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Lecture notes, cheat sheets
Histology. General embryology
Directory / Lecture notes, cheat sheets Table of contents (expand) 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. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Morphology and functions of the nucleus. cell reproduction >> Forward: Human Embryology
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