leads to the replacement of the defect and restoration of the integrity of the muscle fiber. Under conditions of a small defect in the muscle fiber at its ends, due to the regeneration of intracellular organelles, muscle buds are formed that grow towards each other and then merge, leading to the closure of the defect.

Reparative regeneration and restoration of the integrity of muscle fibers can be carried out only under certain conditions: if motor innervation of muscle fibers is preserved and if elements of connective tissue (fibroblasts) do not get into the area of ​​damage. Otherwise, a connective tissue scar is formed at the site of the defect.

At present, the possibility of autotransplantation of muscle tissue, including whole muscles, has been proven under the following conditions:

1) mechanical grinding of the transplant muscle tissue in order to disinhibit satellite cells for their subsequent proliferation;

2) placing the crushed tissue in the fascial bed;

3) suturing the motor nerve fiber to the crushed graft;

4) the presence of contractile movements of antagonist and synergist muscles.

Skeletal muscle innervation

Skeletal muscles receive motor, sensory and trophic (vegetative) innervation. The motor (efferent) innervation of the skeletal muscles of the trunk and limbs is received from the motor neurons of the anterior horns of the spinal cord, and the muscles of the face and head - from the motor neurons of certain cranial nerves.

In this case, either the axon of the motor neuron itself, or its branch, approaches each muscle fiber. In muscles that provide coordinated movements (muscles of the hands, forearm, neck), each muscle fiber is innervated by one motor neuron, which ensures greater accuracy of movements. In the muscles that predominantly maintain the posture, tens and even hundreds of muscle fibers receive motor innervation from one motor neuron through the branching of its axon.

The motor nerve fiber, approaching the muscle fiber, penetrates under the endomysium and basal plate and breaks up into terminals, which, together with the adjacent specific area of ​​the myosymplast, form an axonomuscular synapse (or motor plaque).

Under the influence of a nerve impulse, the depolarization wave propagates further along the T-tubules and, in the region of the triads, is transmitted to the terminal cisterns of the sarcoplasmic reticulum, causing the release of calcium ions and the beginning of the process of contraction of the muscle fiber.

Sensitive innervation of skeletal muscles is carried out by pseudounipolar neurons of the spinal ganglia through various receptor endings in the dendrites of these cells. The receptor endings of skeletal muscles can be divided into two groups:

1) specific receptor devices that are characteristic only for skeletal muscles - muscle spindles and the Golgi tendon complex;

2) non-specific receptor endings of a bushy or tree-like form, distributed in the loose connective tissue of the endo-, peri- and epineurium.

Muscle spindles are complex encapsulated formations. Each muscle contains several to hundreds of muscle spindles. Each muscle spindle contains not only nerve elements, but also 10-12 specific muscle fibers - intrafusal, surrounded by a capsule. These fibers are located parallel to the contractile muscle fibers (extrafusally) and receive not only sensitive, but also special motor innervation. Muscle spindles perceive irritation both when the given muscle is stretched, caused by contraction of the antagonist muscles, and when it contracts, and thereby regulate the degree of contraction and relaxation.

Tendon organs are specialized encapsulated receptors, which include in their structure several tendon fibers surrounded by a capsule, among which the terminal branches of the pseudounipolar neuron dendrite are distributed. When the muscle contracts, the tendon fibers come together and compress the nerve endings. Tendon organs perceive only the degree of contraction of a given muscle. Through muscle spindles and tendon organs, with the participation of spinal centers, automatic movement is ensured, for example, when walking.

Trophic innervation of skeletal muscles is carried out by the autonomic nervous system - its autonomic part and is mainly carried out indirectly through the innervation of blood vessels.

Blood supply

Skeletal muscles are richly supplied with blood. Loose connective tissue (perimysium) contains a large number of arteries and veins, arterioles, venules and arterio-venular anastomoses.

In the endomysium there are capillaries, mostly narrow (4,5 - 7 microns), which provide the trophism of the nerve fiber. The muscle fiber, together with the surrounding capillaries and motor endings, make up the mion. The muscles contain a large number of arteriovenular anastomoses that provide adequate blood supply during various muscle activity.

Topic 16. MUSCLE TISSUES. CARDIAC AND SMOOTH MUSCLE TISSUE

Cardiac muscle tissue

The structural and functional unit of the cardiac striated muscle tissue is the cardiomyocyte. Based on their structure and function, cardiomyocytes are divided into two main groups:

1) typical (or contractile) cardiomyocytes, which together form the myocardium;

2) atypical cardiomyocytes that make up the conduction system of the heart.

A contractile cardiomyocyte is an almost rectangular cell 50–120 µm long and 15–20 µm wide, usually with one nucleus in the center.

Covered on the outside by a basal plate. In the sarcoplasm of the cardiomyocyte, myofibrils are located on the periphery of the nucleus, and between them and near the nucleus there are a large number of mitochondria - sarcosomes. Unlike skeletal muscles, myofibrils of cardiomyocytes are not separate cylindrical formations, but, in essence, a network consisting of anastomosing myofibrils, since some myofilaments seem to split off from one myofibril and continue obliquely into another. In addition, the dark and light disks of neighboring myofibrils are not always located at the same level, and therefore the transverse striation in cardiomyocytes is practically not pronounced compared to striated muscle tissue. The sarcoplasmic reticulum, covering the myofibrils, is represented by dilated anastomosing tubules. Terminal tanks and triads are absent. T-tubules are present, but they are short, wide, and are formed not only by depressions in the plasmalemma, but also in the basal lamina. The mechanism of contraction in cardiomyocytes practically does not differ from the striated skeletal muscles.

Contractile cardiomyocytes, connecting end-to-end with each other, form functional muscle fibers, between which there are numerous anastomoses. Due to this, a network (functional syncytium) is formed from individual cardiomyocytes.

The presence of such slit-like contacts between cardiomyocytes ensures their simultaneous and friendly contraction, first in the atria, and then in the ventricles. The contact areas of neighboring cardiomyocytes are called intercalated discs. In fact, there are no additional structures between cardiomyocytes. Intercalated discs are sites of contact between the cytolemmas of adjacent cardiomyocytes, including simple, desmosomal, and slit-like junctions. Intercalated discs are divided into transverse and longitudinal fragments. In the region of transverse fragments, there are extended desmosomal junctions; actin filaments of sarcomeres are attached to the same place on the inner side of the plasmolemma. Slot-like contacts are localized in the region of longitudinal fragments. Through the intercalated disks, both mechanical, metabolic, and functional connections of cardiomyocytes are provided.

The contractile cardiomyocytes of the atria and the ventricle differ somewhat in morphology and function.

Atrial cardiomyocytes in the sarcoplasm contain fewer myofibrils and mitochondria, T-tubules are almost not expressed in them, and instead of them, vesicles and caveolae, analogues of T-tubules, are detected in a large number under the plasmolemma. In the sarcoplasm of atrial cardiomyocytes, at the poles of the nuclei, specific atrial granules are localized, consisting of glycoprotein complexes. Released from cardiomyocytes into the blood of the atria, these biologically active substances affect the level of pressure in the heart and blood vessels, and also prevent the formation of intra-atrial thrombi. Thus, atrial cardiomyocytes have contractile and secretory functions.

In ventricular cardiomyocytes, contractile elements are more pronounced, and secretory granules are absent.

Atypical cardiomyocytes form the conduction system of the heart, which includes the following structural components:

1) sinus node;

2) atrioventricular node;

3) atrioventricular bundle (His bundle) - trunk, right and left legs;

4) terminal branching of the legs (Purkinje fibers).

Atypical cardiomyocytes provide the generation of biopotentials, their behavior and transmission to contractile cardiomyocytes.

In morphology, atypical cardiomyocytes differ from typical ones:

1) they are larger - 100 microns, thickness - up to 50 microns;

2) the cytoplasm contains few myofibrils, which are randomly arranged, which is why atypical cardiomyocytes do not have transverse striation;

3) the plasmalemma does not form T-tubules;

4) in the intercalated discs between these cells, there are no desmosomes and gap-like junctions.

Atypical cardiomyocytes of different parts of the conducting system differ from each other in structure and function and are divided into three main varieties:

1) P-cells - pacemakers - type I pacemakers;

2) transitional - type II cells;

3) cells of the bundle of His and Purkinje fibers - type III cells.

Type I cells are the basis of the sinoatrial node, and are also contained in a small amount in the atrioventricular node. These cells are able to independently generate bioelectric potentials with a certain frequency, as well as transmit them to type II cells with subsequent transmission to type III cells, from which biopotentials are distributed to contractile cardiomyocytes.

The sources of development of cardiomyocytes are myoepicardial plates, which are certain areas of visceral splanchiotomes.

Innervation of cardiac muscle tissue. Contractile cardiomyocytes receive biopotentials from two sources:

1) from the conducting system (primarily from the sinoatrial node);

2) from the autonomic nervous system (from its sympathetic and parasympathetic parts).

Regeneration of cardiac muscle tissue. Cardiomyocytes regenerate only according to the intracellular type. Proliferation of cardiomyocytes is not observed. There are no cambial elements in cardiac muscle tissue. If significant areas of the myocardium are damaged (for example, necrosis of significant areas in myocardial infarction), the defect is restored due to the growth of connective tissue and the formation of a scar - plastic regeneration. At the same time, the contractile function of this area is absent. The defeat of the conducting system is accompanied by the appearance of rhythm and conduction disturbances.

Smooth muscle tissue of mesenchymal origin

It is localized in the walls of hollow organs (stomach, intestines, respiratory tract, organs of the genitourinary system) and in the walls of blood and lymphatic vessels. The structural and functional unit is the myocyte - a spindle-shaped cell, 30 - 100 microns long (up to 500 microns in the pregnant uterus), 8 microns in diameter, covered with a basal plate.

In the center of the myocyte, an elongated rod-shaped nucleus is localized. Common organelles are located along the poles of the nucleus: mitochondria (sarcosomes), elements of the granular endoplasmic reticulum, lamellar complex, free ribosomes, centrioles. The cytoplasm contains thin (7 nm) and thicker (17 nm) filaments. The thin filaments are made up of the protein actin, and the thick filaments are made up of myosin, and are mostly arranged parallel to the actin filaments. However, together actin and myosin filaments do not form typical myofibrils and sarcomeres, so there is no transverse striation in myocytes. In the sarcoplasm and on the inner surface of the sarcolemma, electron-microscopically, dense bodies are determined, in which actin filaments end and which are considered as analogues of Z-bands in the sarcomeres of skeletal muscle fiber myofibrils. Fixation of myosin components to specific structures has not been established.

Myosin and actin filaments make up the contractile apparatus of the myocyte.

Due to the interaction of actin and myosin filaments, actin filaments slide along myosin filaments, bring together their points of attachment on the dense bodies of the cytolemma, and shorten the length of the myocyte. It has been established that, in addition to actin and myosin filaments, myocytes also contain intermediate ones (up to 10 nm), which are attached to cytoplasmic dense bodies, and with other ends to the cytolemma and transmit the contraction forces of the centrally located contractile filaments to the sarcolemma. With the contraction of the myocyte, its contours become uneven, the shape is oval, and the nucleus twists in a corkscrew shape.

For the interaction of actin and myosin filaments in the myocyte, as well as in the skeletal muscle fiber, energy is needed in the form of ATP, calcium ions and biopotentials. ATP is produced in mitochondria, calcium ions are contained in the sarcoplasmic reticulum, which is presented in a reduced form in the form of vesicles and thin tubules. Under the sarcolemma there are small cavities - caveolae, which are considered as analogues of T-tubules. All these elements ensure the transfer of biopotentials to the vesicles in the tubules, the release of calcium ions, the activation of ATP, and then the interaction of actin and myosin filaments.

The basal plate of the myocyte consists of thin collagen, reticulin and elastic fibers, as well as an amorphous substance, which are the product of the synthesis and secretion of the myocytes themselves. Consequently, the myocyte has not only a contractile, but also a synthetic and secretory function, especially at the stage of differentiation. The fibrillar components of the basal plates of neighboring myocytes connect to each other and thereby unite individual myocytes into functional muscle fibers and functional syncytia. However, between myocytes, in addition to the mechanical connection, there is also a functional connection. It is provided with the help of slot-like contacts, which are located in places of close contact of myocytes. In these places, the basal plate is absent, the cytolemmas of neighboring myocytes approach each other and form slit-like contacts through which ion exchange is carried out. Thanks to mechanical and functional contacts, a friendly contraction of a large number of myocytes that are part of a functional muscle fiber, or syncytium, is ensured.

Efferent innervation of smooth muscle tissue is carried out by the autonomic nervous system. At the same time, the terminal branches of the axons of efferent autonomic neurons, passing over the surface of several myocytes, form small varicose thickenings on them, which somewhat bend the plasmalemma and form myoneural synapses. When nerve impulses enter the synaptic cleft, mediators - acetylcholine and norepinephrine - are released. They cause depolarization of the plasmolemma of myocytes and their contraction. However, not all myocytes have nerve endings. Depolarization of myocytes that do not have autonomic innervation is carried out through slit-like contacts from neighboring myocytes that receive efferent innervation. In addition, excitation and contraction of myocytes can occur under the influence of various biologically active substances (histamine, serotonin, oxytocin), as well as mechanical stimulation of an organ containing smooth muscle tissue. There is an opinion that, despite the presence of efferent innervation, nerve impulses do not induce contraction, but only regulate its duration and strength.

The contraction of smooth muscle tissue is usually prolonged, which ensures the maintenance of the tone of hollow internal organs and blood vessels.

Smooth muscle tissue does not form muscles in the anatomical sense of the word. However, in the hollow internal organs and in the wall of the vessels between the bundles of myocytes, there are layers of loose fibrous connective tissue that form a kind of endomysium, and between layers of smooth muscle tissue - perimysium.

Regeneration of smooth muscle tissue is carried out in several ways:

1) through intracellular regeneration (hypertrophy with increased functional load);

2) through mitotic division of myocytes (proliferation);

3) through differentiation from cambial elements (from adventitial cells and myofibroblasts).

Special smooth muscle tissue

Among special smooth muscle tissues, tissues of neural and epidermal origin can be distinguished.

Tissues of neural origin develop from the neuroectoderm, from the edges of the optic cup, which is a protrusion of the diencephalon. From this source, myocytes develop, forming two muscles of the iris of the eye - the muscle that narrows the pupil, and the muscle that expands the pupil. In their morphology, these myocytes do not differ from mesenchymal ones, but differ in their innervation. Each myocyte has autonomic innervation: the muscle that expands the pupil is sympathetic, and the muscle that narrows is parasympathetic. Due to this, the muscles contract quickly and in a coordinated manner, depending on the power of the light beam.

Tissues of epidermal origin develop from the skin ectoderm and are star-shaped cells located in the terminal sections of the salivary, mammary and sweat glands, outside the secretory cells. In its processes, the myoepithelial cell contains actin and myosin filaments, due to which the processes of the cells contract and contribute to the release of secretions from the terminal sections and small ducts into larger ones. These myocytes also receive efferent innervation from the autonomic nervous system.

Topic 17. NERVE TISSUE

Structural and functional features of the nervous tissue:

1) consists of two main types of cells - neurocytes and neuroglia;

2) there is no intercellular substance;

3) nervous tissue is not divided into morphological subgroups;

4) the main source of origin is the neuroectoderm.

Structural components of the nervous tissue:

1) nerve cells (neurocytes or neurons);

2) glial cells - gliocytes.

Functions of nervous tissue:

1) perception of various stimuli and their transformation into nerve impulses;

2) conduction of nerve impulses, their processing and transmission to the working organs.

These functions are performed by neurocytes - the functionally leading structural components of the nervous tissue. Neuroglial cells contribute to the implementation of these functions.

Sources and stages of development of nervous tissue

The main source is the neuroectoderm. Some cells, glial cells, develop from microglia and from mesenchyme (from blood monocytes).

Stages of development:

1) neural plate;

2) neural groove;

3) neural tube, ganglion plate, neural placodes.

Nervous tissue develops from the neural tube, mainly from the organs of the central nervous system (spinal cord and brain). From the ganglion plate develops the nervous tissue of some organs of the peripheral nervous system (vegetative and spinal ganglia). Cranial nerve ganglia develop from neural placodes. In the process of development of the nervous tissue, two types of cells are first formed:

1) neuroblasts;

2) glioblasts.

Then, various types of neurocytes differentiate from neuroblasts, and various types of macroglial cells (ependymocytes, astrocytes, oligodendrocytes) differentiate from glioblasts.

Characterization of neurocytes

Morphologically, all differentiated neurocytes are process cells. Conventionally, two parts are distinguished in each nerve cell:

1) cell body (pericaryon);

2) processes.

The processes of neurocytes are divided into two types:

1) an axon (neurite), which conducts impulses from the cell body to other nerve cells or working organs;

2) a dendrite that conducts impulses to the cell body.

In any nerve cell there is only one axon, there can be one or more dendrites. The processes of nerve cells end with terminal devices of various types (effector, receptor, synaptic).

The structure of the perikaryon of a nerve cell. In the center, usually one nucleus is localized, containing mainly euchromatin, and 1–2 distinct nucleoli, which indicates a high functional stress of the cell.

The most developed organelles of the cytoplasm are the granular ER and the lamellar Golgi complex.

When staining neurocytes with basic dyes (according to the Nissl method), granular EPS is detected in the form of basophilic clumps (Nissl clumps), and the cytoplasm has a spotted appearance (the so-called tigroid substance).

The processes of nerve cells are elongated sections of nerve cells. They contain neuroplasm, as well as single mitochondria, neurofilaments and neurotubules. In the processes, there is a movement of neuroplasm from the perikaryon to the nerve endings (direct current), as well as from the terminals to the pericarinone (retrograde current). At the same time, direct fast transport (5–10 mm/h) and direct slow transport (1–3 mm/day) are distinguished in axons. Transport of substances in dendrites - 3 mm/h.

The most common method for detecting and studying nerve cells is the silver nitrate impregnation method.

Classification of neurocytes

Nerve cells are classified:

1) by morphology;

2) by function.

According to morphology, according to the number of processes, they are divided into:

1) unipolar (pseudo-unipolar) - with one process;

2) bipolar - with two processes;

3) multipolar - more than two processes.

By function, they are divided into:

1) afferent (sensitive);

2) efferent (motor, secretory);

3) associative (insert);

4) secretory (neuroendocrine).

Structural and functional characteristics of glial cells

Neuroglia cells are auxiliary cells of the nervous tissue and perform the following functions:

1) support;

2) trophic;

3) delimiting;

4) secretory;

5) protective, etc.

Glial cells in their morphology are also process cells, not identical in size, shape, and number of processes. On the basis of size, they are divided primarily into macroglia and microglia. In addition, macroglial cells have an ectodermal source of origin (from the neuroectoderm), microglial cells develop from the mesenchyme.

Ependymocytes have a strictly limited localization: they line the cavities of the central nervous system (central canal of the spinal cord, ventricles and cerebral aqueduct). In their morphology, they somewhat resemble epithelial tissue, since they form the lining of the brain cavities. Ependymocytes have an almost prismatic shape, and they distinguish between apical and basal poles. They are interconnected by their lateral surfaces by means of desmosomal junctions. On the apical surface of each epindimocyte there are cilia, due to the vibrations of which the movement of cerebrospinal fluid in the brain cavities is ensured.

Thus, ependymocytes perform the following functions of the nervous system:

1) delimiter (forming a lining of the brain cavities);

2) secretory;

3) mechanical (ensure the movement of cerebral fluid);

4) support (for neurocytes);

5) barrier (participate in the formation of the superficial glial boundary membrane).

Astrocytes are cells with numerous processes that together resemble the shape of a star, hence their name. According to the structural features of their processes, astrocytes are divided into:

1) protoplasmic (short, but wide and strongly branching processes);

2) fibrous (thin, long, slightly branching processes).

Protoplasmic astrocytes perform supporting and trophic functions for gray matter neurocytes.

Fibrous astrocytes carry out a supporting function for neurocytes and their processes, since their long, thin processes form glial fibers. In addition, the terminal extensions of the processes of fibrous astrocytes form perivascular (circumvascular) glial boundary membranes, which are one of the structural components of the blood-brain barrier.

Oligodendrocytes are small cells, the most common population of gliocytes. They are localized mainly in the peripheral nervous system and, depending on the area of ​​localization, are divided into:

1) mantle gliocytes (surround the bodies of nerve cells in the nerve and autonomic ganglia;

2) lemmocytes, or Schwann cells (surround the processes of nerve cells, together with which they form nerve fibers);

3) terminal gliocytes (accompany the terminal branching of the dendrites of sensitive nerve cells).

All varieties of oligodendrocytes, surrounding the bodies, processes and endings of nerve cells, perform supporting, trophic, and barrier functions for them, isolating nerve cells from lymphocytes.

The fact is that the antigens of nerve cells are foreign to their own lymphocytes. Therefore, nerve cells and their various parts are distinguished from blood lymphocytes and connective tissue:

1) perivascular boundary glial membranes;

2) superficial glial boundary membrane;

3) lemmocytes and terminal gliocytes (on the periphery).

When these barriers are violated, autoimmune reactions occur.

Microglia is represented by small process cells that perform a protective function - phagocytosis. Based on this, they are called glial macrophages. Most researchers believe that glial macrophages (like any other macrophages) are cells of mesenchymal origin.

Nerve fibers

Nerve fibers are not independent structural elements of the nervous tissue, but are complex formations that include the following elements:

1) processes of nerve cells (axial cylinders);

2) glial cells (lemmocytes, or Schwann cells);

3) connective tissue plate (knitting plate).

The main function of nerve fibers is to conduct nerve impulses. In this case, the processes of nerve cells (axial cylinders) conduct nerve impulses, and glial cells (lemmocytes) contribute to this conduction.

According to the structural features and function, nerve fibers are divided into two types:

1) unmyelinated;

2) myelin.

The structure and functional features of an unmyelinated nerve fiber. An unmyelinated nerve fiber is a chain of lemmocytes into which several (5–20) axial cylinders are pressed. Each axial cylinder bends the cytolemma of the lemmocyte and, as it were, sinks into its cytoplasm. In this case, the axial cylinder is surrounded by the cytolemma of the lemmocyte, and its contiguous areas constitute the mesaxon.

Mesaxone in unmyelinated nerve fibers does not play a significant functional role, but is an important structural and functional formation in the myelinated nerve fiber.

In their structure, unmyelinated nerve fibers are cable-type fibers. Despite this, they are thin (5 - 7 microns) and conduct nerve impulses very slowly (1 - 2 m / s).

The structure of the myelinated nerve fiber. The myelinated nerve fiber has the same structural components as the unmyelinated one, but differs in a number of features:

1) the axial cylinder is one and plunges into the central part of the lemmocyte chain;

2) the mesaxon is long and twisted around the axial cylinder, forming a myelin layer;

3) the cytoplasm and nucleus of lemmocytes are shifted to the periphery and constitute the neurolemma of the myelin nerve fiber;

4) the basal plate is located on the periphery.

On the cross section of the myelinated nerve fiber, the following structural elements are visible:

1) axial cylinder;

2) myelin layer;

3) neurolemma;

4) basal plate.

Since the basis of any cytolemma is the bilipid layer, the myelin sheath of the myelin nerve fiber (twisted mesaxon) is formed by layers of lipid layers, intensely stained black with osmic acid.

Along the course of the myelinated nerve fiber, the boundaries of neighboring lemmocytes are visible - nodal intercepts (Ranvier intercepts), as well as areas between two intercepts (internodal segments), each of which corresponds to the length of one lemmocyte. In each internodal segment, myelin notches are clearly visible - transparent areas that contain the cytoplasm of the lemmocyte between the turns of the mesaxon.

The high speed of conduction of nerve impulses along myelinated nerve fibers is explained by the saltatory method of conducting nerve impulses: jumps from one intercept to another.

The reaction of nerve fibers to rupture or intersection. After a rupture or intersection of a nerve fiber, the processes of degeneration and regeneration are carried out in it.

Since the nerve fiber is a combination of nerve and glial cells, after its damage, a reaction is noted (both in nerve and glial cells). After crossing, the most noticeable changes appear in the distal section of the nerve fiber, where the collapse of the axial cylinder is noted, i.e., degeneration of the part of the nerve cell cut off from the body. Lemmocytes surrounding this area of ​​the axial cylinder do not die, but round, proliferate and form a strand of glial cells along the disintegrated nerve fiber. At the same time, these glial cells phagocytize fragments of the disintegrated axial cylinder and its myelin sheath.

In the perikaryon of a nerve cell with a cut-off process, signs of irritation appear: swelling of the nucleus and its shift to the periphery of the cell, expansion of the perinuclear space, degranulation of the membranes of the granular ER, vacuolization of the cytoplasm, etc.

In the proximal part of the nerve fiber at the end of the axial cylinder, an expansion is formed - a growth flask, which gradually grows into the strand of glial cells at the site of the dead distal section of the same fiber. Glial cells surround the growing axial cylinder and gradually transform into lemmocytes. As a result of these processes, the regeneration of the nerve fiber occurs at a rate of 1–4 mm per day. The axial cylinder, growing up to the terminal gliocytes of the disintegrated nerve ending, branches and forms the terminal apparatus (motor or sensory ending) with the help of glial cells. As a result of the regeneration of the nerve fiber and nerve ending, the innervation of the damaged area (reinnervation) is restored, which leads to the restoration of its functions. It should be emphasized that a necessary condition for the regeneration of the nerve fiber is a clear comparison of the proximal and distal sections of the damaged nerve fiber. This is achieved by suturing the end of the cut nerve.

The concepts of "nerve fiber" and "nerve" should not be confused.

The nerve is a complex formation, consisting of:

1) nerve fibers;

2) loose fibrous connective tissue that forms the nerve sheath.

Among the sheaths of the nerve are distinguished:

1) endoneurium (connective tissue surrounding individual nerve fibers);

2) perineurium (connective tissue surrounding bundles of nerve fibers);

3) epineurium (connective tissue surrounding the nerve trunk).

In these membranes are blood vessels that provide trophism of nerve fibers.

Nerve endings (or terminal nerve apparatus). They are the endings of nerve fibers. If the axial cylinder of a nerve fiber is a dendrite of a sensitive nerve cell, then its terminal apparatus forms a receptor. If the axial cylinder is an axon of a nerve cell, then its terminal apparatus forms an effector or synaptic ending. Therefore, nerve endings are divided into three main groups:

1) effector (motor or secretory);

2) prescription (sensitive);

3) synaptic.

The motor nerve ending is the terminal apparatus of the axon on a striated muscle fiber or on a myocyte. A motor nerve ending on a striated muscle fiber is also called a motor plaque. It has three parts:

1) nerve pole;

2) synaptic cleft;

3) muscular pole.

Each terminal branch of the axon contains the following structural elements:

1) presynaptic membrane;

2) synaptic vesicles with a mediator (acetylcholine);

3) accumulation of mitochondria with longitudinal cristae.

The muscle pole (or motor plaque sheets) includes:

1) postsynaptic membrane - a specialized section of the myosymplast plasmolemma containing acetylcholine receptor proteins;

2) a section of the sarcoplasm of the myosymplast, which lacks myofibrils and contains an accumulation of nuclei and sarcosomes.

The synaptic cleft is a 50 nm space between pre- and postsynaptic membranes that contains the enzyme acetylcholinesterase.

Receptor endings (or receptors). They are specialized end devices of dendrites of sensory neurons, mainly pseudo-unipolar nerve cells of the spinal ganglia and cranial nerves, as well as some autonomic neurins (Dogel type II cells).

Receptor nerve endings are classified according to several criteria:

1) by localization:

a) interoroceptors (receptors of internal organs);

b) extrareceptors (perceive external stimuli: repeaters of the skin, sensory organs);

c) proprioceptors (localized in the apparatus of movement);

2) according to the specificity of perception (by modality):

a) chemoreceptors;

b) mechanoreceptors;

c) baroreceptors;

d) thermoreceptors (thermal, cold);

3) by structure:

a) free;

b) non-free (encapsulated, non-encapsulated).

SECTION II. PRIVATE HISTOLOGY

Topic 18. NERVOUS SYSTEM

From an anatomical point of view, the nervous system is divided into central (brain and spinal cord) and peripheral (peripheral nerve nodes, trunks and endings).

The morphological substrate of the reflex activity of the nervous system is reflex arcs, which are a chain of neurons of various functional significance, the bodies of which are located in different parts of the nervous system - both in the peripheral nodes and in the gray matter of the central nervous system.

From a physiological point of view, the nervous system is divided into somatic (or cerebrospinal), which innervates the entire human body, except for internal organs, vessels and glands, and autonomous (or autonomic), which regulates the activity of these organs.

Spinal nodes

The first neuron of each reflex arc is the receptor nerve cell. Most of these cells are concentrated in the spinal nodes located along the posterior roots of the spinal cord. The spinal ganglion is surrounded by a connective tissue capsule. From the capsule, thin layers of connective tissue penetrate into the parenchyma of the node, which forms its skeleton, and blood vessels pass through it in the node.

The dendrites of the nerve cell of the spinal ganglion go as part of the sensitive part of the mixed spinal nerves to the periphery and end there with receptors. Neurites together form the posterior roots of the spinal cord, carrying nerve impulses either to the gray matter of the spinal cord, or along its posterior funiculus to the medulla oblongata.

The dendrites and neurites of the cells in the node and outside it are covered with membranes of lemmocytes. The nerve cells of the spinal nodes are surrounded by a layer of glial cells, which are here called mantle gliocytes. They can be recognized by the round nuclei surrounding the body of the neuron. Outside, the glial sheath of the body of the neuron is covered with a delicate, fine-fibred connective tissue sheath. The cells of this membrane are characterized by an oval-shaped nucleus.

The structure of the peripheral nerves is described in the general histology section.

Spinal cord

It consists of two symmetrical halves, delimited from each other in front by a deep median fissure, and behind by a connective tissue septum.

The inner part of the spinal cord is darker - this is its gray matter. On the periphery of it is a lighter white matter. The gray matter on the cross section of the brain is seen in the form of a butterfly. The protrusions of the gray matter are called horns. There are anterior, or ventral, posterior, or dorsal, and lateral, or lateral, horns.

The gray matter of the spinal cord consists of multipolar neurons, non-myelinated and thin myelinated fibers, and neuroglia.

The white matter of the spinal cord is formed by a set of longitudinally oriented predominantly myelinated fibers of nerve cells.

The bundles of nerve fibers that communicate between different parts of the nervous system are called the pathways of the spinal cord.

In the middle part of the posterior horn of the spinal cord is the own nucleus of the posterior horn. It consists of bundle cells, the axons of which, passing through the anterior white commissure to the opposite side of the spinal cord into the lateral funiculus of the white matter, form the ventral spinocerebellar and spinothalamic pathways and go to the cerebellum and thalamus.

Interneurons are diffusely located in the posterior horns. These are small cells whose axons terminate within the gray matter of the spinal cord of the same (associative cells) or opposite (commissural cells) side.

The dorsal nucleus, or Clark's nucleus, consists of large cells with branched dendrites. Their axons cross the gray matter, enter the lateral funiculus of the white matter of the same side, and ascend to the cerebellum as part of the dorsal spinocerebellar tract.

The medial intermediate nucleus is located in the intermediate zone, the neurites of its cells join the ventral spinocerebellar tract of the same side, the lateral intermediate nucleus is located in the lateral horns and is a group of associative cells of the sympathetic reflex arc. The axons of these cells leave the spinal cord together with the somatic motor fibers as part of the anterior roots and separate from them in the form of white connecting branches of the sympathetic trunk.

The largest neurons of the spinal cord are located in the anterior horns, they also form nuclei from the bodies of nerve cells, the roots of which form the bulk of the fibers of the anterior roots.

As part of the mixed spinal nerves, they enter the periphery and end with motor endings in the skeletal muscles.

The white matter of the spinal cord is composed of myelin fibers running longitudinally. The bundles of nerve fibers that communicate between different parts of the nervous system are called the pathways of the spinal cord.

Brain

In the brain, gray and white matter are also distinguished, but the distribution of these two components is more complicated here than in the spinal cord. The main part of the gray matter of the brain is located on the surface of the cerebrum and cerebellum, forming their cortex. The other (smaller) part forms numerous nuclei of the brain stem.

Brain stem. All nuclei of the gray matter of the brainstem are composed of multipolar nerve cells. They have endings of neurite cells of the spinal ganglia. Also in the brain stem there are a large number of nuclei designed to switch nerve impulses from the spinal cord and brain stem to the cortex and from the cortex to the spinal cord's own apparatus.

The medulla oblongata has a large number of nuclei of its own apparatus of cranial nerves, which are mainly located in the bottom of the IV ventricle. In addition to these nuclei, there are nuclei in the medulla oblongata that switch impulses entering it to other parts of the brain. These kernels include the lower olives.

In the central region of the medulla oblongata is located the reticular substance, in which there are numerous nerve fibers running in different directions and together forming a network. This network contains small groups of multipolar neurons with long few dendrites. Their axons spread in ascending (to the cerebral cortex and cerebellum) and descending directions.

The reticular substance is a complex reflex center associated with the spinal cord, cerebellum, cerebral cortex and hypothalamic region.

The main bundles of myelinated nerve fibers of the white matter of the medulla oblongata are represented by cortico-spinal bundles - pyramids of the medulla oblongata, lying in its ventral part.

The bridge of the brain consists of a large number of transversely running nerve fibers and nuclei lying between them. In the basal part of the bridge, the transverse fibers are separated by pyramidal paths into two groups - posterior and anterior.

The midbrain consists of the gray matter of the quadrigemina and the cerebral peduncles, which are formed by a mass of myelinated nerve fibers coming from the cerebral cortex. The tegmentum contains a central gray matter composed of large multipolar and smaller spindle-shaped cells and fibers.

The diencephalon is basically the optic tubercle. Ventral to it is a hypothalamic (hypothalamic) region rich in small nuclei. The visual hillock contains many nuclei delimited from each other by layers of white matter, they are interconnected by associative fibers. In the ventral nuclei of the thalamic region, ascending sensory pathways end, from which nerve impulses are transmitted to the cortex. Nerve impulses to the visual hillock from the brain go along the extrapyramidal motor pathway.

In the caudal group of nuclei (in the pillow of the thalamus), the fibers of the optic pathway end.

The hypothalamic region is a vegetative center of the brain that regulates the main metabolic processes: body temperature, blood pressure, water, fat metabolism, etc.

Cerebellum

The main function of the cerebellum is to ensure balance and coordination of movements. It has a connection with the brain stem through afferent and efferent pathways, which together form three pairs of cerebellar peduncles. On the surface of the cerebellum there are many convolutions and grooves.

Gray matter forms the cerebellar cortex, a smaller part of it lies deep in the white matter in the form of central nuclei. In the center of each gyrus there is a thin layer of white matter, covered with a layer of gray matter - the bark.

There are three layers in the cerebellar cortex: outer (molecular), middle (ganglionic) and inner (granular).

Efferent neurons of the cerebellar cortex - pear-shaped cells (or Purkinje cells) make up the ganglion layer. Only their neurites, leaving the cerebellar cortex, form the initial link of its efferent inhibitory pathways.

All other nerve cells of the cerebellar cortex are intercalary associative neurons that transmit nerve impulses to pear-shaped cells. In the ganglionic layer, the cells are arranged strictly in one row, their cords, branching abundantly, penetrate the entire thickness of the molecular layer. All branches of the dendrites are located only in one plane, perpendicular to the direction of the convolutions, therefore, with a transverse and longitudinal section of the convolutions, the dendrites of the pear-shaped cells look different.

The molecular layer consists of two main types of nerve cells: basket and stellate.

Basket cells are located in the lower third of the molecular layer. They have thin long dendrites, which branch mainly in a plane located transversely to the gyrus. The long neurites of the cells always run across the gyrus and parallel to the surface above the piriform cells.

The stellate cells are located above the basket cells. There are two forms of stellate cells: small stellate cells, which are equipped with thin short dendrites and weakly branched neurites (they form synapses on the dendrites of pear-shaped cells), and large stellate cells, which have long and highly branched dendrites and neurites (their branches connect with the dendrites of pear-shaped cells). cells, but some of them reach the bodies of pear-shaped cells and are part of the so-called baskets). Together, the described cells of the molecular layer represent a single system.

The granular layer is represented by special cellular forms in the form of grains. These cells are small in size, have 3 - 4 short dendrites, ending in the same layer with terminal branches in the form of a bird's foot. Entering into a synaptic connection with the endings of excitatory afferent (mossy) fibers entering the cerebellum, the dendrites of the granule cells form characteristic structures called cerebellar glomeruli.

The processes of granule cells, reaching the molecular layer, form in it T-shaped divisions into two branches, oriented parallel to the surface of the cortex along the gyri of the cerebellum. These fibers, running in parallel, cross the branching of the dendrites of many pear-shaped cells and form synapses with them and the dendrites of basket cells and stellate cells. Thus, the neurites of the granule cells transmit the excitation they receive from mossy fibers over a considerable distance to many pear-shaped cells.

The next type of cells are spindle-shaped horizontal cells. They are located mainly between the granular and ganglionic layers, from their elongated bodies long, horizontally extending dendrites extend in both directions, ending in the ganglionic and granular layers. Afferent fibers entering the cerebellar cortex are represented by two types: mossy and so-called climbing fibers. Mossy fibers are part of the olivocerebellar and cerebellopontine tracts and have a stimulating effect on the piriform cells. They end in the glomeruli of the granular layer of the cerebellum, where they come into contact with the dendrites of the granule cells.

Climbing fibers enter the cerebellar cortex along the spinocerebellar and vestibulocerebellar pathways. They cross the granular layer, adjoin pear-shaped cells and spread along their dendrites, ending on their surface with synapses. These fibers transmit excitation to pear-shaped cells. When various pathological processes occur in pear-shaped cells, it leads to a disorder in the coordination of movement.

cerebral cortex

It is represented by a layer of gray matter about 3 mm thick. It is very well represented (developed) in the anterior central gyrus, where the thickness of the cortex reaches 5 mm. A large number of furrows and convolutions increases the area of ​​the gray matter of the brain.

The cortex contains about 10 - 14 billion nerve cells.

Different parts of the cortex differ from each other in the location and structure of the cells.

Cytoarchitectonics of the cerebral cortex. The neurons of the cortex are very diverse in form, they are multipolar cells. They are divided into pyramidal, stellate, fusiform, arachnid and horizontal neurons.

Pyramidal neurons make up the bulk of the cerebral cortex. Their bodies have the shape of a triangle, the apex of which faces the surface of the cortex. From the top and side surfaces of the body depart dendrites, ending in different layers of gray matter. Neurites originate from the base of the pyramidal cells, in some cells they are short, forming branches within a given area of ​​the cortex, in others they are long, entering the white matter.

Pyramidal cells of different layers of the cortex are different. Small cells are intercalary neurons, the neurites of which connect separate parts of the cortex of one hemisphere (associative neurons) or two hemispheres (commissural neurons).

Large pyramids and their processes form pyramidal pathways that project impulses to the corresponding centers of the trunk and spinal cord.

In each layer of cells of the cerebral cortex there is a predominance of some types of cells. There are several layers:

1) molecular;

2) external granular;

3) pyramidal;

4) internal granular;

5) ganglionic;

6) a layer of polymorphic cells.

The molecular layer of the cortex contains a small number of small spindle-shaped cells. Their processes run parallel to the surface of the brain as part of the tangential plexus of nerve fibers of the molecular layer. In this case, the bulk of the fibers of this plexus is represented by branching of the dendrites of the underlying layers.

The outer granular layer is a cluster of small neurons that have a different shape (mostly rounded) and stellate cells. The dendrites of these cells rise into the molecular layer, and the axons go into the white matter or, forming arcs, go to the tangential plexus of fibers of the molecular layer.

The pyramidal layer is the largest in thickness, very well developed in the precentral gyrus. The sizes of pyramidal cells are different (within 10 - 40 microns). From the top of the pyramidal cell, the main dendrite departs, which is located in the molecular layer. The dendrites coming from the lateral surfaces of the pyramid and its base are of insignificant length and form synapses with adjacent cells of this layer. In this case, you need to know that the axon of the pyramidal cell always departs from its base. The inner granular layer in some areas of the cortex is very strongly developed (for example, in the visual cortex), but in some areas of the cortex it may be absent (in the precentral gyrus). This layer is formed by small stellate cells, it also includes a large number of horizontal fibers.

The ganglionic layer of the cortex consists of large pyramidal cells, and the region of the precentral gyrus contains giant pyramids, first described by the Kyiv anatomist V. Ya. Bets in 1874 (Bets cells). Giant pyramids are characterized by the presence of large lumps of basophilic substance. The neurites of the cells of this layer form the main part of the cortico-spinal tracts of the spinal cord and terminate in synapses on the cells of its motor nuclei.

The layer of polymorphic cells is formed by spindle-shaped neurons. The neurons of the inner zone are smaller and lie at a great distance from each other, while the neurons of the outer zone are larger. The neurites of the cells of the polymorphic layer go into the white matter as part of the efferent pathways of the brain. Dendrites reach the molecular layer of the cortex.

It must be borne in mind that in different parts of the cerebral cortex, its different layers are represented differently. So, in the motor centers of the cortex, for example, in the anterior central gyrus, layers 3, 5 and 6 are highly developed and layers 2 and 4 are underdeveloped. This is the so-called agranular type of cortex. Descending pathways of the central nervous system originate from these areas. In the sensitive cortical centers, where the afferent conductors coming from the organs of smell, hearing and vision end, the layers containing large and medium pyramids are poorly developed, while the granular layers (2nd and 4th) reach their maximum development. This type is called the granular type of the cortex.

Myeloarchitectonics of the cortex. In the cerebral hemispheres, the following types of fibers can be distinguished: associative fibers (connect individual parts of the cortex of one hemisphere), commissural (connect the cortex of different hemispheres) and projection fibers, both afferent and efferent (connect the cortex with the nuclei of the lower parts of the central nervous system).

The autonomic (or autonomic) nervous system, according to various properties, is divided into sympathetic and parasympathetic. In most cases, both of these species simultaneously take part in the innervation of organs and have an opposite effect on them. So, for example, if irritation of the sympathetic nerves delays intestinal motility, then irritation of the parasympathetic nerves excites it. The autonomic nervous system also consists of central sections, represented by the nuclei of the gray matter of the brain and spinal cord, and peripheral sections - nerve nodes and plexuses. The nuclei of the central division of the autonomic nervous system are located in the middle and medulla oblongata, as well as in the lateral horns of the thoracic, lumbar and sacral segments of the spinal cord. The nuclei of the craniobulbar and sacral divisions belong to the parasympathetic, and the nuclei of the thoracolumbar division belong to the sympathetic nervous system. The multipolar nerve cells of these nuclei are associative neurons of the reflex arcs of the autonomic nervous system. Their processes leave the central nervous system through the anterior roots or cranial nerves and end in synapses on the neurons of one of the peripheral ganglia. These are the preganglionic fibers of the autonomic nervous system. The preganglionic fibers of the sympathetic and parasympathetic autonomic nervous system are cholinergic. The axons of the nerve cells of the peripheral ganglions emerge from the ganglia in the form of postganglionic fibers and form terminal apparatuses in the tissues of the working organs. Thus, morphologically, the autonomic nervous system differs from the somatic one in that the efferent link of its reflex arcs is always binomial. It consists of central neurons with their axons in the form of preganglionic fibers and peripheral neurons located in peripheral nodes. Only the axons of the latter - postganglionic fibers - reach the tissues of the organs and enter into a synaptic connection with them. Preganglionic fibers in most cases are covered with a myelin sheath, which explains the white color of the connecting branches that carry sympathetic preganglionic fibers from the anterior roots to the ganglia of the sympathetic border column. Postganglionic fibers are thinner and in most cases do not have a myelin sheath: these are fibers of gray connecting branches that run from the nodes of the sympathetic border trunk to the peripheral spinal nerves. The peripheral nodes of the autonomic nervous system lie both outside the organs (sympathetic prevertebral and paravertebral ganglia, parasympathetic nodes of the head), and in the wall of organs as part of the intramural nerve plexuses that occur in the digestive tract, heart, uterus, bladder, etc.

Sheaths of the brain and spinal cord

The brain and spinal cord are covered with three types of membranes: soft (directly adjacent to the tissues of the brain), arachnoid and hard (bordering on the bone tissue of the skull and spine). The pia mater covers the brain tissue, it is delimited from it only by the marginal glial membrane. In this shell there are a large number of blood vessels that feed the brain, and numerous nerve fibers, terminal apparatus and single nerve cells. The arachnoid is a very delicate, loose layer of fibrous connective tissue. Between it and the pia mater lies the subarachnoid space, which communicates with the ventricles of the brain and contains cerebrospinal fluid. The dura mater is formed by dense fibrous connective tissue, it consists of a large number of elastic fibers. In the cranial cavity, it is tightly fused with the periosteum. In the spinal canal, the dura mater is delimited from the vertebral periosteum by an epidural space filled with a layer of loose fibrous unformed connective tissue, which provides it with some mobility. The subdural space contains a small amount of fluid.

Topic 19. CARDIOVASCULAR SYSTEM

The heart, blood vessels, and lymphatics together make up the cardiovascular system. Thanks to it, the tissues and organs of the human body are provided with nutrients and biologically active substances, gases, metabolic products and thermal energy.

Blood vessels

These are tubes of various diameters closed in the form of a ring, which carry out a transport function, as well as establishing blood supply to organs and metabolism between blood and surrounding tissues. In the circulatory system, arteries, arterioles, hemocapillaries, venules, veins and arteriolo-venular anastomoses are isolated. Vessels of small caliber in total make up the microvasculature.

Development of blood vessels - angiogenesis

Angiogenesis is the process of formation and growth of blood vessels. It occurs both under normal conditions (for example, in the area of ​​the ovarian follicle after ovulation), and in pathological conditions (during wound healing, tumor growth, during immune responses, observed in neovascular glaucoma, rheumatoid arthritis and other pathological conditions). Cells need oxygen and nutrients to survive. The minimum distance for effective gas diffusion from a blood vessel (source of oxygen) to a cell is 100 - 200 µm. If this value is exceeded, new blood vessels are formed. Angiogenesis causes low pO₂, decrease in pH, hypoglycemia, mechanical stress in the tissue due to cell proliferation, tissue infiltration with immunocompetent or inflammation-supporting cells, mutations (for example, activation of oncogenes or deletion of tumor suppressor genes that control the formation of angiogenic factors).

Angiogenic factors

These factors stimulate the formation of blood vessels. These are growth factors produced by tumors, components of the extracellular matrix, angiogenic factors produced by endothelial cells themselves. Angiogenesis is stimulated by vascular endothelial growth factor (VEGF), angiogenin, fibroblast growth factors (aFGF - acidic and bFGF - alkaline), transforming growth factor (TGFa). All angiogenic factors can be divided into two groups: the first - directly acting on endothelial cells and stimulating their mitosis and motility, and the second - factors of indirect influence that act on macrophages, which, in turn, release growth factors and cytokines. The factors of the second group include, in particular, angiogenin. In response to the action of the angiogenic factor, endothelial cells begin to multiply and change their phenotype. The proliferative activity of cells can increase 100 times. Endothelial cells through their own basement membrane penetrate into the adjacent connective tissue, participating in the formation of the capillary bud. After the end of the action of the angiogenic factor, the phenotype of endothelial cells returns to its original calm state. At later stages of angiogenesis, angiopoietin-1 is involved in vessel remodeling, and its action is also associated with a stabilizing effect on the vessel.

Inhibition of angiogenesis. This process is important and can be considered as a potentially effective method of combating the development of tumors in the early stages, as well as other diseases associated with the growth of blood vessels (for example, neovascular glaucoma, rheumatoid arthritis). Angiogenesis inhibitors - factors that inhibit the proliferation of the main cell types of the vascular wall: angiostatin, endostatin, matrix metalloproteinase inhibitors - α-IFN, r-IFN, γ-IFN, IL-4, IL-12, IL-18, prolactin, plasma coagulation factor blood IV. A natural source of factors that inhibit angiogenesis are tissues that do not contain blood vessels (epithelium, cartilage).

Malignant tumors require an intensive blood supply for growth and reach a noticeable size after the development of a blood supply system in them. Active angiogenesis occurs in tumors associated with the synthesis and secretion of angiogenic factors by tumor cells.

Types of blood vessels and their structure

Arteries are the vessels that carry blood from the heart to the organs. As a rule, this blood is saturated with oxygen, with the exception of the pulmonary artery systems, which carry venous blood. The venous vessels include the vessels through which blood goes to the heart and contains little oxygen, except for the blood in the pulmonary veins. Through the microcirculation vessels (arterioles, hemocapillaries, venules and arteriolo-venular anastomoses) there is an exchange between tissues and blood.

Hemocapillaries connect the arterial link of the circulatory system with the venous one, in addition to networks, the capillaries of which are located either between two arteries (for example, in the glomeruli of the kidney) or between two veins (for example, in the lobules of the liver). The structure of the vessel determines its function, as well as hemodynamic parameters of the blood (blood pressure, blood flow velocity).

All arteries are divided into three types: elastic, muscular and mixed (muscular-elastic). The wall of all arteries and veins consists of three shells: internal, middle and external. Their thickness, tissue composition and functional features are not the same in vessels of different types. The elastic type arteries include large-caliber vessels (aorta and pulmonary artery): blood flows into them under high pressure (120 - 130 mm Hg) and at high speed (0,5 - 1,3 m / s) or directly from the heart, or near it from the aortic arch. The main function of these vessels is transport. High pressure and high speed of flowing blood determine the structure of the walls of the vessels of the elastic type. Thus, the inner shell of large arteries includes the endothelium with a basement membrane, followed by the subendothelial layer and the plexus of elastic fibers. The human endothelium consists of cells of various shapes and sizes. Along the entire length of the vessel, the size and shape of the cells are not the same: sometimes the cells can sometimes reach 500 microns in length and 150 microns in width. As a rule, they are single-core, but there are also multi-core ones. The subendothelial layer is represented by loose, thin-fibrillar connective tissue rich in poorly differentiated stellate cells. The thickness of the subendothelial layer is significant. Occasionally, individual longitudinally directed smooth muscle cells may be seen.

The intercellular substance of the inner membrane of a large vessel, or less often other membranes, contains a large amount of glycosaminoglycans and phospholipids, which are detected with appropriate processing. At the same time, it is known that cholesterol and fatty acids are found in people older than 40-50 years. Of great importance in the trophism of the vessel wall is an amorphous substance. The middle shell of a large vessel consists of a large number of elastic fenestrated membranes connected by elastic fibers. As a result, together with other shells, they form a single elastic frame. Between the membranes lie smooth muscle cells (SMC), which have an oblique direction with respect to the membranes, and a few fibroblasts. Due to this structure in large vessels, the tremors of blood ejected into the vessel during contraction of the left ventricle of the heart are softened, and the tone of the vascular wall is maintained during diastole. The outer shell consists of loose fibrous connective tissue, which has many elastic and collagen fibers with a longitudinal direction.

The structure and functional features of the mixed arteries occupy an intermediate position between the vessels of the muscular and elastic types. These vessels include the carotid and subclavian arteries. Their wall also consists of an inner membrane, a subendothelial layer, and an internal elastic membrane. The middle layer of mixed arteries has the same number of smooth muscle cells, elastic fibers and fenestrated elastic membranes. And in the outer shell of the arteries, two layers are distinguished: the inner, containing separate bundles of smooth muscle cells, and the outer, consisting mainly of longitudinally and obliquely arranged bundles of collagen and elastic fibers and connective tissue cells, vessels and nerve fibers. The arteries of the muscular type mainly include the arteries of the body, limbs and internal organs of medium and small caliber, that is, most of the arteries of the body. Their distinguishing feature is a large number of smooth muscle cells, which provide additional pumping power and regulate blood flow to organs. The inner membrane consists of the endothelium, the subdentelial layer and the internal elastic membrane. From the vessels of the microvasculature, a dense network of anastomoses of precapillary, capillary and postcapillary vessels is formed, and other options are possible with the selection of a preferred channel, for example, precapillary arterioles, etc. Arterioles are small arteries of the muscular type, they gradually pass into capillaries. In the arterioles, three membranes are preserved, which are characteristic of larger arteries, but their degree of severity is small. Under an electron microscope in arterioles, especially in precapillary ones, one can detect perforations in the basement membrane of the endothelium and the internal elastic membrane, due to which there is a direct close contact between endotheliocytes and smooth muscle cells. Blood capillaries are the most numerous and thinnest vessels, but the diameter of their lumen can vary. This is due to both the organ features of the capillaries and the functional state of the vascular system. The cross-sectional area of ​​the cut of the capillary bed in any area is many times greater than the cross-sectional area of ​​the original artery.

In the wall of the capillaries, three thin layers are distinguished as the rudiments of the three membranes of the vessels. Slits (or pores) can be found between the cells of the capillary membranes, which are visible even under a light microscope. Fenestra and crevices facilitate the penetration of various macromolecular and corpuscular substances through the capillary wall. The extensibility of the endothelium and the permeability for colloidal particles in the venous part of the capillary is higher than in the arterial part. The capillary wall is a semipermeable membrane, functionally and morphologically closely related to the surrounding connective tissue and actively regulates the metabolism between blood and other tissues. The venous part of the capillaries begins the discharge section of the microvasculature, they are characterized by larger microvilli on the luminal surface of the endothelium and folds resembling valve leaflets, fenestras are more often found in the endothelium. Blood from the capillary bed is collected in postcapillary venules. The structure of these vessels is characterized by shorter sizes of endothelial cells, roundness of the nuclei, and a pronounced outer connective tissue membrane. The venous section of the microvasculature performs a drainage function, regulating the balance between blood and extravascular fluid, removing metabolic products of tissues. Leukocytes often migrate through the walls of venules. Slow blood flow and low blood pressure, as well as the distensibility of these vessels, create conditions for the deposition of blood.

Arteriovenular anastomoses are connections of vessels carrying arterial and venous blood bypassing the capillary bed. They are present in almost all organs.

There are two groups of anastomoses:

1) true arteriovenular anastomoses (shunts), through which pure arterial blood is discharged;

2) atypical arteriovenular fistulas (semi-shunts), through which mixed blood flows.

The external form of the first group of anastomoses can be different - in the form of straight short anastomoses, loop-like, sometimes in the form of branching connections.

Histostructurally, they are divided into two subgroups:

1) vessels that do not have special locking devices;

2) vessels equipped with special contractile structures.

In the second subgroup, anastomoses may have special contractile sphincters in the form of longitudinal ridges or pillows in the subendothelial layer (arteriovenular anastomoses of the trailing arteries type). The contraction of the muscle pads protruding into the lumen of the anastomosis leads to the cessation of blood flow. Simple anastomoses of the epithelioid type (second subgroup) are characterized by the presence in the middle shell of the inner longitudinal and outer circular layers of smooth muscle cells, which, as they approach the venous end, are replaced by short oval light cells, similar to epithelial cells, capable of swelling and swelling, due to which change in the lumen of the anastomosis. In the venous segment of the arteriovenular anastomosis, its wall sharply becomes thinner. The middle shell here contains only a small number of bands of circularly arranged smooth muscle cells. The outer shell consists of dense connective tissue. Arteriovenular anastomoses, especially of the glomerular type, are richly innervated, and they can periodically contract. Arteriovenular anastomoses play an important role in compensatory reactions of the body in case of circulatory disorders. The venous system is the outlet link of the blood. It begins with postcapillary venules in the vessels of the microvasculature. The structure of veins is closely related to the hemodynamic conditions of their functioning. The number of smooth muscle cells in the wall of the veins is not the same and depends on whether the blood moves in them to the heart under the influence of gravity or against it. Due to the fact that in the lower extremities the blood must be lifted against gravity, there is a strong development of smooth muscle elements in the veins of the lower extremities, in contrast to the veins of the upper extremities, head and neck. Veins, especially subcutaneous veins, have valves. The exception is the veins of the brain and its membranes, veins of internal organs, hypogastric, iliac, hollow and nameless.

According to the degree of development of muscle elements in the wall of the veins, they can be divided into two groups: veins of the non-muscular type and veins of the muscular type. Muscular veins, in turn, are divided into veins with weak development of muscle elements and veins with medium and strong development of muscle elements. In veins, as well as in arteries, three membranes are distinguished: internal, middle and external. At the same time, the degree of expression of these membranes in the veins differs significantly. Veins of the non-muscular type are veins of the dura mater, pia mater, veins of the retina, bones, spleen, and placenta. Under the action of blood, these veins are capable of stretching, but the blood accumulated in them flows relatively easily under the influence of its own gravity into larger venous trunks. Veins of the muscular type are distinguished by the development of muscle elements in them. These veins include the veins of the lower body. Also, in some types of veins there are a large number of valves, which prevents the reverse flow of blood, under the force of its own gravity. In addition, the rhythmic contractions of the circularly arranged muscle bundles also contribute to the movement of blood towards the heart. In addition, a significant role in the promotion of blood towards the heart belongs to the contractions of the skeletal muscles of the lower extremities.

Lymphatic vessels

The lymphatic vessels drain lymph into the veins. Lymphatic vessels include lymphatic capillaries, intra- and extraorganic lymphatic vessels that drain lymph from organs, and lymphatic trunks of the body, which include the thoracic duct and the right lymphatic duct, which flow into the large veins of the neck. Lymphatic capillaries are the beginning of the lymphatic system of vessels, into which metabolic products come from tissues, and in pathological cases - foreign particles and microorganisms. It has also long been proven that cells of malignant tumors can also spread through the lymphatic vessels. Lymphatic capillaries are a system of closed and anastomosing with each other and penetrating the entire body. The diameter of the lymphatic capillaries may be larger than the blood capillaries. The wall of the lymphatic capillaries is represented by endothelial cells, which, unlike similar cells in the blood capillaries, do not have a basement membrane. Cell borders are tortuous. The endothelial tube of the lymphatic capillary is closely connected with the surrounding connective tissue. In the lymphatic vessels that bring the lymphatic fluid to the heart, a distinctive feature of the structure is the presence of valves in them and a well-developed outer shell. This can be explained by the similarity of the lympho- and hemodynamic conditions for the functioning of these vessels: the presence of low pressure and the direction of the fluid flow from the organs to the heart. According to the size of the diameter, all lymphatic vessels are divided into small, medium and large. Like veins, these vessels can be non-muscular and muscular in their structure. Small vessels are mainly intraorganic lymphatic vessels, there are no muscle elements in them, and their endothelial tube is surrounded only by a connective tissue membrane.

Medium and large lymphatic vessels have three well-developed membranes - internal, middle and external. In the inner shell, covered with endothelium, there are longitudinally and obliquely directed bundles of collagen and elastic fibers. There are valves on the inner lining of the vessels. They consist of a central connective tissue plate covered with endothelium on the inner and outer surfaces. The boundary between the inner and middle membranes of the lymphatic vessel is not always clearly defined internal elastic membrane. The middle membrane of the lymphatic vessels is poorly developed in the vessels of the head, upper body and upper limbs. In the lymphatic vessels of the lower extremities, on the contrary, it is expressed very clearly. In the wall of these vessels there are bundles of smooth muscle cells that have a circular and oblique direction. The muscular layer of the wall of the lymphatic vessel reaches good development in the collectors of the iliac lymphatic plexus, near the aortic lymphatic vessels and the cervical lymphatic trunks accompanying the jugular veins. The outer shell of the lymphatic vessels is formed by loose, fibrous, unformed connective tissue, which, without sharp boundaries, passes into the surrounding connective tissue.

Vascularization. All large and medium-sized blood vessels have their own system for their nutrition, which is called "vascular vessels". These vessels are necessary to feed the very wall of a large vessel. In the arteries, the vessels of the vessels penetrate to the deep layers of the middle shell. The inner lining of the arteries receives nutrients directly from the blood flowing in this artery. Protein-mucopolysaccharide complexes, which are part of the main substance of the walls of these vessels, play an important role in the diffusion of nutrients through the inner lining of the arteries. The innervation of the vessels is obtained from the autonomic nervous system. The nerve fibers of this part of the nervous system, as a rule, accompany the vessels and end in their wall. By structure, vascular nerves are either myelinated or unmyelinated. Sensory nerve endings in capillaries are diverse in shape. Arteriovenular anastomoses have complex receptors located simultaneously on the anastomosis, arteriole and venule. The terminal branches of nerve fibers end on smooth muscle cells with small thickenings - neuromuscular synapses. Effectors on arteries and veins are of the same type. Along the vessels, especially large ones, there are individual nerve cells and small ganglia of a sympathetic nature. Regeneration. Blood and lymphatic vessels have a high ability to recover both after injuries and after various pathological processes occurring in the body. Recovery of defects in the vascular wall after its damage begins with the regeneration and growth of its endothelium. Already after 1-2 days, a massive amitotic division of endothelial cells is observed at the site of the former injury, and on the 3rd-4th day, a mitotic type of reproduction of endothelial cells appears. The muscle bundles of the damaged vessel, as a rule, recover more slowly and incompletely compared to other tissue elements of the vessel. In terms of the rate of recovery, the lymphatic vessels are somewhat inferior to the blood vessels.

Vascular afferents

pO changes₂, рСО₂ blood, the concentration of H+, lactic acid, pyruvate and a number of other metabolites have both a local effect on the vascular wall and are recorded by chemoreceptors embedded in the vascular wall, as well as baroreceptors that respond to pressure in the lumen of the vessels. These signals reach the centers of regulation of blood circulation and respiration. The responses of the central nervous system are realized by motor autonomic innervation of the smooth muscle cells of the vascular wall and myocardium. In addition, there is a powerful system of humoral regulators of smooth muscle cells of the vascular wall (vasoconstrictors and vasodilators) and endothelial permeability. Baroreceptors are especially numerous in the aortic arch and in the wall of large veins close to the heart. These nerve endings are formed by the terminals of the fibers passing through the vagus nerve. The reflex regulation of blood circulation involves the carotid sinus and carotid body, as well as similar formations of the aortic arch, pulmonary trunk, and right subclavian artery.

Structure and function of the carotid sinus. The carotid sinus is located near the bifurcation of the common carotid artery. This is an expansion of the lumen of the internal carotid artery immediately at the place of its branch from the common carotid artery. In the area of ​​expansion, the middle shell is thinned, while the outer one, on the contrary, is thickened. Here, in the outer shell, there are numerous baroreceptors. Considering that the median sheath of the vessel within the carotid sinus is relatively thin, it is easy to imagine that the nerve endings in the outer sheath are highly sensitive to any changes in blood pressure. From here, information enters the centers that regulate the activity of the cardiovascular system. The nerve endings of the baroreceptors of the carotid sinus are the terminals of the fibers passing through the sinus nerve, a branch of the glossopharyngeal nerve.

carotid body. The carotid body responds to changes in the chemical composition of the blood. The body is located in the wall of the internal carotid artery and consists of cell clusters immersed in a dense network of wide sinusoid-like capillaries. Each glomerulus of the carotid body (glomus) contains 2–3 glomus cells (or type I cells), and 1–3 type II cells are located on the periphery of the glomerulus. Afferent fibers for the carotid body contain substance P and peptides related to the calcitonin gene.

Type I cells form synaptic contacts with afferent fiber terminals. Type I cells are characterized by an abundance of mitochondria, light, and electron-dense synaptic vesicles. Type I cells synthesize acetylcholine, contain an enzyme for the synthesis of this neurotransmitter (choline acetyltransferase), as well as an efficient choline uptake system. The physiological role of acetylcholine remains unclear. Type I cells have H- and M-cholinergic receptors. Activation of any of these types of cholinergic receptors causes or facilitates the release of another neurotransmitter, dopamine, from type I cells. With a decrease in pO₂ secretion of dopamine from type I cells increases. Type I cells can form synapse-like contacts with each other.

Efferent innervation

On the glomus cells, the fibers that pass as part of the sinus nerve (Hering) and the postganglionic fibers from the superior cervical sympathetic ganglion end. The terminals of these fibers contain light (acetylcholine) or granular (catecholamines) synaptic vesicles.

Function

The carotid body registers changes in pCO₂ and ro₂, as well as shifts in blood pH. Excitation is transmitted through synapses to afferent nerve fibers, through which impulses enter the centers that regulate the activity of the heart and blood vessels. Afferent fibers from the carotid body pass through the vagus and sinus nerves (Hering).

The main cell types of the vascular wall

Smooth muscle cell. The lumen of the blood vessels decreases with contraction of the smooth muscle cells of the middle membrane or increases with their relaxation, which changes the blood supply to the organs and the magnitude of blood pressure.

Vascular smooth muscle cells have processes that form numerous gap junctions with neighboring SMCs. Such cells are electrically coupled, through the contacts, excitation (ionic current) is transmitted from cell to cell. This circumstance is important, since only MMCs located in the outer layers of t are in contact with motor terminals. media. SMC walls of blood vessels (especially arterioles) have receptors for various humoral factors.

Vasoconstrictors and vasodilators. The effect of vasoconstriction is realized by the interaction of agonists with α-adrenergic receptors, serotonin receptors, angiotensin II, vasopressin, thromboxane. Stimulation of α-adrenergic receptors leads to contraction of vascular smooth muscle cells. Norepinephrine is predominantly an α-adrenergic receptor antagonist. Adrenaline is an antagonist of α- and β-adrenergic receptors. If the vessel has smooth muscle cells with a predominance of α-adrenergic receptors, then adrenaline causes a narrowing of the lumen of such vessels.

Vasodilators. If α-adrenergic receptors predominate in the SMC, then adrenaline causes the expansion of the lumen of the vessel. Antagonists that in most cases cause relaxation of the SMC: atriopeptin, bradykinin, VIP, histamine, peptides related to the calcitonin gene, prostaglandins, nitric oxide NO.

Motor autonomic innervation. The autonomic nervous system regulates the size of the lumen of the vessels.

Adrenergic innervation is regarded as predominantly vasoconstrictor. Vasoconstrictive sympathetic fibers abundantly innervate small arteries and arterioles of the skin, skeletal muscles, kidneys and celiac region. The density of innervation of the veins of the same name is much less. The vasoconstrictor effect is realized with the help of norepinephrine, an antagonist of α-adrenergic receptors.

cholinergic innervation. Parasympathetic cholinergic fibers innervate the vessels of the external genitalia. With sexual arousal, due to the activation of parasympathetic cholinergic innervation, there is a pronounced expansion of the vessels of the genital organs and an increase in blood flow in them. The cholinergic vasodilating effect has also been observed in relation to the small arteries of the pia mater.

Proliferation

The size of the SMC population of the vascular wall is controlled by growth factors and cytokines. Thus, cytokines of macrophages and B-lymphocytes (transforming growth factor IL-1) inhibit the proliferation of SMCs. This problem is important in atherosclerosis, when SMC proliferation is enhanced by growth factors produced in the vascular wall (platelet growth factor [PDGF], alkaline fibroblast growth factor, insulin-like growth factor 1 [IGF-1], and tumor necrosis factor).

Phenotypes of MMC

There are two variants of SMC of the vascular wall: contractile and synthetic.

Contractile phenotype. SMCs have numerous myofilaments and respond to vasoconstrictors and vasodilators. The granular endoplasmic reticulum in them is expressed moderately. Such SMCs are not capable of migration and do not enter mitoses, since they are insensitive to the effects of growth factors.

synthetic phenotype. SMCs have a well-developed granular endoplasmic reticulum and the Golgi complex, cells synthesize components of the intercellular substance (collagen, elastin, proteoglycan), cytokines and factors. SMCs in the area of ​​atherosclerotic lesions of the vascular wall are reprogrammed from a contractile to a synthetic phenotype. In atherosclerosis, SMCs produce growth factors (eg, platelet-derived factor PDGF), alkaline fibroblast growth factor [bFGF], which enhance the proliferation of neighboring SMCs.

Regulation of the SMC phenotype. The endothelium produces and secretes heparin-like substances that maintain the contractile phenotype of SMC. Paracrine regulatory factors produced by endothelial cells control vascular tone. Among them are derivatives of arachidonic acid (prostaglandins, leukotrienes and thromboxanes), endothelin-1, nitric oxide NO, etc. Some of them cause vasodilation (for example, prostacyclin, nitric oxide NO), others cause vasoconstriction (for example, endothelin-1, angiotensin -II). Insufficiency of NO causes an increase in blood pressure, the formation of atherosclerotic plaques, an excess of NO can lead to collapse.

endothelial cell

The wall of a blood vessel reacts very subtly to changes in hemodynamics and chemical composition of the blood. A peculiar sensitive element that captures these changes is the endothelial cell, which on the one hand is washed by the blood, and on the other hand is turned to the structures of the vascular wall.

Restoration of blood flow in thrombosis.

The effect of ligands (ADP and serotonin, thrombin thrombin) on the endothelial cell stimulates the secretion of NO. His targets are located nearby the MMC. As a result of relaxation of the smooth muscle cell, the lumen of the vessel in the area of ​​the thrombus increases, and blood flow can be restored. Activation of other endothelial cell receptors leads to a similar effect: histamine, M-cholinergic receptors, α2-adrenergic receptors.

Blood clotting. The endothelial cell is an important component of the hemocoagulation process. On the surface of endothelial cells, prothrombin can be activated by clotting factors. On the other hand, the endothelial cell exhibits anticoagulant properties. The direct participation of the endothelium in blood coagulation is the secretion of certain plasma coagulation factors (for example, von Willebrand factor) by endothelial cells. Under normal conditions, the endothelium weakly interacts with blood cells, as well as with blood coagulation factors. The endothelial cell produces prostacyclin PGI2, which inhibits platelet adhesion.

growth factors and cytokines. Endothelial cells synthesize and secrete growth factors and cytokines that influence the behavior of other cells in the vascular wall. This aspect is important in the mechanism of atherosclerosis development, when, in response to the pathological effects of platelets, macrophages, and SMCs, endothelial cells produce platelet-derived growth factor (PDGF), alkaline fibroblast growth factor (bFGF), and insulin-like growth factor-1 (IGF-1). ), IL-1, transforming growth factor. On the other hand, endothelial cells are targets for growth factors and cytokines. For example, endothelial cell mitosis is induced by alkaline fibroblast growth factor (bFGF), while endothelial cell proliferation is stimulated by platelet-derived endothelial cell growth factor. Cytokines from macrophages and B-lymphocytes - transforming growth factor (TGFp), IL-1 and α-IFN - inhibit the proliferation of endothelial cells.

hormone processing. The endothelium is involved in the modification of hormones and other biologically active substances circulating in the blood. So, in the endothelium of the vessels of the lungs, angiotensin-I is converted to angiotensin-II.

Inactivation of biologically active substances. Endothelial cells metabolize norepinephrine, serotonin, bradykinin, prostaglandins.

Breakdown of lipoproteins. In endothelial cells, lipoproteins are broken down to form triglycerides and cholesterol.

Homing of lymphocytes. Venules in the paracortical zone of the lymph nodes, tonsils, Peyer's patches of the ileum, containing an accumulation of lymphocytes, have a high endothelium that expresses on its surface a vascular addressin, recognizable by the CD44 molecule of lymphocytes circulating in the blood. In these areas, lymphocytes attach to the endothelium and are removed from the bloodstream (homing).

barrier function. The endothelium controls the permeability of the vascular wall. This function is most clearly manifested in the blood-brain and hematothymic barriers.

Heart

Development

The heart is laid on the 3rd week of intrauterine development. In the mesenchyme, between the endoderm and the visceral layer of the splanchiotoma, two endocardial tubes lined with endothelium are formed. These tubes are the rudiment of the endocardium. The tubes grow and are surrounded by a visceral splanchiotome. These areas of the splanchiotome thicken and give rise to myoepicardial plates. As the intestinal tube closes, both anlages approach and grow together. Now the common bookmark of the heart (heart tube) looks like a two-layer tube. The endocardium develops from its endocardial part, and the myocardium and epicardium develop from the myoepicardial plate. Cells migrating from the neural crest are involved in the formation of the efferent vessels and valves of the heart (neural crest defects are the cause of 10% of congenital heart defects, such as transposition of the aorta and pulmonary trunk).

Within 24 - 26 days, the primary heart tube quickly lengthens and acquires an s-shape. This is possible due to local changes in the shape of the cells of the heart tube. At this stage, the following sections of the heart are distinguished: the venous sinus is a chamber at the caudal end of the heart, large veins flow into it. Cranial to the venous sinus is an expanded part of the heart tube, which forms the region of the atrium. From the middle curved part of the heart tube develops the ventricle of the heart. The ventricular loop bends caudally, which moves the future ventricle, which was cranial to the atrium, to the definitive position. The area of ​​narrowing of the ventricle and its transition to the arterial trunk is a cone. An opening is visible between the atrium and the ventricle - the atrioventricular canal.

Division into right and left heart. Immediately after the formation of the atrium and ventricle, there are signs of the division of the heart into the right and left halves, which occurs at the 5th and 6th weeks. At this stage, the interventricular septum, interatrial septum and endocardial cushions are formed. The interventricular septum grows from the wall of the primary ventricle in the direction from the apex to the atrium. Simultaneously with the formation of the interventricular septum in the narrowed part of the heart tube between the atrium and the ventricle, two large masses of loosely organized tissue are formed - endocardial pads. Endocardial cushions, consisting of dense connective tissue, are involved in the formation of the right and left atrioventricular canals.

At the end of the 4th week of intrauterine development, a median septum in the form of a semicircular fold appears on the cranial wall of the atrium - the primary interatrial septum.

One arc of the fold runs along the ventral wall of the atria, and the other along the dorsal. The arcs merge near the atrioventricular canal, but the primary interatrial opening remains between them. Simultaneously with these changes, the venous sinus moves to the right and opens into the atrium to the right of the atrial septum. In this place, venous valves are formed.

Complete division of the heart. Complete separation of the heart occurs after the development of the lungs and their vasculature. When the primary septum fuses with the endocardial cushions of the atrioventricular valve, the primary atrial opening closes. Mass death of cells in the cranial part of the primary septum leads to the formation of many small holes that form the secondary interatrial opening. It controls the even flow of blood to both halves of the heart. Soon, a secondary atrial septum forms between the venous valves and the primary atrial septum in the right atrium. Its concave edge is directed upwards to the confluence of the sinus, and later - the inferior vena cava. A secondary opening is formed - an oval window. The remnants of the primary atrial septum, which close the foramen ovale in the secondary atrial septum, form a valve that distributes blood between the atria.

Direction of blood flow

Since the outlet of the inferior vena cava lies near the foramen ovale, blood from the inferior vena cava enters the left atrium. When the left atrium contracts, blood presses the cusp of the primary septum against the foramen ovale. As a result, blood does not flow from the right atrium to the left, but moves from the left atrium to the left ventricle.

The primary septum functions as a one-way valve in the foramen ovale of the secondary septum. Blood enters from the inferior vena cava through the foramen ovale into the left atrium. Blood from the inferior vena cava mixes with blood entering the right atrium from the superior vena cava.

Fetal blood supply. Oxygenated placental blood with a relatively low concentration of CO2 enters the liver through the umbilical vein, and from the liver into the inferior vena cava. Part of the blood from the umbilical vein through the venous duct, bypassing the liver, immediately enters the system of the inferior vena cava. In the inferior vena cava, the blood is mixed. CO high blood₂ enters the right atrium from the superior vena cava, which collects blood from the upper body. Through the foramen ovale, part of the blood flows from the right atrium to the left. With atrial contraction, the valve closes the foramen ovale, and blood from the left atrium enters the left ventricle and then into the aorta, that is, into the systemic circulation. From the right ventricle, blood is directed to the pulmonary trunk, which is connected with the aorta by an arterial or botallic duct. Consequently, small and large circles of blood circulation are communicated through the ductus arteriosus. In the early stages of fetal development, the need for blood in the immature lungs is still small, blood from the right ventricle enters the pool of the pulmonary artery. Therefore, the level of development of the right ventricle will be determined by the level of development of the lung.

As the lungs develop and their volume increases, more and more blood is sent to them and less passes through the ductus arteriosus. The ductus arteriosus closes shortly after birth as the lungs take all the blood from the right heart. After birth, they cease to function and are reduced, turning into connective tissue cords and other vessels - the umbilical cord, the venous duct. The foramen ovale also closes shortly after birth.

The heart is the main organ that moves blood through the blood vessels, a kind of "pump".

The heart is a hollow organ consisting of two atria and two ventricles. Its wall consists of three membranes: internal (endocardium), middle, or muscular (myocardium) and external, or serous (epicardium).

The inner shell of the heart - the endocardium - from the inside covers all the chambers of the heart, as well as the valves of the heart. In different areas, its thickness is different. It reaches its largest size in the left chambers of the heart, especially on the interventricular septum and at the mouth of large arterial trunks - the aorta and pulmonary artery. While on tendon threads it is much thinner.

The endocardium is made up of several types of cells. So, on the side facing the cavity of the heart, the endocardium is lined with endothelium, consisting of polygonal cells. Next comes the subendothelial layer, formed by a connective tissue rich in poorly differentiated cells. Muscles are located deeper.

The deepest layer of the endocardium, lying on the border with the myocardium, is called the outer connective tissue layer. It consists of connective tissue containing thick elastic fibers. In addition to elastic fibers, the endocardium contains long, tortuous collagen and reticular fibers.

The nutrition of the endocardium is carried out mainly diffusely due to the blood in the chambers of the heart.

Next comes the muscle layer of cells - the myocardium (its properties were described in the chapter on muscle tissue). Myocardial muscle fibers are attached to the supporting skeleton of the heart, which is formed by fibrous rings between the atria and ventricles and dense connective tissue at the mouths of large vessels.

The outer shell of the heart, or epicardium, is a visceral sheet of the pericardium, similar in structure to the serous membranes.

Between the pericardium and the epicardium there is a slit-like cavity, in which there is a small amount of fluid, due to which, when the heart contracts, the friction force decreases.

Valves are located between the atria and ventricles of the heart, as well as the ventricles and large vessels. However, they have specific names. So, the atrioventricular (atrioventricular) valve in the left half of the heart is bicuspid (mitral), in the right - tricuspid. They are thin plates of dense fibrous connective tissue covered with endothelium with a small number of cells.

In the subendothelial layer of the valves, thin collagen fibrils were found, which gradually pass into the fibrous plate of the valve leaflet, and at the site of attachment of the bi- and tricuspid valves - into the fibrous rings. A large amount of glycosaminoglycans was found in the ground substance of the valve leaflets.

In this case, you need to know that the structure of the atrial and ventricular sides of the valve leaflets is not the same. So, the atrial side of the valve, smooth from the surface, has a dense plexus of elastic fibers and bundles of smooth muscle cells in the subendothelial layer. The number of muscle bundles markedly increases at the base of the valve. The ventricular side is uneven, equipped with outgrowths from which tendon filaments begin. Elastic fibers in a small amount are located on the ventricular side only directly under the endothelium.

There are also valves on the border between the ascending aortic arch and the left ventricle of the heart (aortic valves), between the right ventricle and the pulmonary trunk there are semilunar valves (so named because of the specific structure).

On a vertical section in the leaflet of the valve, three layers can be distinguished - inner, middle and outer.

The inner layer, facing the ventricle of the heart, is a continuation of the endocardium. In it, under the endothelium, elastic fibers run longitudinally and transversely, followed by a mixed elastic-collagen layer.

The middle layer is thin, consists of loose fibrous connective tissue rich in cellular elements.

The outer layer, facing the aorta, contains collagen fibers that originate from the annulus fibrosus around the aorta.

The heart receives nutrients from the system of coronary arteries.

Blood from the capillaries is collected in the coronary veins, which flow into the right atrium, or venous sinus. Lymphatic vessels in the epicardium accompany the blood vessels.

Innervation. Several nerve plexuses and small nerve ganglia are found in the membranes of the heart. Among the receptors, there are both free and encapsulated endings located in the connective tissue, on muscle cells and in the wall of the coronary vessels. The bodies of sensory neurons lie in the spinal nodes (C7 - Th6), and their axons, covered with a myelin sheath, enter the medulla oblongata. There is also an intracardiac conduction system - the so-called autonomous conduction system, which generates impulses to contract the heart.

Topic 20. ENDOCRINE SYSTEM

The endocrine system together with the nervous system have a regulatory effect on all other organs and systems of the body, forcing it to function as a single system.

The endocrine system includes glands that do not have excretory ducts, but release highly active biological substances into the internal environment of the body, acting on cells, tissues and organs of substances (hormones), stimulating or weakening their functions.

Cells in which the production of hormones becomes the main or predominant function are called endocrine. In the human body, the endocrine system is represented by the secretory nuclei of the hypothalamus, pituitary, epiphysis, thyroid, parathyroid glands, adrenal glands, endocrine parts of the sex and pancreas, as well as individual glandular cells scattered in other (non-endocrine) organs or tissues.

With the help of hormones secreted by the endocrine system, the body functions are regulated and coordinated and brought into line with its needs, as well as with irritations received from the external and internal environment.

By chemical nature, most hormones belong to proteins - proteins or glycoproteins. Other hormones are derivatives of amino acids (tyrosine) or steroids. Many hormones, entering the bloodstream, bind to serum proteins and are transported throughout the body in the form of such complexes. The connection of the hormone with the carrier protein, although it protects the hormone from premature degradation, but weakens its activity. The release of the hormone from the carrier occurs in the cells of the organ that perceives this hormone.

Since hormones are released into the blood stream, an abundant blood supply to the endocrine glands is a prerequisite for their functioning. Each hormone acts only on those target cells that have specific chemical receptors in their plasma membranes.

The target organs, usually classified as non-endocrine, include the kidney, in the juxtaglomerular complex of which renin is produced; salivary and prostate glands, in which special cells are found that produce a factor that stimulates the growth of nerves; as well as special cells (enterinocytes) localized in the mucous membrane of the gastrointestinal tract and producing a number of enteric (intestinal) hormones. Many hormones (including endorphins and enkephalins), which have a wide spectrum of action, are produced in the brain.

Relationship between the nervous and endocrine systems

The nervous system, sending its efferent impulses along the nerve fibers directly to the innervated organ, causes directed local reactions that come on quickly and stop just as quickly.

Distant hormonal influences play a predominant role in the regulation of such general body functions as metabolism, somatic growth, and reproductive functions. The joint participation of the nervous and endocrine systems in ensuring the regulation and coordination of body functions is determined by the fact that the regulatory influences exerted by both the nervous and endocrine systems are implemented by fundamentally the same mechanisms.

At the same time, all nerve cells exhibit the ability to synthesize protein substances, as evidenced by the strong development of the granular endoplasmic reticulum and the abundance of ribonucleoproteins in their perikarya. The axons of such neurons, as a rule, end in capillaries, and the synthesized products accumulated in the terminals are released into the blood, with the current of which they are carried throughout the body and, unlike mediators, have not a local, but a distant regulatory effect, similar to the hormones of the endocrine glands. Such nerve cells are called neurosecretory, and the products produced and secreted by them are called neurohormones. Neurosecretory cells, perceiving, like any neurocyte, afferent signals from other parts of the nervous system, send their efferent impulses through the blood, that is, humorally (like endocrine cells). Therefore, neurosecretory cells, physiologically occupying an intermediate position between nervous and endocrine cells, unite the nervous and endocrine systems into a single neuroendocrine system and thus act as neuroendocrine transmitters (switches).

In recent years, it has been established that the nervous system contains peptidergic neurons, which, in addition to mediators, secrete a number of hormones that can modulate the secretory activity of the endocrine glands. Therefore, as noted above, the nervous and endocrine systems act as a single regulatory neuroendocrine system.

Classification of the endocrine glands

At the beginning of the development of endocrinology as a science, endocrine glands were tried to be grouped according to their origin from one or another embryonic rudiment of the germ layers. However, further expansion of knowledge about the role of endocrine functions in the body showed that the commonality or proximity of embryonic anlages does not at all prejudge the joint participation of the glands developing from such rudiments in the regulation of body functions.

According to modern concepts, the following groups of endocrine glands are distinguished in the endocrine system: neuroendocrine transmitters (secretory nuclei of the hypothalamus, pineal gland), which, with the help of their hormones, switch information entering the central nervous system to the central link in the regulation of adenohypophysis-dependent glands (adenohypophysis) and the neurohemal organ (posterior pituitary, or neurohypophysis). The adenohypophysis, thanks to the hormones of the hypothalamus (liberins and statins), secretes an adequate amount of tropic hormones that stimulate the function of the adenohypophysis-dependent glands (adrenal cortex, thyroid and gonads). The relationship between the adenohypophysis and the endocrine glands dependent on it is carried out according to the feedback principle (or plus or minus). The neurohemal organ does not produce its own hormones, but accumulates the hormones of the large cell nuclei of the hypothalamus (oxytocin, ADH-vasopressin), then releases them into the bloodstream and thus regulates the activity of the so-called target organs (uterus, kidneys). In functional terms, the neurosecretory nuclei, the pineal gland, the adenohypophysis, and the neurohemal organ constitute the central link of the endocrine system, while the endocrine cells of non-endocrine organs (digestive system, airways and lungs, kidneys and urinary tract, thymus), adenohypophysis-dependent glands (thyroid gland, adrenal cortex , gonads) and adenohypophysis-independent glands (parathyroid glands, adrenal medulla) are peripheral endocrine glands (or target glands).

Summarizing all of the above, we can say that the endocrine system is represented by the following main structural components.

1. Central regulatory formations of the endocrine system:

1) hypothalamus (neurosecretory nuclei);

2) pituitary gland;

3) epiphysis.

2. Peripheral endocrine glands:

1) thyroid gland;

2) parathyroid glands;

3) adrenal glands:

a) cortical substance;

b) the adrenal medulla.

3. Organs that combine endocrine and non-endocrine functions:

1) gonads:

a) testis;

b) ovary;

2) placenta;

3) pancreas.

4. Single hormone-producing cells:

1) neuroendocrine cells of the POPA group (APUD) (nervous origin);

2) single hormone-producing cells (not of nervous origin).

Hypothalamus

The hypothalamus occupies the basal region of the diencephalon and borders the lower part of the third ventricle of the brain. The cavity of the third ventricle continues into the funnel, the wall of which becomes the pituitary stalk and at its distal end gives rise to the posterior lobe of the pituitary gland (or neurohypophysis).

In the gray matter of the hypothalamus, its nuclei (over 30 pairs) are isolated, which are grouped in the anterior, middle (mediobasal or tuberal) and posterior sections of the hypothalamus. Some of the hypothalamic nuclei are clusters of neurosecretory cells, while others are formed by a combination of neurosecretory cells and neurons of the usual type (mainly adrenergic).

In the nuclei of the middle hypothalamus, hypothalamic adenohypophysotropic hormones are produced, which regulate the secretion (and probably also production) of hormones in the anterior and middle lobes of the pituitary gland. Adenohypophysotropic hormones are low molecular weight proteins (oligopeptides) that either stimulate (liberins) or inhibit (statins) the corresponding hormone-forming functions of the adenohypophysis. The most important nuclei of this part of the hypothalamus are localized in the gray tubercle: the arcuate, or infundibular, nucleus and the ventromedial nucleus. The ventromedial nucleus is large and turns out to be the main site for the production of adenohypophysotropic hormones, but along with it, this function is also inherent in the arcuate nucleus. These nuclei are formed by small neurosecretory cells in combination with adrenergic neurons of the usual type. The axons of both small neurosecretory cells of the mediobasal hypothalamus and adjacent adrenergic neurons are directed to the medial emission, where they end at the loops of the primary capillary network.

Thus, the neurosecretory formations of the hypothalamus are divided into two groups: cholinergic (large cell nuclei of the anterior hypothalamus) and adrenergic (small neurosecretory cells of the mediobasal hypothalamus).

The division of neurosecretory formations of the hypothalamus into peptidocholinergic and peptidoadrenergic reflects their belonging, respectively, to the parasympathetic or sympathetic part of the hypothalamus.

The connection of the anterior hypothalamus with the posterior pituitary gland, and the mediobasal hypothalamus with the adenohypophysis allows us to divide the hypothalamic-pituitary complex into the hypothalamic-neurohypophyseal and hypothalamic-adenohypophyseal systems. The significance of the posterior lobe of the pituitary lies in the fact that it accumulates and releases into the blood the neurohormones produced by the large-celled peptidocholinergic nuclei of the anterior hypothalamus. Consequently, the posterior lobe of the pituitary gland is not a gland, but is an auxiliary neurohemal organ of the hypothalamic-neurohypophyseal system.

A similar neurohemal organ of the hypothalamic-adenohypophyseal system is medial emission, in which adenohypophysotropic hormones (liberins and statins) are accumulated and enter the blood, produced by peptidoadrenergic neurosecretory cells of the mediobasal hypothalamus.

Pituitary

There are several lobes in the pituitary gland: adenohypophysis, neurohypophysis.

In the adenohypophysis, the anterior, middle (or intermediate) and tuberal parts are distinguished. The anterior part has a trabecular structure. Trabeculae, strongly branching, are woven into a narrow-loop network. The gaps between them are filled with loose connective tissue, through which numerous sinusoidal capillaries pass.

In each trabecula, several types of glandular cells (adenocytes) can be distinguished. Some of them, located along the periphery of the trabeculae, are larger in size, contain secretory granules and are intensely stained on histological preparations, therefore these cells are called chromophilic. Other cells are chromophobic, occupying the middle of the trabeculae, differ from the chromophilic cells by a weakly staining cytoplasm. Due to the quantitative predominance of chromophobic cells in the composition of trabeculae, they are sometimes called the main ones.

Chromophilic cells are divided into basophilic and acidophilic. Basophilic cells, or basophils, produce glycoprotein hormones, and their secretory granules on histological preparations are stained with basic colors.

Among them, two main varieties are distinguished - gonadotropic and thyrotropic.

Some of the gonadotropic cells produce follicle-stimulating hormone (follitropin), while others are attributed to the production of luteinizing hormone (lutropin).

If the body is deficient in sex hormones, the production of gonadotropins, especially follitropin, is so enhanced that some gonadotropic cells hypertrophy and are strongly stretched by a large vacuole, as a result of which the cytoplasm takes the form of a thin rim, and the nucleus is pushed to the edge of the cell ("castration cells").

The second variety - a thyrotropic cell that produces thyrotropic hormone (thyrotropin) - is distinguished by an irregular or angular shape. In case of insufficiency of the thyroid hormone in the body, the production of thyrotropin increases, and thyrotropocytes are partially transformed into thyroidectomy cells, which are characterized by larger sizes and a significant expansion of the cisterns of the endoplasmic reticulum, as a result of which the cytoplasm takes the form of coarse foam. In these vacuoles, aldehyde fuchsinophilic granules are found, larger than the secretory granules of the original thyrotropocytes.

For acidophilic cells, or acidophiles, large dense granules are characteristic, stained on preparations with acidic dyes. Acidophilic cells are also divided into two varieties: somatotropic, or somatotropocytes that produce somatotropic hormone (somatotropin), and mammotropic, or mammotropocytes that produce lactotropic hormone (prolactin).

The function of these cells is similar to basophilic ones.

A corticotropic cell in the anterior pituitary gland produces adrenocorticotropic hormone (ACTH or corticotropin), which activates the adrenal cortex.

The middle part of the adenohypophysis is a narrow strip of stratified epithelium, homogeneous in structure. Adenocytes of the middle lobe are able to produce a protein secret, which, accumulating between neighboring cells, leads to the formation of follicle-like cavities (cysts) in the middle lobe.

In the middle part of the adenohypophysis, melanocyte-stimulating hormone (melanotropin) is produced, which affects pigment metabolism and pigment cells, as well as lipotropin, a hormone that enhances the metabolism of fat-lipoid substances.

The tuberal part is a section of the adenohypophyseal parenchyma adjacent to the pituitary stalk and in contact with the lower surface of the medial hypothalamic emission.

The functional properties of the tuberal part are not sufficiently elucidated.

The posterior lobe of the pituitary gland - neurohypophysis - is formed by neuroglia. Glial cells of this lobe are represented mainly by small process or spindle-shaped cells - pituicites. The axons of the neurosecretory cells of the supraoptic and paraventricular nuclei of the anterior hypothalamus enter the posterior lobe. In the posterior lobe, these axons terminate in expanded terminals (storage bodies, or Herring bodies) that are in contact with the capillaries.

The posterior pituitary gland accumulates antidiuretic hormone (vasopressin) and oxytocin produced by neurosecretory cells of the supraoptic and paraventricular nuclei of the anterior hypothalamus. It is possible that pituicytes are involved in the transfer of these hormones from storage bodies into the blood.

Innervation. The pituitary gland, as well as the hypothalamus and pineal gland, receive nerve fibers from the cervical ganglia (mainly from the upper ones) of the sympathetic trunk. Extirpation of the upper cervical sympathetic ganglia or transection of the cervical sympathetic trunk leads to an increase in the thyrotropic function of the pituitary gland, while irritation of the same ganglia causes its weakening.

Blood supply. The superior pituitary arteries enter the medial emission, where they break up into the primary capillary network. Its capillaries form loops and glomeruli that penetrate into the medial emission ependyma. The axons of peptidoadrenergic cells of the mediobasal hypothalamus approach these loops, forming axovasal synapses (contacts) on the capillaries, in which the hypothalamic liberins and statins are transferred into the blood stream. Then the capillaries of the primary network are collected in the portal veins, which run along the pituitary stalk to the parenchyma of the adenohypophysis, where they again break up into a secondary capillary network, the sinusoidal capillaries of which, branching, braid the trabeculae. Finally, the sinusoids of the secondary network merge into the efferent veins, which divert blood enriched with adenohypophyseal hormones into the general circulation.

Thyroid gland

The thyroid gland has two lobes (right and left, respectively) and an isthmus.

Outside, it is surrounded by a dense connective tissue capsule, from which partitions extend into the gland. Composing the stroma of the gland, they branch and divide the thyroid parenchyma into lobules.

The functional and structural unit of the thyroid gland are follicles - closed spherical or rounded formations of varying sizes with a cavity inside. Sometimes the walls of the follicles form folds, and the follicles become irregular in shape. In the lumen of the follicles, a secretory product accumulates - a colloid, which during life has the consistency of a viscous liquid and consists mainly of thyroglobulin.

In addition, in the connective tissue layers there are always lymphocytes and plasma cells, the number of which in a number of diseases (thyrotoxicosis, autoimmune thyroiditis) increases dramatically up to the appearance of lymphoid accumulations and even lymphoid follicles with reproduction centers. In the same interfollicular layers, parafollicular cells are found, as well as mast cells (tissue basophils).

Thyrocytes - glandular cells of the thyroid gland, which make up the wall (lining) of the follicles and are located in one layer on the basement membrane, limit the follicle from the outside. The shape, volume and height of thyrocytes change in accordance with shifts in the functional activity of the thyroid gland.

When the body's needs for thyroid hormone increase and the functional activity of the thyroid gland increases (hyperfunctional state), the thyrocytes of the follicular lining increase in volume and height and take on a prismatic shape.

The intrafollicular colloid becomes more liquid, numerous vacuoles appear in it, and on histological preparations it takes the form of foam.

The apical surface of the thyrocyte forms microvilli protruding into the lumen of the follicle. As the functional activity of the thyroid gland increases, the number and size of microvilli increase. At the same time, the basal surface of thyrocytes, which is almost flat during the period of functional rest of the thyroid gland, becomes folded when it is activated, which leads to an increase in the contact of thyrocytes with the pericapillary spaces.

The secretory cycle of any glandular cell consists of the following phases: the absorption of the starting materials, the synthesis of the hormone and its release.

production phase. The production of thyroglobulin (and, consequently, thyroid hormone) begins in the cytoplasm of the basal part of the thyrocyte and ends in the cavity of the follicle on its apical surface (on the border with the intrafollicular colloid). The initial products (amino acids, salts), brought to the thyroid gland by the blood and absorbed by thyrocytes through their base, are concentrated in the endoplasmic reticulum, and the synthesis of the polypeptide chain, the basis of the future thyroglobulin molecule, takes place on the ribosomes. The resulting product accumulates in the cisterns of the endoplasmic reticulum and then moves to the zone of the lamellar complex, where thyroglobulin condenses (but not yet iodinated) and small secretory vesicles are formed, which then move to the upper part of the thyrocyte. Iodine is taken up by thyrocytes from the blood in the form of iodide, and thyroxine is synthesized.

Elimination phase. It is carried out by reabsorption of intrafollicular colloid. Depending on the degree of activation of the thyroid gland, endocytosis occurs in different forms. Excretion of the hormone from the gland, which is in a state of functional rest or weak excitation, proceeds without the formation of apical pseudopodia and without the appearance of drops of intracellular colloid inside thyrocytes. It is carried out by proteolysis of thyroglobulin, which takes place in the peripheral layer of the intrafollicular colloid at the border with microvilli, and subsequent micropinocytosis of the products of this cleavage.

Parafollicular cells (calcitoninocytes), found in the thyroid parenchyma, differ sharply from thyrocytes in their lack of ability to absorb iodine. As mentioned above, they produce a protein hormone - calcitonin (thyrocalcitonin), which lowers the level of calcium in the blood and is an antagonist of parathyrin (parathyroid hormone).

Parathyroid glands (parathyroid glands)

It is believed that at each of the poles of the thyroid gland there are parathyroid glands (there are 4-6 of them in total).

Each parathyroid gland is surrounded by a thin connective tissue capsule. Their parenchyma is formed by epithelial strands (trabeculae) or accumulations of glandular cells (parathyrocytes) separated by thin layers of loose connective tissue with numerous capillaries.

Among parathyrocytes, there are main, intermediate and acidophilic (oxyphilic) cells, which, however, should not be considered as separate types of glandular cells of the parathyroid glands, but as functional or age-related states of parathyrocytes.

During the increase in the secretory activity of the parathyroid glands, the chief cells swell and increase in volume, the endoplasmic reticulum and the lamellar complex hypertrophy in them. The release of parathyrin from the glandular cells into the intercellular gaps is carried out by exocytosis. The released hormone enters the capillaries and is carried out into the general circulation.

The blood supply to the thyroid and parathyroid glands comes from the superior and inferior thyroid arteries.

Adrenal

Paired organs formed by a combination of two independent glands of different origin and different physiological significance: cortical and cerebral (medullary). Adrenal hormones are involved in the protective and adaptive reactions of the body, the regulation of metabolism and the activity of the cardiovascular system.

In the adrenal glands, there are: a cortical layer and a medulla.

The adrenal cortex is divided into three zones: glomerular, fascicular and reticular.

The glomerular (external) zone is formed by elongated glandular cells (adrenocorticocytes), which are layered on top of each other, forming rounded clusters, which determines the name of this zone.

In the cells of the glomerular zone, there is a high content of ribonucleoproteins and a high activity of enzymes involved in steroidogenesis.

The zona glomeruli produces aldosterone, a hormone that regulates the level of sodium in the body and prevents the body from losing this element in the urine. Therefore, aldosterone can be called the mineralocorticoid hormone. Mineralocorticoid function is indispensable for life, and therefore the removal or destruction of both adrenal glands, which captures their zona glomeruli, is fatal. At the same time, mineralocorticoids accelerate the course of inflammatory processes and promote the formation of collagen.

The middle part of the cortical substance is occupied by the largest beam zone in width. Adrenocorticocytes of this zone are large and cubic or prismatic in shape, their axis is oriented along the epithelial cord.

The fascicular zone of the adrenal cortex produces glucocorticoid hormones - corticosterone, cortisol (hydrocortisone) and cortisone. These hormones affect the metabolism of carbohydrates, proteins and lipids, enhance the processes of phosphorylation and promote the formation of substances that store and release energy in the cells and tissues of the body. Glucocorticoids promote gluconeogenesis (i.e., the formation of glucose at the expense of proteins), the deposition of glycogen in the liver and myocardium, and the mobilization of tissue proteins. Glucocorticoid hormones increase the body's resistance to the action of various damaging agents of the environment, such as severe injuries, poisoning with poisonous substances and intoxication with bacterial toxins, as well as in other extreme conditions, mobilizing and enhancing the protective and compensatory reactions of the body.

At the same time, glucocorticoids increase the death of lymphocytes and eosinophils, leading to lymphocytopenia and blood eosinopenia, and weaken both inflammatory processes and immunogenesis (antibody formation).

In the inner reticular zone, the epithelial strands lose their correct location and, branching out, form a loose network, in connection with which this zone of the cortex got its name. Adrenocorticocytes in this zone decrease in volume and become diverse in shape (cubic, round or polygonal).

In the reticular zone, androgenic hormone is produced (male sex hormone, similar in chemical nature and physiological properties to testosterone testis). Therefore, tumors of the adrenal cortex in women are often the cause of the development of male secondary sexual characteristics, such as mustaches and beards. In addition, female sex hormones (estrogen and progesterone) are also formed in the reticular zone, but in small quantities.

The medulla of the adrenal glands is separated from the cortical part by a thin, in some places interrupted, internal connective tissue capsule. The adrenal medulla is formed by an accumulation of relatively large cells, mostly round in shape, located between the blood vessels. These cells are modified sympathetic neurons, they contain catecholamines (norepinephrine and adrenaline).

Both catecholamines are similar in physiological action, but norepinephrine is a mediator that mediates the transmission of a nerve impulse from a postganglionic sympathetic neuron to an innervated effector, while adrenaline is a hormone and does not have a mediator property. Norepinephrine and epinephrine exhibit a vasoconstrictive effect and increase blood pressure, but the vessels of the brain and striated muscles expand under the influence of adrenaline. Adrenaline increases the level of glucose and lactic acid, increasing the breakdown of glycogen in the liver, and this is less common for norepinephrine.

The blood supply to the adrenal gland comes from the adrenal arteries.

The innervation of the adrenal glands is represented mainly by the fibers of the celiac and vagus nerves.

Topic 21. DIGESTIVE SYSTEM

The human digestive system is a digestive tube with glands located next to it, but outside it (salivary glands, liver and pancreas), the secret of which is involved in the process of digestion. Sometimes the digestive system is called the gastrointestinal tract.

The process of digestion is the process of chemical and mechanical processing of food, followed by the absorption of its breakdown products.

The role of the gastrointestinal tract in the human body is very large: from it comes the supply of substances that provide the body with the necessary energy and building materials to restore its constantly collapsing structures.

The entire digestive tract is very conditionally divided into three main sections - anterior, middle and posterior.

The anterior section includes the oral cavity with all its structural components, the pharynx and esophagus. In the anterior section, mainly the mechanical processing of food occurs.

The middle section includes the stomach, small and large intestines, liver and pancreas. In this department, the chemical processing of food takes place, the absorption of its breakdown products and the formation of feces.

The posterior section includes the caudal part of the rectum, which performs the function of evacuating undigested food residues from the alimentary canal.

Development of the digestive system

Tissue sources of development

Endoderm. In the early stages (4-week embryo), the rudiment of the digestive tract looks like an enterodermal tube (primary intestine), closed at both ends. In the middle part, the primary intestine communicates with the yolk sac by means of a yolk stalk. At the anterior end, a gill apparatus is formed.

Ectoderm. The invaginations of the ectoderm directed towards the blind ends of the primary intestine form the oral cavity and the anal bay.

The oral bay (stomodeum) is separated from the anterior end of the primary intestine by the oral (drain) plate.

The anal bay (proctodeum) is separated from the hindgut by a cloacal membrane.

Mesenchyme. The composition of the digestive wall includes derivatives of the mesenchyme - layers of connective tissue, smooth muscle cells and blood vessels.

The mesoderm forms the mesothelium of the serous integument, striated muscle fibers.

Neuroectoderm. Derivatives of the neuroectoderm (especially the neural crest) are an essential part of the gastrointestinal tract (enteric nervous system, part of the endocrine cells).

Development of the anterior gastrointestinal tract

Development of the face and mouth. Ectoderm, mesenchyme, neuroectoderm (neural crest and ectodermal placodes) are involved in the development of the face and oral cavity.

The ectoderm gives rise to stratified squamous epithelium of the skin, glands, and integumentary epithelium of the oral mucosa.

Mesenchyme. Derivatives of the mesenchyme of the head develop from several primordia.

The mesenchyme of the somites and the lateral plate of the head of the embryo forms the voluntary muscles of the craniofacial region, the skin itself, and the connective tissue of the dorsal region of the head.

The mesenchyme of the neural crest forms the structures of the face and pharynx - cartilage, bones, tendons, the skin itself, dentin, and the connective tissue stroma of the glands.

Ectodermal placodes. Some of the sensory neurons of the trigeminal ganglion (ganglion trigeminale) and the ganglion of the geniculi (ganglion geniculi) of the intermediate nerve originate from ectodermal placodes. From the same source, all neurons VIII (spiral ganglion, ganglion spirale cochleae), x (nodular ganglion, ganglion nodosum), IX (petrosal ganglion, ganglion petrosum) of the cranial nerve ganglia develop.

The face develops from seven rudiments: two early fused mandibular processes, two maxillary processes, two lateral nasal processes, and a medial nasal process. The maxillary and mandibular processes originate from the first gill arch.

In the facial region, by the 4th week, a frontal protrusion is formed, located along the midline and covering the forebrain. The frontal protrusion gives rise to the medial and lateral nasal processes. The emerging olfactory pits separate the medial nasal process from the lateral ones. Towards the midline, the maxillary processes grow, which together with the mandibular process form the corners of the mouth. Thus, the entrance to the oral cavity is limited by the medial nasal process, the paired maxillary processes, and the mandibular process.

By the 5th week, the maxillary processes are separated from the lateral nasal processes by the nasolacrimal groove, from which the nasolacrimal canal later develops. On the 6th week, during the formation of the upper jaw, the maxillary processes growing towards the midline bring together the nasal processes, which simultaneously increase and gradually cover the lower part of the frontal protrusion. At week 7, the maxillary and medial nasal processes fuse to form the philtrum. From the material of the fused maxillary processes, a maxillary segment is formed, from which the primary palate and the premaxillary part of the dental arch develop. The bone structures of the face are formed at the end of the 2nd - beginning of the 3rd month of development.

Development of the hard palate. The developing secondary palate separates the primary oral cavity into the nasal and secondary (final) oral cavity. On the inner surface of the maxillary processes, palatine processes are formed. On the 6th - 7th week, their edges are directed obliquely down and lie along the bottom of the oral cavity on the sides of the tongue. As the lower jaw develops and the volume of the oral cavity increases, the tongue descends, and the edges of the palatine processes rise up to the midline. After the fusion of the palatine processes and the formation of the secondary palate, the nasal chambers communicate with the nasopharynx through the final choanae.

With non-closure of the medial and lateral nasal processes, a gap of the upper lip is observed. The oblique facial fissure runs from the upper lip to the eye along the junction of the maxillary and lateral nasal processes. With incomplete connection of the maxillary and mandibular processes, an abnormally wide mouth develops - macrostomia. In addition to cosmetic defects, these malformations of the maxillofacial region cause serious respiratory and nutritional disorders in a child in the first days of life. With underdevelopment of the palatine processes, a cleft of the hard and soft palate is observed. Sometimes the cleft is only present in the soft palate.

Gill apparatus and its derivatives. In the initial section of the foregut, the branchial apparatus is formed, which is involved in the formation of the face, organs of the oral cavity and the cervical region. The gill apparatus consists of five pairs of pharyngeal pouches and the same number of gill arches and slits.

Development and role of pharyngeal pouches and gill slit. From the structures of the gill apparatus, the pharyngeal pockets are the first to appear. These are protrusions of the endoderm in the region of the lateral walls of the pharyngeal section of the primary intestine.

Towards the pharyngeal pockets of the endoderm, invaginations of the ectoderm of the cervical region grow, which are called gill slits.

Gill arches. The material between adjacent pharyngeal pouches and slits is called gill arches. There are four of them, the fifth gill arch is a rudimentary formation. Gill arches on the anterolateral surface of the neck form a ridge-like elevation. The mesenchymal base of each gill arch is penetrated by blood vessels (aortic arches) and nerves. Soon, muscles and a cartilaginous skeleton develop in each of them. The largest is the first gill arch, extramaxillary. The second gill arch is called the hyoid arch. The smaller third, fourth and fifth arches do not reach the median line and grow together with those located above. From the lower edge of the second gill arch, a gill fold (operculum) grows, covering the outside of the lower gill arches. This fold grows together with the skin of the neck, forming the anterior wall of the deep fossa (sinus cervicalis), at the bottom of which the lower gill arches are located. This sinus first communicates with the external environment, and then the hole above it overgrows. When the cervical sinus is not closed, a fistulous tract remains on the child's neck, communicating with the pharynx, if the second gill arch breaks through.

Development of the vestibule of the oral cavity. On the 7th week of development near the outer part of the jaw, in parallel with the formation of the epithelial dental plate, another growth of the epithelium occurs, called the labio-gingival plate (lamina labio-gingivalis). It forms a furrow that separates the rudiments of the upper and lower jaws from the lip.

Language development. The tongue develops from several rudiments that look like tubercles and are located at the bottom of the primary oral cavity in the region of the ventral gill arches. On the 8th - 9th week, the development of papillae on the upper surface of the anterior body of the tongue begins, while the lymphoid tissue develops in the back of the mucous membrane of the tongue. The muscles of the tongue originate from the myotomes of the upper (anterior) somites.

The material of all four gill arches is involved in the laying of the tongue. Two large lateral lingual tubercles and an unpaired lingual tubercle (tuberculum impar) originate from the first gill arch. The root of the tongue develops from a staple that originates from the second, third, and fourth gill arches. From the material between the unpaired lingual tubercle and the bracket, the thyroid gland is laid. The excretory duct (linguothyroid duct) of its rudiment opens on the surface of the rudiment of the tongue with a blind opening.

On the 4th week, an unpaired lingual tubercle (tuberculum impar) appears, located in the midline between the first and second gill arches. From this tubercle develops a small part of the back of the tongue, lying anterior to the blind basking (foramen coecum). In addition, on the inside of the first gill arch, two paired thickenings are formed, called lateral lingual tubercles. From these three protrusions, a significant part of the body of the tongue and its tip are formed.

The root of the tongue arises from a thickening of the mucous membrane lying behind the blind opening, at the level of the second, third and fourth gill arches. This is a bracket (copula).

The unpaired tubercle flattens rather quickly. All the rudiments of the tongue grow together, forming a single organ.

The boundary between the root and the body of a language. In the future, the boundary between the root and the body of the tongue is the line of location of the grooved papillae. At the top of this angle is a blind hole, the mouth of the lingual-thyroid duct. From the remnants of this duct, epithelial cysts can develop in the thickness of the tongue.

The digestive tube, despite the morphological and physiological features of its departments, has a general structural plan. Its wall consists of a mucous membrane lining the tube from the inside, a submucosa, a muscular membrane and an outer membrane, which is represented by a serous or adventitious membrane.

Mucous membrane. It got its name due to the fact that its surface is constantly moistened with mucus secreted by the glands. This membrane consists, as a rule, of three plates: the epithelium, the lamina propria of the mucosa and the muscular lamina of the mucosa. The epithelium in the anterior and posterior sections of the digestive tube (in the oral cavity, pharynx, esophagus, caudal part of the rectum) is stratified flat, and in the middle section, that is, in the stomach and intestines, it is single-layer cylindrical. Glands are located either endoepithelially (for example, goblet cells), or exoepithelially (in the lamina propria and in the submucosa), or outside the alimentary canal (in the liver, pancreas).

The composition of the mucous membrane includes its own plate, which lies under the epithelium, is separated from it by a basal membrane and is represented by loose fibrous unformed connective tissue. Blood and lymphatic vessels, nerve elements, accumulations of lymphoid tissue pass through it.

The location of the muscularis mucosa is the border with the submucosa. This plate consists of several layers formed by smooth muscle cells.

The relief of the mucous membrane throughout the entire alimentary canal is heterogeneous. It can be both smooth (lips, cheeks), and form indentations (pits in the stomach, crypts in the intestines), folds, villi (in the small intestine).

The submucosa is represented by a loose fibrous, unformed connective tissue, as it were, it connects the mucous membrane with the underlying formations (muscle membrane or bone base). Thanks to it, the mucous membrane has mobility and can form folds.

The muscular membrane is made up of smooth muscle tissue, in this case, the arrangement of muscle fibers can be circular (inner layer) and longitudinal (outer layer).

These layers are separated by connective tissue, which contains the blood and lymphatic vessels and the intermuscular nerve plexus. When the muscle membrane contracts, food is mixed and promoted during digestion.

Serous membrane. The bulk of the gastrointestinal tract is covered with a serous membrane - the visceral sheet of the peritoneum. The peritoneum consists of a connective tissue base, in which there are vessels and nerve elements, and of the mesothelium that surrounds it from the outside. At the same time, in relation to this shell, organs can be in several states: intraperitoneally (the organ is covered by it for the entire diameter), mesoperitoneally (the organ is covered by it only by 2/3) and extraperitoneally (the organ is covered by it from only one side).

Some sections (esophagus, part of the rectum) do not contain a serous membrane. In such places, the alimentary canal is covered on the outside with an adventitial membrane consisting of connective tissue.

The blood supply of the gastrointestinal tract is very abundant.

The most powerful plexuses are in the submucosal layer, they are closely related to the arterial plexuses that lie in the lamina propria of the mucous membrane. In the small intestine, arterial plexuses are also formed in the muscular membrane. Capillary networks are formed under the epithelium of the mucous membrane, around the glands, crypts, gastric pits, inside the villi, papillae of the tongue and in the muscle layers. Veins also form plexuses of the submucosa and mucosa.

Lymphatic capillaries take part in the formation of a network under the epithelium, around the glands in the lamina propria, as well as in the submucosa and muscularis.

The efferent innervation of all digestive organs comes from the ganglia of the autonomic nervous system, located either outside the digestive tube (extramural sympathetic ganglia) or in its thickness (intramural parasympathetic ganglia).

Afferent innervation is carried out by the endings of the dendrites of sensitive nerve cells, occurs due to the intramural ganglia, in which the endings are dendrites from the spinal ganglia. Sensitive nerve endings are located in the muscles, epithelium, fibrous connective tissue and nerve ganglia.

Oral cavity

The mucous membrane lining the oral cavity is distinguished by the following features: the presence of stratified squamous epithelium, the complete absence or weak development of the muscularis mucosa, and the absence of a submucosal layer in some areas. At the same time, there are places in the oral cavity where the mucous membrane is firmly fused with the underlying tissues and lies directly on the muscles (for example, in the back of the tongue) or on the bones (in the gums and hard palate). The mucous membrane can form folds in which accumulations of lymphoid tissue are located. Such areas are called tonsils.

In the mucous membrane there are many small blood vessels that shine through the epithelium and give it a characteristic pink color. A well-moistened epithelium is able to pass many substances into the underlying blood vessels, therefore, in medical practice, the introduction of drugs such as nitroglycerin, validol, and others through the oral mucosa is often used.

Lips. Three parts are distinguished in the lip - skin, transitional (or red) and mucous. In the thickness of the lip there is a striated muscle. The skin part of the lip has the structure of the skin. It is covered with stratified squamous keratinized epithelium and is supplied with sebaceous, sweat glands and hair. The epithelium of this part is located on the basal membrane, under which lies a loose fibrous connective tissue that forms high papillae that protrude into the epithelium.

The transitional (or red) part of the lip, in turn, consists of two zones: the outer (smooth) and the inner (villous). In the outer zone, the stratum corneum of the epithelium is preserved, but becomes thinner and more transparent. There is no hair in this area, the sweat glands gradually disappear, and only the sebaceous glands remain, opening with their ducts to the surface of the epithelium. There are more sebaceous glands in the upper lip, especially in the corner of the mouth. The lamina propria is a continuation of the connective tissue part of the skin, its papillae in this area are low. The inner zone in newborns is covered with epithelial papillae, which are sometimes called villi. These epithelial papillae, as the organism develops, gradually smooth out and become inconspicuous. The inner zone of the transitional part of the lip of an adult is characterized by a very high epithelium, devoid of the stratum corneum. In this zone, as a rule, sebaceous glands are absent. The lamina propria, protruding into the epithelium, forms very high papillae, in which there are numerous capillaries. The blood circulating in them shines through the epithelium and gives this area a reddish tint. The papillae contain a huge number of nerve endings, so the red edge of the lip is very sensitive.

The mucous part of the lip is covered with stratified squamous non-keratinized epithelium, but sometimes a small amount of keratin grains can still be detected in the cells of the surface layer of the epithelium.

The lamina propria also forms papillae here, but they are less high than in the adjacent villous zone of the lip. The muscular lamina of the mucous membrane is absent, therefore, its own lamina, without a sharp border, passes into the submucosa, adjacent directly to the striated muscles. In the submucosal base of the mucous part of the lip are the secretory sections of the salivary labial glands. Their excretory ducts open on the surface of the epithelium. The glands are quite large, sometimes reaching the size of a pea. By structure, these are complex alveolar-tubular glands. By the nature of the secret, they belong to the mixed mucous-protein glands. Their excretory ducts are lined with stratified squamous non-keratinized epithelium. In the submucosa of the mucous part of the lip, large arterial trunks pass, and there is also an extensive venous plexus, which also extends into the red part of the lip.

The cheeks are a muscular formation, which is covered on the outside with skin, and on the inside with a mucous membrane. Three zones are distinguished in the mucous membrane of the cheek - the upper (maxillary), middle (intermediate) and lower (mandibular). At the same time, a distinctive feature of the cheeks is that there is no muscular plate in the mucous membrane.

The maxillary part of the cheek has a structure similar to the structure of the mucous part of the lip. It is covered with stratified squamous non-keratinized epithelium, the papillae of the lamina propria are small in size. In these areas there are a large number of salivary glands of the cheek.

The middle (intermediate) zone of the cheek goes from the corner of the mouth to the branch of the lower jaw. The papillae of the lamina propria here, as in the transitional part of the lip, are large. There are no salivary glands. All these features indicate that the intermediate zone of the cheek, like the transitional part of the lip, is the zone of transition of the skin into the mucous membrane of the oral cavity.

The submucosa contains many blood vessels and nerves. The muscular membrane of the cheek is formed by the buccal muscle, in the thickness of which lie the buccal salivary glands. Their secretory sections are represented by mixed protein-mucous and purely mucous glands.

Gums are formations covered with a mucous membrane, tightly fused with the periosteum of the upper and lower jaws. The mucous membrane is lined with stratified squamous epithelium, which can become keratinized. The lamina propria forms long papillae, which consist of loose connective tissue. The papillae become lower in the part of the gum that is directly adjacent to the teeth. The lamina propria contains blood and lymph vessels. The gum is richly innervated. The epithelium contains free nerve endings, and the lamina propria contains encapsulated and non-encapsulated nerve endings.

Solid sky. It consists of a bone base covered with a mucous membrane.

The mucous membrane of the hard palate is lined with stratified squamous non-keratinizing epithelium, while the submucosa is absent.

The lamina propria of the mucous membrane of the hard palate is formed by fibrous unformed connective tissue.

The lamina propria has one peculiarity: bundles of collagen fibers are strongly intertwined and woven into the periosteum, this is especially pronounced in those places where the mucous membrane is tightly fused with the bone (for example, in the area of ​​​​the seam and the zone of transition to the gums).

The soft palate and uvula are represented by a tendon-muscle base covered with a mucous membrane. In the soft palate and uvula, oral (anterior) and nasal (posterior) surfaces are distinguished.

The mucous membrane of the oral part of the soft palate and uvula is covered with stratified squamous non-keratinized epithelium. The lamina propria, consisting of loose fibrous unformed connective tissue, forms high narrow papillae that protrude deeply into the epithelium. Deeper there is a pronounced submucosal base formed by loose fibrous unformed connective tissue with a large number of fatty elements and mucous salivary glands. The excretory ducts of these glands open on the oral surface of the soft palate and uvula.

The mucous membrane of the nasal surface of the soft palate is covered with a single-layer prismatic multi-row ciliated epithelium with a large number of goblet cells.

The human tongue, in addition to participating in taste perception, mechanical processing of food and the act of swallowing, performs an important function of the organ of speech. The basis of the tongue is striated muscle tissue, the contraction of which is arbitrary.

The relief of the mucous membrane covering it is different on the lower, lateral and upper surfaces of the tongue. The epithelium on the underside of the tongue is multi-layered, flat, non-keratinized, of small thickness. The mucous membrane of the upper and lateral surfaces of the tongue is fixedly fused with its muscular body. It contains special formations - papillae.

On the surface of the tongue there are four types of papillae: filiform, mushroom-shaped, surrounded by a shaft and leaf-shaped.

Most of the filiform papillae of the tongue. In size, they are the smallest among the papillae of the tongue. These papillae may be either filiform or conical in shape. In some forms of diseases, the process of rejection of superficial keratinizing epithelial cells can slow down, and epithelial cells, accumulating in large quantities on the tops of the papillae, thus form a film (plaque).

The second place in frequency of occurrence is occupied by fungiform papillae of the tongue, they are located on the back of the tongue among the filiform papillae (most of all on the tip of the tongue and along its edges). Most of them are mushroom-shaped.

Grooved papillae of the tongue (papillae of the tongue surrounded by a shaft) are located on the upper surface of the tongue in an amount of 6 to 12. They are located between the body and the root of the tongue along the boundary line. Unlike adults, the foliate papillae of the tongue are well developed only in children; they are located on the right and left edges of the tongue.

The mucous membrane of the root of the tongue does not have papillae. Elevations of the epithelium are formed due to the fact that in the own plate of the mucous membrane there are accumulations of lymphoid tissue, sometimes reaching 0,5 cm in diameter. Between these clusters, the epithelium forms depressions - crypts. The ducts of numerous mucous glands flow into the crypts. The collection of accumulations of lymphoid tissue in the root of the tongue is called the lingual tonsil.

The muscles of the tongue form the body of this organ, they are represented by a striated type of bundles, and are located in three mutually perpendicular directions.

The salivary glands of the tongue, according to the nature of the secret they secrete, can be divided into three types - proteinaceous, mucous and mixed.

The blood supply to the tongue is carried out by the lingual arteries.

The muscles of the tongue are innervated by branches of the hypoglossal nerve and the chorda tympani.

Sensitive innervation of the anterior 2/3 of the tongue is carried out by the branches of the trigeminal nerve, the posterior 1/3 by the branches of the glossopharyngeal nerve.

Salivary glands. In the oral cavity there are openings of the excretory ducts of three pairs of large salivary glands - parotid, submandibular and sublingual.

All salivary glands are complex alveolar or alveolar-tubular glands. They include the secretory ends of the departments and ducts that remove the secret.

Secretory sections according to the structure and nature of the secreted secret are of three types - lateral (serous), mucous and mixed (i.e., protein-mucous).

The excretory ducts of the salivary glands are divided into intercalary, striated, intralobular, interlobular excretory ducts and the common excretory duct.

The salivary glands perform exocrine and endocrine functions.

The exocrine function consists in the regular separation of saliva into the oral cavity. Saliva consists of water (about 99%), protein substances, including enzymes, non-protein substances (salts), inorganic substances, as well as cellular elements (epithelial cells, leukocytes).

The endocrine function of the salivary glands is ensured by the presence in saliva of biologically active substances such as hormones (kallikrein and bradykinin, an insulin-like substance, nerve growth factor, epithelial growth factor, thymocyte-transforming factor, lethality factor, etc.).

Teeth are the main part of the chewing apparatus. There are several types of teeth: first, falling (milk) teeth are formed, and then permanent ones. In the holes of the jaw bones, the teeth are strengthened by a dense connective tissue - periodontium, which forms a circular dental ligament in the region of the neck of the tooth. Collagen fibers of the dental ligament have a predominantly radial direction, while on the one hand they penetrate into the cement of the tooth root, and on the other - into the alveolar bone. The periodontium performs not only a mechanical, but also a trophic function, since blood vessels pass through it, feeding the root of the tooth.

Development of teeth. The laying of milk teeth begins at the end of the 2nd month of intrauterine development. The following structures are involved in the formation of the tooth germ: dental plate, enamel organ, dental papilla and dental sac.

The dental plate appears on the 7th week of intrauterine development as a thickening of the epithelium of the upper and lower jaws. At the 8th week, the dental lamina grows into the underlying mesenchyme.

Enamel organ - a local accumulation of cells of the dental plate, corresponding to the position of the tooth, determines the shape of the crown of the future tooth. The cells of the organ form the outer and inner enamel epithelium. Between them is localized loose mass of cells - enamel pulp. The cells of the inner enamel epithelium differentiate into cylindrical cells that form enamel - ameloblasts (enameloblasts). The enamel organ is connected to the dental plate, and then (on the 3rd - 5th month of intrauterine development) is completely separated from it.

Ameloblastoma is a benign but locally invasive tumor of the oral cavity originating from remnants of the epithelium of the enamel organ.

The dental papilla is a collection of mesenchymal cells originating from the neural crest and located within the goblet enamel organ. The cells form a dense mass that takes the shape of the crown of the tooth. Peripheral cells differentiate into odontoblasts.

dental pouch

The dental sac is the mesenchyme that surrounds the tooth germ. Cells that come into contact with root dentin differentiate into cementoblasts and deposit cementum. The outer cells of the dental sac form the periodontal connective tissue.

Milk tooth development. In a two-month-old fetus, the tooth rudiment is represented only by a formed dental plate in the form of an epithelial outgrowth into the underlying mesenchyme. The end of the dental plate is expanded. The enamel organ will develop from it in the future. In a three-month-old fetus, the formed enamel organ is connected to the dental plate with the help of a thin epithelial cord - the neck of the enamel organ. In the enamel organ, internal enamel cells of a cylindrical shape (ameloblasts) are visible. Along the edge of the enamel organ, the inner enamel cells pass into the outer ones, which lie on the surface of the enamel organ and have a flattened shape. The cells of the central part of the enamel organ (pulp) acquire a stellate shape. Part of the pulp cells, adjacent directly to the layer of enameloblasts, forms an intermediate layer of the enamel organ, consisting of 2-3 rows of cubic cells. The dental sac surrounds the enamel organ and then merges at the base of the tooth germ with the mesenchyme of the dental papilla. The dental papilla grows in size even deeper into the enamel organ. It is penetrated by blood vessels.

On the surface of the dental papilla, cells with dark basophilic cytoplasm differentiate from mesenchymal cells, arranged in several rows. This layer is separated from the ameloblasts by a thin basement membrane. In the circumference of the tooth germ, the crossbars of the bone tissue of the dental alveoli are formed. At the 6th month of development, the nuclei of ameloblasts move in the direction opposite to their original position. Now the nucleus is located in the former apical part of the cell, bordering the pulp of the enamel organ. In the dental papilla, a peripheral layer of regularly located pear-shaped odontoblasts is determined, the long process of which faces the enamel organ. These cells form a narrow strip of non-mineralized predentin, outside of which there is some mature mineralized dentin. On the side facing the dentin layer, a strip of organic matrix of enamel prisms is formed. The formation of dentin and enamel extends from the apex of the crown to the root, which is fully formed after the crown has erupted.

Laying of permanent teeth. Permanent teeth are laid at the end of the 4th month of intrauterine development. From the common dental plate behind each rudiment of a milk tooth, a rudiment of a permanent tooth is formed. First, the milk and permanent teeth are in a common alveolus. Later, a bony septum separates them. By the age of 6-7, osteoclasts destroy this septum and the root of the falling milk tooth.

Change of teeth. The first set of teeth (milk teeth) consists of 10 in the upper jaw and 10 in the lower jaw. The eruption of milk teeth in a child begins at the 6-7th month of life. The central (medial) and lateral incisors erupt first on both sides of the midline in the upper and lower jaws. In the future, canines appear lateral to the incisors, behind which two molars erupt. A full set of milk teeth is formed at about two years of age. Milk teeth serve for the next 4 years. The change of milk teeth occurs in the range from 6 to 12 years. Permanent front teeth (canines, small molars) replace the corresponding milk teeth and are called replacement permanent teeth. Premolars (permanent small molars) replace milk molars (large molars). The germ of the second large molar tooth is formed in the 1st year of life, and the third molar (wisdom tooth) - by the 5th year. The eruption of permanent teeth begins at the age of 6-7 years. The large molar (first molar) erupts first, then the central and lateral incisors. At 9-14 years old, premolars, canines and the second molar erupt. Wisdom teeth erupt later than all - at the age of 18 - 25 years.

The structure of the tooth. It includes two parts: hard and soft. In the hard part of the tooth, enamel, dentin and cement are isolated, the soft parts of the tooth are represented by the so-called pulp. Enamel is the outer shell and covers the crowns of the tooth. The thickness of the enamel is 2,5 mm along the cutting edge or in the region of the masticatory tubercles of the molars and decreases as it approaches the neck.

In the crown, under the enamel, there is a characteristically striated dentin, continuing in a continuous mass into the root of the tooth. Enamel formation (synthesis and secretion of the components of its organic matrix) involves cells that are absent in mature enamel and an erupted tooth - enameloblasts (ameloblasts), so enamel regeneration during caries is impossible.

Enamel has a high refractive index - 1,62, enamel density - 2,8 - 3,0 g per square centimeter of area.

Enamel is the hardest tissue in the body. However, enamel is fragile. Its permeability is limited, although there are pores in the enamel through which aqueous and alcoholic solutions of low molecular weight substances can penetrate. Relatively small water molecules, ions, vitamins, monosaccharides, amino acids can slowly diffuse in the enamel substance. Fluorides (drinking water, toothpaste) are included in the crystals of enamel prisms, increasing the resistance of enamel to caries. The permeability of enamel increases under the action of acids, alcohol, with a deficiency of calcium, phosphorus, fluorine.

Enamel is formed by organic substances, inorganic substances, water. Their relative content in weight percent: 1 : 96 : 3. By volume: organic matter 2%, water - 9%, inorganic matter - up to 90%. Calcium phosphate, which is part of hydroxyapatite crystals, makes up 3/4 of all inorganic substances. In addition to phosphate, calcium carbonate and fluoride are present in small amounts - 4%. Of the organic compounds, there is a small amount of protein - two fractions (soluble in water and insoluble in water and weak acids), a small amount of carbohydrates and lipids was found in the enamel.

The structural unit of enamel is a prism with a diameter of about 5 microns. The orientation of the enamel prisms is almost perpendicular to the boundary between enamel and dentine. Neighboring prisms form parallel beams. On sections parallel to the surface of the enamel, the prisms have the shape of a key nest: the elongated part of the prism of one row lies in the other row between the two bodies of adjacent prisms. Due to this shape, there are almost no spaces between the prisms in the enamel. There are prisms and a different (in cross section) shape: oval, irregular shape, etc. Perpendicular to the surface of the enamel and the enamel-dentin border, the course of the prisms has s-shaped bends. We can say that the prisms are helically curved.

There are no prisms on the border with dentin, as well as on the enamel surface (prismless enamel). The material surrounding the prism also has other characteristics and is called the "prism shell" (the so-called gluing (or soldering) substance), the thickness of such a shell is about 0,5 microns, in some places the shell is absent.

Enamel is an exceptionally hard tissue, which is explained not only by the high content of calcium salts in it, but also by the fact that calcium phosphate is found in enamel in the form of hydroxyapatite crystals. The ratio of calcium and phosphorus in crystals normally varies from 1,3 to 2,0. With an increase in this coefficient, the stability of the enamel increases. In addition to hydroxyapatite, other crystals are also present. The ratio of different types of crystals: hydroxyapatite - 75%, carbonate apatite - 12%, chlorine apatite - 4,4%, fluorapatite - 0,7%.

Between the crystals there are microscopic spaces - micropores, the totality of which is the medium in which diffusion of substances is possible. In addition to micropores, there are spaces between prisms in enamel - pores. Micropores and pores are the material substrate of enamel permeability.

There are three types of lines in the enamel, reflecting the uneven nature of enamel formation in time: transverse striation of enamel prisms, Retzius lines and the so-called neonatal line.

The transverse striation of enamel prisms has a period of about 5 µm and corresponds to the daily periodicity of prism growth.

Due to differences in optical density due to lower mineralization, Retzius lines are formed at the boundary between the elementary units of enamel. They look like arches arranged in parallel at a distance of 20 - 80 microns. The lines of Retzius may be interrupted, there are especially many of them in the neck area. These lines do not reach the surface of the enamel in the region of the masticatory tubercles and along the cutting edge of the tooth. The elementary units of enamel are rectangular spaces delimited from each other by vertical lines - the boundaries between prisms and horizontal lines (transverse striation of prisms). In connection with the unequal rate of enamel formation at the beginning and at the end of amelogenesis, the value of elementary units, which differs between the surface and deep layers of enamel, is also important. Where the Retzius lines reach the surface of the enamel, there are furrows - perichyma, running in parallel rows along the surface of the tooth enamel.

The neonatal line delimits the enamel formed before and after birth, it is visible as an oblique strip, clearly visible against the background of prisms and passing at an acute angle to the tooth surface. This line consists predominantly of prismless enamel. The neonatal line is formed as a result of changes in the mode of enamel formation at birth. These enamels are found in the enamel of all temporary teeth and, as a rule, in the enamel of the first premolar.

The surface areas of the enamel are denser than its underlying parts, the concentration of fluorine is higher here, there are grooves, pits, elevations, prismatic areas, pores, micro-holes. Various layers may appear on the surface of the enamel, including colonies of microorganisms in combination with amorphous organic matter (dental plaques). When inorganic substances are deposited in the plaque area, tartar is formed.

Huntero-Schreger bands in enamel are clearly visible in polarized light in the form of alternating bands of different optical density, directed from the border between the dentin almost perpendicular to the surface of the enamel. The stripes reflect the fact that the prisms deviate from the perpendicular position with respect to the enamel surface or to the enamel-dentin border. In some areas, enamel prisms are cut longitudinally (light stripes), in others - transversely (dark stripes).

Dentin is a type of mineralized tissue that makes up the bulk of the tooth. Dentin in the area of ​​the crown is covered with enamel, in the area of ​​the root - with cement. Dentin surrounds the cavity of the tooth in the area of ​​the crown, and in the area of ​​the root - the root canal.

Dentin is denser than bone tissue and cementum, but much softer than enamel. Density - 2,1 g/cm³. The permeability of dentin is much greater than the permeability of enamel, which is associated not so much with the permeability of the dentin substance itself, but with the presence of tubules in the mineralized dentin substance.

Composition of dentin: organic matter - 18%, inorganic matter - 70%, water - 12%. By volume - organic matter is 30%, inorganic matter - 45%, water - 25%. Of the organic substances, the main component is collagen, much less chondroitin sulfate and lipids. Dentin is highly mineralized, the main inorganic component being hydroxyapatite crystals. In addition to calcium phosphate, calcium carbonate is present in dentin.

Dentin is permeated with tubules. The direction of the tubules is from the border between the pulp and dentin to the dentin-enamel and dentin-cement junctions. Dentinal tubules are parallel to each other, but have a tortuous course (S-shaped on vertical sections of the tooth). The diameter of the tubules is from 4 µm closer to the pulpal edge of the dentin to 1 µm along the periphery of the dentin. Closer to the pulp, the tubules account for up to 80% of the volume of dentin, closer to the dentin-enamel junction - about 4%. In the root of the tooth, closer to the dentin-cement border, the tubules not only branch, but also form loops - the region of the granular layer of Toms.

On a section running parallel to the enamel-dentin junction, heterogeneities of dentin mineralization are visible. The lumen of the tubules is covered by a double concentric cuff with a dense periphery - peritubular dentin, dental (or Neumann) sheaths. The dentin of the Neumann sheaths is more mineralized than the intertubular dentin. The outermost and innermost parts of the peritubular dentin are less mineralized than the median part of the cuff. There are no collagen fibrils in peritubular dentin, and hydroxyapatite crystals are organized differently in peritubular and intertubular dentin. Closer to the predentin, the peritubular dentin is practically absent. Peritubular dentin is constantly formed, therefore, in adults, peritubular dentin is significantly larger than in children, respectively, the permeability of dentin in children is higher.

In different parts of the tooth, dentin is heterogeneous.

Primary dentin is formed during mass dentinogenesis. In the mantle (superficial) and near-pulp dentin, the orientation of collagen fibers is different. The mantle dentin is less mineralized than the peripulpal dentin. Raincoat dentin is located on the border with enamel. The peripulpal dentin is the bulk of the dentin.

Granular and hyaline layers of dentin. In the root of the tooth, between the main mass of dentin and acellular cement, there are granular and hyaline layers of dentin. In the hyaline layer, the orientation of the fibers is felt-like. The granular layer consists of alternating areas of hypo- or completely non-mineralized dentin (interglobular spaces) and fully mineralized dentin in the form of spherical formations (dentinal balls or calcospherites).

Secondary dentin (or irritant dentin) is deposited between the bulk of dentin (primary dentin) and predentin. Irritation dentin is constantly formed throughout life by abrasion of chewing surfaces or destruction of dentin.

Regular dentin (organized dentin) is located in the region of the root of the tooth.

Irregular irritation dentin (disorganized dentin) is located at the apex of the tooth cavity.

Predentin (or non-mineralized dentin) is located between the layer of odontoblasts and dentin. Predentin is newly formed and non-mineralized dentin. Between the predentin and the peripulpal dentin there is a plate of mineralizing predentin - an intermediate dentin of calcification.

There are several types of breaklines in dentin. The lines are perpendicular to the dentinal tubules. The following main types of lines are distinguished: the Schreger and Owen lines associated with the bends of the dentinal tubules, the Ebner lines and the mineralization lines associated with uneven mineralization, violations of mineralization and its rhythm. In addition, there is a neonatal line.

Owen's lines are visible in polarized light and are formed when the secondary bends of the dentinal tubules are superimposed on each other. Owen's contour lines are quite rare in primary dentin, they are more often located on the border between primary and secondary dentin.

These lines are located perpendicular to the tubules at a distance of about 5 µm from each other.

Lines of mineralization are formed due to the uneven rate of calcification during dentinogenesis. Since the mineralization front is not necessarily strictly parallel to the predentin, the course of the lines can be tortuous.

The neonatal lines, as in enamel, reflect the fact of a change in the mode of dentinogenesis at birth. These lines are expressed in milk teeth and in the first permanent molar.

The cement covers the root dentin with a thin layer, thickening towards the root apex. The cement located closer to the neck of the tooth does not contain cells and is called acellular. The top of the root is covered with cement containing cells - cementocytes (cellular cement). Acellular cement consists of collagen fibers and an amorphous substance. Cell cement resembles coarse fibrous bone tissue, but does not contain blood vessels.

The pulp is the soft part of the tooth, represented by loose connective tissue and consists of peripheral, intermediate and central layers. The peripheral layer contains odontoblasts - analogues of bone osteoblasts - high cylindrical cells with a process extending from the apical pole of the cell to the border between dentin and enamel. Odontoblasts secrete collagen, glycosaminoglycans (chondroitin sulfate) and lipids that are part of the organic matrix of dentin. With the mineralization of predentin (non-calcified matrix), the processes of odontoblasts become immured in the dentinal tubules. The intermediate layer contains odontoblast precursors and emerging collagen fibers. The central layer of the pulp is a loose fibrous connective tissue with many anastomosing capillaries and nerve fibers, the terminals of which branch out in the intermediate and peripheral layers. In the elderly, in the pulp, irregularly shaped calcified formations - denticles are often found. True denticles consist of dentin surrounded on the outside by odontoblasts. False denticles are concentric deposits of calcified material around necrotic cells.

Pharynx

This is the intersection of the respiratory and digestive tracts. According to the functional conditions in the pharynx, three sections are distinguished, which have a different structure - nasal, oral and laryngeal. All of them differ in the structure of the mucous membrane, which is represented by various types of epithelium.

The mucous membrane of the nasal part of the pharynx is covered with multi-row ciliated epithelium, contains mixed glands (respiratory type of mucous membrane).

The mucous membrane of the oral and laryngeal sections is lined with stratified squamous epithelium, located on the lamina propria of the mucous membrane, in which there is a well-defined layer of elastic fibers.

Esophagus

The esophagus is a hollow tube that consists of the mucosa, submucosa, muscularis and adventitia.

The mucous membrane, together with the submucosa, forms 7–10 longitudinally located folds in the esophagus, protruding into its lumen.

The mucous membrane of the esophagus consists of the epithelium, its own and muscular plates. The epithelium of the mucous membrane is multilayered, flat, non-keratinizing.

The lamina propria of the esophageal mucosa is a layer of loose, fibrous, unformed connective tissue that protrudes into the epithelium in the form of papillae.

The muscular plate of the mucous membrane of the esophagus consists of bundles of smooth muscle cells located along it, surrounded by a network of elastic fibers.

The submucosa of the esophagus, formed by loose fibrous unformed connective tissue, provides greater mobility of the mucosa in relation to the muscular membrane. Together with the mucosa, it forms numerous longitudinal folds, which straighten out during the swallowing of food. In the submucosa are the own glands of the esophagus.

The muscular membrane of the esophagus consists of an inner circular and outer longitudinal layers, separated by a layer of loose fibrous unformed connective tissue. At the same time, in the upper part of the esophagus muscles belong to striated tissue, on average - to striated tissue and smooth muscles, and in the lower part - only to smooth.

The adventitial membrane of the esophagus consists of loose fibrous unformed connective tissue, which, on the one hand, is associated with layers of connective tissue in the muscular membrane, and on the other hand, with the connective tissue of the mediastinum surrounding the esophagus.

The abdominal esophagus is covered with a serous membrane.

The blood supply of the esophagus is produced from the artery entering the esophagus, and plexuses are formed in the submucosa (large-loop and small-loop), from which blood enters the large-loop plexus of the lamina propria.

Innervation. The intramural nervous apparatus is formed by three interconnected plexuses: adventitious (most developed in the middle and lower thirds of the esophagus), subadventitial (lying on the surface of the muscular membrane and well expressed only in the upper parts of the esophagus), intermuscular (located between the circular and longitudinal muscle layers).

Stomach

The main function of the stomach is secretory. It consists in the production of gastric juice by the glands. It consists of the enzymes pepsin (promoting the breakdown of proteins), chymosin (contributing to the curdling of milk), lipase (promoting the breakdown of lipids), as well as hydrochloric acid and mucus.

The mechanical function of the stomach is to mix food with gastric juice and push the processed food into the duodenum.

Also, the wall of the stomach produces an anti-anemic factor, which promotes the absorption of vitamin B¹².

The endocrine function of the stomach consists in the production of a number of biologically active substances - gastrin, histamine, serotonin, motilin, enteroglucagon, etc. Together, these substances have a stimulating or inhibitory effect on the motility and secretory activity of the glandular cells of the stomach and other parts of the digestive tract.

Structure. The wall of the stomach consists of the mucous membrane, submucosa, muscular and serous membranes.

The mucous membrane of the stomach has an uneven surface due to the presence of three types of formations in it - folds, fields and pits.

The epithelium lining the surface of the gastric mucosa and pits is single-layer cylindrical. The peculiarity of this epithelium is its glandular character: all epithelial cells constantly secrete a mucoid (mucus-like) secret. Each glandular cell is clearly divided into two parts: basal and apical.

The lamina propria of the gastric mucosa is represented by loose, fibrous, unformed connective tissue. In it, in greater or lesser quantities, there are always accumulations of lymphoid elements in the form of either diffuse infiltrates or solitary (single) lymphatic follicles.

The muscular plate of the gastric mucosa is located on the border with the submucosa. It consists of three layers formed by smooth muscle tissue: inner and outer circular and middle longitudinal. Each of these layers is made up of bundles of smooth muscle cells.

The glands of the stomach in its various departments have an unequal structure. There are three types of gastric glands: own gastric, pyloric and cardiac.

Own glands of the stomach contain several types of glandular cells - the main, parietal (cooking), mucous, cervical and endocrine (argyrophilic).

The main cells of their own glands are located mainly in the region of their bottom and bodies. They distinguish between the basal and apical parts. The basal part of the cell is located at the base on the basal membrane, bordering on the lamina propria, and has a well-defined basophilia. Granules of protein secretion are found in the apical part of the cell. Chief cells secrete pepsinogen, a proenzyme that, in the presence of hydrochloric acid, is converted to its active form, pepsin. It is believed that chymosin, which breaks down milk proteins, is also produced by chief cells.

The parietal cells of the own glands are located outside the main and mucous cells, tightly adhering to their basal ends. In size they are larger than the main cells, their shape is irregularly rounded.

The main role of the parietal cells of the own glands of the stomach is the production of chlorides, from which hydrochloric acid is formed.

The mucous cells of the own glands of the stomach are represented by two types. Some are located in the body of their own glands and have a compacted nucleus in the basal part of the cells.

In the apical part of these cells, many round or oval granules, a small amount of mitochondria, and a lamellar complex were found. Other mucous cells (cervical) are located only in the neck of their own glands.

The pyloric glands of the stomach are located in a small area near its exit into the duodenum. The secret produced by the pyloric glands is alkaline. In the neck of the glands there are also intermediate (cervical) cells, which have already been described in the own glands of the stomach.

Cardiac glands of the stomach are simple tubular glands with highly branched terminal sections. Apparently, the secretory cells of these glands are identical to the cells lining the pyloric glands of the stomach and the cardiac glands of the esophagus.

Endocrine argyrophilic cells. Several types of endocrine cells have been identified in the stomach according to morphological, biochemical and functional characteristics.

EC cells - the largest group of cells, located in the area of ​​the bottom of the glands between the main cells. These cells secrete serotonin and melatonin.

G-cells (gastrin-producing) are located mainly in the pyloric glands, as well as in the cardiac glands, located in the area of ​​\uXNUMXb\uXNUMXbtheir body and bottom, sometimes the neck. The gastrin secreted by them stimulates the secretion of pepsinogen by the chief cells and hydrochloric acid by parietal cells, as well as gastric motility.

P-cells secrete bombesin, which stimulates the release of hydrochloric acid and enzyme-rich pancreatic juice, and also increases the contraction of the smooth muscles of the gallbladder.

ECX cells (enterochromaffin-like) are characterized by a variety of shapes and are located mainly in the body and bottom of the fundic glands. These cells produce histamine, which regulates the secretory activity of parietal cells that produce hydrochloric acid.

The submucosa of the stomach consists of loose fibrous irregular connective tissue containing a large number of elastic fibers. This layer contains arterial and venous plexuses, a network of lymphatic vessels and a submucosal nerve plexus.

The muscular coat of the stomach is characterized by weak development in the region of its bottom, good expression in the body and the achievement of the greatest development in the pylorus. In the muscular membrane of the stomach, there are three layers formed by smooth muscle tissue.

The serous membrane of the stomach forms the outer part of its wall. It is based on loose fibrous unformed connective tissue adjacent to the muscular membrane of the stomach. From the surface, this connective tissue layer is covered with a single-layer squamous epithelium - mesothelium.

The arteries that feed the wall of the stomach pass through the serous and muscular membranes, giving them the corresponding branches, and then pass into a powerful plexus in the submucosa. The main sources of nutrition include the right and left ventricular arteries. From the stomach, blood flows into the portal vein.

Innervation. The stomach has two sources of efferent innervation - parasympathetic (from the vagus nerve) and sympathetic (from the borderline sympathetic trunk).

In the wall of the stomach there are three nerve plexuses - intermuscular, submucosal and subserous.

Small intestine

In the small intestine, all kinds of nutrients - proteins, fats and carbohydrates - undergo chemical processing. Protein digestion involves the enzymes enterokinase, kinasogen and trypsin, which break down simple proteins, erepsin (a mixture of peptidases), which breaks down peptides into amino acids, and nuclease, which digests complex proteins (nucleoproteins). Digestion of carbohydrates occurs due to amylase, maltose, sucrose, lactose and phosphatase, and fats - the enzyme lipase.

In the small intestine, the process of absorption of the breakdown products of proteins, fats and carbohydrates into the blood and lymphatic vessels also takes place.

Also, the small intestine performs a mechanical function: it pushes the chyme in the caudal direction.

The endocrine function, performed by special secretory cells, consists in the production of biologically active substances - serotonin, histamine, motilin, secretin, enteroglucagon, cholecystokinin, pancreozymin, gastrin and gastrin inhibitor.

Structure. The wall of the small intestine consists of a mucous membrane, submucosa, muscular and serous membranes.

The relief due to the presence of a number of formations (folds, villi and crypts) is very specific for the mucous membrane of the small intestine.

These structures increase the overall surface of the small intestine mucosa, which contributes to the performance of its main functions.

From the surface, each intestinal villus is lined with a single-layer cylindrical epithelium. In the epithelium, three types of cells are distinguished - border, goblet and endocrine (argyrophilic).

Enterocytes with a striated border make up the bulk of the epithelial layer covering the villus. They are characterized by a pronounced polarity of the structure, which reflects their functional specialization - ensuring the resorption and transport of substances from food.

On the apical surface of the cells, a border formed by many microvilli is visible. Due to such a large number of villi, the absorption surface of the intestine increases by 30-40 times.

It was revealed that the breakdown of nutrients and their absorption most intensively occur in the region of the striated border. This process is called parietal digestion, in contrast to the cavity, which takes place in the lumen of the intestinal tube, and intracellular.

Goblet intestinal. By structure, these are typical mucous cells. They show cyclical changes associated with the accumulation and subsequent secretion of mucus.

Beneath the epithelium of the villi is a weakly expressed basement membrane, followed by a loose, fibrous, unformed connective tissue of the lamina propria.

In the stroma of the villi, there are always separate smooth muscle cells: derivatives of the muscular layer of the mucous membrane. Bundles of smooth muscle cells are wrapped in a network of reticular fibers that connect them to the stroma of the villus and the basement membrane.

The contraction of myocytes promotes the absorption of food hydrolysis products into the blood and lymph of the intestinal villi.

Intestinal crypts of the small intestine are tubular depressions of the epithelium, lying in its own plate of its mucous membrane, and the mouth opens into the lumen between the villi.

The epithelial lining of intestinal crypts contains the following types of cells: bordered, borderless intestinal cells, goblet, endocrine (argyrophilic) and intestinal cells with acidophilic granularity (Paneth cells). Intestinal enterocytes with a striated border make up the bulk of the epithelial lining of the crypts.

The lamina propria of the small intestine mucosa mainly consists of a large number of reticular fibers. They form a dense network throughout the lamina propria and, approaching the epithelium, participate in the formation of the basement membrane. Process cells with a pale oval nucleus are closely associated with reticular fibers. In appearance, they resemble the reticular cells of the hematopoietic organs.

The mucosa contains many single lymphatic follicles and aggregates of follicles. Single (solitary) lymphatic follicles are found throughout the small intestine. Large follicles lying in the distal small intestine penetrate into the muscularis mucosa and are partially located in the submucosa. Larger accumulations of lymphoid tissue - aggregates (or group lymphatic follicles (Peyer's patches)), as a rule, are located in the ileum, but sometimes occur in the jejunum and duodenum.

The submucosa contains blood vessels and nerve plexuses.

The muscular coat is represented by two layers of smooth muscle tissue - internal (circular) and external (longitudinal).

The serous membrane covers the intestine from all sides, with the exception of the duodenum, which is covered by the peritoneum only in front.

The blood supply to the small intestine is carried out at the expense of the arteries entering the wall of the small intestine with the formation of a plexus in it in all layers of the intestinal membrane.

The lymphatic vessels of the small intestine are represented by a very widely branched network. In each intestinal villus there is a centrally located, blindly ending at its top, a lymphatic capillary.

Innervation. The small intestine is innervated by sympathetic and parasympathetic nerves.

Afferent innervation is carried out by a sensitive musculo-intestinal plexus formed by sensitive nerve fibers of the spinal ganglia and their receptor endings.

Efferent parasympathetic innervation is carried out due to the musculo-intestinal and submucosal nerve plexuses. The muscular-intestinal plexus is most developed in the duodenum, where numerous, densely located large ganglia are observed.

Colon

In the large intestine, water is absorbed from the chyme and feces are formed. A significant amount of mucus is secreted in the large intestine, which facilitates the movement of contents through the intestines and helps to glue undigested food particles. Excretion processes also take place in the large intestine. A number of substances are released through the mucous membrane of this intestine, for example, calcium, magnesium, phosphates, salts of heavy metals, etc. There is also evidence that vitamin K is produced in the large intestine, and the bacterial flora that is constantly present in the intestine takes part in this. Bacteria in the large intestine help digest fiber.

The large intestine is divided into the colon and the rectum.

Colon. The wall of the colon, as well as the entire gastrointestinal tract, consists of a mucous membrane, submucosa, muscular and serous membranes.

The mucous membrane has a large number of folds and crypts, which significantly increase its surface, but there are no villi.

Folds are formed on the inner surface of the intestine from the mucous membrane and submucosa. They are located across and have a crescent shape (hence the name - crescent folds). Crypts in the colon are better developed than in the small intestine. At the same time, the epithelium is single-layer prismatic, it consists of cells of the intestinal epithelium with a striated border, goblet and intestinal cells without a border.

The lamina propria consists of loose, fibrous, unformed connective tissue. Its thin layers are visible between the intestinal crypts.

The muscular plate of the mucous membrane is more pronounced than in the small intestine, and consists of two strips. Its inner strip is denser, formed mainly by circularly located bundles of smooth muscle cells. The outer strip is represented by bundles of smooth muscle cells, oriented partly longitudinally, partly obliquely with respect to the axis of the intestine.

The submucosa consists of loose fibrous irregular connective tissue, in which there are many fat cells. Here are the vascular and nerve submucosal plexuses. There are always a lot of lymphatic follicles in the submucosa of the colon, they spread here from the lamina propria.

The muscular coat is represented by two layers of smooth muscle tissue: internal (or circular) and external (or longitudinal), which forms three ribbons stretching along the entire length of the intestine.

In the parts of the intestine lying between the ribbons, only a thin layer is found, consisting of a small amount of longitudinally arranged bundles of smooth muscle cells. These areas form swellings - gaustra.

The serous membrane covers the colon, however, there are sections covered with a serous membrane on all sides, and there are sections covered only on three sides - mesoperitoneally (ascending and descending sections of the colon).

The appendix is ​​a rudimentary formation of the large intestine, it contains large accumulations of lymphoid tissue. The mucous membrane of the appendix has crypts that are located radially with respect to its lumen.

The epithelium of the mucous membrane is cylindrical, bordered, with a small number of goblet cells.

The lamina propria of the mucosa consists of loose, fibrous, unformed connective tissue, which, without a sharp border (due to the weak development of the muscular mucosal lamina), passes into the submucosa.

In the submucosal base of the appendix, formed by loose fibrous unformed connective tissue, blood vessels and the nerve submucosal plexus lie.

The muscular coat is also formed by two layers.

The appendix performs a protective function. It has been established that differentiation of B-lymphocytes occurs in the follicles.

Rectum. The rectum is a continuation of the colon.

In the anal part of the intestine, three zones are distinguished - columnar, intermediate and skin. In the columnar zone, the longitudinal folds form the anal columns.

The mucous membrane of the rectum consists of the epithelium, its own and muscular plates. The epithelium in the upper section of the rectum is single-layered, cylindrical, in the columnar zone of the lower section - multi-layered, cubic, in the intermediate - multi-layered, flat, non-keratinizing, in the skin - multi-layered, flat, keratinizing. The transition from stratified, cuboidal epithelium to stratified, squamous epithelium stands out as a zigzag line.

The lamina propria is made up of loose, fibrous, unformed connective tissue. She takes part in the formation of the folds of the rectum. Here are single lymphatic follicles and vessels. In the region of the columnar zone in this plate lies a network of thin-walled blood lacunae, the blood from which flows into the hemorrhoidal veins.

In the intermediate zone of the rectum, the lamina propria contains a large number of elastic fibers, elements of lymphoid tissue.

In the skin area surrounding the anus, hair joins the sebaceous glands. Sweat glands in the lamina propria of the mucous membrane appear at a distance of 1 - 1,5 cm from the anus, they are tubular glands.

The muscular plate of the mucous membrane, as in other parts of the large intestine, consists of two strips.

The submucosa is represented by loose fibrous unformed connective tissue. It contains the vascular and nerve plexuses. In the submucosa lies the plexus of hemorrhoidal veins. In case of violation of the tone of the walls of these vessels, varicose expansions appear.

The muscular coat is formed by smooth muscle tissue and consists of two layers - inner (circular) and outer (longitudinal). The circular layer at different levels of the rectum forms two thickenings, which stand out as separate anatomical formations - sphincters.

The serous membrane covers the rectum in its upper part, in the lower sections the rectum has a connective tissue membrane.

Liver

The liver is one of the major glands of the digestive tract, performing numerous functions.

The following processes take place in it:

1) neutralization of various metabolic products;

2) destruction of various biologically active substances;

3) destruction of sex hormones;

4) various protective reactions of the organism;

5) it takes part in the formation of glycogen (the main source of glucose);

6) the formation of various proteins;

7) hematopoiesis;

8) it accumulates vitamins;

9) formation of bile.

Structure. The liver is an unpaired organ located in the abdominal cavity, covered with peritoneum on all sides. It has several lobes, 8 segments.

The main structural and functional unit of the liver is the hepatic lobule. It is a hexagonal prism of liver cells (hepatocytes collected in the form of beams). Each lobule is covered with a connective tissue membrane, in which the bile ducts and blood vessels pass. From the periphery of the lobule (through the system of capillaries of the portal vein and the hepatic artery) to its center, the blood passes through the blood vessels, being cleansed, and through the central vein of the hepatic lobule enters the collecting veins, then into the hepatic veins and into the inferior vena cava.

Bile capillaries pass between the rows of hepatocytes that form the beam of the hepatic lobule. These capillaries do not have their own wall. Their wall is formed by contiguous surfaces of hepatocytes, on which there are small depressions that coincide with each other and together form the lumen of the bile capillary.

Summarizing the above, we can conclude that the hepatocyte has two surfaces: one is capillary (facing the blood vessel), the other is biliary (facing the lumen of the bile capillary).

At the same time, you need to know that the lumen of the bile capillary does not communicate with the intercellular gap due to the fact that the membranes of neighboring hepatocytes in this place fit tightly to each other, forming end plates, which, in turn, prevents the penetration of bile into the blood vessels. In these cases, bile spreads throughout the body and stains its tissues yellow.

Basic cell types

Hepatocytes form hepatic plates (strands), contain in abundance almost all organelles. The nucleus has 1 - 2 nucleoli and is most often located in the center of the cell. 25% of hepatocytes have two nuclei. The cells are characterized by polyploidy: 55-80% of hepatocytes are tetraploid, 5-6% are octaploid and only 10% are diploid. The granular and smooth endoplasmic reticulum is well developed. Elements of the Golgi complex are present in various parts of the cell. The number of mitochondria in a cell can reach 2000. Cells contain lysosomes and peroxisomes. The latter have the form of a bubble surrounded by a membrane with a diameter of up to 0,5 μm. Peroxisomes contain oxidative enzymes - amino oxidase, urate oxidase, catalase. As in mitochondria, oxygen is utilized in peroxisomes. Direct relation to the formation of these organelles has a smooth endoplasmic reticulum. Numerous inclusions, mainly of glycogen, are present in the cytoplasm. Each hepatocyte has two poles - sinusoidal and bile (or biliary).

The sinusoidal pole faces the space of Disse. It is covered with microvilli, which are involved in the transport of substances from the blood to hepatocytes and vice versa. Microvilli of hepatocytes are in contact with the surface of endothelial cells. The biliary pole also has microvilli, which facilitates the excretion of bile components. Bile capillaries are formed at the point of contact of the biliary poles of two hepatocytes.

Cholangiocytes (or epithelial cells of the intrahepatic bile ducts) make up 2-3% of the total liver cell population. The total length of the intrahepatic bile ducts is approximately 2,2 km, which plays an important role in the formation of bile. Cholangiocytes are involved in the transport of proteins and actively secrete water and electrolytes.

stem cells. Hepatocytes and cholangiocytes are among the growing cell populations of the endodermal epithelium. The stem cells for both are oval cells located in the bile ducts.

Sinusoid cells of the liver. Four cell types are known and intensively studied that are constantly present in the sinusoids of the liver: endothelial cells, Kupffer stellate cells, Ito cells and pit cells. According to the data of morphometric analysis, sinusoid cells occupy about 7% of the liver volume.

Endothelial cells contact with the help of numerous processes, separating the lumen of the sinusoid from the space of Disse. The nucleus is located along the cell membrane from the space of Disse. The cells contain elements of a granular and smooth endoplasmic reticulum. The Golgi complex is located between the nucleus and the lumen of the sinusoid. The cytoplasm of endothelial cells contains numerous pinocytic vesicles and lysosomes. Fenestra, not tightened by diaphragms, occupy up to 10% of the endothelium and regulate the entry of particles larger than 0,2 in diameter into the space of Disse, for example, chylomicrons. Endothelial cells of sinusoids are characterized by endocytosis of all types of molecules and particles with a diameter of not more than 0,1 μm. The absence of a typical basement membrane, the capacity for endocytosis, and the presence of fenestrations distinguish the endothelium of the sinusoids from the endothelium of other vessels.

Kupffer cells belong to the system of mononuclear phagocytes and are located between endothelial cells as part of the wall of the sinusoid. The main site of localization of Kupffer cells are the periportal areas of the liver. Their cytoplasm contains lysosomes with high peroxidase activity, phagosomes, iron inclusions, and pigments. Kupffer cells remove foreign material from the blood, fibrin, an excess of activated blood coagulation factors, participate in the phagocytosis of aging and damaged red blood cells, hemoglobin and iron metabolism. Iron from destroyed erythrocytes or from the blood accumulates in the form of hemosiderin for subsequent use in the synthesis of Hb. Metabolites of arachidonic acid, platelet activating factor cause the activation of Kupffer cells. Activated cells, in turn, begin to produce a complex of biologically active substances, such as oxygen radicals, plasminogen activator, tumor necrosis factor TNF, IL-1, IL-6, transforming growth factor, which can cause toxic damage to hepatocytes.

Pit cells (Pit-cells) - lymphocytes located on endothelial cells or between them. It is suggested that pit cells may be NK cells and act against tumor and virus-infected cells. Unlike Kupffer cells, which require activation, the cytolytic effect of pit cells appears spontaneously, without prior activation from other cells or biologically active substances.

Fat-accumulating cells (lipocytes, Ito cells) have a process shape, are localized in the space of Disse or between hepatocytes. Ito cells play an important role in the metabolism and accumulation of retinoids. About 50 - 80% of vitamin A in the body accumulates in the liver, and up to 90% of all liver retinoids are deposited in fat drops of Ito cells. Retinol esters enter hepatocytes as part of chylomicrons. In hepatocytes, retinol esters are converted to retinol and a complex of vitamin A with retin-binding protein is formed. The complex is secreted into the space of Disse, from where it is deposited by Ito cells. In vitro, Ito cells have been shown to be able to synthesize collagen, which suggests their involvement in the development of cirrhosis and fibrosis of the liver.

The main functions of the liver

Secretion of bile. Hepatocytes produce and secrete bile through the biliary pole into the bile capillaries. Bile is an aqueous solution of electrolytes, bile pigments, bile acids. Bile pigments are the end products of the metabolism of Hb and other porphyrins. Hepatocytes take up free bilirubin from the blood, conjugate it with glucuronic acid, and secrete non-toxic, conjugated bilirubin into the bile capillaries. Bile acids are the end product of cholesterol metabolism and are essential for the digestion and absorption of lipids. Physiologically active substances, such as conjugated forms of glucocorticoids, are also excreted from the body with bile. As part of the bile, class A immunoglobulins from the spaces of Disse enter the intestinal lumen.

Synthesis of proteins. Hepatocytes secrete albumins (fibrinogen, prothrombin, factor III, angiotensinogen, somatomedins, thrombopoietin, etc.) into the space of Disse. Most plasma proteins are produced by hepatocytes.

Metabolism of carbohydrates. Excess glucose in the blood that occurs after a meal is absorbed by hepatocytes with the help of insulin and stored in the form of glycogen. With glucose deficiency, glucocorticoids stimulate gluconeogenesis in hepatocytes (the conversion of amino acids and lipids into glucose).

lipid metabolism. Chylomicrons from the spaces of Disse enter hepatocytes, where they are stored as triglycerides (lipogenesis) or secreted into the blood as lipoproteins.

Storage. Triglycerides, carbohydrates, iron, copper are stored in hepatocytes. Ito cells accumulate lipids and up to 90% of retinoids deposited in the liver.

Detoxification. Inactivation of Hb metabolic products, proteins, xenobiotics (eg, drugs, drugs, industrial chemicals, toxic substances, metabolic products of bacteria in the intestine) occurs with the help of enzymes during oxidation, methylation and binding reactions. A non-toxic form of bilirubin is formed in hepatocytes, urea is synthesized from ammonia (the end product of protein metabolism), which is to be excreted through the kidneys, and sex hormones are degraded.

Body protection. Kupffer cells remove microorganisms and their waste products from the blood. Pit cells are active against tumor and virus-infected cells. Hepatocytes transport IgA from the space of Disse to the bile and then to the intestinal lumen.

Hematopoietic. The liver is involved in prenatal hematopoiesis. In the postnatal period, thrombopoietin is synthesized in hepatocytes.

The bile ducts are a system of bile vessels that transport bile from the liver to the lumen of the duodenum. Allocate intrahepatic and extrahepatic bile ducts. The intrahepatic ones include the interlobular bile ducts, and the extrahepatic ones include the right and left hepatic ducts, the common hepatic, cystic and common bile ducts (choledochus).

The gallbladder is a hollow organ with a thin wall (about 1,5 - 2 mm). It holds 40 - 60 ml of bile. The wall of the gallbladder consists of three membranes: mucous, muscular and adventitial. The latter from the side of the abdominal cavity is covered with a serous membrane.

The mucous membrane of the gallbladder forms folds that anastomose with each other, as well as crypts or sinuses in the form of pockets.

In the region of the neck of the bladder, there are alveolar-tubular glands in it that secrete mucus. The epithelium of the mucous membrane has the ability to absorb water and some other substances from the bile that fills the bladder cavity. In this regard, cystic bile is always thicker in consistency and darker in color than bile that comes directly from the liver.

The muscular coat of the gallbladder consists of smooth muscle cells (arranged in a network in which their circular direction predominates), which are especially well developed in the region of the neck of the gallbladder. Here are the sphincters of the gallbladder, contributing to the retention of bile in the lumen of the gallbladder.

The adventitia of the gallbladder is composed of dense fibrous connective tissue.

Innervation. In the capsule of the liver there is a vegetative nerve plexus, the branches of which, accompanying the blood vessels, continue into the interlobular connective tissue.

Pancreas

The pancreas is an organ of the digestive system, which includes exocrine and endocrine parts. The exocrine part is responsible for the production of pancreatic juice containing digestive enzymes (trypsin, lipase, amylase, etc.), which enters the duodenum through the excretory ducts, where its enzymes are involved in the breakdown of proteins, fats and carbohydrates to final products. In the endocrine part, a number of hormones are synthesized (insulin, glucagon, somatostatin, pancreatic polypeptide), which are involved in the regulation of carbohydrate, protein and fat metabolism in tissues.

Structure. The pancreas is an unpaired organ of the abdominal cavity, on the surface covered with a connective tissue capsule, fused with the visceral sheet of the peritoneum. Its parenchyma is divided into lobules, between which connective tissue strands pass. They contain blood vessels, nerves, intramural nerve ganglia, lamellar bodies (Vater-Pacini bodies) and excretory ducts.

The acinus is a structural and functional unit. It consists of cells of the pancreas, includes a secretory section and an insertion section, from which the ductal system of the gland begins.

Acinar cells perform a secretory function, synthesizing the digestive enzymes of pancreatic juice. They have the shape of a cone with a narrowed apex and a wide base lying on the basement membrane of the acinus.

The secretion of hormones occurs cyclically. The secretion phases are the same as those of other glands. However, secretion according to the merocrine type occurs depending on the physiological needs of the body for digestive enzymes, this cycle can be reduced or, conversely, increased.

The released secret passes through the ducts (intercalary, interacinar, intralobular), which, uniting, flow into the Wirsung duct.

The walls of these ducts are lined with a single layer of cuboidal epithelium. Their cytolemma forms internal folds and microvilli.

The endocrine part of the pancreas is in the form of islets (round or oval) lying between the acini, while their volume does not exceed 3% of the volume of the entire gland.

Islets consist of endocrine insular cells - insulocytes. Between them are fenestrated blood capillaries. The capillaries are surrounded by a pericapillary space. Hormones secreted by the insular cells first enter this space and then through the capillary wall into the blood.

There are five main types of insular cells: B cells (basophilic), A cells (acidophilic), D cells (dendritic), D1 cells (argyrophilic), and PP cells.

B cells make up the bulk of islet cells (about 70-75%). Granules of B-cells consist of the hormone insulin, A-cells make up approximately 20 - 25% of the total mass of insular cells. In the islets, they occupy a predominantly peripheral position.

The hormone glucagon was found in the A-cell granules. It acts as an insulin antagonist.

The number of D-cells in the islets is small - 5 - 10%.

D cells secrete the hormone somatostatin. This hormone delays the release of insulin and glucagon by A- and B-cells, and also inhibits the synthesis of enzymes by pancreatic acinar cells.

PP cells (2 - 5%) produce a pancreatic polypeptide that stimulates the secretion of gastric and pancreatic juice.

These are polygonal cells with very small grains in the cytoplasm (the size of the granules is not more than 140 nm). PP cells are usually localized along the periphery of the islets in the head of the gland, and also occur outside the islets among the exocrine compartments and ducts.

The blood supply to the pancreas comes from the branches of the celiac trunk. Venous blood flows from the pancreas into the portal vein.

Innervation. The efferent innervation of the pancreas is carried out by the vagus and sympathetic nerves.

Topic 22. RESPIRATORY SYSTEM

The respiratory system includes various organs that perform air conduction and respiratory (gas exchange) functions: the nasal cavity, nasopharynx, larynx, trachea, extrapulmonary bronchi and lungs.

The main function of the respiratory system is external respiration, i.e., the absorption of oxygen from the inhaled air and the supply of blood to it, as well as the removal of carbon dioxide from the body (gas exchange is carried out by the lungs, their acini). Internal, tissue respiration occurs in the form of oxidative processes in the cells of organs with the participation of blood. Along with this, the respiratory organs perform a number of other important non-gas exchange functions: thermoregulation and humidification of the inhaled air, cleansing it of dust and microorganisms, deposition of blood in a richly developed vascular system, participation in maintaining blood clotting due to the production of thromboplastin and its antagonist (heparin), participation in the synthesis of certain hormones and in water-salt, lipid metabolism, as well as in voice formation, smell and immunological protection.

Development

On the 22nd - 26th day of intrauterine development, a respiratory diverticulum - the rudiment of the respiratory organs - appears on the ventral wall of the foregut. It is separated from the foregut by two longitudinal esophagotracheal (tracheoesophageal) grooves, which protrude into the lumen of the foregut in the form of ridges. These ridges, coming together, merge, and the esophagotracheal septum is formed. As a result, the foregut is divided into a dorsal part (esophagus) and a ventral part (trachea and pulmonary buds). As it separates from the foregut, the respiratory diverticulum, lengthening in the caudal direction, forms a structure lying in the midline - the future trachea; it ends in two sac-like protrusions. These are pulmonary buds, the most distal parts of which constitute the respiratory rudiment. Thus, the epithelium lining the tracheal primordium and pulmonary buds is of endodermal origin. The mucous glands of the airways, which are derivatives of the epithelium, also develop from the endoderm. Cartilage cells, fibroblasts and SMCs are derived from the splanchic mesoderm surrounding the foregut. The right pulmonary kidney is divided into three, and the left - into two main bronchi, predetermining the presence of three lobes of the lung on the right and two on the left. Under the inductive influence of the surrounding mesoderm, branching continues, eventually forming the bronchial tree of the lungs. By the end of the 6th month there are 17 branches. Later, 6 more additional branchings occur, the branching process ends after birth. At birth, the lungs contain about 60 million primary alveoli, their number increases rapidly in the first 2 years of life. Then the growth rate slows down, and by 8 to 12 years the number of alveoli reaches approximately 375 million, which is equal to the number of alveoli in adults.

Stages of development. Differentiation of the lungs goes through the following stages - glandular, tubular and alveolar.

The glandular stage (5-15 weeks) is characterized by further branching of the airways (the lungs take on the appearance of a gland), the development of cartilage of the trachea and bronchi, and the appearance of bronchial arteries. The epithelium lining the respiratory bud consists of cylindrical cells. On the 10th week, goblet cells appear from the cells of the cylindrical epithelium of the airways. By the 15th week, the first capillaries of the future respiratory department are formed.

The tubular stage (16-25 weeks) is characterized by the appearance of respiratory and terminal bronchioles lined with cubic epithelium, as well as tubules (prototypes of alveolar sacs) and the growth of capillaries to them.

The alveolar (or terminal sac stage (26-40 weeks)) is characterized by massive transformation of tubules into sacs (primary alveoli), an increase in the number of alveolar sacs, differentiation of type I and II alveolocytes, and the appearance of surfactant. By the end of the 7th month, a significant part of the cells of the cubic epithelium of the respiratory bronchioles differentiates into flat cells (type I alveolocytes), closely connected by blood and lymphatic capillaries, and gas exchange becomes possible. The rest of the cells remain cuboidal (type II alveolocytes) and begin to produce surfactant. During the last 2 months of prenatal and several years of postnatal life, the number of terminal sacs is constantly increasing. Mature alveoli before birth are absent.

lung fluid

At birth, the lungs are filled with fluid containing large amounts of chlorides, protein, some mucus from the bronchial glands, and surfactant.

After birth, lung fluid is rapidly resorbed by the blood and lymph capillaries, and a small amount is removed through the bronchi and trachea. The surfactant remains as a thin film on the surface of the alveolar epithelium.

Malformations

Tracheoesophageal fistula occurs as a result of incomplete splitting of the primary intestine into the esophagus and trachea.

Principles of organization of the respiratory system

The lumen of the airways and alveoli of the lung is the external environment. In the airways and on the surface of the alveoli - there is a layer of epithelium. The epithelium of the airways performs a protective function, which is performed, on the one hand, by the very fact of the presence of the layer, and on the other hand, due to the secretion of a protective material - mucus. It is produced by the goblet cells present in the epithelium. In addition, under the epithelium there are glands that also secrete mucus, the excretory ducts of these glands open to the surface of the epithelium.

The airways function as an air junction unit. The characteristics of the external air (temperature, humidity, contamination with different types of particles, the presence of microorganisms) vary quite significantly. But the air that meets certain requirements must enter the respiratory department. The function of bringing air to the required conditions is played by the airways.

Foreign particles are deposited in the mucosal film located on the surface of the epithelium. Further, contaminated mucus is removed from the airways with its constant movement towards the exit from the respiratory system, followed by coughing. Such a constant movement of the mucous film is ensured by synchronous and undulating oscillations of the cilia located on the surface of the epithelial cells directed towards the exit from the airways. In addition, by moving the mucus to the exit, it is prevented from reaching the surface of the alveolar cells, through which diffusion of gases occurs.

Conditioning of the temperature and humidity of the inhaled air is carried out with the help of blood located in the vascular bed of the airway wall. This process occurs mainly in the initial sections, namely in the nasal passages.

The mucous membrane of the airways is involved in protective reactions. The epithelium of the mucous membrane contains Langerhans cells, while its own layer contains a significant number of various immunocompetent cells (T- and B-lymphocytes, plasma cells synthesizing and secreting IgG, IgA, IgE, macrophages, dendritic cells).

Mast cells are very numerous in their own mucosal layer. Mast cell histamine causes bronchospasm, vasodilation, hypersecretion of mucus from the glands, and mucosal edema (as a result of vasodilation and increased permeability of the wall of postcapillary venules). In addition to histamine, mast cells, along with eosinophils and other cells, secrete a number of mediators, the action of which leads to inflammation of the mucous membrane, damage to the epithelium, reduction of SMC and narrowing of the airway lumen. All of the above effects are characteristic of bronchial asthma.

The airways do not collapse. The clearance is constantly changing and adjusting in connection with the situation. The collapse of the lumen of the airways prevents the presence in their wall of dense structures formed in the initial sections by bone, and then by cartilage tissue. The change in the size of the lumen of the airways is provided by the folds of the mucous membrane, the activity of smooth muscle cells and the structure of the wall.

Regulation of MMC tone. The tone of the SMC of the airways is regulated by neurotransmitters, hormones, metabolites of arachidonic acid. The effect depends on the presence of the corresponding receptors in the SMC. SMC walls of the airways have M-cholinergic receptors, histamine receptors. Neurotransmitters are secreted from the terminals of the nerve endings of the autonomic nervous system (for the vagus nerve - acetylcholine, for neurons of the sympathetic trunk - norepinephrine). Bronchoconstriction is caused by choline, substance P, neurokinin A, histamine, thromboxane TXA2, leukotrienes LTC4, LTD4, LTE4. Bronchodilation is caused by VIP, epinephrine, bradykinin, prostaglandin PGE2. The reduction of MMC (vasoconstriction) is caused by adrenaline, leukotrienes, angiotensin-II. Histamine, bradykinin, VIP, prostaglandin PG have a relaxing effect on the SMC of blood vessels.

The air entering the respiratory tract is subjected to chemical examination. It is carried out by the olfactory epithelium and chemoreceptors in the wall of the airways. Such chemoreceptors include sensitive endings and specialized chemosensitive cells of the mucous membrane.

airways

The airways of the respiratory system include the nasal cavity, nasopharynx, larynx, trachea, and bronchi. When the air moves, it is purified, moistened, the temperature of the inhaled air approaches the body temperature, the reception of gas, temperature and mechanical stimuli, as well as the regulation of the volume of inhaled air.

In addition, the larynx is involved in sound production.

Nasal cavity

It is divided into the vestibule and the nasal cavity itself, consisting of the respiratory and olfactory regions.

The vestibule is formed by a cavity, located under the cartilaginous part of the nose, covered with stratified squamous epithelium.

Under the epithelium in the connective tissue layer there are sebaceous glands and bristle hair roots. Bristle hairs perform a very important function: they retain dust particles from the inhaled air in the nasal cavity.

The inner surface of the nasal cavity proper in the respiratory part is lined with a mucous membrane consisting of a multi-row prismatic ciliated epithelium and a connective tissue proper plate.

The epithelium consists of several types of cells: ciliated, microvillous, basal and goblet. Intercalated cells are located between the ciliated cells. Goblet cells are unicellular mucous glands that secrete their secret on the surface of the ciliated epithelium.

The lamina propria is formed by a loose, fibrous, unformed connective tissue containing a large number of elastic fibers. It contains the terminal sections of the mucous glands, the excretory ducts of which open on the surface of the epithelium. The secret of these glands, like the secret of goblet cells, moisturizes the mucous membrane.

The mucous membrane of the nasal cavity is very well supplied with blood, which contributes to the warming of the inhaled air in the cold season.

Lymphatic vessels form a dense network. They are associated with the subarachnoid space and perivascular sheaths of various parts of the brain, as well as with the lymphatic vessels of the major salivary glands.

The mucous membrane of the nasal cavity has abundant innervation, numerous free and encapsulated nerve endings (mechano-, thermo- and angioreceptors). Sensitive nerve fibers originate from the semilunar ganglion of the trigeminal nerve.

In the region of the superior nasal concha, the mucous membrane is covered with a special olfactory epithelium containing receptor (olfactory) cells. The mucous membrane of the paranasal sinuses, including the frontal and maxillary sinuses, has the same structure as the mucous membrane of the respiratory part of the nasal cavity, with the only difference that their own connective tissue plate is much thinner.

Larynx

The organ of the air-bearing section of the respiratory system, complex in structure, is involved not only in air conduction, but also in sound production. The larynx in its structure has three membranes - mucous, fibrocartilaginous and adventitial.

The mucous membrane of the human larynx, in addition to the vocal cords, is lined with multi-row ciliated epithelium. The mucosal lamina propria, formed by loose fibrous unformed connective tissue, contains numerous elastic fibers that do not have a specific orientation.

In the deep layers of the mucous membrane, elastic fibers gradually pass into the perichondrium, and in the middle part of the larynx they penetrate between the striated muscles of the vocal cords.

In the middle part of the larynx there are folds of the mucous membrane, forming the so-called true and false vocal cords. The folds are covered by stratified squamous epithelium. Mixed glands lie in the mucous membrane. Due to the contraction of the striated muscles embedded in the thickness of the vocal folds, the size of the gap between them changes, which affects the pitch of the sound produced by the air passing through the larynx.

The fibrocartilaginous membrane consists of hyaline and elastic cartilages surrounded by dense fibrous connective tissue. This shell is a kind of skeleton of the larynx.

The adventitia is composed of fibrous connective tissue.

The larynx is separated from the pharynx by the epiglottis, which is based on elastic cartilage. In the region of the epiglottis, there is a transition of the mucous membrane of the pharynx into the mucous membrane of the larynx. On both surfaces of the epiglottis, the mucous membrane is covered with stratified squamous epithelium.

Trachea

This is an air-conducting organ of the respiratory system, which is a hollow tube consisting of a mucous membrane, submucosa, fibrocartilaginous and adventitious membranes.

The mucous membrane, with the help of a thin submucosa, is connected with the underlying dense parts of the trachea and, due to this, does not form folds. It is lined with multi-row prismatic ciliated epithelium, in which ciliated, goblet, endocrine and basal cells are distinguished.

Ciliated prismatic cells flicker in the direction opposite to the inhaled air, most intensively at the optimum temperature (18 - 33 ° C) and in a slightly alkaline environment.

Goblet cells - unicellular endoepithelial glands, secrete a mucous secretion that moisturizes the epithelium and creates conditions for adherence of dust particles that enter with air and are removed when coughing.

The mucus contains immunoglobulins secreted by immunocompetent cells of the mucous membrane, which neutralize many microorganisms that enter with the air.

Endocrine cells have a pyramidal shape, a rounded nucleus and secretory granules. They are found both in the trachea and in the bronchi. These cells secrete peptide hormones and biogenic amines (norepinephrine, serotonin, dopamine) and regulate the contraction of airway muscle cells.

Basal cells are cambial cells that are oval or triangular in shape.

The submucosa of the trachea consists of loose fibrous unformed connective tissue, without a sharp border passing into dense fibrous connective tissue of the perichondrium of open cartilaginous semirings. In the submucosa there are mixed protein-mucous glands, the excretory ducts of which, forming flask-shaped extensions on their way, open on the surface of the mucous membrane.

The fibrocartilaginous membrane of the trachea consists of 16-20 hyaline cartilaginous rings, not closed on the posterior wall of the trachea. The free ends of these cartilages are connected by bundles of smooth muscle cells attached to the outer surface of the cartilage. Due to this structure, the posterior surface of the trachea is soft, pliable. This property of the posterior wall of the trachea is of great importance: when swallowing, food boluses passing through the esophagus, located directly behind the trachea, do not encounter obstacles from its cartilaginous skeleton.

The adventitial membrane of the trachea consists of loose, fibrous, irregular connective tissue that connects this organ to the adjacent parts of the mediastinum.

The blood vessels of the trachea, just as in the larynx, form several parallel plexuses in its mucous membrane, and under the epithelium - a dense capillary network. Lymphatic vessels also form plexuses, of which the superficial is directly below the network of blood capillaries.

The nerves approaching the trachea contain spinal (cerebrospinal) and autonomic fibers and form two plexuses, the branches of which end in its mucous membrane with nerve endings. The muscles of the posterior wall of the trachea are innervated from the ganglia of the autonomic nervous system.

Lungs

The lungs are paired organs that occupy most of the chest and constantly change their shape depending on the phase of breathing. The surface of the lung is covered with a serous membrane (visceral pleura).

Structure. The lung consists of branches of the bronchi, which are part of the airways (bronchial tree), and a system of pulmonary vesicles (alveoli), which act as the respiratory sections of the respiratory system.

The composition of the bronchial tree of the lung includes the main bronchi (right and left), which are divided into extrapulmonary lobar bronchi (large bronchi of the first order), and then into large zonal extrapulmonary (4 in each lung) bronchi (bronchi of the second order). Intrapulmonary segmental bronchi (10 in each lung) are subdivided into bronchi of III-V orders (subsegmental), which are medium in diameter (2-5 mm). The middle bronchi are subdivided into small (1-2 mm in diameter) bronchi and terminal bronchioles. Behind them, the respiratory sections of the lung begin, performing a gas exchange function.

The structure of the bronchi (although not the same throughout the bronchial tree) has common features. The inner shell of the bronchi - the mucous membrane - is lined like the trachea with ciliated epithelium, the thickness of which gradually decreases due to a change in the shape of the cells from high prismatic to low cubic. Among epithelial cells, in addition to ciliated, goblet, endocrine and basal, in the distal sections of the bronchial tree, secretory cells (Clara cells), bordered (brush), and non-ciliated cells are found in humans and animals.

Secretory cells are characterized by a dome-shaped top, devoid of cilia and microvilli and filled with secretory granules. They contain a rounded nucleus, a well-developed endoplasmic reticulum of an agranular type, and a lamellar complex. These cells produce enzymes that break down the surfactant that coats the respiratory compartments.

Ciliated cells are found in bronchioles. They are prismatic in shape. Their apical end rises somewhat above the level of adjacent ciliated cells.

The apical part contains accumulations of glycogen granules, mitochondria, and secretion-like granules. Their function is not clear.

Border cells are distinguished by their ovoid shape and the presence of short and blunt microvilli on the apical surface. These cells are rare. They are believed to function as chemoreceptors.

The lamina propria of the bronchial mucosa is rich in longitudinally directed elastic fibers, which provide stretching of the bronchi during inhalation and their return to their original position during exhalation. The mucous membrane of the bronchi has longitudinal folds due to the contraction of oblique bundles of smooth muscle cells that separate the mucous membrane from the submucosal connective tissue base. The smaller the diameter of the bronchus, the relatively thicker is the muscular plate of the mucosa. In the mucous membrane of the bronchi, especially large ones, there are lymphatic follicles.

In the submucosal connective base, the terminal sections of the mixed mucosal-protein glands lie. They are located in groups, especially in places that are devoid of cartilage, and the excretory ducts penetrate the mucous membrane and open on the surface of the epithelium. Their secret moisturizes the mucous membrane and promotes adhesion, enveloping of dust and other particles, which are subsequently released to the outside. Mucus has bacteriostatic and bactericidal properties. In the bronchi of small caliber (diameter 1 - 2 mm) glands are absent.

The fibrocartilaginous membrane, as the caliber of the bronchus decreases, is characterized by a gradual change of open cartilage rings in the main bronchi by cartilaginous plates (lobar, zonal, segmental, subsegmental bronchi) and islets of cartilaginous tissue (in medium-sized bronchi). In medium-sized bronchi, hyaline cartilage tissue is replaced by elastic cartilage tissue. In the bronchi of small caliber, the fibrocartilaginous membrane is absent.

The outer adventitial membrane is built of fibrous connective tissue, passing into the interlobar and interlobular connective tissue of the lung parenchyma. Among the connective tissue cells, tissue basophils are found, which are involved in the regulation of the composition of the intercellular substance and blood coagulation.

The terminal (terminal) bronchioles are about 0,5 mm in diameter. Their mucous membrane is lined with a single layer of cubic ciliated epithelium, in which brush cells and secretory Clara cells occur. In the lamina propria of the mucous membrane of these bronchioles, longitudinally extending elastic fibers are located, between which individual bundles of smooth muscle cells lie. As a result, the bronchioles are easily distensible during inhalation and return to their original position during exhalation.

Respiratory department. The structural and functional unit of the respiratory section of the lung is the acinus. It is a system of alveoli located in the wall of the respiratory bronchiole, alveolar ducts and sacs that carry out gas exchange between the blood and air of the alveoli. The acinus begins with a respiratory bronchiole of the 12st order, which is dichotomously divided into respiratory bronchioles of the 18nd, and then of the XNUMXrd order. In the lumen of the bronchioles, the alveoli open, which in this regard are called alveolar. Each respiratory bronchiole III order, in turn, is divided into alveolar passages, and each alveolar passage ends with two alveolar sacs. At the mouth of the alveoli of the alveolar ducts there are small bundles of smooth muscle cells, which are visible in transverse sections in the form of button-like thickenings. The acini are separated from each other by thin connective tissue layers, XNUMX-XNUMX acini form the lung lobule. Respiratory bronchioles are lined with a single layer of cuboidal epithelium. The muscular plate becomes thinner and breaks up into separate, circularly directed bundles of smooth muscle cells.

On the walls of the alveolar passages and alveolar sacs there are several dozen alveoli. Their total number in adults reaches an average of 300 - 400 million. The surface of all alveoli with a maximum breath in an adult can reach 100 m², and when exhaling, it decreases by 2 - 2,5 times. Between the alveoli are thin connective tissue septa, through which the blood capillaries pass.

Between the alveoli there are messages in the form of holes with a diameter of about 10 - 15 microns (alveolar pores).

The alveoli look like an open vesicle. The inner surface is lined by two main types of cells: respiratory alveolar cells (type I alveolocytes) and large alveolar cells (type II alveolocytes). In addition, in animals, type III cells exist in the alveoli - kamchatye.

Type I alveolocytes have an irregular, flattened, elongated shape. On the free surface of the cytoplasm of these cells, there are very short cytoplasmic outgrowths facing the cavity of the alveoli, which significantly increases the total area of ​​air contact with the surface of the epithelium. Their cytoplasm contains small mitochondria and pinocytic vesicles.

An important component of the air-blood barrier is the surfactant alveolar complex. It plays an important role in preventing the collapse of the alveoli on exhalation, as well as in preventing them from penetrating through the wall of the alveoli of microorganisms from the inhaled air and transuding fluid from the capillaries of the interalveolar septa into the alveoli. Surfactant consists of two phases: membrane and liquid (hypophase). Biochemical analysis of the surfactant showed that it contains phospholipids, proteins and glycoproteins.

Type II alveolocytes are somewhat larger in height than type I cells, but their cytoplasmic processes, on the contrary, are short. In the cytoplasm, larger mitochondria, a lamellar complex, osmiophilic bodies, and an endoplasmic reticulum are revealed. These cells are also called secretory because of their ability to secrete lipoprotein substances.

In the wall of the alveoli, brush cells and macrophages containing trapped foreign particles and an excess of surfactant are also found. The cytoplasm of macrophages always contains a significant amount of lipid droplets and lysosomes. The oxidation of lipids in macrophages is accompanied by the release of heat, which warms the inhaled air.

Surfactant

The total amount of surfactant in the lungs is extremely small. 1 m² alveolar surface accounts for about 50 mm³ surfactant. The thickness of its film is 3% of the total thickness of the air-blood barrier. The components of the surfactant enter the type II alveolocytes from the blood.

Their synthesis and storage in lamellar bodies of these cells is also possible. 85% of the surfactant components are recycled and only a small amount is resynthesized. Removal of surfactant from the alveoli occurs in several ways: through the bronchial system, through the lymphatic system and with the help of alveolar macrophages. The main amount of surfactant is produced after the 32nd week of pregnancy, reaching a maximum amount by the 35th week. Before birth, an excess of surfactant is formed. After birth, this excess is removed by alveolar macrophages.

Respiratory distress syndrome of the newborn develops in preterm infants due to the immaturity of type II alveolocytes. Due to the insufficient amount of surfactant secreted by these cells to the surface of the alveoli, the latter are unexpanded (atelectasis). As a result, respiratory failure develops. Due to alveolar atelectasis, gas exchange occurs through the epithelium of the alveolar ducts and respiratory bronchioles, which leads to their damage.

Compound. Pulmonary surfactant is an emulsion of phospholipids, proteins and carbohydrates, 80% glycerophospholipids, 10% cholesterol and 10% proteins. The emulsion forms a monomolecular layer on the surface of the alveoli. The main surface active component is dipalmitoylphosphatidylcholine, an unsaturated phospholipid that makes up more than 50% of the surfactant's phospholipids. The surfactant contains a number of unique proteins that promote the adsorption of dipalmitoylphosphatidylcholine at the interface between two phases. Among the surfactant proteins, SP-A, SP-D are isolated. Proteins SP-B, SP-C and surfactant glycerophospholipids are responsible for reducing surface tension at the air-liquid interface, while SP-A and SP-D proteins are involved in local immune responses by mediating phagocytosis.

SP-A receptors are present in type II alveolocytes and in macrophages.

Production regulation. The formation of surfactant components in the fetus is facilitated by glucocorticosteroids, prolactin, thyroid hormones, estrogens, androgens, growth factors, insulin, cAMP. Glucocorticoids enhance the synthesis of SP-A, SP-B and SP-C in the lungs of the fetus. In adults, surfactant production is regulated by acetylcholine and prostaglandins.

Surfactant is a component of the lung defense system. Surfactant prevents direct contact of alveolocytes with harmful particles and infectious agents that enter the alveoli with inhaled air. The cyclic changes in surface tension that occur during inhalation and exhalation provide a breath-dependent cleaning mechanism. Enveloped by the surfactant, dust particles are transported from the alveoli to the bronchial system, from which they are removed with mucus.

Surfactant regulates the number of macrophages migrating into the alveoli from the interalveolar septa, stimulating the activity of these cells. Bacteria entering the alveoli with air are opsonized by surfactant, which facilitates their phagocytosis by alveolar macrophages.

The surfactant is present in bronchial secretions, coating the ciliated cells, and has the same chemical composition as lung surfactant. Obviously, surfactant is needed to stabilize the distal airways.

immune protection

macrophages

Macrophages make up 10-15% of all cells in the alveolar septa. Many microfolds are present on the surface of macrophages. The cells form rather long cytoplasmic processes that allow macrophages to migrate through the interalveolar pores. Being inside the alveolus, the macrophage can attach itself to the surface of the alveolus with the help of processes and capture particles. Alveolar macrophages secrete α1-antitrypsin, a glycoprotein from the family of serine proteases that protects alveolar elastin from: splitting of leukocytes by elastase. Mutation of the α1-antitrypsin gene leads to congenital emphysema (damage to the elastic framework of the alveoli).

Migration paths. Cells loaded with phagocytosed material can migrate in various directions: up the acinus and into the bronchioles, where macrophages enter the mucous membrane, which is constantly moving along the surface of the epithelium towards the exit from the airways; inside - into the internal environment of the body, i.e., into the interalveolar septa.

Function. Macrophages phagocytize microorganisms and dust particles that enter with the inhaled air, have antimicrobial and anti-inflammatory activity mediated by oxygen radicals, proteases and cytokines. In lung macrophages, the antigen presenting function is poorly expressed. Moreover, these cells produce factors that inhibit the function of T-lymphocytes, which reduces the immune response.

Antigen presenting cells

Dendritic cells and Langerhans cells belong to the system of mononuclear phagocytes, they are the main antigen-presenting cells of the lung. Dendritic cells and Langerhans cells are numerous in the upper respiratory tract and trachea. With a decrease in the caliber of the bronchi, the number of these cells decreases. As antigen-presenting pulmonary Langerhans cells and dendritic cells express MHC class 1 molecules. These cells have receptors for the Fc fragment of IgG, the fragment of the C3b complement component, IL-2, they synthesize a number of cytokines, including IL-1, IL-6, tumor necrosis factor, stimulate T-lymphocytes, showing increased activity against the antigen that first appeared in the body.

Dendritic cells

Dendritic cells are found in the pleura, interalveolar septa, peribronchial connective tissue, and in the lymphoid tissue of the bronchi. Dendritic cells, differentiating from monocytes, are quite mobile and can migrate in the intercellular substance of the connective tissue. They appear in the lungs before birth. An important property of dendritic cells is their ability to stimulate the proliferation of lymphocytes. Dendritic cells have an elongated shape and numerous long processes, an irregularly shaped nucleus, and typical cell organelles in abundance. There are no phagosomes, since the cells practically do not have phagocytic activity.

Langerhans cells

Langerhans cells are present only in the epithelium of the airways and absent in the alveolar epithelium. Langerhans cells differentiate from dendritic cells, and such differentiation is possible only in the presence of epithelial cells. Connecting with cytoplasmic processes penetrating between epitheliocytes, Langerhans cells form a developed intraepithelial network. Langerhans cells are morphologically similar to dendritic cells. A characteristic feature of Langerhans cells is the presence in the cytoplasm of specific electron-dense granules with a lamellar structure.

Metabolic lung function

In the lungs, it metabolizes a number of biologically active substances.

Angiotensins. Activation is only known for angiotensin I, which is converted to angiotensin II. The conversion is catalyzed by an angiotensin-converting enzyme localized in the endothelial cells of the alveolar capillaries.

Inactivation. Many biologically active substances are partially or completely inactivated in the lungs. So, bradykinin is inactivated by 80% (with the help of angiotensin-converting enzyme). In the lungs, serotonin is inactivated, but not with the participation of enzymes, but by excretion from the blood, part of the serotonin enters the platelets. Prostaglandins PGE, PGE2, PGE2a and norepinephrine are inactivated in the lungs with the help of appropriate enzymes.

Pleura

The lungs are covered on the outside with a pleura called the pulmonary (or visceral). The visceral pleura fuses tightly with the lungs, its elastic and collagen fibers pass into the interstitial tissue, so it is difficult to isolate the pleura without injuring the lungs. The visceral pleura contains smooth muscle cells. In the parietal pleura, which lines the outer wall of the pleural cavity, there are fewer elastic elements, and smooth muscle cells are rare.

Blood supply in the lung is carried out through two vascular systems. On the one hand, the lungs receive arterial blood from the systemic circulation through the bronchial arteries, and on the other hand, they receive venous blood for gas exchange from the pulmonary arteries, that is, from the pulmonary circulation. The branches of the pulmonary artery, accompanying the bronchial tree, reach the base of the alveoli, where they form a capillary network of the alveoli. Through the alveolar capillaries, the diameter of which varies between 5 - 7 microns, erythrocytes pass in 1 row, which creates an optimal condition for the implementation of gas exchange between erythrocyte hemoglobin and alveolar air. The alveolar capillaries gather into postcapillary venules, which merge to form the pulmonary veins.

Bronchial arteries depart directly from the aorta, nourish the bronchi and lung parenchyma with arterial blood. Penetrating into the wall of the bronchi, they branch out and form arterial plexuses in their submucosa and mucous membrane. In the mucous membrane of the bronchi, the vessels of the large and small circles communicate by anastomosis of the branches of the bronchial and pulmonary arteries.

The lymphatic system of the lung consists of superficial and deep networks of lymphatic capillaries and vessels. The superficial network is located in the visceral pleura. The deep network is located inside the pulmonary lobules, in the interlobular septa, lying around the blood vessels and bronchi of the lung.

Innervation is carried out by sympathetic and parasympathetic nerves and a small number of fibers coming from the spinal nerves. Sympathetic nerves conduct impulses that cause bronchial dilatation and constriction of blood vessels, parasympathetic - impulses that, on the contrary, cause bronchial constriction and dilation of blood vessels. The ramifications of these nerves form a nerve plexus in the connective tissue layers of the lung, located along the bronchial tree and blood vessels. In the nerve plexuses of the lung, large and small ganglia are found, from which nerve branches depart, innervating, in all likelihood, the smooth muscle tissue of the bronchi. Nerve endings were identified along the alveolar ducts and alveoli.

Topic 23. LEATHER AND ITS DERIVATIVES

The skin forms the outer covering of the body, the area of ​​​​which in an adult reaches 1,5 - 2 m². Of the appendages of the skin, a person has hair, nails, sweat and sebaceous glands.

Leather

The function of the skin is to protect the underlying parts of the body from damage. Healthy skin is impervious to microorganisms, many poisonous and harmful substances. The skin is involved in water and heat exchange with the external environment. During the day, about 500 ml of water is excreted through the human skin, which is 1% of its total amount in the body. In addition to water, various salts, mainly chlorides, as well as lactic acid and products of nitrogen metabolism, are excreted through the skin with sweat. About 82% of all body heat loss occurs through the skin surface. In cases of violation of this function (for example, during prolonged work in rubber overalls), overheating of the body and heat stroke may occur. Vitamin D is synthesized in the skin under the action of ultraviolet rays. Its absence in the body causes rickets, a serious disease. The skin is in a certain ratio with the sex glands of the body. As a result, most of the secondary sexual characteristics appear in the skin. The presence in the skin of an abundant vascular network and arteriolo-venular anastomoses determines its significance as a blood depot. In an adult, up to 1 liter of blood can linger in the vessels of the skin. Due to the abundant innervation, the skin appears as a receptor field, consisting of tactile, temperature and pain nerve endings. In some areas of the skin, for example, on the head and hands, 1 cm² its surface has up to 300 sensitive points.

skin development

The two main components of the skin have different origins. The epidermis develops from the ectoderm, and the skin itself develops from the mesenchyme.

development of the epidermis. The early embryo is covered with a single layer of ectodermal cells. At the beginning of the 2nd month of development, flat surface cells and the underlying basal layer of cuboidal epithelial cells responsible for the formation of new cells are distinguished in the emerging epidermis. Later, an intermediate layer forms between the superficial and basal layers. By the end of the 4th month in the epidermis, the basal layer, a wide layer of spiny cells, granular and stratum corneum are distinguished. During the first 3 months of development, migrants from the neural crest colonize the epidermis. Later, cells of bone marrow origin appear.

The development of the skin itself. The skin itself (dermis) is of mesenchymal origin. Its formation involves cells that migrate from the somite dermatome. On the 3rd - 4th month, outgrowths of connective tissue protruding into the epidermis are formed - papillae of the skin.

Lubrication of the skin. The skin of the fetus is covered with a white lubricant, consisting of the secretion of the sebaceous glands, fragments of epidermal cells and hair. The lubricant protects the skin from the effects of amniotic fluid.

Structure

The skin consists of two parts - epithelial and connective tissue.

The epithelium of the skin is called the cuticle (or epidermis), and the connective tissue base is called the dermis (or the skin itself). The connection of the skin with the underlying parts of the body occurs through a layer of adipose tissue - subcutaneous tissue (or hypodermis). The thickness of the skin in different parts of the body varies from 0,5 to 5 mm. The epidermis is composed of keratinized squamous epithelium. Its thickness is from 0,03 to 1,5 mm or more. The thickest epidermis on the palms and soles, consisting of many layers of cells. These cells consist of 5 main layers, which include basal, spiny, granular, shiny and horny. Directly on the basement membrane, which separates the epithelium from the dermis, are the cells that make up the basal layer. Among them, basal epidermocytes, melanocytes (pigment cells) are distinguished, the quantitative ratio between which is approximately 10: 1. The shape of basal epidermocytes can be cylindrical or oval, with the presence of basophilic cytoplasm and a rounded nucleus saturated with chromatin. They revealed organelles of general importance, tonofibrils and granules of dark brown or black pigment (melanin). Their connection with each other and with overlying cells occurs through desmosomes, and with the basement membrane - through hemidesmosomes.

Melanocytes on preparations stained with hematoxylineosin have the appearance of light cells. Melanocytes do not have desmosomes and lie freely. Their cytoplasm contains large amounts of melanin grains, but organelles are poorly developed and tonofibrils are absent. Above the basal cells in 5-10 layers are polygonal-shaped cells forming a prickly layer. Numerous short cytoplasmic processes ("bridges") are clearly visible between the cells, at the meeting point of which there are desmosomes. Desmosomes end with tonofibrils. In addition to epidermocytes, white process cells (Langerhans cells) are observed in the spinous layer. They lack tonofibrils and do not form desmosomes. There are many lysosomes in their cytoplasm, and there are melanin granules captured from the processes of melanocytes. Currently, many authors regard these cells as epidermal macrophages migrating into the epidermis from the mesenchyme during embryogenesis. A feature of the basal and deep levels of the spinous layer of the epidermis is the ability of epidermocytes to reproduce by mitotic division. Therefore, they are often combined under the name of the germinal layer. Thanks to him, the renewal of the epidermis occurs in various parts of the human skin within 10 - 30 days (physiological regeneration). The granular layer consists of 3-4 layers of relatively flat cells. Their cytoplasm contains ribosomes, mitochondria, lysosomes and their variety - keratinosomes (in the form of layered bodies), as well as bundles of fragmented tonofibrils and large keratohyalin granules lying next to them. Staining of granules occurs through the use of basic dyes, consisting of polysaccharides, lipids and proteins, characterized by a high content of basic amino acids (proline, arginine), as well as a sulfur-containing amino acid (cystine). The presence in the cells of the granular layer of the complex of keratohyalin with tonofibrils indicates the beginning of keratinization processes, since, according to many authors, it is the initial stage in the formation of keratin (keratin). The next layer (shiny) also consists of 3-4 layers of flat cells, in which the nuclei cease to stain due to their death, and the cytoplasm is diffusely impregnated with a protein substance - eleidin, which, on the one hand, is not stained with dyes, and on the other hand, refracts light well . Because of this, the structure of cells in the shiny layer of the border is imperceptible, and the entire layer looks like a shiny stripe. It is believed that eleidin is formed from the proteins of tonofibrils and keratohyalin by oxidation of their sulfhydryl groups. Eleidin itself is regarded as a precursor of keratin.

The stratum corneum is represented by many horny scales. The scales contain keratin and air bubbles. Keratin is a protein rich in sulfur (up to 5%), characterized by resistance to various chemical agents (acids, alkalis, etc.). Inside the cells are keratin fibrils. In rare cases, there are remains of tonofibrils, representing a delicate network and a cavity formed at the site of the dead nucleus. The horny scales that are on the surface are constantly falling off, sloughing off and being replaced by new ones coming from the layers lying below. During desquamation, keratinosomes are of great importance, which leave the cells, concentrating in the intercellular spaces. As a result, lysis (dissolution) of desmosomes and separation of horny cells from each other is observed. The value of the stratum corneum is determined by the fact that it has great elasticity and poor thermal conductivity. Thus, a number of cell components are involved in the process of keratinization of the epidermis of the skin: tonofibrils, keratohyalin, keratinosomes, desmosomes. Compared to the skin of the palms and soles, the epidermis is much thinner in other areas of the skin. Its thickness, for example, on the scalp does not exceed 170 microns. The shiny layer is absent in it, and the horny layer is represented by only 2-3 rows of keratinized cells (scales). In all likelihood, keratinization in this case proceeds according to a shortened cycle. Consequently, most of the skin has an epidermis, which consists of 3 main layers - sprout, granular and horny. Moreover, each of them is much thinner than the corresponding layers of the epidermis of the skin of the palms and soles. Under the influence of some external and internal factors, the nature of the epidermis can change significantly. So, for example, with strong mechanical influences, with A-avitaminosis, under the influence of hydrocortisone, the processes of keratinization sharply increase.

The concept of a proliferative unit. A proliferative unit is a clone that combines different stages of differon, cells of different degrees of differentiation and originating from a single stem cell located in the basal layer and in contact with the basement membrane. As cells differentiate, they move to the surface of the layer.

Differentiation. The stem cell is in contact with the basement membrane. As cells differentiate and multiply, they move to the surface of the epidermis, forming together a proliferative unit of the epidermis, which, in the form of a column, occupies a certain area of ​​it. Keratinocytes that have completed their life cycle are exfoliated from the surface of the stratum corneum. Proliferative unit - a structure formed by keratinocytes of different layers of the epidermis, of varying degrees of differentiation and originating from one stem cell of the basal layer.

The nature of the population. Keratinocytes are referred to as a renewing cell population. Their maximum mitotic activity is observed at night, and life expectancy is 2 - 4 weeks.

The concept of hard and soft keratin. By physical and chemical properties, hard and soft keratin are distinguished. Solid keratin is present in the cortex and cuticle of the hair. This type of keratin is found in human hair and nails. It is more durable and chemically more resistant. Soft keratin is the most abundant, present in the epidermis, localized in the hair medulla and in the inner root sheath, and contains less cystine and disulfide bonds than hard keratin.

Influence of hormones and growth factors on the layers of the epidermis. Keratinocytes serve as targets for numerous hormones and growth factors. The epidermal growth factor (EGF), keratinocyte growth factor, fibroblast growth factor, growth factor FGF7, transforming growth factor (TGFoc), which stimulate keratinocyte mitoses, are of the greatest importance. Substance P, released from the terminals of sensitive nerve fibers, has a similar effect. 1a,25-dihydroxycholecalciferol inhibits secretion and DNA synthesis in keratinocytes and stimulates terminal differentiation.

Application: 1a,25-dihydroxycholecalciferol is used in psoriasis, when the process of differentiation of keratinocytes is disturbed and their proliferation is enhanced, it gives a positive therapeutic effect.

melanocytes. Melanocytes are located in the basal layer, their number varies significantly in different areas of the skin. Melanocytes originate from the neural crest and synthesize pigments (melanins) enclosed in special vesicles - melanosomes.

Tyrosinase. Melanocytes are characterized by a copper-containing and ultraviolet-sensitive enzyme - tyrosinase (tyrosine hydroxylase), which catalyzes the conversion of tyrosine to DOPA. Insufficiency of tyrosinase or its blocking in melanocytes leads to the development of various forms of albinism.

Melanosomes. Tyrosinase after synthesis on the ribosomes of the granular endoplasmic reticulum enters the Golgi complex, where it is "packed" into vesicles, which then merge with premelanosomes. Melanin is produced in melanosomes.

DOPA is oxidized by DOPA oxidase and converted into melanin during chemical reactions. The histochemical reaction to DOPA makes it possible to identify melanocytes among other skin cells.

Melanin. Long processes of melanocytes go into the spiny layer. Melanosomes are transported along them, the contents of which (melanin) are released from melanocytes and captured by keratinocytes. Here, melanin undergoes degradation under the action of lysosome enzymes. Melanin protects the underlying structures from exposure to ultraviolet radiation. The acquisition of a tan indicates an increase in the production of melanin under the influence of ultraviolet radiation. There are two types of melanins in human skin - eumelanin (black pigment) and pheomelanin (red pigment). Eumelanin is a photoprotector, pheomelanin, on the contrary, can contribute to ultraviolet damage to the skin due to the formation of free radicals in response to irradiation. People with brown (red) hair, light eyes, and skin contain predominantly pheomelanin in their hair and skin, have a reduced ability to produce eumelanin, develop a slight tan, and are at risk of UV overexposure.

Melanocortins. Of the melanocortins, α-melanotropin regulates the ratio of eumelanin and pheomelanin in the skin. In particular, α-melanotropin stimulates the synthesis of eumelanin in melanocytes. Specific agouti protein blocks the action of melanotropins through melanocortin receptors, which helps to reduce the production of eumelanin.

Langerhans cells. They make up 3% of all epidermal cells. These antigen-presenting cells carry class I and class II MHC proteins on the cell membrane and are involved in the immune response. They originate from the bone marrow and belong to the mononuclear phagocyte system. Differentiation of Langerhans cells from CD34+ pluripotent stem cells is supported by TGF^β1, TNF^α and GM-CSF. In the epidermis, these cells are located mainly in the spinous layer. The cells contain an irregularly shaped nucleus with invaginations, a moderately developed granular endoplasmic reticulum, a Golgi complex, a small number of microtubules, and elongated Birbeck cytoplasmic granules with longitudinal striation. The Langerhans cell marker is the glycoprotein langerin.

Actually the skin, or dermis, has a thickness of 0,5 to 5 mm, the largest - on the back, shoulders, hips. The dermis consists of 2 layers (papillary and reticular), which do not have a clear boundary between them. The papillary layer is located directly under the epidermis and consists of loose fibrous unformed connective tissue responsible for trophic function. This layer was named due to the presence of numerous papillae protruding into the epithelium. The various parts that make up the skin vary in size and quantity. The main part of the papillae (up to 0,2 mm high) is concentrated in the skin of the palms and soles. Facial papillae are poorly developed and may disappear with age. The pattern on the surface of the skin is determined by the papillary layer of the dermis, which has a strictly individual character. The connective tissue of the papillary layer consists of thin collagen, elastic and reticular fibers, cells with the most common fibroblasts, macrophages, tissue basophils (mast cells), etc. In addition, there are smooth muscle cells, in some places collected in small bundles. Many of them are related to the muscles that raise the hair, but there are muscle bundles that have no connection with them. A particularly large number of them are concentrated in the skin of the head, cheeks, forehead and dorsal surface of the limbs. The reduction of these cells causes the appearance of the so-called goose bumps. At the same time, blood flow to the skin decreases, as a result of which the heat transfer of the body decreases. The reticular layer consists of a dense, irregular connective tissue with powerful bundles of collagen fibers running either parallel to the skin surface or obliquely, and a network of elastic fibers. Together they form a network where, by means of the functional load on the skin, its structure is determined. In areas of the skin that experience strong pressure (skin of the foot, fingertips, elbows, etc.), a wide-looped, rough network of collagen fibers is well developed. In the same areas where the skin is significantly stretched (the area of ​​​​the joints, the back of the foot, the face, etc.), there is a narrow-loop collagen network in the mesh layer. The course of elastic fibers basically coincides with the course of collagen bundles. Their number predominates in areas of the skin that are often stretched (in the skin of the face, joints, etc.). Reticular fibers are found in small numbers. They are usually found around blood vessels and sweat glands. The cellular elements of the reticular layer are represented mainly by fibroblasts. In most parts of the human skin, its reticular layer contains sweat and sebaceous glands, as well as hair roots. The structure of the mesh layer is fully consistent with its function - to ensure the strength of the entire skin.

Bundles of collagen fibers from the reticular layer of the dermis pass into the layer of subcutaneous tissue. Between them there are significant gaps filled with lobules of adipose tissue. Subcutaneous tissue softens the effect of various mechanical factors on the skin, so it is especially well developed in places such as fingertips, feet, etc. Here, complete preservation of subcutaneous tissue is observed, despite the extreme degree of exhaustion of the body. In addition, the subcutaneous layer provides some mobility of the skin compared to the underlying parts, which leads to its protection from ruptures and other mechanical damage. Finally, subcutaneous tissue is the most extensive fat depot of the body, and also provides its thermoregulation.

Skin pigment, with very few exceptions, is found in the skin of all people. People whose body is devoid of pigment are called albinos. Skin pigment belongs to the group of melanins. Melanin is formed during the oxidation of the amino acid tyrosine under the influence of the enzyme tyrosinase and DOPA oxidase. In the skin dermis, the pigment is located in the cytoplasm of dermal melanocytes (process-shaped cells), however, unlike epidermal melanocytes, they do not give a positive DOPA reaction. Because of this, the pigment cells of the dermis contain but do not synthesize the pigment. How the pigment enters these cells is not exactly known, but it is assumed that it comes from the epidermis. Dermal melanocytes are of mesenchymal origin. Relatively often they are found only in certain places of the skin - in the anus and in the areola. Pigment metabolism in the skin is closely related to the content of vitamins in it, and also depends on endocrine factors. With a lack of B vitamins, melanogenesis in the epidermis decreases, and a lack of vitamins A, C and PP causes the opposite effect. Hormones of the pituitary, adrenal, thyroid and sex glands have a direct effect on the level of melanin pigmentation of the skin. Blood vessels are involved in the formation of plexuses in the skin, from which the news depart, participating in the nutrition of its various parts. The vascular plexuses are located in the skin at different levels. There are deep and superficial arterial plexuses, as well as one deep and two superficial venous plexuses. Skin arteries originate from a wide-loop vascular network located between the muscular fascia and subcutaneous fatty tissue (fascial arterial network). Vessels depart from this network, which, after passing through the layer of subcutaneous adipose tissue, branch out, forming a deep skin arterial network, from which there are branches involved in the blood supply to the fatty lobules, sweat glands and hair. From the deep skin arterial network, arteries begin, which, after passing through the reticular layer of the dermis at the base of the papillary layer, break up into arterioles involved in the formation of the subpapillary (superficial) arterial network, from which branches branch, which in the papillae break up into capillaries, shaped like hairpins not long more than 0,4 mm. Short arterial branches extending from the subpapillary network supply blood to the papillary groups. It is characteristic that they do not anastomose with each other. This may explain why sometimes redness or blanching of the skin occurs in patches. From the subpapillary network, arterial vessels branch towards the sebaceous glands and hair roots.

The capillaries of the papillary layer, sebaceous glands and hair roots are collected in veins that flow into the subpapillary venous plexus. There are two papillary plexuses, lying one after the other, from which the blood is directed to the skin (deep) venous plexus, lying between the dermis and subcutaneous fatty tissue. Blood is sent to the same plexus from the fat lobules and sweat glands. The connection of the skin plexus with the fascial occurs through the venous plexus, from which larger venous trunks depart. Arteriovenular anastomoses (glomus) are widespread in the skin, especially numerous at the tips of the fingers and toes and in the area of ​​the nail bed. They are directly related to the process of thermoregulation. The lymphatic vessels of the skin form two plexuses - a superficial one, lying slightly below the subpapillary venous plexus, and a deep one, located in the subcutaneous fatty tissue.

The innervation of the skin occurs both through the branches of the cerebrospinal nerves and through the nerves of the autonomic system. The cerebrospinal nervous system includes numerous sensory nerves that form a huge number of sensory nerve endings in the skin. The nerves of the autonomic nervous system innervate blood vessels, smooth myocytes and sweat glands in the skin. Nerves in the subcutaneous adipose tissue form the main nerve plexus of the skin, from which numerous stems depart, which play a major role in the creation of new plexuses located around the hair roots, sweat glands, fatty lobules and in the papillary dermis. The dense nerve plexus of the papillary layer is involved in the transfer to the connective tissue and to the epidermis of myelinated and unmyelinated nerve fibers involved in the formation of many sensory nerve endings that are unevenly distributed in the skin. A large number of them are observed in areas of the skin with hypersensitivity, for example, on the palms and soles, on the face, in the genital area. They are also a large group of non-free nerve endings, such as lamellar nerve bodies, terminal flasks, tactile bodies, genital bodies and tactile discs. It is believed that the feeling of pain is transmitted by free nerve endings located in the epidermis, reaching the granular layer, as well as by nerve endings lying in the papillary dermis. The sense of touch (touch) is perceived by the tactile bodies and discs, as well as the nerve plexuses (cuffs) of the hair. The first are located in the papillary layer of the dermis, the second - in the germ layer of the epidermis. Nerve cuffs are nerve networks that wrap around the hair roots to the level at which the sebaceous glands are located. In the epidermis, in addition, there are tactile cells (Merkel cells) that are in contact with the tactile discs. These are large, round or elongated cells with a light vacuolated cytoplasm, in which osmophilic granules are present. Merkel cells are thought to be of glial origin. The feeling of pressure is associated with the presence of lamellar nerve bodies in the skin. These are the largest nerve endings (up to 2 mm in diameter) that lie deep in the skin. The feeling of warmth is probably perceived by free nerve endings, and the feeling of cold by Merkel cells.

Hair

Hair covers almost the entire surface of the skin. The highest density of their location is on the head, where their total number can reach 100 thousand. The length of the hair varies from a few millimeters to 1,5 m, the thickness is from 0,005 to 0,6 mm.

There are three types of hair: long (hair of the head, beard, mustache, and also located in the armpits and on the pubis), bristly (hair of the eyebrows, eyelashes, and also growing in the external auditory canal and on the eve of the nasal cavity); vellus (hair covering the rest of the skin).

Structure. Hair is an epithelial appendage of the skin. There are two parts in the hair - the shaft and the root. The hair shaft is above the surface of the skin. The hair root is hidden in the thickness of the skin and reaches the subcutaneous fatty tissue. The hair shaft is formed by the cortex and cuticle. The root of long and bristly hair consists of cortical substance, medulla and cuticle, in vellus hair - only of cortical substance and cuticle.

The hair root is located in the hair follicle (or follicle), the wall of which consists of the inner and outer epithelial (root) sheaths and the connective tissue hair follicle.

The hair root ends with an extension (hair follicle). Both epithelial sheaths merge with it. From below, connective tissue with capillaries in the form of a hair papilla protrudes into the hair follicle. At the point of transition of the hair root to the shaft, the epidermis of the skin forms a small depression - a hair funnel. Here, the hair, coming out of the funnel, appears above the surface of the skin. The growth layer of the funnel epidermis passes into the outer epithelial sheath. The internal epithelial sheath ends at this level. The duct of one or more sebaceous glands opens into the hair funnel. Below the sebaceous glands in an oblique direction passes the muscle that lifts the hair.

The hair follicle is the hair matrix, that is, the part of the hair from which it grows. It consists of epithelial cells capable of reproduction. Reproducing, the cells of the hair follicle move into the medulla and cortex of the hair root, its cuticle and into the inner epithelial sheath. Thus, due to the cells of the hair follicle, the growth of the hair itself and its inner epithelial (root) sheath occurs. The hair follicle is nourished by vessels located in the hair papilla. As the cells of the hair bulb pass into the medulla and cortex, into the hair cuticle and the inner epithelial sheath, they move further and further away from their source of nutrition - from the vessels of the hair papilla. In this regard, irreversible changes and the processes of keratinization associated with them slowly increase in them. In areas more distant from the hair bulb, the cells die and turn into horny scales. Therefore, the structure of the hair root, its cuticle and the inner epithelial sheath is not the same at different levels.

The process of keratinization of cells occurs most intensively in the cortex and cuticle of the hair. As a result, hard keratin is formed in them, which differs in physical and chemical properties from soft keratin. Hard keratin is more durable. In humans, nails are also built from it. Hard keratin is poorly soluble in water, acids and alkalis, it contains more sulfur-containing amino acids cystine than in soft keratin.

During the formation of solid keratin, there are no intermediate stages - the accumulation of keratohyalin and eleidin grains in the cells.

In the medulla of the hair and the inner epithelial sheath, keratinization processes proceed in the same way as in the epidermis of the skin, i.e., keratohyalin (trichogialin) grains first appear in the cells, which then turn into soft keratin.

The medulla of the hair is well expressed only in long and bristly hair. It is absent in vellus hair. The medulla consists of polygonal-shaped cells lying on top of each other in the form of coin columns. They contain acidophilic shiny granules of trichohyalin, small gas bubbles and a small amount of pigment grains. The pigment is formed in the hair follicle by melanocytes, which are located directly around the hair papilla. The processes of keratinization in the medulla proceed slowly, therefore, approximately to the level of the ducts of the sebaceous glands, the medulla consists of incompletely keratinized cells, in which compacted nuclei or their remains are found. Only above this level, the cells undergo complete keratinization.

Trichohyalin differs from keratohyalin in that it is stained not with basic, but with acidic dyes.

With age, the processes of keratinization in the medulla of the hair intensify, the amount of pigment in the cells decreases and the number of air bubbles increases - the hair turns gray.

The cortical substance of the hair makes up its bulk. The processes of keratinization in the cortical substance proceed intensively and without intermediate stages. Throughout most of the root and the entire hair shaft, the cortical substance consists of flat horny scales. Only in the region of the neck of the hair bulb in this substance are not completely keratinized cells with oval nuclei found. The horny scales contain hard keratin, the remains of nuclei in the form of thin plates, pigment grains and gas bubbles.

The better the cortical substance is developed in the hair, the stronger, more elastic and less brittle it is. By old age, in the horny scales of the cortical substance, as in the medulla, the number of gas bubbles increases.

The hair cuticle is directly adjacent to the cortex. Closer to the hair follicle, it is represented by cylindrical cells lying perpendicular to the surface of the cortex. In the more superficial areas of the hair root, these cells acquire an inclined position and turn into horny scales, overlapping each other in the form of tiles. These scales contain hard keratin, but are completely devoid of pigment and the remainder of the nuclei.

The internal root sheath is a derivative of the hair follicle. In the lower sections of the hair root, it passes into the substance of the hair bulb, and in the upper sections at the level of the ducts of the sebaceous glands it disappears. In the lower parts of the internal root sheath, three layers are distinguished: the cuticle, the granular epithelial layer (Huxley's layer), and the pale epithelial layer (Henle's layer). In the middle and upper sections of the hair root, all these 3 layers merge, and here the inner root sheath consists only of completely keratinized cells containing soft keratin.

The outer root sheath is formed from the germ layer of the epidermis of the skin, which continues up to the hair follicle. At the same time, it gradually becomes thinner and at the point of transition to the hair follicle consists of only 1 - 2 layers of cells. The cells have a light vacuolated cytoplasm due to the presence of a significant amount of glycogen in it.

The hair follicle is the connective tissue sheath of the hair. It distinguishes the outer longitudinal layer of fibers, the inner and circular layers of fibers and the basement membrane.

The raising hair muscle is made up of smooth muscle cells. In bristly, vellus hair, beard hair and armpits, it is absent or poorly developed. The muscle lies in an oblique direction and is woven into the hair follicle of the hair at one end, and into the papillary dermis with the other. When it is reduced, the root takes a perpendicular direction to the surface of the skin and as a result of this, the hair shaft rises slightly above the skin (the hair stands on end). Muscle contraction also causes some compression of the skin and the blood vessels lying in its upper layers (goosebumps). As a result, the body's heat transfer through the skin is reduced.

Hair change. The lifespan of a hair is from several months to 2-4 years, so there is a periodic change of hair throughout life. This process consists in the fact that the hair papilla of the hair is reduced, the cells in the hair follicle lose their ability to multiply and undergo keratinization, which leads to the formation of the so-called hair bulb, and hair growth stops. The hair flask is separated from the hair papilla and, along the case formed by the outer root sheath, moves upward to the attachment site of the muscle that lifts the hair. In this place, a small invagination is formed in the wall of the hair follicle - the hair bed. A hair flask is placed in it. The desolate part of the epithelial sheath collapses and turns into a cell cord. At the end of this strand, the hair papilla subsequently re-forms. It grows into the end of the epithelial cord and gives rise to a new hair follicle. This is where the new hair starts to grow. The new hair grows along the epithelial strand, which at the same time turns into its outer epithelial sheath.

As the new hair grows further, it displaces the old hair from its hair bed, and the process ends with the loss of the old and the appearance of a new hair on the surface of the skin.

Nails

Nails are a derivative of the epidermis of the skin. They develop in the 3rd month of the intrauterine period. Before the nail appears, the so-called nail bed is formed at the site of its future bookmark. At the same time, the epithelium covering the dorsal surfaces of the terminal phalanges of the fingers and toes thickens and somewhat sinks into the underlying connective tissue. In a later stage, the nail itself begins to grow from the epithelium of the proximal part of the nail bed. Due to slow growth (about 0,25 - 1 mm per week), only by the last month of pregnancy does the nail reach the tip of the finger. Nail - a dense horny plate lying on the nail bed. The nail bed from the sides and at the base is limited by skin folds (or nail folds), posterior and lateral. Between the nail bed and the nail folds there are nail gaps (posterior and lateral). The nail (horny) plate protrudes into these cracks with its edges. The nail plate is divided into root, body and edge. The root of the nail is called the back of the nail plate, lying in the back of the nail gap. Only a small part of the root protrudes from the posterior nail fissure (from under the posterior nail fold) in the form of a whitish area of ​​a semilunar shape (lunula of the nail). The rest of the nail plate, located on the nail bed, makes up the body of the nail. The free end of the nail plate, protruding beyond the nail bed, is called the edge (protrusion) of the nail. The formation of the nail plate occurs due to the horny scales adjacent to each other, which contain hard keratin. The nail bed consists of epithelium and connective tissue. The epithelium of the nail bed is represented by the growth layer of the epidermis. The nail plate lying directly on it is its stratum corneum. The connective tissue of the bed contains a large number of fibers, some of which are parallel to the nail plate, and some are perpendicular to it. The latter reach the bone phalanx of the finger and connect to its periosteum. The connective tissue of the nail bed forms longitudinal folds in which blood vessels pass. The area of ​​the epithelium of the nail bed, on which the root of the nail lies, is the place of its growth and is called the nail matrix. In the nail matrix, the process of reproduction and keratinization of cells is constantly taking place. The resulting horny scales are displaced into the nail (horny) plate, which as a result of this increases in size, i.e., the nail grows. The connective tissue of the nail matrix forms papillae, in which numerous blood vessels lie. Nail folds are skin folds. The growth layer of their epidermis passes into the epithelium of the nail bed, and the stratum corneum partially - into the nail plate, and partially moves over it from above (especially at its base), forming the so-called supraungual skin.

skin glands

There are three types of glands in the human skin - milk, sweat and sebaceous. The surface of the glandular epithelium of the sweat and sebaceous glands is approximately 600 times greater than the surface of the epidermis. These skin glands provide thermoregulation (about 20% of heat is given off by the body by evaporation of sweat), protection of the skin from damage (fatty lubrication protects the skin from drying out, as well as from maceration by water and moist air), excretion of some metabolic products from the body (urea, urinary acids, ammonia, etc.). Sweat glands are found in almost all areas of the skin. Their number reaches 2 - 2,5 million. The skin of the pads of the fingers and toes, palms and soles, axillary and inguinal folds is the richest in sweat glands. In these places for 1 cm² more than 300 glands open on the surface of the skin, while in other parts of the skin there are 120-200 glands. The secretion of sweat glands (sweat) is a liquid with a low relative density, it contains 98% water and 2% solid residue. About 500 - 600 ml of sweat is released per day. Sweat glands can be subdivided into merocrine and apocrine glands. Apocrine glands are located only in certain places of the skin, for example, in the armpits, the anus, the skin of the forehead, and the labia majora. Apocrine glands develop during puberty and are somewhat larger. Their secret is richer in protein substances, which, when decomposed on the surface of the skin, give it a special, pungent smell. A variety of apocrine sweat glands are glands of the eyelids and glands that secrete earwax. Sweat glands have a simple tubular structure. They consist of a long excretory duct, running straight or slightly meandering, and of an equally long terminal section, twisted in the form of a ball. The diameter of the glomerulus is about 0,3 - 0,4 mm. The end sections are located in the deep parts of the reticular layer on its border with the subcutaneous fatty tissue, and the excretory ducts, having passed through both layers of the dermis and the epidermis, open on the surface of the skin, the so-called sweat pore. The excretory ducts of many apocrine glands do not form sweat pores, but flow together with the excretory ducts of the sebaceous glands into the hair funnels. The terminal sections of the merocrine sweat glands have a diameter of about 30 - 35 microns. They are lined with a single-layer epithelium, the cells of which, depending on the phase of secretion, can have a cubic or cylindrical shape. Drops of fat, glycogen granules and pigment grains are constantly found in the weakly basophilic cytoplasm of secretory cells. They usually contain highly active alkaline phosphatase. In addition to secretory cells, myoepithelial cells are located on the basement membrane of the terminal sections. By their contraction, they contribute to the secretion. The terminal sections of the apocrine glands are larger: their diameter reaches 150 - 200 microns. Secretory cells have an oxyphilic cytoplasm and do not have high alkaline phosphatase activity. In the process of secretion, the apical ends of the cells are destroyed and become part of the secret. The function of the apocrine sweat glands is associated with the function of the sweat glands - in the premenstrual and menstrual periods and during pregnancy, the secretion of the apocrine glands increases. The transition of the terminal section into the excretory duct is made abruptly. The wall of the excretory duct consists of a two-layer cubic epithelium, the cells of which are stained more intensely. Passing through the epidermis, the excretory duct acquires a corkscrew-like course. Here its wall is formed by flat cells. There are indications that when acetylcholine is introduced into the body, the metabolism of not only the cells of the terminal sections, but also the excretory ducts, increases.

The sebaceous glands reach their greatest development during puberty. Unlike sweat glands, sebaceous glands are almost always associated with hair. Only where there is no hair (lips, nipples, etc.), they lie on their own. Most of the sebaceous glands are on the head, face and upper back. They are absent on the palms and soles. The secret of the sebaceous glands (sebum) serves as a fatty lubricant for the hair and epidermis of the skin. During the day, the human sebaceous glands secrete about 20 g of sebum. It softens the skin, gives it elasticity and facilitates the friction of the contacting surfaces of the skin, and also prevents the development of microorganisms on it. Unlike the sweat glands, the sebaceous glands are located more superficially - in the border sections of the papillary and reticular layers of the dermis. Near one hair root you can find 1 - 3 glands. The sebaceous glands in structure are simple alveolar with branched terminal sections. They secrete according to the holocrine type. The terminal sections, the diameter of which ranges from 0,2 to 2 mm, consist of two types of cells - poorly differentiated cells capable of mitotic division, and cells in different stages of fatty degeneration. The first type of cells forms the outer germ layer of the terminal section. Inside of it are larger cells, in the cytoplasm of which drops of fat appear. Gradually, the process of obesity intensifies, and at the same time the cells are shifted towards the excretory duct. Finally, obesity goes so far that there is cell death, which breaks down and forms the secretion of the gland. The excretory duct is short, opening into the hair funnel. Its wall consists of stratified squamous epithelium. Closer to the end section, the number of layers in the wall of the duct decreases, and it passes into the outer growth layer of the end section.

Topic 24. EXTRACTIVE SYSTEM

The excretory system includes the kidneys, ureters, bladder and urethra.

Development of the excretory system

The urinary and reproductive systems develop from the intermediate mesoderm. In this case, the pronephros, mesonephros and metanephros are successively formed. The pronephros is rudimentary and does not function, the mesonephros acts in the early stages of intrauterine development, the metanephros forms the permanent kidney.

Pronephros. At the end of the 3rd - beginning of the 4th week of development, the intermediate mesoderm of the cervical region separates from the somites and forms segmented cell clusters that have the shape of a stalk with an internal cavity - nephrotomes growing in the lateral direction. Nephrotomes give rise to nephric tubules, the medial ends of which open into the body cavity, and the lateral ends grow in the caudal direction. The nephric tubules of adjacent segments unite and form paired longitudinal ducts growing towards the cloaca (primary renal duct). Small branches separate from the dorsal aorta, one of which penetrates into the wall of the nephritic tubule, and the other into the wall of the coelomic cavity, forming, respectively, the inner and outer glomeruli. The glomeruli consist of a spherical plexus of capillaries and together with tubules form excretory units (nephrons). As subsequent nephrotomes appear, degeneration of the previous ones occurs. By the end of the 4th week of intrauterine development, all signs of nephrotomes are absent.

Mesonephros. As the pronephros degenerates, the first tubules of the mesonephros appear more caudally. They lengthen, forming an s-shaped loop, the medial end of which reaches the capillary glomerulus. The glomerulus is embedded in the wall of the tubule, and in this place the tubule forms an epithelial capsule. The capsule and glomerulus form the renal corpuscle. The lateral end of the tubule drains into the primary renal duct, now called the Wolffian (mesonephric duct). In the future, the tubules lengthen, becoming more and more tortuous. They are surrounded by a plexus of capillaries formed by postglomerular vessels. By the middle of the 2nd month, the mesonephros reaches its maximum value. It is a large ovoid organ located on either side of the midline. On its medial side is the rudiment of the gonads. The elevation formed by both organs is known as the urogenital ridge. When the caudal tubules of the mesonephros are still being formed, the cranial tubules and glomeruli are already degenerating; by the end of the 2nd month, most of them disappear. A small portion of the caudal tubules and the mesonephric duct, however, are preserved in the male fetus. A number of structures of the male reproductive system are subsequently formed from the tubules of the mesonephros. With the beginning of the degeneration of the mesonephros, the formation of the metanephros begins.

The function of the mesonephros is similar to the function of the tubules of the nephron of the definitive kidney. The blood filtrate from the glomerulus enters the capsule, then into the tubule, then into the mesonephric duct. At the same time, a number of substances are reabsorbed in the tubule. However, urine is poorly concentrated in the mesonephros, which is associated with the absence of medulla structures necessary for water retention.

The metanephros (or permanent kidney) develops from a metanephrogenic blastoma, the source of the nephron tubules, and a metanephric diverticulum, the source of the collecting ducts and larger urinary tracts. Metanephros appears during the 5th week of development. Its tubules develop similarly to how it happened in the mesonephros.

Metanephric diverticulum and metanephrogenic blastoma. When it flows into the cloaca, the mesonephric duct forms an outgrowth - a metanephric diverticulum. This outgrowth is embedded in the caudal part of the intermediate mesoderm, which thickens around the diverticulum, forming a metanephrogenic blastoma. Further, the diverticulum dichotomously divides, forming a system of collecting ducts, gradually deepening into the tissue of the metanephros. The derivative of the metanephric diverticulum - the collecting duct - is covered at the distal end with a "cap" of the metanephrogenic blastoma.

Under the inductive influence of the tubules, small bubbles form from this tissue, giving rise to tubules. In turn, the developing tubules induce further branching of the collecting ducts. The tubules, uniting with the capillary glomerulus, form the nephrons. The proximal end of the nephron forms a capsule into which the glomerulus is deeply embedded. The distal end connects to one of the collecting ducts. Further, the tubule lengthens, resulting in the formation of the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule. First, the kidney is located in the pelvic area. In the future, it moves more cranially. The apparent rise of the kidney is associated with a decrease in the curvature of the body during the development of the fetus and its growth in the lumbar and sacral regions.

Functions in the fetus. Fetal urine is hypotonic relative to plasma, slightly acidic (pH 6,0). Maintaining the volume of amniotic fluid is one of the main functions of the fetal urinary system. Beginning at about the 9th week of development, the fetus excretes urine into the amniotic cavity (10 ml/kg/h) and also absorbs up to 0,5 liters of amniotic fluid per day. Nitrogenous residues from the body of the fetus are removed by diffusion through the placenta into the mother's blood.

Kidney of a newborn. In a newborn, the kidney has a pronounced lobular appearance. Lobulation subsequently disappears as a result of growth, but not the formation of new nephrons. Nephrogenesis is completed by the 36th week of development, by which time there are about 1 million nephrons in each kidney.

Kidneys

They are a urinary organ. The rest of the organs make up the urinary tract, through which urine is excreted from the body. Together with urine, over 80% of the end products of metabolism are excreted. The kidneys are paired organs that continuously produce urine. They are located on the inner surface of the posterior abdominal wall and are bean-shaped. Their concave surface is called the gate. The renal arteries enter the gates of the kidneys and the renal veins and lymphatic vessels exit. Here the urinary tract begins - the renal calyces, renal pelvis and ureters.

Structure. The kidney is covered with a connective tissue capsule and a serous membrane. The substance of the kidney is divided into cortical and medulla. The cortex is dark red in color, located in a common layer under the capsule. The medulla is lighter in color, divided into 8 - 12 pyramids. The tops of the pyramids, or papillae, protrude freely into the renal calyces. In the process of kidney development, its cortical substance, increasing in mass, penetrates between the bases of the pyramids in the form of renal columns. In turn, the medulla grows into the cortical substance with thin rays, forming brain rays. The kidney is supported by loose connective tissue rich in reticular cells and reticular fibers. The parenchyma of the kidney is represented by epithelial renal tubules, which, with the participation of blood capillaries, form nephrons. There are about 1 million of them in each kidney. Nephron is the structural and functional unit of the kidney. The length of its tubules is from 18 to 50 mm, and of all nephrons, on average, about 100 km. The nephron begins with the renal corpuscle, which includes a capsule enclosing the glomerulus of blood capillaries. At the other end, the nephron passes into the collecting duct. The collecting duct continues into the papillary canal, which opens at the top of the pyramid into the cavity of the renal calyx. There are four main sections in the nephron - the renal corpuscle, the proximal section, the nephron loop with descending and ascending parts, and the distal section. The proximal and distal sections are represented by convoluted tubules of the nephron. The descending and ascending parts of the loop are the direct tubules of the nephron. About 80% of nephrons are located almost entirely in the cortex, and only the knees of their loops are in the medulla. They are called cortical nephrons. The remaining 20% ​​of nephrons are located in the kidney so that their renal corpuscles, proximal and distal parts lie in the cortex on the border with the medulla, while the loops go deep into the medulla. These are the pericerebral (juxtamedullary) nephrons. The collecting ducts into which the nephrons open begin in the cortex, where they form part of the brain rays. Then they pass into the medulla and at the top of the pyramids flow into the papillary canal. Thus, the cortical and medulla of the kidney is formed by different parts of the nephrons. The cortex consists of renal corpuscles, proximal and distal nephrons, which look like convoluted tubules.

The medulla consists of straight descending and ascending parts of the nephron loops, as well as the terminal sections of the collecting ducts and papillary canals. Blood is brought to the kidneys through the renal arteries, which, having entered the kidneys, break up into interlobar arteries that run between the cerebral pyramids. At the border between the cortical and medulla, they branch into arcuate arteries, from which the direct arteries branch into the medulla, and the interlobular arteries into the cortex. Afferent arterioles diverge from the interlobular arteries. The upper ones go to the cortical nephrons, the lower ones go to the juxtamedullary nephrons. In this regard, in the kidneys, the cortical circulation, serving the cortical nephrons, and the juxtamedullary circulation, associated with the pericerebral nephrons, are conditionally distinguished. In the cortical circulatory system, the afferent arterioles break up into capillaries that form the vascular glomeruli of the renal corpuscles of the cortical nephrons. There is a collection of glomerular capillaries into efferent arterioles, which are approximately 2 times smaller in diameter than the afferent arterioles. Due to this, in the capillaries of the glomeruli of the cortical nephrons, the blood pressure is unusually high (70 - 90 mm Hg). This is the cause of the first phase of urination, which has the character of the process of filtering substances from the blood plasma into the nephron. The efferent arterioles, having passed a short path, again break up into capillaries, braiding the tubules of the nephron and forming a peritubular capillary network. In these secondary capillaries, the blood pressure, on the contrary, is relatively low (about 10 - 12 mm Hg), which contributes to the second phase of urination, which is in the nature of a process of reabsorption of a number of substances from the nephron into the blood. From the secondary capillaries, blood is collected in the upper sections of the cortex, first into the stellate veins, and then into the interlobular veins, in the middle sections of the cortical substance - directly into the interlobular veins. The interlobular veins flow into the arcuate veins, which pass into the interlobar veins, which form the renal veins that exit the renal hilum. Thus, cortical nephrons, as a result of the characteristics of the cortical circulation (high blood pressure in the capillaries of the vascular glomeruli and the presence of a peritubular network of capillaries with low blood pressure), are actively involved in urination.

In the juxtamedullary circulatory system, the afferent and efferent arterioles of the vascular glomeruli of the renal bodies of the paracerebral nephrons are almost the same in size or the efferent arterioles are even somewhat larger, due to which the blood pressure in the capillaries of these glomeruli does not exceed 40 mm Hg. Art., i.e., significantly lower than in the glomeruli of cortical nephrons. The efferent arterioles do not break up into a wide peritubular network of capillaries, which is typical for cortical nephrons, but, by the type of arteriovenular anastomoses, they pass into straight veins that flow into arcuate venous vessels. Therefore, pericerebral nephrons, in contrast to cortical ones, are less active when participating in urination. At the same time, the juxtamedullary circulation plays the role of a shunt, i.e., a short and easy path, which is the place where blood passes through the kidneys under conditions of their strong blood supply, for example, when a person performs hard physical work. The nephron begins with the renal corpuscle, represented by the vascular glomerulus and its capsule. The vascular glomerulus consists of more than 100 blood capillaries. Their endothelial cells have numerous fenestrae (possibly, in addition, pores). Endothelial cells of capillaries are located on the inner surface of a thick, three-layer basement membrane. On the outer side, the epithelium of the inner leaf of the glomerular capsule lies on it. The capsule of the glomerulus in shape resembles a double-walled cup, in which, in addition to the inner leaf, there is an outer leaf, and between them there is a slit-like cavity - the cavity of the capsule, passing into the lumen of the proximal tubule of the nephron. The inner leaf of the capsule penetrates between the capillaries of the vascular glomerulus and covers them from almost all sides. It is formed by large (up to 30 microns) irregularly shaped epithelial cells - podocytes.

From the bodies of podocytes, several large wide processes depart - cytotrabeculae, from which, in turn, numerous small processes (cytopodia) begin, which are attached to the three-layer basement membrane. Narrow slits are located between the cytopodia, communicating through the gaps between the bodies of podocytes with the cavity of the capsule. The three-layer basement membrane, which is common to the endothelium of the blood capillaries and podocytes of the inner leaf of the capsule, includes the outer and inner layers (less dense (light)) and the middle layer (more dense (dark)). In the middle layer of the membrane there are microfibrils that form a mesh with a cell diameter of up to 7 nm. All three of these components (the wall of the glomerular capillaries, the inner sheet of the capsule and the three-layer basement membrane common to them) constitute a biological barrier through which the components of the blood plasma that form the primary urine are filtered from the blood into the cavity of the capsule. Thus, in the composition of the renal corpuscles there is a renal filter. He participates in the first phase of urination, which has the character of a filtration process. The renal filter has a selective permeability, retaining everything that is larger than the size of the cells in the middle layer of the basement membrane. Normally, blood cells and some blood plasma proteins with the largest molecules do not pass through it: immune bodies, fibrinogen, etc. If the filter is damaged in cases of kidney disease (for example, with nephritis), they can be found in the urine of patients. In the vascular glomeruli of the renal corpuscles, in those places where the podocytes of the inner leaf of the capsule cannot penetrate between the capillaries, there is another type of cell - mesangial cells. After endotheliocytes and podocytes, they are the third type of cellular elements of the renal bodies, forming their mesangium. Mesangiocytes, like capillary pericytes, have a process shape capable of phagocytosis, and in pathological conditions, in addition, to fiber formation. The outer sheet of the glomerular capsule is represented by a single layer of flat and low cubic epithelial cells located on the basement membrane. The epithelium of the outer leaf of the capsule passes into the epithelium of the proximal nephron.

The proximal part has the appearance of a convoluted tubule with a diameter of up to 60 microns with a narrow, irregularly shaped lumen. The wall of the tubule is formed by high cylindrical border epithelium. It carries out obligate reabsorption - reverse absorption into the blood (into the capillaries of the peritubular network) from the primary urine of a number of substances contained in it. The mechanism of this process is associated with the histophysiology of proximal epithelial cells. The surface of these cells is covered with a brush border with a high activity of alkaline phosphatase, which is involved in the complete reabsorption of glucose. In the cytoplasm of cells, pinocytic vesicles are formed and there are lysosomes rich in proteolytic enzymes, with the help of which complete reabsorption of proteins is carried out. The cells have a basal striation formed by the inner folds of the cytolemma and mitochondria located between them. Mitochondria containing succinate dehydrogenase and other enzymes play an important role in the active reabsorption of some electrolytes, and cytolemma folds are of great importance for the passive reabsorption of some of the water. As a result of obligate reabsorption, primary urine undergoes significant qualitative changes: sugar and protein completely disappear from it. In kidney diseases, these substances can be found in the final urine of the patient due to damage to the proximal nephrons. The nephron loop consists of a descending thin portion and an ascending thick portion. The descending part is a straight tubule with a diameter of about 13 - 15 microns. Its wall is formed by flat epithelial cells, the nucleated parts of which swell into the lumen of the tubule.

The cytoplasm of the cells is light, poor in organelles. The cytolemma forms deep internal folds. Passive absorption of water into the blood occurs through the wall of this tubule. The ascending part of the loop also looks like a straight epithelial tubule, but with a larger diameter - up to 30 microns. In structure and role in reabsorption, this tubule is close to the distal nephron. The distal nephron is a convoluted tubule. Its wall is formed by a cylindrical epithelium, which is involved in facultative reabsorption: the reabsorption of electrolytes into the blood. The epithelial cells of the tubule lack a brush border, but due to the active transfer of electrolytes, they have a pronounced basal striation - the accumulation of a large number of mitochondria in the basal regions of the cytoplasm. Facultative reabsorption is a key link in the entire process of urination, since the amount and concentration of urine excreted depend on it. The mechanism of this process, called countercurrent-multiplier, seems to be as follows: with the reverse absorption of electrolytes in the distal section, the osmotic pressure in the blood and in the connective tissue surrounding the nephron changes, and the level of passive reverse absorption of water from the tubules of the nephron depends on this. The collecting ducts in the upper cortical part are lined with a single layer of cuboidal epithelium, and in the lower brain part - with a single layer of low cylindrical epithelium. In the epithelium, light and dark cells are distinguished. Light cells are poor in organelles, their cytoplasm forms internal folds. Dark cells in their ultrastructure resemble parietal cells of the gastric glands that secrete hydrochloric acid. In the collecting ducts, with the help of light cells, passive reabsorption of part of the water from the urine into the blood is completed. In addition, acidification of urine occurs, which is probably associated with the secretory activity of dark epithelial cells.

Thus, urination is a complex process that takes place in the nephrons. In the renal corpuscles of nephrons, the first phase of this process, or filtration, occurs, resulting in the formation of primary urine (more than 100 liters per day). In the tubules of nephrons, the second phase of urination occurs, i.e., reabsorption (obligatory and facultative), resulting in a qualitative and quantitative change in urine. Sugar and protein completely disappear from it, and its amount also decreases (up to 1,5 - 2 liters per day), which leads to a sharp increase in the concentration of excreted toxins in the final urine: creatine bodies - 75 times, ammonia - 40 times and etc. The final (third) secretory phase of urination is carried out in the collecting ducts, where the urine reaction becomes slightly acidic. All phases of urine formation are biological processes, that is, the result of the vigorous activity of nephron cells. The juxtaglomerular apparatus of the kidneys (JGA), or the periglomerular apparatus, secretes renin into the blood, which is a catalyst for the formation of angiotensins in the body, which have a strong vasoconstrictive effect, and also stimulates the production of the hormone aldosterone in the adrenal glands.

In addition, it is possible that JGA plays an important role in the production of erythropoietins. JGA consists of juxtaglomerular cells, macula densa, and Gurmagtig cells. The location of juxtaglomerular cells is the wall of afferent and efferent arterioles under the endothelium. They have an oval or polygonal shape, and in the cytoplasm there are large secretory (renin) granules that are not stained by conventional histological methods, but give a positive PAS reaction. A dense spot is a section of the wall of the distal nephron where it passes next to the renal corpuscle between the afferent and efferent arterioles. In the dense patch, epithelial cells are taller, almost devoid of basal folding, and their basement membrane is extremely thin (according to some sources, it is completely absent). It is assumed that the macula, like a sodium receptor, detects changes in the sodium content in the urine and affects the periglomerular cells that secrete renin. Gurmagtig cells lie in a triangular space between the afferent and efferent arterioles and the macula densa. Their shape may be oval or irregular, they form stretching processes that have a connection with the cells of the mesangium of the glomerulus. Fibrillar structures are revealed in their cytoplasm. Some authors also classify mesangial cells of vascular glomeruli as JGA. It is suggested that Gurmagtig and mesangium cells are involved in renin production when juxtaglomerular cells are depleted. Inpersitial cells (IC) of the kidneys of mesenchymal origin are located in the stroma of the cerebral pyramids in a horizontal direction. Their elongated body has processes, some of which are woven into tubules of the nephron loop, while others are blood capillaries. In the cytoplasm of IC, organelles are well developed and there are lipid (osmiophilic) granules.

There are two hypotheses about the role of these cells:

1) participation in the work of the countercurrent-multiplier system;

2) the production of one of the types of prostaglandins, which has an antihypertensive effect, i.e., lowers blood pressure.

Thus, JGA and IC are the endocrine complex of the kidneys, which regulates the general and renal circulation, through which it influences urination. Aldosterone (adrenal glands) and vasopressin, or antidiuretic hormone (hypothalamus), directly affect nephron function. Under the influence of the first hormone, sodium reabsorption in the distal nephrons is enhanced, and under the influence of the second, water reabsorption in the nephron tubules and in the collecting ducts is enhanced. The lymphatic system of the kidney is represented by a network of capillaries surrounding the tubules of the cortex and renal corpuscles. There are no lymphatic capillaries in the vascular glomeruli. Lymph from the cortex flows through a sheath-shaped network of lymphatic capillaries surrounding the interlobular arteries and veins to the first order lymphatic vessels that surround the arcuate arteries and veins. Lymphatic capillaries of the medulla surrounding the direct arteries and veins flow into these plexuses of lymphatic vessels. Lymphatic vessels of the XNUMXst order form larger lymphatic collectors of the XNUMXnd, XNUMXrd and XNUMXth order, which flow into the interlobar sinuses of the kidney. From these vessels, lymph enters the regional lymph nodes. The kidney is innervated by efferent sympathetic and parasympathetic nerves and afferent posterior root nerve fibers. The distribution of nerves in the kidney is different. Some of them are related to the vessels of the kidney, others - to the renal tubules. The renal tubules are supplied by the nerves of the sympathetic and parasympathetic systems. Their endings are localized under the epithelium membrane. However, according to some reports, nerves can pass through the basement membrane and terminate on the epithelial cells of the renal tubules. In structure, these nerves resemble secretory nerve endings. Polyvalent endings are also described, when one branch of the nerve ends on the renal tubule, and the other on the capillary.

Urinary tract

The urinary tract includes the renal calyces and pelvises, the ureters, the bladder and the urethra, which in men simultaneously performs the function of removing seminal fluid from the body and therefore will be described in the chapter on the reproductive system. The structure of the walls of the renal calyces and pelvis, ureters and bladder is similar in general terms. They distinguish between the mucous membrane, consisting of the transitional epithelium and the lamina propria, the submucosa, the muscular and outer membranes. In the wall of the renal calyces and renal pelvis, after the transitional epithelium, there is a lamina propria of the mucous membrane, imperceptibly passing into the connective tissue of the submucosa. The muscular coat consists of two thin layers of smooth muscle cells - inner (longitudinal) and outer (circular). However, only one circular layer of smooth muscle cells remains around the papillae of the renal pyramids. The outer shell without sharp boundaries passes into the connective tissue surrounding the large renal vessels. The ureters have a pronounced ability to stretch due to the presence of deep longitudinal folds of the mucous membrane in them. The submucosa of the lower part of the ureters has small alveolar-tubular glands, similar in structure to the prostate gland. The muscular membrane of the ureters in the upper half consists of two layers - the inner (longitudinal) and the outer (circular). The muscular membrane of the lower part of the ureters has three layers - the inner and outer layers of the longitudinal direction and the middle layer - circular. In the muscular membrane of the ureters, in the places where they pass through the wall of the bladder, the bundles of smooth muscle cells run only in the longitudinal direction. Contracting, they open the opening of the ureter, regardless of the state of the smooth muscles of the bladder.

Outside, the ureters are covered with a connective tissue adventitial membrane. The mucous membrane of the bladder consists of a transitional epithelium and its own plate. In it, small blood vessels are especially close to the epithelium. In a collapsed or moderately distended state, the bladder mucosa has many folds. They are absent in the anterior section of the bottom of the bladder, where the ureters flow into it and the urethra exits. This section of the bladder wall, which has the shape of a triangle, is devoid of a submucosa, and its mucous membrane is tightly fused with the muscular membrane. Here, in the own plate of the mucous membrane, glands are laid, similar to the glands of the lower part of the ureters. The muscular membrane of the bladder consists of three limited layers - inner, outer with a longitudinal arrangement of smooth muscle cells and the middle - circular. Smooth muscle cells often resemble split spindles. Layers of connective tissue divide the muscle tissue in this sheath into separate large bundles. In the neck of the bladder, the circular layer forms the muscular sphincter. The outer shell on the upper-posterior and partially on the lateral surfaces of the bladder is characterized by a sheet of peritoneum (serous membrane), in the rest of it it is adventitious. The wall of the bladder is richly supplied with blood and lymphatic vessels. The bladder is innervated by both sympathetic and parasympathetic and spinal (sensory) nerves. In addition, a significant number of nerve ganglia and scattered neurons of the autonomic nervous system were found in the bladder. There are especially many neurons at the place where the ureters enter the bladder. In the serous, muscular and mucous membranes of the bladder there are also a large number of receptor nerve endings.

Topic 25. REGENERAL SYSTEM

Development of the sex organs

The sources of development of the genital organs are the genital ridges and primary germ cells.

Sexual (or gonadal) ridges are indifferent gonads, the rudiments of future sexual future organs (both male and female) - testicles and ovaries.

Sexual rollers are formed already at the 4th week of intrauterine development, however, at this time it is impossible to identify male or female rudiments. After laying the indifferent gonads are populated by the primary germ cells of the cortex and medulla.

Primary sex cells are formed in the wall of the yolk sac, after which they migrate to the sex gonads. After migration and sexual differentiation, the primary germ cells, under the influence of certain factors, turn into spermatogonia in the testicles and into oogonia in the ovaries. However, for the final differentiation into spermatozoa and eggs, germ cells must go through the stages of reproduction, growth, maturation and formation.

Until the 8th week of intrauterine development, it is impossible to find differences in the male and female genital organs. 45 - 50th day (8 weeks) - a critical period in the development of the embryo, it is at this time that sexual differentiation occurs.

During fertilization, chromosomal determination occurs, while the Y chromosome ensures the subsequent genetic development of the male. The Y chromosome encodes the regulatory factor TDF, one of the inducers of the male reproductive system, a factor that determines the development of male gonads. Under the influence of the TDF factor, the testicles develop from the primary gonads, and the development of further sexual structures is provided by male sex hormones and the Müllerian inhibitory factor, also produced in the testicles.

The indifferent gonads consist of cortex and medulla. In the female body, the cortical substance develops in the gonads, and the male substance atrophies; in the male body, on the contrary, the cortical substance atrophies, and the medullary substance develops. At the 8th week of embryogenesis, the testicles are located at the level of the upper lumbar vertebrae, and a supporting ligament stretches from their lower pole, which stretches down and acts as a conductor for the testicles from the abdominal cavity to the scrotum. The final descent of the testicles occurs by the end of the 1st month of life.

The extragonadal genital ducts originate from the mesonephric (Wolffian) and paramesonephric (Müllerian) ducts, the external genital organs differentiate from the urogenital sinus, genital tubercle and genital ridges.

The primary kidney of the embryo is drained by the mesonephric (or wolffian) duct. In boys, under the influence of the male sex hormone testosterone, it forms the testicular network, epididymis, seminal vesicles and vas deferens. In women, due to a different hormonal background, these ducts are obliterated.

In the testicles of boys, there are Sertoli cells that synthesize the Müllerian inhibitory factor. It leads to obliteration and regression of the paramesonephric (or Müllerian) ducts.

The paramesonephric duct (or female duct) is a thin tube that runs parallel to the mesonephric duct along the primary kidney. In the proximal (cranial) section, the paramesonephric ducts run separately, parallel to each other, and in the distal (or caudal) section they merge and open into the urogenital sinus.

The cranial section of the paramesonephric ducts differentiates into the fallopian tubes and uterus, and the caudal section into the upper part of the vagina. Differentiation is carried out in the absence of the Müllerian inhibitory factor, regardless of whether female sex (ovarian) hormones are present or not. In the male body, under the influence of the Müllerian inhibitory factor, the paramesonephric ducts undergo degeneration.

Differentiation of the external genital organs is carried out from the urogenital sinus, genital tubercle, genital folds and genital folds. The development of the external genital organs is determined by sex hormones.

In boys, under the influence of testosterone, the prostate gland and bulbourethral glands develop from the urogenital sinus. The formation of other external genital organs - the penis and scrotum is carried out under the influence of dihydrotestosterone at the 12th - 14th week of intrauterine development.

The development of the external genital organs according to the female type occurs in the absence of male sex hormones (androgens). The genitourinary sinus gives rise to the lower part of the vagina, the genital tubercle turns into the clitoris, and the genital ridges and genital folds into the labia majora and labia minora.

Gametogenesis

Spermatogenesis

The process of formation of male germ cells goes through four stages - reproduction, growth, maturation and formation.

Stage of reproduction and growth. After formation, the primary germ cells migrate to the rudiments of the gonads, where they divide and differentiate into spermatogonia. In the spermatogonia stage, the germ cells are at rest until the period of sexual reproduction. Under the influence of male sex hormones and, above all, testosterone, the reproduction of spermatogonia begins. Testosterone is synthesized by Leydig cells. Their activity, in turn, is regulated by the hypothalamus, where gonadoliberins are synthesized, which activate the secretion of gonadotropic hormones of the adenohypophysis, which affect the secretion of Leydig cells. At the stage of reproduction, there are two types of spermatogonia - A and B.

Type A spermatogonia differ in the degree of chromatin condensation into light and dark. Dark spermatogonia are reservoir cells and rarely enter mitosis, light spermatogonia are semi-stem cells, they constantly and very actively divide, and interphase is replaced by mitosis. Mitosis of type A clear cells can proceed symmetrically (with the formation of two type B spermatogonia) and asymmetrically, in which one type B spermatogonium and one type A clear cell are formed.

Type B spermatogonia have a round nucleus and condensed chromatin. They enter mitosis, but at the same time remain connected to each other with the help of cytoplasmic bridges. After passing through several successive mitotic divisions, type B spermatogonia differentiate into first-order spermatocytes. First-order spermatocytes move from the basal space to the adluminal space and enter the growth stage.

At the growth stage, there is an increase in the size of first-order spermatocytes by about 4 times.

The maturation stage includes the meiotic division of first-order spermatocytes with the formation, first, from the 1st cell of two second-order spermatocytes, and then 4 spermatids containing a haploid set of chromosomes - 22 autosomes each plus an X or Y chromosome. The spermatid is 4 times smaller than the first-order spermatocyte. After formation, they are located near the lumen of the tubule.

The last stage of spermatogenesis is the formation stage. It is absent in ovogenesis. At this stage, the morphological differentiation of spermatids and the formation of spermatozoa occur. At this stage, the spermatozoa acquire their final form - a tail is formed, energy reserves. The nucleus compaction occurs, the centrioles migrate to one of the poles of the nucleus, organizing the axoneme. Mitochondria are arranged spirally, forming a sheath around the axoneme. The Golgi complex develops into an acrosome.

The process of spermatogenesis from spermatogonia to the formation of a mature spermatozoon lasts about 65 days, but the final differentiation of spermatozoa occurs in the duct of the epididymis for another 2 weeks.

Only after this, the spermatozoa become completely mature and acquire the ability to move independently in the female genital tract.

At the stages of reproduction, growth and maturation, spermatogenic cells form cell associations. For example, light type A spermatogonia form a syncytium in which cells are linked by cytoplasmic bridges prior to the formation stage. The cell association in its development from the stage of spermatogonia to the spermatozoon passes through six stages, each of which is characterized by a certain combination of spermatogenic cells.

Ovogenesis

Unlike spermatogenesis, oogenesis includes three stages - the stages of reproduction, growth and maturation.

The reproduction stage occurs in the female body during intrauterine development. By the 7th month of embryogenesis, oogonia stop dividing. At this time, in the ovaries of a female fetus there are up to 10 million first-order oocytes.

After completion of the growth stage, oocytes of the first order in the prophase of the first division of meiosis acquire a membrane of follicular cells, after which they fall into a long state of rest, ending in the period of sexual development.

The ovaries of a newborn girl contain about 2 million first-order oocytes.

The maturation stage occurs during puberty, after the establishment of the ovarian-menstrual cycle. At the level of luteinizing hormone, the first division of meiosis is completed, after which the first-order oocyte enters the fallopian tube. The second meiotic division occurs only under the condition of fertilization, with the formation of one second-order oocyte and a polar (or directional) body. A mature egg contains a haploid set of chromosomes - 22 autosomes and one X chromosome.

Male reproductive system

The male reproductive system includes the sex glands - testicles, a collection of ducts (efferent tubules, epididymal duct, vas deferens, ejaculatory duct), accessory sex glands (seminal vesicles, prostate and bulbourethral glands) and the penis.

Unlike the ovaries, which are located in the small pelvis (in the abdominal cavity), the testicles are located outside the body cavities - in the scrotum. This arrangement can be explained by the need for a certain temperature (not higher than 34 ° C) for the normal course of spermatogenesis.

Outside, the testicle is covered with a connective tissue plate or tunica albuginea. The inner layer of the membrane, rich in blood vessels, forms the choroid. The albuginea forms a thickening, which on one side protrudes into the parenchyma of the testis, thereby forming the testicular mediastinum (or Gaimar's body). From the Gaimar body, the albuginea passes into the testicle, piercing the partitions that divide the parenchyma into conical lobules. Each lobule contains from one to four convoluted seminiferous tubules lined with spermatogenic epithelium. Convoluted seminiferous tubules perform the main function of the testicle - spermatogenesis.

Loose connective tissue is located between the seminiferous tubules. It contains interstitial Leydig cells. Leydig cells can be attributed to the cells of the endocrine system. They synthesize male sex hormones - androgens. Leydig cells are characterized by a highly developed synthetic apparatus - a smooth endoplasmic reticulum, numerous mitochondria and vacuoles.

Among the male sex hormones that are synthesized in Leydig cells, testosterone and dihydrotestosterone are isolated. Stimulation of the synthesis of these hormones is carried out under the influence of lutropin, a hormone that has a stimulating effect on interstitial cells. After isolation from Leydig cells, testosterone enters the bloodstream, where it binds to plasma transport proteins, and when it enters the testicular tissue, to androgen-binding protein.

The function of the androgen-binding protein is to maintain a high (necessary for spermatogenesis) level of testosterone in the spermatogenic epithelium by transporting testosterone in the lumen of the seminiferous tubules.

As they approach the mediastinum of the testis, the convoluted seminiferous tubules become straight. The wall of the straight tubules is lined with cuboidal epithelium located on the basement membrane. The straight tubules form a testicular network - a system of anastomosing tubules, which then continue into the efferent tubules of the epididymis.

The structure of the convoluted seminiferous tubules and Sertoli cells. The convoluted seminiferous tubules are internally lined with spermatogenic epithelium, which contains two types of cells - gametes at various stages of development (spermatogonia, first and second order spermatocytes, spermatids and spermatozoa), as well as supporting Sertoli cells.

Outside, the convoluted seminiferous tubules are surrounded by a thin connective tissue sheath.

Sertoli cells (or supporting cells) are located on the basement membrane, with their wide base located on the membrane, and the apical part facing the lumen of the tubule. Sertoli cells divide the spermatogenic epithelium into basal and adluminal spaces.

Only spermatogonia are located in the basal space, and spermatocytes of the first and second orders, spermatids and spermatozoa are located in the adluminal space.

Functions of Sertoli cells:

1) secretion of androgen-binding protein, which regulates the level of testosterone in the spermatogenic epithelium of the convoluted seminiferous tubules;

2) trophic function. Sertoli cells provide the developing gametes with nutrients;

3) transport. Sertoli cells provide the secretion of fluid necessary for the transport of a spermatozoa in the seminiferous tubules;

4) phagocytic. Sertoli cells phagocytize the remnants of the cytoplasm of the emerging spermatozoa, absorb various metabolic products and degenerating sex cells;

5) secretion of the SCF factor (stem cell factor), which ensures the survival of spermatogonia.

Hormonal regulation of spermatogenesis. In the hypothalamus, gonadoliberins are secreted, which activate the synthesis and secretion of gonadotropic hormones of the pituitary gland. They, in turn, affect the activity of Leydig and Sertoli cells. The testicles produce hormones that regulate the synthesis of releasing factors on the feedback principle. Thus, the secretion of gonadotropic hormones from the pituitary gland is stimulated by GnRH, and inhibited by testicular hormones.

Gonadoliberin enters the bloodstream from the axons of neurosecretory cells in a pulsating mode, with peak intervals of about 2 hours. Gonadotropic hormones also enter the bloodstream in a pulsating mode, at intervals of 90-120 minutes.

Gonadotropic hormones include lutropin and follitropin. The targets of these hormones are the testicles, and Sertoli cells have receptors for follitropin, and Leydig cells for lutropin.

In Sertoli cells, under the influence of follitropin, the synthesis and secretion of androgen-binding protein, inhibin (a substance that inhibits the synthesis of follitropin in its excess), estrogens, and plasminogen activators are activated.

Under the influence of lutropin, the synthesis of testosterone and estrogen is stimulated in Leydig cells. Leydig cells synthesize about 80% of all estrogen produced in the male body (the remaining 20% ​​are synthesized by cells of the fascicular and reticular cortex zones of the adrenal cortex and Sertoli cells). The function of estrogens is to suppress the synthesis of testosterone.

The structure of the epididymis. The epididymis consists of a head, body, and tail. The head consists of 10 - 12 efferent tubules, the body and tail are represented by the duct of the appendage, into which the vas deferens opens.

The efferent tubules of the appendage are lined with garland epithelium - its cells have different heights. There are tall cylindrical cells, equipped with cilia, which facilitate the movement of spermatozoa, and a low cuboidal epithelium, which contains microvilli and lysosomes, whose function is to reabsorb the fluid formed in the testicles.

The duct of the body of the appendage is lined with a multi-row cylindrical epithelium, in which two types of cells are distinguished - basal intercalary and high cylindrical. Cylindrical cells are equipped with stereocilia glued together in the form of a cone - the plasma epithelium. Between the bases of cylindrical cells are small intercalated cells, which are their precursors. Under the epithelial layer is a layer of circularly oriented muscle fibers. The muscular layer becomes more pronounced towards the vas deferens.

The main role of the muscles is the promotion of spermatozoa into the vas deferens.

The structure of the vas deferens. The wall of the vas deferens is quite thick and is represented by three layers - mucous, muscular and adventitious membranes.

The mucous membrane consists of its own layer and multilayer epithelium. In the proximal part, it is similar in structure to the epithelium of the duct of the appendage. The muscular layer has three layers - inner longitudinal, middle circular and outer longitudinal. On the value of the muscular membrane - the release of sperm during ejaculation. Outside, the duct is covered with an adventitial membrane, consisting of fibrous connective tissue with blood vessels, nerves, and groups of smooth muscle cells.

The structure of the prostate. The development of the prostate gland is carried out under the influence of testosterone. Before puberty, the volume of the gland is insignificant. With the activation of the synthesis of male sex hormones in the body, its active differentiation, growth and maturation begin.

The prostate gland consists of 30-50 branched tubular alveolar glands. It is covered on the outside with a connective tissue capsule containing smooth muscle cells. Connective tissue partitions extend from the capsule deep into the gland, dividing the gland into lobules. In addition to the connective tissue, these partitions include well-developed smooth muscles.

The mucous membrane of the secretory sections is formed by a single layer of cuboidal or cylindrical epithelium, which depends on the phase of secretion.

The excretory ducts of the gland are lined with multi-row prismatic epithelium, which becomes transitional in the distal sections. Each lobule of the gland has its own excretory duct, which opens into the lumen of the urethra.

The secretory cells of the prostate produce a fluid that is secreted into the urethra by contraction of smooth muscle. The secret of the gland is involved in the liquefaction of sperm and promotes its movement through the urethra during ejaculation.

In the secret of the prostate gland there are lipids that perform a trophic function, enzymes - fibrinolysin, which prevent spermatozoa from sticking together, as well as acid phosphatase.

Seminal vesicles are bulbourethral glands. The seminal vesicles are two symmetrical, highly convoluted tubes, up to 15 cm long. They open into the ejaculatory duct immediately after the vas deferens.

The wall of the seminal vesicles consists of three membranes - internal mucosa, middle muscular and external connective tissue.

The mucous membrane is formed by a single layer of multi-row cylindrical epithelium containing secretory and basal cells. It has numerous folds.

The muscular coat consists of two layers - the inner circular and the outer longitudinal.

The seminal vesicles secrete a yellowish liquid. It consists of fructose, ascorbic and citric acids, prostaglandins. All these substances provide the energy supply of spermatozoa and increase their survival in the female genital tract. The secret of the seminal vesicles is ejected into the ejaculatory duct during ejaculation.

The bulbourethral glands (or Cooper's glands) have a tubular-alveolar structure. The mucous membrane of the secretory cells of the glands is lined with cubic and cylindrical epithelium. The value of glandular secretions is to lubricate the urethra before ejaculation. The secret is released during sexual arousal and prepares the urethral mucosa for the movement of sperm.

The structure of the male penis. The male penis consists of three cavernous bodies. The cavernous bodies are paired and cylindrical and are located on the dorsal side of the organ. On the ventral side along the midline is the spongy body of the urethra, which forms the glans penis at the distal end. Cavernous bodies are formed by an anastomosing network of septa (trabeculae) of connective tissue and smooth muscle cells. Capillaries open into the free spaces between the endothelium-covered septa.

The head of the penis is formed by dense fibrous connective tissue containing a network of large tortuous veins.

The cavernous bodies are surrounded on the outside by a dense connective tissue protein membrane, consisting of two layers of collagen fibers - the inner circular and the outer longitudinal. There is no albuginea on the head.

The head is covered with thin skin, in which there are many sebaceous glands.

The cavernous bodies are united by the fascia of the penis.

The foreskin is called a circular fold of skin covering the head.

In a relaxed state, the large arteries of the penis, which pass in the septa of the cavernous bodies, are spirally twisted. These arteries are muscular type vessels, as they have a thick muscular membrane. A longitudinal thickening of the inner membrane, consisting of bundles of smooth muscle cells and collagen fibers, bulges into the lumen of the vessel and serves as a valve that closes the lumen of the vessel. A significant proportion of these arteries open directly into the intertrabecular space.

The veins of the penis have numerous smooth muscle elements. In the middle shell there is a circular layer of smooth muscle fibers, in the inner and outer shells there are longitudinal layers of smooth muscle tissue.

During an erection, the smooth muscle tissue of the septa and spiral arteries relax. Due to the relaxation of smooth muscle tissue, blood enters the free spaces of the corpora cavernosa almost without resistance. Simultaneously with the relaxation of the smooth muscles of the septa and arteries of the spiral type, the smooth muscle cells of the veins contract, as a result of which resistance to the outflow of blood from the intertrabecular spaces overflowing with it develops.

Relaxation of the penis (or detumescence) occurs as a result of the reverse process - relaxation of the smooth muscles of the veins and contraction of the muscles of the spiral-type arteries, as a result of which the outflow of blood from the intertrabecular spaces improves and the inflow becomes more difficult.

The innervation of the penis is carried out as follows.

The skin and choroid plexus of the head, the fibrous membranes of the cavernous bodies, the mucous membrane and muscular membrane of the membranous and prostatic parts of the urethra are strong reflexogenic zones saturated with various receptors.

Each of these zones plays its role during sexual intercourse, being a reflexogenic zone that underlies unconditioned reflexes - erection, ejaculation, orgasm.

Among the nerve elements in the penis, one can distinguish - free nerve endings, bodies of Vater - Pacini, Meissner, Krause flasks.

The structure of the male urethra. The urethra in men is a tube about 12 cm long, passing through the prostate, perforating the fascia of the urogenital diaphragm, penetrating the spongy body of the urethra and opening with the external opening of the urethra on the glans penis.

In the male urethra, respectively, there are:

1) the prostatic part;

2) membranous part;

3) spongy part;

In the prostatic part, the lumen of the urethra has a v-shape. This shape is due to the v-shaped protrusion of the wall of the crest of the urethra. Along the crest are two sinuses into which the ducts of the main and submucosal glands open. On either side of the ridge, ejaculatory channels open. In the region of the internal opening of the urethra, smooth muscle cells of the outer circular layer are involved in the formation of the sphincter of the bladder.

The external sphincter of the bladder is formed by the skeletal muscles of the pelvic diaphragm. If the prostatic part of the urethra was characterized by transitional epithelium, then in the membranous part it is replaced by a multilayer cylindrical epithelium. The mucous and muscular membranes of both the prostatic and membranous parts have a powerful receptor innervation.

During ejaculation, strong periodic contractions of smooth muscle cells occur, causing irritation of sensitive endings and orgasm.

After passing through the bulbs of the spongy substance of the penis, the urethra expands, forming the bulb of the urethra. An enlargement of the urethra at the head of the penis is called the navicular fossa. Before the scaphoid fossa, the mucous membrane of the urethra was lined with stratified columnar epithelium, and after it it is replaced by a stratified squamous keratinizing one and covers the glans penis.

Topic 26. FEMALE REGENERAL SYSTEM

The female reproductive system consists of paired ovaries, uterus, fallopian tubes, vagina, vulva, and paired mammary glands.

The main functions of the female reproductive system and its individual organs:

1) the main function is reproductive;

2) the ovaries perform a germinal function, participating in the processes of oogenesis and ovulation, as well as an endocrine function; estrogen is produced in the ovaries; during pregnancy, the corpus luteum is formed in the ovaries, which synthesizes progesterone;

3) the uterus is intended for bearing the fetus;

4) the fallopian tubes communicate between the ovaries and the uterine cavity to advance the fertilized egg into the uterine cavity, followed by implantation;

5) the cervical canal and vagina form the birth canal;

6) mammary glands synthesize milk for feeding a newborn baby.

The body of a non-pregnant woman is constantly undergoing cyclic changes, which is associated with cyclic changes in the hormonal background. Such a complex of changes in a woman's body is called the "ovarian-menstrual cycle".

The ovarian cycle is the cycle of ovogenesis, i.e., the phases of growth and maturation, ovulation and the formation of the corpus luteum. The ovarian cycle is under the influence of follicle-stimulating and luteinizing hormones.

The menstrual cycle is a change in the mucous membrane of the uterus, the purpose of which is to prepare the most favorable conditions for the implantation of the embryo, and in its absence, they end with the rejection of the epithelium, manifested by menstruation.

The average duration of the ovarian-menstrual cycle is about 28 days, but the duration can be purely individual.

Female sex hormones

All female sex hormones can be divided into two groups - estrogens and progestins.

Estrogens are produced by follicular cells, corpus luteum and placenta.

There are the following hormones estrogen:

1) estradiol - a hormone formed from testosterone, with the help of aromatization of the latter under the influence of the enzymes aromatase and estrogen synthetase. The formation of these enzymes is induced by follitropin. It has significant estrogenic activity;

2) estrol is formed by aromatization of androstenedione, has little estrogenic activity, is excreted in the urine of pregnant women. It is also found in the follicular fluid of growing ovarian follicles and in the placenta;

3) estriol - a hormone formed from estrol, excreted in the urine of pregnant women, found in a significant amount in the placenta.

Progestins include the hormone progesterone. It is synthesized by the cells of the corpus luteum during the luteal phase of the ovarian-menstrual cycle. Synthesis of progesterone is also carried out by chorion cells during pregnancy. The formation of this hormone is stimulated by lutropin and human chorionic gonadotropin. Progesterone is the pregnancy hormone.

The structure of the ovary

Outside, the ovary is covered with a single layer of cuboidal epithelium. Under it is a thick connective tissue plate (or albuginea) of the ovary. The transverse section shows that the ovary consists of a cortex and a medulla.

The medulla of the ovary is formed by loose connective tissue, it contains many elastic fibers, blood vessels and nerve plexuses.

The ovarian cortex contains primordial follicles, growing primary and secondary follicles, corpus luteum and white, and atretic follicles.

ovarian cycle. Features of the structure of the primary, secondary and tertiary follicles

The ovarian cycle has two halves:

1) follicular phase. In this phase, under the influence of follicle-stimulating hormone, the development of primordial follicles occurs;

2) luteal phase. Under the influence of luteal hormone, the corpus luteum of the ovary is formed from the cells of the Graafian body, which produces progesterone.

Between these two phases of the cycle, ovulation occurs.

The development of the follicle is carried out as follows:

1) primordial follicle;

2) primary follicle;

3) secondary follicle;

4) tertiary follicle (or Graafian vesicle).

During the ovarian cycle, there are changes in the level of hormones in the blood.

Structure and development of primordial follicles. Primordial follicles are located under the ovarian albuginea in the form of compact groups. The primordial follicle consists of one first-order oocyte, which is covered by a single layer of flat follicular cells (granulomatous tissue cells) and surrounded by a basement membrane.

After birth, a girl’s ovaries contain about 2 million primordial follicles. During the reproductive period, about 98% of them die, the remaining 2% reach the stage of primary and secondary follicles, but only no more than 400 follicles develop into the Graafian vesicle, after which ovulation occurs. During one ovarian-menstrual cycle, 1, extremely rarely 2 or 3 first-order oocytes ovulate.

With a long lifespan of the first-order oocyte (up to 40-50 years in the mother's body), the risk of various gene defects increases significantly, which is associated with the effect of environmental factors on the follicle.

During one ovarian-menstrual cycle, from 3 to 30 primordial follicles, under the influence of follicle-stimulating hormone, enter the growth phase, resulting in the formation of primary follicles. All follicles that have begun their growth but have not reached the stage of ovulation undergo atresia.

Atrezated follicles consist of a dead oocyte, a wrinkled transparent membrane that is surrounded by degenerated follicular cells. Between them are fibrous structures.

In the absence of folliculotropic hormone, primordial follicles develop only to the stage of the primary follicle. This is possible during pregnancy, before puberty, as well as when using hormonal contraceptives. Thus, the cycle will be anovulatory (no ovulation).

Structure of primary follicles. After the growth stage and its formation, the flat-shaped follicular cell turns into a cylindrical one and begins to actively divide. During division, several layers of follicular cells are formed that surround the first-order oocyte. Between the oocyte of the first order and the resulting environment (follicular cells) there is a fairly thick transparent membrane. The outer shell of the growing follicle is formed from the elements of the ovarian stroma.

In the outer shell, one can distinguish the inner layer containing interstitial cells that synthesize androgens, a rich capillary network and the outer layer, which is formed by connective tissue. The inner cell layer is called the theca. The resulting follicular cells have receptors for follicle-stimulating hormone, estrogen and testosterone.

Follicle-stimulating hormone promotes the synthesis of aromatase in granulose cells. It also stimulates the formation of estrogens from testosterone and other steroids.

Estrogens stimulate the proliferation of follicular cells, while the number of granulose cells increases significantly, and the follicle increases in size, they also stimulate the formation of new receptors for follicle-stimulating hormone and steroids. Estrogens enhance the effect of follitropin on follicular cells, thereby preventing follicular atresia.

Interstitial cells are cells of the parenchyma of the ovary, they have the same origin as the cells of the theca. The functions of interstitial cells are the synthesis and secretion of androgens.

Norepinephrine acts on granulose cells through α2-adrenergic receptors, stimulates the formation of steroids in them, facilitates the action of gonadotropic hormones on steroid production, and thereby accelerates the development of the follicle.

The structure of the secondary follicle. With the growth of the primary follicle between the follicular cells, rounded cavities filled with fluid are formed. Secondary follicles are characterized by further growth, while a dominant follicle appears, which is ahead of the rest in its development, the theca is most pronounced in its composition.

Follicular cells increase estrogen production. Estrogens by an autocrine mechanism increase the density of follitropin recipes in the membranes of follicular cells.

Follitropin stimulates the appearance of lutropin receptors in the membrane of follicular cells.

The high content of estrogen in the blood blocks the synthesis of follitropin, which inhibits the development of other primary follicles and stimulates the secretion of LH.

At the end of the follicular stage of the cycle, the level of lutropin rises, luteinizing hormone is formed, which stimulates the formation of androgens in the theca cells.

Androgens from the theca through the basement membrane (vitreous membrane at later stages of follicle development penetrate deep into the follicle, into granulose cells, where they are converted into estrogens with the help of aromatase.

The structure of the tertiary follicle. The tertiary follicle (or Graafian vesicle) is a mature follicle. It reaches 1 - 2,5 cm in diameter, primarily due to the accumulation of fluid in its cavity. A mound of follicular cells protrudes into the cavity of the Graaffian vesicle, inside which the egg is located. The egg at the stage of the oocyte of the first order is surrounded by a transparent membrane, outside of which follicular cells are located.

Thus, the wall of the Graafian vesicle consists of a transparent and granular membrane, as well as theca.

24 - 36 hours before ovulation, the increasing level of estrogen in the body reaches its maximum values.

The content of LH increases until the middle of the cycle. 12-14 hours after the onset of the peak of estrogen, its content also increases significantly.

Lutropin stimulates luteinization of granulosa and theca cells (in this case, accumulation of lipids, yellow pigment occurs) and induces preovulatory synthesis of progesterone. This increase facilitates the reverse positive effect of estrogens, and also induces a preovulatory follitropin peak by enhancing the pituitary response to GnRH.

Ovulation occurs 24 to 36 hours after the estrogen peak or 10 to 12 hours after the LH peak. Most often on the 11th - 13th day of a 28-day cycle. However, theoretically, ovulation is possible from 8 to 20 days.

Under the influence of prostaglandins and the proteolytic action of granulose enzymes, thinning and rupture of the follicle wall occur.

A first order oocyte undergoes the first meiotic division, resulting in a second order oocyte and a polar body. The first meiosis is completed already in the mature follicle before ovulation against the background of the LH peak.

The second meiosis is completed only after fertilization.

The structure and functions of the corpus luteum. Under the influence of LH in the luteal stage of the ovarian-menstrual cycle, the menstrual corpus luteum forms at the site of the burst follicle. It develops from the Graafian vesicle and consists of luteinized follicles and theca cells, between which sinusoidal capillaries are located.

In the luteal stage of the cycle, the menstrual corpus luteum functions, which maintains a high level of estrogen and progesterone in the blood and prepares the endometrium for implantation.

Subsequently, the development of the corpus luteum is stimulated by chorionic gonadotropin (only under the condition of fertilization). If fertilization does not occur, then the corpus luteum undergoes involution, after which the levels of progesterone and estrogen in the blood decrease significantly.

The menstrual corpus luteum functions until the completion of the cycle before implantation. The maximum level of progesterone is observed 8 - 10 days after ovulation, which approximately corresponds to the time of implantation.

Under the condition of fertilization and implantation, the further development of the corpus luteum occurs under the stimulating effect of chorionic gonadotropin, which is produced in the trophoblast, resulting in the formation of the corpus luteum of pregnancy.

During pregnancy, trophoblast cells secrete chorionic gonadotropin, which through LH receptors stimulates the growth of the corpus luteum. It reaches a size of 5 cm and stimulates the synthesis of estrogens.

A high level of progesterone, which is formed in the corpus luteum, and estrogen allows you to keep the pregnancy.

In addition to progesterone, cells of the corpus luteum synthesize relaxin, a hormone of the insulin family, which reduces the tone of the myometrium and reduces the density of the pubic articulation, which are also very important factors for maintaining pregnancy.

The corpus luteum of pregnancy functions most actively in the first and early second trimesters, then its function gradually fades away, and the synthesis of progesterone begins to be carried out by the formed placenta. After degeneration of the corpus luteum, a connective tissue scar, called the white body, is formed in its original place.

Hormonal regulation of the ovarian-menstrual cycle The ovarian-menstrual cycle is regulated by the pituitary hormones - follicle-stimulating hormone and luteinizing hormone. The regulation of the synthesis of these hormones is under the influence of releasing factors of the hypothalamus. Ovarian hormones - estrogens, progesterone, inhibin - affect the synthesis of hormones of the hypothalamus and pituitary gland according to the feedback principle.

Gonadoliberin. The secretion of this hormone is carried out in a pulsating manner: within a few minutes there is an increased secretion of the hormone, which is replaced by several hours of interruptions with low secretory activity (usually the interval between secretion peaks is 1-4 hours). The regulation of GnRH secretion is under the control of estrogen and progesterone levels.

At the end of each ovarian-menstrual cycle, there is an involution of the corpus luteum of the ovary. Accordingly, the concentration of estrogen and progesterone decreases significantly. According to the feedback principle, a decrease in the concentration of these hormones stimulates the activity of neurosecretory cells of the hypothalamus, which leads to the release of GnRH with peaks lasting several minutes and with intervals between them of about 1 hour.

Initially, the hormone is secreted from the pool stored in neurosecretory cell granules, and then immediately after secretion. The active mode of GnRH secretion activates the gonadotropic cells of the adenohypophysis.

In the luteal stage of the ovarian-menstrual cycle, the corpus luteum is actively functioning. There is a constant synthesis of progesterone and estrogens, the concentration of which in the blood is significant. In this case, the interval between the peak of secretory activity of the hypothalamus increases to 2-4 hours. Such secretion is insufficient for the activation of gonadotropic hormones of the adenohypophysis.

Follitropin. The secretion of this hormone is carried out in the follicular stage, at the very beginning of the ovarian-menstrual cycle, against the background of a reduced concentration of estrogens and progesterone in the blood. Stimulation of secretion is carried out under the influence of gonadoliberin. Estrogens, the peak of which is observed a day before ovulation, and inhibin suppress the secretion of follicle-stimulating hormone.

Follitropin has an effect on follicular cells. Estradiol and follicle-stimulating hormone increase the number of receptors on the membranes of granulose cells, which enhances the effect of follitropin on follicular cells.

Follitropin has a stimulating effect on the follicles, causing their growth. The hormone also activates aromatase and estrogen secretion.

Lutropin. The secretion of lutropin occurs at the end of the follicular stage of the cycle. Against the background of a high concentration of estrogens, the release of follitropin is blocked and the secretion of lutropin is stimulated. The highest concentration of lutropin is observed 12 hours before ovulation. A decrease in the concentration of lutropin is observed during the secretion of progesterone by granulose cells.

Lutropin interacts with specific receptors located on the membranes of theca and granulose cells, while luteinization of follicular cells and theca cells occurs.

The main action of lutropin is the stimulation of androgen synthesis in theca cells and the induction of progesterone by granulose cells, as well as the activation of proteolytic enzymes of granulose cells. At the peak of lutropin, the first meiotic division is completed.

Estrogen and progesterone. Estrogens are secreted by granulosa cells. Secretion gradually increases in the follicular stage of the cycle and reaches a peak one day before ovulation.

The production of progesterone begins in the granulosa cells before ovulation, and the main source of progesterone is the corpus luteum of the ovary. The synthesis of estrogen and progesterone is greatly enhanced during the luteal stage of the cycle.

Sex hormones (estrogens) interact with specific receptors located on the membranes of neurosecretory cells of the hypothalamus, gonadotrophic cells of the adenohypophysis, ovarian follicular cells, alveolar cells of the mammary glands, mucous membranes of the uterus, fallopian tubes and vagina.

Estrogens and progesterone have a regulatory effect on the synthesis of GnRH. With a simultaneously high concentration of estrogen and progesterone in the blood, the peaks of secretion of gonadotropic hormones increase to 3-4 hours, and at their low concentration they decrease to 1 hour.

Estrogens control the proliferative phase of the menstrual cycle - they contribute to the restoration of the functionally active epithelium of the uterus (endometrium). Progesterone controls the secretory phase - it prepares the endometrium for implantation of a fertilized egg.

A simultaneous decrease in the concentration of progesterone and estrogens in the blood leads to rejection of the functional layer of the endometrium, the development of uterine bleeding - the menstrual phase of the cycle.

Under the influence of estrogens, progesterone, prolactin, as well as chorionic somatomammotropin, the differentiation of secretory cells of the mammary gland is stimulated.

The structure and function of the fallopian tubes

In the wall of the fallopian tube (oviduct), three membranes can be distinguished - internal mucosa, middle muscular and external serous. There is no mucous membrane in the intrauterine section of the tube.

The mucous membrane of the fallopian tube surrounds its lumen. It forms a huge number of branching folds. The epithelium of the mucous membrane is represented by a single layer of cylindrical cells, among which ciliated and secretory cells are distinguished. The lamina propria of the mucosa consists of loose fibrous unformed connective tissue, rich in blood vessels.

Secretory cells of the mucous membrane have a pronounced granular endoplasmic reticulum and the Golgi complex. In the apical part of such cells there is a significant amount of secretory granules. Cells are more active during the secretory stage of the ovarian-menstrual cycle and carry out mucus production. The direction of mucus movement is from the fallopian tube to the uterine cavity, which contributes to the movement of a fertilized egg.

Ciliated cells have cilia on their apical surface that move towards the uterus. These cilia help move the fertilized egg from the distal fallopian tube, where fertilization occurs, to the uterine cavity.

The muscular membrane of the fallopian tube is represented by two layers of smooth muscles - the outer circular and the inner longitudinal. Between the layers is a layer of connective tissue, which has a large number of blood vessels. The contraction of smooth muscle cells also promotes the movement of the fertilized egg.

The serous membrane covers the surface of the fallopian tube facing the abdominal cavity.

Uterus

The wall of the uterus consists of three layers - mucous, muscular and serous.

The mucous membrane of the uterus (endometrium) is formed by a single-layer cylindrical epithelium, which lies on its own plate of the mucosa, represented by loose fibrous unformed connective tissue. Epithelial cells can be divided into secretory and ciliated. In the lamina propria of the mucous membrane there are uterine glands (crypts) - long curved simple tubular glands that open into the lumen of the uterus.

The muscular layer (myometrium) consists of three layers of smooth muscle tissue. The outer layer is represented by longitudinal fibers, the middle layer is circular, and the inner layer is also longitudinal. The middle layer contains a large number of blood vessels. During pregnancy, the thickness of the muscle membrane increases significantly, as well as the size of smooth muscle fibers.

Outside, the uterus is covered with a serous membrane, represented by connective tissue.

The structure of the cervix. The cervix is ​​the lower segment of the organ, partly protruding into the vagina. Allocate the supravaginal and vaginal parts of the cervix. The supravaginal part of the cervix is ​​located above the place of attachment of the walls of the vagina and opens into the lumen of the uterus with the internal uterine os. The vaginal part of the cervix opens with the external uterine os. Outside, the vaginal part of the cervix is ​​covered with stratified squamous epithelium. This epithelium is completely renewed every 4 to 5 days by desquamation of the superficial and proliferation of basal cells.

The cervix is ​​a narrow canal, slightly expanding in the middle part.

The wall of the cervix consists of dense connective tissue, among the collagen and elastic fibers of which there are separate smooth muscle elements.

The mucous membrane of the cervical canal is represented by a single-layer cylindrical epithelium, which in the area of ​​​​the external pharynx passes into a stratified squamous epithelium, and its own layer. In the epithelium, glandular cells that produce mucus and cells that have cilia are distinguished. In the lamina propria there are numerous branched tubular glands that open into the lumen of the cervical canal.

There are no spiral arteries in the own layer of the mucous membrane of the cervix, therefore, during the menstrual stage of the cycle, the mucous membrane of the cervix is ​​not rejected like the endometrium of the body of the uterus.

Vagina

This is a fibromuscular tube, consisting of three layers - mucous, muscular and adventitious.

The mucosa is represented by stratified squamous epithelium and lamina propria.

The stratified squamous epithelium consists of basal, intermediate, and superficial cells.

Basal cells are germ cells. Due to them, there is a constant renewal of the epithelium and its regeneration. The epithelium undergoes partial keratinization - keratohyalin granules can be found in the surface layers. The growth and maturation of the epithelium is under hormonal control. During menstruation, the epithelium becomes thinner, and during the reproductive period, it increases due to division.

In its own layer of the mucous membrane there are lymphocytes, granular leukocytes, sometimes lymphatic follicles can be found. During menstruation, leukocytes can easily enter the lumen of the vagina.

The muscular coat consists of two layers - the inner circular and the outer longitudinal.

The adventitia is composed of fibrous connective tissue and connects the vagina to the surrounding structures.

The structure of the external genitalia

Large labia

The labia majora are two skin folds located on the sides of the genital slit. From the outside, the labia majora are covered with skin that has sebaceous and sweat glands. There are no hair follicles on the inner surface.

In the thickness of the labia majora there are venous plexuses, fatty tissue and Bartholin's glands of the vestibule. Bartholin's glands are paired formations, have a size no larger than a pea and are located on the border of the anterior and middle thirds of the labia.

The glands are tubular-alveolar structures that open into the vestibule of the vagina. Their secret moisturizes the mucous membrane of the vestibule and the entrance to the vagina during sexual arousal.

Small labia

The labia minora are located medially from the large ones and are normally hidden by the large ones. The labia minora do not have adipose tissue. They are composed of numerous elastic fibers, as well as blood vessels in the form of plexuses. Pigmented skin contains sebaceous and small mucous glands that open into the vestibule of the vagina.

Clitoris

The clitoris is analogous to the dorsal surface of the male penis. It consists of two cavernous bodies that form the head at the distal end of the clitoris. The clitoris has a mucous membrane outside, consisting of a stratified squamous epithelium with weak keratinization (no hair, sebaceous and sweat glands). The skin contains numerous free and encapsulated nerve endings.

Menstrual cycle

Cyclic changes in the lining of the uterus are called the menstrual cycle.

During each cycle, the endometrium goes through menstrual, proliferative, and secretory phases. The endometrium is divided into functional and basal layers. The basal layer of the endometrium is supplied with blood from the rectus arteries and is preserved in the menstrual phase of the cycle. The functional layer of the endometrium, which is shed during menstruation, is supplied with blood from the spiral arteries that sclerosis during the menstrual phase, resulting in ischemia of the functional layer.

After menstruation and rejection of the functional layer of the endometrium, a proliferative phase develops, which lasts until ovulation. At this time, there is an active growth of the follicle and at the same time, under the influence of estrogens, the proliferation of cells of the basal layer of the endometrium. The epithelial cells of the glands of the basal layer migrate to the surface, proliferate and form a new epithelial lining of the mucosa. New uterine glands are formed in the endometrium, new spiral arteries grow from the basal layer.

After ovulation and until the onset of menstruation, the secretory phase lasts, depending on the total length of the cycle, it can vary from 12 to 16 days. In this phase, the corpus luteum functions in the ovary, which produces progesterone and estrogens.

Due to the high level of progesterone, favorable conditions are created for implantation.

In this stage, the uterine glands expand, they become tortuous. Glandular cells stop dividing, hypertrophy and begin to secrete glycogen, glycoproteins, lipids and mucin. This secret rises to the mouth of the uterine glands and is released into the lumen of the uterus.

In the secretory phase, the spiral arteries become more tortuous and approach the mucosal surface.

The number of connective tissue cells increases in the surface of the compact layer, and glycogen and lipids accumulate in the cytoplasm. Collagen and reticular fibers are formed around the cells, which are formed by collagen types I and III.

The stromal cells acquire the features of placental decidual cells.

Thus, two zones are created in the endometrium - compact, facing the lumen of the uterine cavity, and spongy - deeper.

The menstrual phase of the ovarian-menstrual cycle is the rejection of the functional layer of the endometrium, which is accompanied by uterine bleeding.

If fertilization and implantation occur, then the menstrual corpus luteum undergoes involution, and the level of ovarian hormones - progesterone and estrogens - increases significantly in the blood. This leads to twisting, sclerosis and a decrease in the lumen of the spiral arteries that supply blood to two-thirds of the functional layer of the endometrium. As a result of these changes, a change occurs - a deterioration in the blood supply to the functional layer of the endometrium. During menstruation, the functional layer is completely rejected, and the basal layer is preserved.

The duration of the ovarian-menstrual cycle is about 28 days, but it is subject to significant variations. The duration of menstruation is from 3 to 7 days.

Changes in the vagina during the ovarian-menstrual cycle.

During the onset of the follicular stage, the vaginal epithelium is thin and pale. Under the influence of estrogens, the proliferation of the epithelium occurs, which reaches its maximum thickness. At the same time, a significant amount of glycogen used by the vaginal microflora accumulates in the cells. The resulting lactic acid prevents the development of pathogenic microorganisms. The epithelium shows signs of keratinization.

In the luteal stage, the growth and maturation of epithelial cells is blocked. Leukocytes and horny scales appear on the surface of the epithelium.

The structure of the mammary gland

The mammary gland is a derivative of the epidermis and belongs to the skin glands. The development of the gland depends on sex - on the type of sex hormones.

In prenatal development, milk lines are laid - epidermal ridges that lie on both sides of the body from the armpit to the groin.

In the midthoracic region, the epithelial cords of the ridges grow into the skin itself and subsequently differentiate into complex tubular alveolar glands.

The histological structure of the mammary gland depends on the degree of its maturity. There are cardinal differences between the juvenile mammary gland, mature inactive and active glands.

The juvenile mammary gland is represented by interlobular and intralobular ducts separated by connective tissue septa. There are no secretory sections in the juvenile gland.

A mature inactive gland is formed during puberty. Under the influence of estrogens, its volume increases significantly. The excretory ducts become more branched, and adipose tissue accumulates among the connective tissue bridges. Secretory departments are absent.

The lactating gland is formed under the influence of progesterone in combination with estrogens, prolactin and chorionic somatomammotropin. Under the action of these hormones, the differentiation of the secretory sections of the mammary gland is induced.

At the 3rd month of pregnancy, kidneys are formed from the growing terminal sections of the intralobular ducts, which differentiate into secretory sections - alveoli. They are lined with cuboidal, secretory epithelium. Outside, the wall of the alveoli and excretory ducts is surrounded by numerous myoepithelial cells. The intralobular ducts are lined with single-layered cuboidal epithelium, which in the milk ducts becomes stratified squamous.

In the lactating gland, the connective tissue septa that separate the lobules of the mammary gland are less pronounced compared to the juvenile and functionally inactive glands.

Secretion and excretion of milk is carried out in the glands under the influence of prolactin. The greatest secretion is carried out in the morning hours (from 2 to 5 am). Under the influence of prolactin in the membranes of alveolar cells, the density of receptors for both prolactin and estrogens increases.

During pregnancy, the concentration of estrogen is high, which blocks the action of prolactin. After the birth of a child, the level of estrogen in the blood decreases significantly, and then prolactin increases, which allows it to induce milk secretion.

In the first 2-3 days after birth, the mammary gland secretes colostrum. The composition of colostrum differs from milk. It has more protein, but less carbohydrates and fats. In colostrum, cell fragments can be found, and sometimes whole cells containing nuclei - colostrum bodies.

During active lactation, alveolar cells secrete fats, casein, lactoferrin, serum albumin, lysozyme, and lactose. Milk also contains fat and water, salts and class A immunoglobulins.

The secretion of milk is carried out according to the apocrine type. The main components of milk are isolated by exocytosis. The only exceptions are fats, which are released by a section of the cell membrane.

The hormones that regulate lactation include prolactin and oxytocin.

Prolactin maintains lactation during breastfeeding. The maximum secretion of prolactin is carried out at night - from 2 to 5 in the morning. The secretion of prolactin is also stimulated by the sucking of the breast by the child, while within half an hour the concentration of the hormone in the blood increases sharply, after which the active secretion of milk by the alveolar cells for the next feeding begins. Against the background of lactation, the secretion of gonadotropic hormones is suppressed. This is due to an increase in the level of endorphins, which block the release of GnRH by neurosecretory cells of the hypothalamus.

Oxytocin is a hormone from the posterior pituitary gland that stimulates the contraction of myoepithelial cells, which promotes the movement of milk in the ducts of the gland.

Topic 27. ORGANIZATION OF VIEW

Sense organs are organs that perceive information from the environment, after which it is analyzed and human actions are corrected.

The sense organs form sensory systems. The sensory system consists of three sections:

1) receptors. These are the peripheral nerve endings of the afferent nerves that receive information from the environment. Receptors include, for example, rods and cones in the organ of vision, neurosensory cells of the organ of Corti - in the organ of hearing, taste buds and buds of the tongue - in the organ of taste.

2) a pathway that includes the afferent processes of the neuron, along which the electrical impulse generated as a result of receptor stimulation is transmitted to the third section.

3) the cortical center of the analyzer.

Organ of vision

The organ of vision, like any analyzer, consists of three departments:

1) the eyeball, in which receptors are located - rods and cones;

2) conducting apparatus - the 2nd pair of cranial nerves - the optic nerve;

3) the cortical center of the analyzer, located in the occipital lobe of the cerebral cortex.

Development of the organ of vision

The rudiment of the eye appears in a 22-day-old embryo in the form of paired shallow intussusceptions - ophthalmic grooves in the forebrain. After the closure of the neuropores, the intussusceptions enlarge and optic vesicles form. Cells that are involved in the formation of the sclera and ciliary muscle are evicted from the neural crest, and also differentiate into endothelial cells and corneal fibroblasts.

The eye vesicles are connected to the fetal brain by means of eye stalks. The eye vesicles come into contact with the ectoderm of the future facial part of the head and induce the development of the lens in it. Invagination of the optic vesicle wall leads to the formation of a two-layer optic cup.

The outer layer of the eyecup forms the pigment layer of the retina. The inner layer forms the retina. The axons of differentiating ganglion cells grow into the optic stalk, after which they become part of the optic nerve.

The choroid is formed from the mesenchymal cells surrounding the eye cup.

The corneal epithelium develops from the ectoderm.

The lens placode separates from the ectoderm and forms a lens vesicle, over which the ectoderm closes. With the development of the lens vesicle, the thickness of its walls changes, in connection with which a thin anterior epithelium and a complex of densely packed elongated spindle-shaped epithelial cells appear - lens fibers located on the posterior surface.

The lens fibers elongate and fill the cavity of the vesicle. In the epithelial cells of the lens, proteins special for the lens are synthesized - crystallins. At the initial stages of lens differentiation, a small amount of alpha and beta crystallins are synthesized. As the lens develops, in addition to these two proteins, gamma crystallins begin to be synthesized.

The structure of the eyeball

The wall of the eyeball consists of three shells - outer - fibrous shell (in the back surface it is an opaque sclera, which in front of the eyeball passes into a transparent cornea), middle shell - vascular, inner shell - retina.

The structure of the cornea

The cornea is the anterior wall of the eyeball, transparent. Posteriorly, the transparent cornea passes into the opaque sclera. The boundary of their transition into each other is called the limb. On the surface of the cornea is a film consisting of the secret of the lacrimal and mucous glands, which includes lysozyme, lactoferrin and immunoglobulins. The surface of the cornea is covered with stratified squamous nonkeratinized epithelium.

The anterior limiting membrane (or Bowman's membrane) is a layer having a thickness of 10 to 16 microns, not containing cells. The anterior limiting membrane consists of the ground substance, as well as thin collagen and reticular fibers that take part in maintaining the shape of the cornea.

The corneal proper substance consists of regularly arranged collagen plates, flattened fibroblasts embedded in a matrix of complex sugars, including keratin and chondroetin sulfate.

The posterior limiting membrane (or Descement's membrane) is a transparent layer of the cornea, it is located between the corneal own substance and the endothelium of the posterior surface of the cornea. This layer consists of collagen fibers of the seventh type and an amorphous substance. The corneal endothelium limits the anterior chamber of the eye in front.

The structure of the sclera

The sclera is the opaque outer layer of the eyeball. The sclera consists of dense strands of collagen fibers, between which are flattened fibroblasts. At the junction of the sclera and cornea, there are small, communicating with each other cavities, which together form the Schlemm canal (or venous sinus) of the sclera, which ensures the outflow of intraocular fluid from the anterior chamber of the eye.

The sclera of an adult has a fairly high resistance to increased intraocular pressure. However, there are separate areas of thinning of the sclera, especially in the limbus.

In children, the sclera is poorly resistant to stretching, therefore, with an increase in intraocular pressure, the size of the eyeball increases significantly.

The thinnest place of the sclera is the region of the ethmoid sinus. The bundles of optic nerve fibers pass through the opening of the cribriform plate. The optic nerve fibers pass through holes in the lamina cribrosa.

The structure of the choroid

The main function of the choroid is to nourish the retina.

The choroid consists of several layers - supravascular, choriocapillary and basal plates.

The supravascular membrane is located on the border with the sclera and consists of loose fibrous connective tissue with numerous pigment cells.

The choroid plate contains plexuses of arteries and veins, consists of loose connective tissue, in which pigment cells and smooth muscle fibers are located.

The choriocapillary plate is formed by a plexus of sinusoidal capillaries.

The basal plate is located on the border of the choroid and the retina. In front of the eye, the choroid forms the iris and the ciliary body.

The structure of the iris

The iris is a continuation of the choroid, located between the cornea and the lens, separating the anterior and posterior chambers of the eye.

The iris consists of several layers - the endothelial (or anterior), vascular outer, and inner boundary layers, as well as the pigment layer.

The endothelium is a continuation of the endothelium of the cornea.

The outer and inner boundary layers have a similar structure, contain fibroblasts, melonocytes, immersed in the ground substance.

The vascular layer is a loose fibrous connective tissue that contains numerous vessels and melanocytes.

The posterior pigment layer passes into the two-layer retinal epithelium, which covers the ciliary body.

The iris contains muscles that constrict and dilate the pupil. When the parasympathetic nerve fibers are irritated, the pupil constricts, and when the sympathetic nerves are irritated, it expands.

The structure of the ciliary body

In the region of the corner of the eye, the choroid thickens, forming the ciliary body.

On the cut, it has the form of a triangle with its base turned into the anterior chamber of the eye.

The ciliary body consists of muscle fibers - the ciliary muscle involved in the regulation of accommodation of the eye. Smooth muscle fibers located in the ciliary muscle run in three mutually perpendicular directions.

Ciliary processes extend from the ciliary body towards the lens of the eye. They contain a mass of capillaries, covered with two layers of epithelium - pigment and ciliary secretory, which produces aqueous humor. The ligament of cinnamon is attached to the ciliary processes. When the ciliary muscle contracts, the zinn ligament relaxes and the lens convexity increases.

The structure of the lens

The lens is a biconvex lens. The anterior surface of the lens is formed by a single layer of cuboidal epithelium, which becomes higher towards the equator. There are slit-like junctions between the epithelial cells of the lens. The lens consists of thin lens fibers that make up its bulk and contain crystallins. Outside, the lens is covered with a capsule - a thick basement membrane with a significant content of reticular fibers.

The chambers of the eye, the movement of intraocular fluid

The eye has two chambers - anterior and posterior. The anterior chamber of the eye is a space bounded in front by the cornea, behind by the iris, and in the pupil area by the central part of the anterior surface of the lens. The depth of the anterior chamber of the eye is greatest in the central part, where it reaches 3 mm. The angle between the posterior surface of the peripheral part of the cornea and the anterior surface of the root of the iris is called the angle of the anterior chamber of the eye. It is located in the transition region of the sclera to the cornea, as well as the iris - to the ciliary body.

The posterior chamber of the eye is the space behind the iris, bounded by the lens, ciliary and vitreous body.

Intraocular fluid is formed in the posterior chamber of the eye from the capillaries and epithelium of the ciliary processes. From the posterior chamber of the eye between the iris and the lens, it passes into the anterior chamber. In terms of composition, intraocular fluid consists of blood plasma proteins, depolymerized hyaluronic acid, is hypertonic in relation to blood plasma and does not contain fibrinogen.

From the elements of the iris, cornea and vitreous body, a trabecula is formed, which forms the posterior wall of the Schlemm's canal. It is extremely important for the outflow of moisture from the anterior chamber of the eye. From the trabecular meshwork, moisture flows into the canal of Schlemm and is then absorbed into the venous vessels of the eye.

The balance between the formation and absorption of aqueous humor forms and determines the amount of intraocular pressure.

A hematotissue barrier is formed between the blood and tissues of the eye. The cells of the ciliary epithelium are tightly interconnected by strong contacts and do not allow macromolecules to pass through.

The structure of the vitreous body

Between the lens and the retina there is a cavity filled with one of the transparent media of the eye - the vitreous body. According to its structure, the vitreous body is a gel consisting of water, collagen, the second, ninth and eleventh types, vitrein protein and hyaluronic acid.

The vitreous body is enclosed in a vitreous membrane, which is an accumulation of collagen fibers that form the vitreous capsule.

A canal passes through the vitreous body in the direction from the lens to the retina - the remnant of the embryonic system of the eye.

Structure, functions of the retina

The retina (or retina) is the inner lining of the eye. It consists of two sections - the visual, where the photoreceptors are located, and the blind. At the posterior edge of the optical axis of the eye, the retina has a round yellow spot about 2 mm in diameter. The central fovea of ​​the retina is located in the middle part of the macula. This is the place of the best perception of the image by the eye. The optic nerve exits the retina medially to the macula, forming the optic papilla. There are no photoreceptors at the point of exit of the optic nerve in the retina, the perception of the image in this place of the retina does not occur, therefore it is called the blind spot.

In the center of the optic nerve head, you can see a recess in which the retinal supply vessels exiting the optic nerve can be seen.

The pigment layer of the retina is the outermost, facing the vitreous body, contains polygonal cells adjacent to the choroid.

One cell of the pigment epithelium interacts with the outer segments of a dozen photoreceptor cells - rods and cones. The cells of the pigment epithelium contain reserves of vitamin A, participate in its transformations and transfer its derivatives to photoreceptors for the formation of visual pigment.

The outer nuclear layer includes the nucleated parts of the photoreceptor cells. Cones are most concentrated in the area of ​​the macula and provide color vision. In this case, the eyeball is arranged in such a way that the central part of the light displayed from any object falls on the cones.

Along the periphery of the retina are rods, the main function of which is the perception of signals in twilight lighting.

The outer reticular layer is the point of contact between the inner segments of rods and cones and the processes of bipolar cells.

inner nuclear layer. The bodies of bipolar cells are located in this layer. Bipolar cells have two processes. With the help of one - short - they communicate between the bodies and photoreceptors, and with the help of long ones - with ganglion cells. Thus, bipolar cells are the link between photoreceptors and ganglion cells.

This layer also contains horizontal and amacrine cells.

The inner reticular layer is the layer in which the processes of bipolar and ganglion cells contact, while amacrine cells act as intercalary neurons. It is currently believed that one type of bipolar cell transmits information to 16 types of ganglion cells with the participation of 20 types of amacrine cells.

The ganglion layer contains ganglion cell bodies.

It has been established that many photoreceptor cells transmit a signal to one bipolar cell, and several bipolar cells to one ganglion cell, i.e., the number of cells in the layers of the retina gradually decreases, and the amount of information received by one cell increases.

The photoreceptors in the retina include rods and cones.

It has been established that cones are predominantly located in the region of the macula and the central fossa of the retina. In this case, one cone makes one connection with one bipolar cell, which ensures the reliability of the transmission of the visual signal.

The photoreceptors contain the visual pigment. In rods it is rhodopsin, and in cones it is red, green and blue pigments.

Photoreceptors have outer and inner segments.

The outer segment contains the visual pigment and faces the choroid.

The inner segment is filled with mitochondria and contains a basal body, from which 9 pairs of microtubules extend into the outer segment.

The main function of the cones is the perception of color, while there are three types of visual pigment, the main function of the rods is the perception of the shape of an object.

The theory of color vision was proposed in 1802 by Thomas Young. At the same time, color vision in humans in this theory was explained by the presence of three types of visual pigment. This ability to distinguish any color, determined by the presence of three types of cones in the retina, is called trichromasia.

In humans, defects in color perception are possible, dichromasia from colors is not perceived by the photoreceptors of the retina.

The structure of retinal neurons and glial cells

Retinal neurons synthesize acetylcholine, dopamine, glycine, α-aminobutyric acid. Some neurons contain serotonin and its analogues.

The layers of the retina contain horizontal and amacrine cells.

Horizontal cells are located in the outer part of the inner nuclear layer, and the processes of these cells enter the area of ​​synapses between photoreceptors and bipolar cells. Horizontal cells receive information from the cones and pass it on to the cones as well. Neighboring horizontal cells are interconnected by slot-like junctions.

Amacrine cells are located in the inner part of the inner nuclear layer, in the area of ​​synapses between bipolar and ganglion cells, while amacrine cells function as intercalary neurons.

Bipolar cells respond to image contrast. Some of these cells respond more strongly to color than to black and white contrast. Some bipolar cells receive information mainly from rods, while others, on the contrary, receive information mainly from cones.

In addition to neurons, the retina also contains large cells of radial glia - Müller cells.

Their nuclei are located at the level of the central part of the inner nuclear layer.

The outer processes of these cells end in villi, thus forming a boundary layer.

The internal processes have an extension (or stalk) in the inner boundary layer at the border with the vitreous body. Glial cells play an important role in the regulation of retinal ion homeostasis. They reduce the concentration of potassium ions in the extracellular space, where their concentration significantly increases when irritated by light. The plasma membrane of the Müllerian cell in the region of the stem is characterized by high permeability to potassium ions leaving the cell. The Müllerian cell captures potassium from the outer layers of the retina and directs the flow of these ions through its stalk into the vitreous fluid.

Mechanism of photoperception

When a light quantum hits the outer segments of photoreceptor cells, the following reactions sequentially occur: activation of rhodopsin and photoisomerization, catalytic reaction of G-protein by rhodopsin, activation of phosphodiesterase upon binding to a protein, hydrolysis of cGMP, transition of cGMP-dependent sodium channels from an open state to a closed state, as a result resulting in hyperpolarization of the plasmolemma of the photoreceptor cell and signal transmission to bipolar cells. An increase in the activity of cGMP-phosphodiestrase reduces the concentration of cGMP, which leads to the closure of ion channels and hyperpolarization of the plasmolemma of the photoreceptor cell. This serves as a signal for a change in the nature of transmitter secretion in the synapse between the inner segment of the receptor cell and the dendrite of the bipolar cell. In the dark, ion channels in the cell membrane of receptor cells are maintained open by binding of ion channel proteins to cyclic GMP. The ducts of sodium and calcium ions into the cell through open channels provide a dark current.

The structure of the lacrimal gland

The lacrimal gland is an auxiliary apparatus of the eye. The gland is surrounded by a group of complex tubular-alveolar glands, the secretory sections are surrounded by myoepithelial cells. The secret of the gland (tear fluid) through 6-12 ducts enters the fornix of the conjunctiva. From the lacrimal sac through the nasolacrimal canal, the lacrimal fluid enters the lower nasal passage.

Topic 28. ORGANS OF TASTE AND SMELL

The olfactory analyzer consists, like any other, of the central and peripheral sections.

The peripheral part of the olfactory analyzer is represented by the olfactory field - the olfactory lining, which is located on the middle part of the superior nasal concha and the corresponding section of the mucous membrane of the nasal septum.

The olfactory epithelium contains receptor cells. Their central processes - axons - transmit information to the olfactory bulb. The olfactory receptors are the first neuron of the olfactory pathway and are surrounded by supporting cells.

The body of the olfactory cell contains numerous mitochondria, cisterns of the endoplasmic reticulum with ribosomes, elements of the Golgi complex, and lysosomes. Olfactory cells, in addition to the central one, also have a short peripheral process - a dendrite, ending on the surface of the olfactory epithelium with a spherical thickening - an olfactory club with a diameter of 1 - 2 mm. It contains mitochondria, small vacuoles and basal bodies, several olfactory hairs up to 10 mm long extending from the top of the club, having the structure of typical cilia.

The subepithelial connective tissue contains the terminal sections of the Bowman glands, blood vessels, and bundles of unmyelinated nerve fibers of the olfactory nerve. The mucus that is secreted by the Bowman glands covers the surface of the olfactory lining.

Olfactory cilia immersed in mucus are involved in the process of chemosensing.

The olfactory nerve is a collection of thin olfactory filaments that pass through a hole in the ethmoid bone into the brain to the olfactory bulbs. In addition to non-myelinated fibers, separate myelinated fibers of the trigeminal nerve pass through the connective tissue layer of the olfactory lining.

The receptor cells of the olfactory lining register 25 - 35 odors.

Their combinations form many millions of perceived odors. Olfactory receptor neurons depolarize in response to adequate stimulation. The cAMP-dependent gate ion channels are built into the plasmolemma of the olfactory cilia, which open when interacting with cAMP.

cAMP-dependent gate channels are activated as a result of a sequence of events - interaction with the receptor protein in the olfactory cilia plasmolemma, G-protein activation, increased adenylate cyclase activity, and increased cAMP levels.

The inositol triphosphate system is also related to the mechanism of chemosensing in the olfactory organ. Under the action of certain odorous substances, the level of inositol triphosphate rapidly increases, which interacts with calcium channels in the plasmolemma of olfactory receptor neurons. Thus, the cAMP and inositol triphosphate second messenger systems interact with each other, providing a better perception of various odors.

Through cAMP-dependent gate ion channels, not only monovalent cations pass into the cell, but also calcium ions, which binds to calmodulin. The resulting calcium-calmodulin complex interacts with the channel, which prevents cAMP activation, as a result of which the receptor cell becomes insensitive to the action of odorous irritants.

The lifespan of olfactory cells is about 30 - 35 days. Olfactory receptors are an exception among all other neurons; they are updated by precursor cells - the basal cells of the epithelium of the olfactory lining.

Support cells. Among them, tall cylindrical and smaller cells that do not reach the surface of the receptor layer are distinguished. Cylindrical cells on the apical surface contain microvilli 3–5 µm long. In addition to well-developed organelles of general importance, supporting cells in the apical part contain many secretory granules.

The taste analyzer, as well as the olfactory one, consists of a central and peripheral sections. The peripheral part of the taste analyzer is represented by taste buds, which are found in the epithelium of the oral cavity, anterior pharynx, esophagus, and larynx. Their main localization is the chemosensitive papillae of the tongue (mushroom-shaped, trough-shaped and foliate). In children, taste buds are also found in the epithelium of the mucous membrane of the lips, epiglottis, and vocal cords.

The taste bud is elliptical in shape, 27-115 µm high and 16-70 µm wide. In their apical region there is a taste canal filled with an amorphous substance, which opens on the surface of the epithelium with a taste pore.

The kidney is formed by 30 - 80 elongated cells, closely adjacent to one another. Most of these cells come into contact with nerve fibers penetrating the kidney from the subepithelial nerve plexus, which contains myelinated and unmyelinated nerve fibers. All cell types of the taste bud form afferent synapses with nerve terminals.

The development of the taste buds of the tongue proceeds in parallel with the germination of nerve fibers in the epithelium. The differentiation of the kidneys begins simultaneously with the appearance of clusters of unmyelinated nerve fibers directly under the location of the future kidney.

Taste bud cells are morphologically heterogeneous. There are four types of cells.

Type I cells in the apical part have up to 40 microvilli protruding into the cavity of the taste canal. The apical part of the cells contains a large number of electron-dense granules. The cytoskeleton is represented by well-defined bundles of microfilaments and microtubules. Some of these structures form a compact bundle, the narrowed end of which is connected to a pair of centrioles. The Golgi complex, which is related to the formation of electron-dense granules, is located above the nucleus. In the basal part of the cell there are small dense mitochondria. A well-developed granular endoplasmic reticulum is concentrated in the same area.

Type II cells have lighter colored cytoplasm. Along with vacuoles varying in size, it contains expanded cisterns of a smooth endoplasmic reticulum. The apical part of the cell contains sparse and small microvilli. There are multivesicular bodies, lysosomes.

Type III cells contain low microvilli, centrioles, and a small amount of vesicles up to 120 nm in diameter in the apical part. The granular endoplasmic reticulum is poorly developed. Numerous flattened cisterns and vesicles form a well-defined smooth endoplasmic reticulum. A characteristic feature of cells is the presence in the cytoplasm of granular vesicles with a diameter of 80 - 150 nm, as well as light vesicles with a diameter of 30 - 60 nm. These vesicles, primarily light ones, are related to afferent synapses. Granular vesicles are located in other parts of the cell, but are always present in the area of ​​synapses.

Type IV cells are located in the basal part of the taste bud and do not reach the taste duct. They contain a large nucleus and bundles of microfilaments. The function of these cells remains unclear. It is possible that type IV cells are precursors for all types of taste bud cells.

chemoreceptor cells. Although contacts with afferent fibers form all types of cells, the function of chemosensing is associated mainly with type III cells. In the presynaptic region of taste cells, granular vesicles contain serotonin, a mediator of the afferent synapse. Sweet stimuli activate adenylate cyclase in taste receptor cells, which leads to an increase in cAMP levels. Bitters act through a G-protein called gastducin, which through an increase in phosphodiesterase activity leads to a decrease in cAMP levels.

In the taste bud, there is a constant renewal of cells. From the peripheral region of the taste bud, cells move to its central part at a rate of 0,06 µm/h. The average lifespan of the cells of the taste organ is 250 ± 50 hours. After damage to the nerves that innervate the taste buds, the latter degenerate, and when the nerves regenerate, they are restored. The results of these studies suggest that taste buds are under neurotrophic control.

Topic 29. STRUCTURE OF THE ORGANS OF HEARING AND BALANCE

Development of the organ of hearing and balance

In a 22-day-old embryo at the level of the rhomboid brain, paired thickenings of the ectoderm appear - auditory placodes. By invagination and subsequent separation from the ectoderm, the auditory vesicle is formed. On the medial side, the rudimentary auditory ganglion is adjacent to the auditory vesicle, from which the ganglion of the vestibule and the ganglion of the cochlea subsequently differentiate. As it develops, two parts appear in the auditory vesicle - an elliptical sac (utriculus with semicircular canals) and a spherical sac (sacculus) with the rudiment of the cochlear canal.

The structure of the organ of hearing

The outer ear includes the auricle, the external auditory meatus and the tympanic membrane, which transmits sound vibrations to the auditory ossicles of the middle ear. The auricle is formed by elastic cartilage covered with thin skin. The external auditory meatus is lined with skin containing hair follicles, typical sebaceous glands, and ceruminous glands, modified sebaceous glands that produce earwax. The outer surface of the eardrum is covered with skin. From the inside, from the side of the tympanic cavity, the tympanic membrane is lined with a single-layer cubic epithelium, which is separated from the outer layer by a thin connective tissue plate.

The middle ear contains the auditory ossicles - the hammer, anvil and stirrup, which transmit vibrations from the tympanic membrane to the membrane of the oval window. The tympanic cavity is lined with stratified epithelium, which turns into a single-layer cylindrical ciliated at the opening of the auditory tube. Between the epithelium and the bone is a layer of dense fibrous connective tissue. The bone of the medial wall of the tympanic cavity has two windows - oval and round, which separate the tympanic cavity from the bony labyrinth of the inner ear.

The inner ear is formed by the bony labyrinth of the temporal bone, which contains a membranous labyrinth that repeats its relief. Bone labyrinth - a system of semicircular canals and a cavity that communicates with them - the vestibule. The membranous labyrinth is a system of thin-walled connective tissue tubes and sacs located inside the bony labyrinth. In the bone ampullae, the membranous canals expand. In the vestibule, the membranous labyrinth forms two interconnected sacs: the ulus (elliptical sac), into which the membranous canals open, and the sacculus (spherical sac). The membranous semicircular canals and sacs of the vestibule are filled with endolymph and communicate with the cochlea, as well as with the endolymphatic sac located in the cranial cavity, where the endolymph is resorbed. The epithelial lining of the endolymphatic sac contains cylindrical cells with dense cytoplasm and irregularly shaped nuclei, as well as cylindrical cells with light cytoplasm, high microvilli, numerous pinocytic vesicles and vacuoles. Macrophages and neutrophils are present in the lumen of the sac.

The structure of the snail. The cochlea is a spirally twisted bony canal that developed as an outgrowth of the vestibule. The cochlea forms 2,5 whorls about 35 mm long. The basilar (basic) and vestibular membranes located inside the cochlear canal divide its cavity into three parts: the scala tympani, the scala vestibularis, and the membranous cochlear canal (the middle scala or cochlear duct). Endolymph fills the membranous canal of the cochlea, and perilymph fills the vestibular and tympanic scala. The scala tympani and the vestibular scala communicate at the top of the cochlea through an opening (helicotrema). In the membranous canal of the cochlea on the basilar scala there is a receptor apparatus - a spiral (or Corti) organ.

The concentration of K+ in the endolymph is 100 times greater than in the perilymph; Na+ concentration in endolymph is 10 times less than in perilymph.

Perilymph is close in chemical composition to blood plasma and siuid and occupies an intermediate position between them in terms of protein content.

The structure of the organ of Corti. The organ of Corti contains several rows of hair cells associated with the tectorial (integumentary) membrane. There are inner and outer hair and supporting cells.

Hair cells - receptor, form synaptic contacts with peripheral processes of sensory neurons of the spiral ganglion. Internal hair cells form one row, have an expanded base, 30-60 immobile microvilli (stereocilia) passing through the cuticle in the apical part. Stereocilia are located in a semicircle, open towards the external structures of the organ of Corti. Inner hair cells are primary sensory cells that are excited in response to a sound stimulus and transmit excitation to the afferent fibers of the auditory nerve. Displacement of the integumentary membrane causes deformation of stereocilia, in the membrane of which mechanosensitive ion channels open and depolarization occurs. In turn, depolarization promotes the opening of voltage-sensitive Ca²+ and K+ channels embedded in the basolateral membrane of the hair cell. The resulting increase in the concentration of Ca in the cytosol²+ initiates secretion (most likely glutamate) from synaptic vesicles with its subsequent action on the postsynaptic membrane as part of the afferent terminals of the auditory nerve.

The outer hair cells are arranged in 3-5 rows, have a cylindrical shape and stereocilia. Myosin is distributed along the stereocilia of the fibrous cell.

supporting cells. Supporting cells include inner phalangeal cells, inner pillar cells, outer phalanx cells of Deiters, outer pillar cells, Hensen cells, and Boettcher cells. The phalangeal cells come into contact with the hair cells on the basement membrane. The processes of the outer phalangeal cells run parallel to the outer hair cells, without touching them for a considerable length, and at the level of the apical part of the hair cells come into contact with them. Supporting cells are connected by gap junctions formed by the gap junction membrane protein connexin-26. Gap junctions are involved in restoring the level of K+ in the endolymph during trace reactions after excitation of hair cells.

The way of transmission of auditory irritation

The sound pressure transmission chain is as follows: the tympanic membrane, then the auditory ossicles - the hammer, anvil, stirrup, then - the oval window membrane, the perilymph basilar and tectorial membranes and the round window membrane.

When the stirrup is displaced, the particles of relymph move along the vestibular scala and then through the helicotrema along the scala tympani to the round window.

The fluid shifted by the displacement of the membrane of the foramen ovale creates excess pressure in the vestibular canal. Under the influence of this pressure, the basal part of the main membrane will be mixed towards the scala tympani. An oscillatory reaction in the form of a wave propagates from the basal part of the main membrane to the helicotrema. The displacement of the tectorial membrane relative to the hair cells under the action of sound causes their excitation. The displacement of the membrane relative to the sensory epithelium deflects the stereocilia of the hair cells, which opens mechanosensing channels in the cell membrane and leads to cell depolarization. The resulting electrical reaction, called the microphone effect, follows the shape of the audio signal in its form.

The structure and functioning of the organ of balance

In the ampullar extension of the semicircular canal are cristae (or scallops). The sensitive areas in the sacs are called patches.

The composition of the epithelium of spots and cristae includes sensitive hair and supporting cells. In the epithelium of spots, kinocilia are distributed in a special way. Here the hair cells form groups of several hundred units. Within each group, the kinocilia are oriented in the same way, but the orientation of the groups themselves is different. The epithelium of the spots is covered with an otolithic membrane. Otoliths are crystals of calcium carbonate. The epithelium of the cristae is surrounded by a gelatinous transparent dome.

Hair cells are present in each ampulla of the semicircular canals and in the maculae of the sacs of the vestibule. There are two types of hair cells. Type I cells are usually located in the center of the scallops, while type II cells are located at the periphery. Cells of both types in the apical part contain 40-110 immobile hairs (stereocilia) and one cilium (kinocilium) located on the periphery of the bundle of stereocilia. The longest stereocilia are located near the kinocilium, while the length of the rest decreases with distance from the kinocilium.

Hair cells are sensitive to the direction of the stimulus (direction sensitivity). When the stimulus is directed from the stereocilium to the kinocilium, the hair cell is excited. With the opposite direction of the stimulus, the response is suppressed. Type I cells are amphora-shaped with a rounded bottom and housed in the goblet cavity of the afferent nerve ending. Efferent fibers form synaptic endings on afferent fibers associated with type I cells. Type II cells have the form of cylinders with a rounded base. A characteristic feature of these cells is their innervation: the nerve endings here can be both afferent (most) and efferent.

With superthreshold sound stimulation (acoustic trauma) and under the action of certain ototoxic drugs (antibiotics streptomycin, gentamicin), hair cells die. The possibility of their regeneration from progenitor cells of the neurosensory epithelium is of great practical importance; it is considered established for birds and intensively studied in mammals.

The vestibular nerve is formed by processes of bipolar neurons in the vestibular ganglion. The peripheral processes of these neurons approach the hair cells of each semicircular canal, utriculus and sacculus, and the central ones go to the vestibular nuclei of the medulla oblongata.

Topic 30

The organs of hematopoiesis and immunological protection include red bone marrow, thymus gland (thymus), lymph nodes, spleen, as well as lymphatic follicles of the digestive tract (tonsils, lymphatic follicles of the intestine) and other organs. They form a single system with blood.

They are divided into central and peripheral organs of hematopoiesis and immunological protection.

The central organs include the red bone marrow, the thymus gland, and an analogue of the Bag of Fabricius, which is still unknown in mammals. In the red bone marrow, stem cells produce erythrocytes, granulocytes, platelets (platelets), B-lymphocytes, and precursors of T-lymphocytes. In the thymus, T-lymphocyte precursors are converted into T-lymphocytes. In the central organs, antigen-independent reproduction of lymphocytes occurs.

In the peripheral hematopoietic organs (lymph nodes, hemolymph nodes, spleen), T- and B-lymphocytes brought here from the central organs multiply and differentiate under the influence of antigens into effector cells that provide immunological protection. In addition, there is a culling of dying blood cells.

The hematopoietic organs function in a friendly manner and ensure the maintenance of the morphological composition of the blood and immunological homeostasis in the body.

Despite differences in the specialization of hematopoietic organs, they all have similar structural and functional features. They are based on reticular connective, and sometimes epithelial tissue (in the thymus gland), which, together with fibroblasts and macrophages, forms the stroma of organs and plays the role of a specific microenvironment for developing cells. In these organs, the reproduction of hematopoietic cells, the temporary deposition of blood or lymph occurs. Hematopoietic organs, due to the presence in them of special phagocytic and immunocompetent cells, also carry out a protective function and are able to purify the blood or lymph from foreign particles, bacteria and the remains of dead cells.

Bone marrow

The bone marrow is the central hematopoietic organ, where a self-sustaining population of stem cells is located, where both myeloid and lymphoid cells are formed.

Structure. In the adult human body, red and yellow bone marrow are distinguished.

Red bone marrow is the hematopoietic part of the bone marrow. It fills the spongy substance of flat bones and epiphyses of tubular bones, and in an adult organism it averages about 4-5% of the total body weight. Red bone marrow is dark red in color and has a semi-liquid consistency, making it easy to prepare thin smears on glass.

The reticular tissue of the structural basis of the bone marrow has a low proliferative activity. The stroma is penetrated by many blood vessels of the microvasculature, between which are located hematopoietic cells: stem cells, semi-stem cells (morphologically unidentifiable), various stages of maturation of erythroblasts and myelocytes, megakaryoblasts, megakaryocytes, lymphoblasts, B-lymphocytes, macrophages and mature blood cells. Lymphocytes and macrophages take part in the protective reactions of the body. The most intense hematopoiesis occurs near the endosteum, where the concentration of stem hematopoietic cells is approximately 3 times greater than in the center of the bone marrow cavity.

Hematopoietic cells are arranged in islets. In the process of maturation, erythroblasts surround a macrophage containing iron of phagocytosed erythrocytes, and receive from it a molecule of this metal to build the heme part of hemoglobin. Macrophages serve as a kind of feeder for erythroblasts, which are gradually enriched with iron at their expense. Macrophages phagocytize cell debris and defective cells. Immature erythroid cells are surrounded by glycoproteins. As cells mature, the amount of these biopolymers decreases.

Granulocytopoietic cells are also located in the form of islands, but are not associated with macrophages. Immature cells of the granulocytic series are surrounded by protein glycans. In the process of maturation, granulocytes are deposited in the red bone marrow, where they are approximately 3 times more than erythrocytes, and 20 times more than granulocytes in peripheral blood.

Megakaryoblasts and megakaryocytes are located in close contact with the sinuses so that the peripheral part of their cytoplasm penetrates into the lumen of the vessel through the pores. The separation of fragments of the cytoplasm in the form of platelets occurs directly into the bloodstream.

Among the islets of myeloid cells there are small clusters of bone marrow lymphocytes (null lymphocytes, B-lymphocytes) and monocytes, which usually surround the blood vessel in dense rings. Experiments with the transplantation of bone marrow lymphocytes into the spleen of animals irradiated with a lethal dose showed the presence among them of stem, semi-stem and unipotent hematopoietic cells.

During the differentiation of B-lymphocytes, the structural and regulatory genes of immunoglobulins are depressed, the synthesis of immunoglobulins inside the cell and their appearance on the membrane of B-lymphocytes in the form of antigen-recognizing receptors.

Under normal physiological conditions, only mature blood cells penetrate through the wall of the sinuses of the bone marrow. Myelocytes and normoblasts enter the blood only in pathological conditions of the body. The reasons for such selective permeability of the sinus wall remain insufficiently clear, but the fact of the penetration of immature cells into the bloodstream is always a sure sign of a disorder in bone marrow hematopoiesis.

The cells released into the bloodstream perform their functions either in the vessels of the microvasculature (erythrocytes, platelets), or when they enter the connective tissue (lymphocytes, leukocytes) and peripheral lymphoid organs (lymphocytes). In particular, lymphocyte precursors (null lymphocytes) and mature B-lymphocytes migrate to the thymus-independent zones of the spleen, where they are cloned into immunological memory cells and cells that directly differentiate into antibody-producing cells (plasma cells) already during the primary immune response.

Yellow bone marrow in adults is located in the diaphysis of tubular bones. It is a regenerated reticular tissue, the cells of which contain fatty inclusions. Due to the presence of pigments such as lipochromes in fat cells, the bone marrow in the diaphysis has a yellow color, which determines its name. Under normal conditions, the yellow bone marrow does not carry out a hematopoietic function, but in the case of large blood loss or in case of toxic poisoning of the body, foci of myelopoiesis appear in it due to the differentiation of stem and semi-stem cells brought here with blood.

There is no sharp boundary between yellow and red bone marrow. A small number of fat cells are constantly found in the red bone marrow. The ratio of yellow and red bone marrow may vary depending on age, nutritional conditions, nervous, endocrine and other factors.

Vascularization. The bone marrow is supplied with blood through vessels that penetrate through the periosteum into special openings in the compact substance of the bone. Entering the bone marrow, the arteries branch into ascending and descending branches, from which arterioles depart radially, which first pass into narrow capillaries (2–4 microns), and then in the endosteal region continue into wide thin-walled sinusoidal capillaries (or sinuses) with slit-like pores. with a diameter of 10 - 14 microns. Blood is collected from the sinuses into the central venule.

Thymus (or thymus) gland (thymus)

The thymus gland is the central organ of lymphocytopoiesis and immunogenesis. From the bone marrow precursors of T-lymphocytes, antigen-independent differentiation occurs in it into T-lymphocytes, varieties of which carry out cellular immunity reactions and regulate humoral immunity reactions.

The thymus gland is an unpaired organ, not completely divided into lobules, which is based on a process epithelial tissue that has invaginated during development so that the basal layer of the epithelium with the basement membrane faces outward and borders on the surrounding connective tissue, which forms a connective tissue capsule. Partitions extend from it inside, dividing the gland into lobules. In each lobule, a cortex and a medulla are distinguished.

The cortical substance of the lobules is infiltrated with T-lymphocytes, which densely fill the gaps of the reticular epithelial backbone, giving this part of the lobule a characteristic appearance and a dark color on the preparations. In the subcapsular zone of the cortical substance there are large lymphoid cells - lymphoblasts, which, under the influence of hematopoietic factors (thymosin), secreted by stromal epithelial cells, proliferate. These T cell precursors migrate here from the red bone marrow. New generations of lymphocytes appear in the thymus gland every 6-9 hours. T-lymphocytes of the cortical substance migrate into the bloodstream without entering the medulla. These lymphocytes differ in the composition of markers and receptors from the T-lymphocytes of the medulla. With the blood flow, they enter the peripheral organs of lymphocytopoiesis - the lymph nodes and the spleen.

The cells of the cortical substance are in a certain way delimited from the blood by a hematotissue barrier that protects the differentiating lymphocytes of the cortical substance from an excess of antigens. It consists of endothelial cells of hemocapillaries with a basement membrane, a pericapillary space with single lymphocytes, macrophages and intercellular substance, as well as epithelial cells with their basement membrane.

The medulla of the lobule on the preparations has a lighter color, since it contains a smaller number of lymphocytes compared to the cortical substance. The lymphocytes of this zone represent the recirculating pool of T-lymphocytes and can enter and exit the bloodstream through postcapillary venules and lymphatic vessels. A feature of the ultramicroscopic structure of process epithelial cells is the presence in the cytoplasm of grape-shaped vacuoles and intracellular tubules, the surface of which forms microoutgrowths. The basement membrane is reduced.

Vascularization. Inside the organ, the arteries branch into interlobular and intralobular, which form arcuate branches. From them, almost at a right angle, blood capillaries depart, forming a dense network, especially in the cortical zone. The capillaries of the cortical substance are surrounded by a continuous basement membrane and a layer of epithelial cells that delimit the pericapillary space (barrier). In the pericapillary space filled with liquid contents, lymphocytes and macrophages are found. Most of the cortical capillaries pass directly into the subcapsular venules.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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