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).

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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