EMBRYOLOGY. Chapter 21. BASICS OF HUMAN EMBRYOLOGY

EMBRYOLOGY. Chapter 21. BASICS OF HUMAN EMBRYOLOGY

Embryology (from the Greek. embryon- the embryo, logos- doctrine) - the science of the laws of development of embryos.

Medical embryology studies the patterns of development of the human embryo. Particular attention is paid to embryonic sources and regular processes of tissue development, metabolic and functional features of the mother-placenta-fetus system, critical periods of human development. All this is of great importance for medical practice.

Knowledge of human embryology is essential for all doctors, especially those working in the field of obstetrics and pediatrics. This helps in making a diagnosis for violations in the mother-fetus system, identifying the causes of deformities and diseases of children after birth.

Currently, knowledge of human embryology is used to disclose and eliminate the causes of infertility, transplantation of fetal organs, development and use of contraceptives. In particular, the problems of egg culture, in vitro fertilization and embryo implantation into the uterus have become topical.

The process of human embryonic development is the result of long evolution and, to a certain extent, reflects the developmental features of other representatives of the animal world. Therefore, some of the early stages of human development are very similar to the analogous stages of embryogenesis in lower-organized chordates.

Human embryogenesis is a part of his ontogenesis, which includes the following main stages: I - fertilization and formation of a zygote; II - crushing and formation of blastula (blastocyst); III - gastrulation - the formation of germ layers and a complex of axial organs; IV - histogenesis and organogenesis of embryonic and extraembryonic organs; V - systems genesis.

Embryogenesis is closely related to progenesis and the early postembryonic period. Thus, tissue development begins in the embryonic period (embryonic histogenesis) and continues after the birth of a child (postembryonic histogenesis).

21.1. PROGENESIS

This is the period of development and maturation of sex cells - eggs and sperm. As a result of progenez, a haploid set of chromosomes appears in mature germ cells, structures are formed that provide the ability to fertilize and develop a new organism. The process of development of germ cells is discussed in detail in the chapters on the male and female reproductive systems (see Chapter 20).

Figure: 21.1.The structure of the male reproductive cell:

I - head; II - tail. 1 - receptor;

2 - acrosome; 3 - "boot"; 4 - proximal centriole; 5 - mitochondrion; 6 - a layer of elastic fibrils; 7 - axonema; 8 - terminal ring; 9 - circular fibrils

The main characteristics of mature human germ cells

Male reproductive cells

Human spermatozoa are formed during the entire active sexual period in large quantities. For a detailed description of spermatogenesis, see chapter 20.

Sperm motility is due to the presence of flagella. The speed of movement of spermatozoa in humans is 30-50 microns / s. Purposeful movement is facilitated by chemotaxis (movement to or from a chemical stimulus) and rheotaxis (movement against the flow of fluid). 30-60 minutes after intercourse, sperm cells are found in the uterine cavity, and after 1.5-2 hours - in the distal (ampullar) part of the fallopian tube, where they meet with the egg and fertilize. Sperm retain their fertilizing capacity for up to 2 days.

Structure.Human male reproductive cells - sperm,or sperm,about 70 microns long, have a head and a tail (Fig. 21.1). The plasmolemma of the sperm in the head region contains a receptor through which it interacts with the egg.

Sperm head (caput spermatozoidi)includes a small dense nucleus with a haploid set of chromosomes. The anterior half of the kernel is covered with a flat pouch that makes up casesperm. It houses acrosome(from the Greek. acron- top, soma- body). The acrosome contains a set of enzymes, among which hyaluronidase and proteases play an important role, which are capable of dissolving the membranes covering the egg during fertilization. The cap and acrosome are derivatives of the Golgi complex.

Figure: 21.2.The cellular composition of human ejaculate is normal:

I - male reproductive cells: A - mature (according to L. F. Kurilo and others); B - immature;

II - somatic cells. 1, 2 - typical spermatozoon (1 - full face, 2 - profile); 3-12 - the most common forms of sperm atypia; 3 - macrohead; 4 - microhead; 5 - elongated head; 6-7 - anomaly in the shape of the head and acrosome; 8-9 - flagellar anomaly; 10 - biflagular spermatozoon; 11 - fused heads (two-headed sperm); 12 - anomaly of the sperm neck; 13-18 - immature male reproductive cells; 13-15 - primary spermatocytes in the prophase of the 1st division of meiosis - proleptothene, pachytene, diplotene, respectively; 16 - primary spermatocyte in the metaphase of meiosis; 17 - typical spermatids (and- early; b- late); 18 - atypical binucleated spermatid; 19 - epithelial cells; 20-22 - leukocytes

The nucleus of the human sperm contains 23 chromosomes, one of which is sex (X or Y), the rest are autosomes. 50% of sperm contain the X chromosome, 50% - the Y chromosome. The mass of the X chromosome is slightly greater than the mass of the Y chromosome, therefore, apparently, spermatozoa containing the X chromosome are less motile than spermatozoa containing the Y chromosome.

There is an annular narrowing behind the head, passing into the tail section.

Tail section (flagellum)the sperm cell consists of a connecting, intermediate, main and terminal parts. In the connecting part (pars conjungens),or neck (cervix),there are centrioles - proximal, adjacent to the nucleus, and remnants of the distal centriole, striated columns. This is where the axial thread starts (axonema),continuing in the intermediate, main and terminal parts.

Intermediate part (pars intermedia)contains 2 central and 9 pairs of peripheral microtubules, surrounded by spiral mitochondria (mitochondrial vagina - vagina mitochondrialis).From the microtubules there are paired protrusions, or "handles," consisting of another protein, dynein, which has ATPase activity (see Chapter 4). Dynein breaks down the ATP produced by mitochondria and converts chemical energy into mechanical energy, due to which the movement of sperm is carried out. In the case of a genetically determined absence of dynein, sperm are immobilized (one of the forms of male sterility).

Among the factors influencing the speed of sperm movement, temperature, pH of the environment, etc. are of great importance.

main part (pars principalis)the structure of the tail resembles a cilium with a characteristic set of microtubules in the axoneme (9 × 2) +2, surrounded by circularly oriented fibrils, giving elasticity, and a plasmolemma.

Terminal,or final, partsperm (pars terminalis)contains an axoneme, which ends in disconnected microtubules and a gradual decrease in their number.

The tail movements are whiplike, which is due to the sequential contraction of microtubules from the first to the ninth pair (the first is considered a pair of microtubules, which lies in a plane parallel to the two central ones).

In clinical practice, when examining sperm, various forms of spermatozoa are counted by counting their percentage (spermiogram).

According to the World Health Organization (WHO), the following indicators are normal characteristics of human sperm: sperm concentration - 20-200 million / ml, content in ejaculate is more than 60% of normal forms. Along with the latter, abnormal ones are always present in human sperm - biflagellates, with defective head sizes (macro- and microforms), with an amorphous head, with accrete

heads, immature forms (with remnants of cytoplasm in the neck and tail), with flagellar defects.

In the ejaculate of healthy men, typical spermatozoa predominate (Fig. 21.2). The number of different types of atypical spermatozoa should not exceed 30%. In addition, there are immature forms of germ cells - spermatids, spermatocytes (up to 2%), as well as somatic cells - epithelial cells, leukocytes.

Among the spermatozoa in the ejaculate of living cells, there should be 75% or more, and actively motile - 50% or more. The established normative parameters are necessary to assess deviations from the norm in various forms of male infertility.

In an acidic environment, sperm cells quickly lose their ability to move and fertilize.

Female reproductive cells

Egg cells,or oocytes(from lat. ovum- egg), ripen in immeasurably less quantity than sperm. During the sexual cycle (24-28 days), a woman usually matures one egg. Thus, during the childbearing period, about 400 eggs are formed.

The release of an oocyte from the ovary is called ovulation (see chapter 20). The oocyte released from the ovary is surrounded by a crown of follicular cells, the number of which reaches 3-4 thousand. The oocyte has a spherical shape, the volume of cytoplasm is larger than that of sperm, and does not have the ability to move independently.

Oocyte classification is based on evidence of presence, quantity and distribution yolk (lecithos),which is a protein-lipid inclusion in the cytoplasm used to feed the embryo. Distinguish yolkless(alecitic), low yolk(oligolecite), middle yolk(mesolecital), poly-yolk(polyilecital) eggs. Yolk oocytes are subdivided into primary (in noncranial, for example, lancelet) and secondary (in placental mammals and humans).

As a rule, in yolk oocytes, yolk inclusions (granules, plates) are evenly distributed, therefore they are called isocytic(Greek. isos- equal). Human ovum secondary isocytic type(as in other mammals) contains a small amount of yolk granules, located more or less evenly.

In humans, the presence of a small amount of yolk in the egg is due to the development of the embryo in the mother's body.

Structure.The human egg cell has a diameter of about 130 microns. A transparent (shiny) zone is adjacent to the plasma lemma (zona pellucida- Zp) and then a layer of follicular epithelial cells (Fig.21.3).

The nucleus of the female reproductive cell has a haploid set of chromosomes with the X-sex chromosome, a well-defined nucleolus, and there are many pore complexes in the membrane of the nucleus. During the period of oocyte growth, intensive processes of mRNA and rRNA synthesis occur in the nucleus.

Figure: 21.3.The structure of the female reproductive cell:

1 - core; 2 - plasmolemma; 3 - follicular epithelium; 4 - radiant crown; 5 - cortical granules; 6 - yolk inclusions; 7 - transparent area; 8 - Zp3 receptor

In the cytoplasm, the protein synthesis apparatus (endoplasmic reticulum, ribosomes) and the Golgi complex are developed. The number of mitochondria is moderate, they are located near the nucleus, where intensive yolk synthesis takes place, the cell center is absent. In the early stages of development, the Golgi complex is located near the nucleus, and in the process of maturation of the egg it shifts to the periphery of the cytoplasm. Here are the derivatives of this complex - cortical granules (granula corticalia),the number of which reaches 4000, and the size is 1 micron. They contain glycosaminoglycans and various enzymes (including proteolytic ones), participate in the cortical reaction, protecting the egg from polyspermy.

Of the inclusions of ovoplasm deserve special attention yolk granules,containing proteins, phospholipids and carbohydrates. Each yolk granule is surrounded by a membrane, has a dense central part, consisting of phosphovitin (phosphoprotein), and a looser peripheral part, consisting of lipovitellin (lipoprotein).

Transparent zone (zona pellucida- Zp) consists of glycoproteins and glycosaminoglycans - chondroitinsulfuric, hyaluronic and sialic acids. Glycoproteins are represented by three fractions - Zpl, Zp2, Zp3. Fractions Zp2 and Zp3 form filaments 2-3 μm long and 7 nm thick, which

interconnected by means of the Zpl fraction. Fraction Zp3 is receptorsperm cells, and Zp2 inhibits polyspermy. The transparent zone contains tens of millions of Zp3 glycoprotein molecules, each of which has more than 400 amino acid residues linked to many oligosaccharide branches. Follicular epithelial cells take part in the formation of the transparent zone: the processes of follicular cells penetrate through the transparent zone, heading to the plasmolemma of the egg. The plasmolemma of the egg, in turn, forms microvilli located between the processes of follicular epithelial cells (see Fig. 21.3). The latter perform trophic and protective functions.

21.2. Embryogenesis

Intrauterine human development lasts on average 280 days (10 lunar months). It is customary to distinguish three periods: initial (1st week), embryonic (2-8th weeks), fetal (from the 9th week of development to the birth of the child). By the end of the embryonic period, the laying of the main embryonic rudiments of tissues and organs is completed.

Fertilization and zygote formation

Fertilization (fertilisatio)- the fusion of male and female germ cells, as a result of which the diploid set of chromosomes characteristic of a given animal species is restored, and a qualitatively new cell appears - a zygote (a fertilized egg, or a unicellular embryo).

In humans, the volume of ejaculate - ejaculated semen - is normally about 3 ml. To ensure fertilization, the total number of sperm in the sperm must be at least 150 million, and the concentration - 20-200 million / ml. In the genital tract of a woman, after copulation, their number decreases in the direction from the vagina to the ampullar part of the fallopian tube.

In the process of fertilization, three phases are distinguished: 1) distant interaction and rapprochement of gametes; 2) contact interaction and activation of the egg; 3) the penetration of the sperm into the egg and subsequent fusion - syngamy.

First phase- distant interaction - provided by chemotaxis - a combination of specific factors that increase the likelihood of meeting of germ cells. An important role in this is played by gamones- chemicals produced by the sex cells (Fig. 21.4). For example, eggs release peptides that help attract sperm.

Immediately after ejaculation, sperm are not capable of penetrating into the egg until capacitation occurs - the acquisition of fertilizing ability by sperm under the influence of the secretion of the female genital tract, which lasts 7 hours. seminal plasma, which promotes the acrosomal reaction.

Figure: 21.4.Distant and contact interaction of sperm and ovum: 1 - sperm and its receptors on the head; 2 - separation of carbohydrates from the surface of the head during capacitation; 3 - binding of sperm receptors with egg receptors; 4 - Zp3 (the third fraction of the transparent zone glycoproteins); 5 - oocyte plasmolemma; ГГI, ГГII - gynogamons; AGI, AGII - androgamones; Gal - glycosyltransferase; NAG - N-acetylglucosamine

In the mechanism of capacitation, hormonal factors are of great importance, primarily progesterone (a hormone of the corpus luteum), which activates the secretion of glandular cells of the fallopian tubes. During capacitation, the plasma membrane cholesterol is bound by the albumin of the female reproductive tract and the receptors of the germ cells are exposed. Fertilization takes place in the ampullar part of the fallopian tube. Fertilization is preceded by insemination - interaction and rapprochement of gametes (distant interaction) due to chemotaxis.

Second phasefertilization - contact interaction. Numerous sperm cells approach the egg and come into contact with its membrane. The ovum begins to rotate around its axis at a speed of 4 revolutions per minute. These movements are caused by the beating of the sperm tails and last for about 12 hours. Sperm, upon contact with the egg, can bind tens of thousands of Zp3 glycoprotein molecules. In this case, the start of the acrosomal reaction is noted. The acrosomal reaction is characterized by an increase in the permeability of the sperm plasmolemma to Ca 2 + ions, its depolarization, which contributes to the fusion of the plasmolemma with the anterior membrane of the acrosome. The transparent zone is in direct contact with acrosomal enzymes. Enzymes destroy it, sperm passes through the transparent zone and

Figure: 21.5.Fertilization (according to Wasserman with changes):

1-4 - stages of the acrosome reaction; five - zona pellucida(transparent area); 6 - perivi-telline space; 7 - plasma membrane; 8 - cortical granule; 8a - cortical reaction; 9 - penetration of sperm into the egg; 10 - zone reaction

enters the perivitelline space located between the transparent zone and the ovum plasmolemma. After a few seconds, the properties of the ovum plasmolemma change and a cortical reaction begins, and after a few minutes the properties of the transparent zone change (zone reaction).

The initiation of the second phase of fertilization occurs under the influence of sulfated polysaccharides of the glittering zone, which cause the flow of calcium and sodium ions into the head, sperm, their replacement of potassium and hydrogen ions and rupture of the acrosome membrane. The attachment of sperm to the egg occurs under the influence of the carbohydrate group of the glycoprotein fraction of the transparent zone of the egg. Sperm receptors are a glycosyltransferase enzyme located on the surface of the head acrosome, which

Figure: 21.6. Fertilization phases and the beginning of cleavage (diagram):

1 - ovoplasm; 1a - cortical granules; 2 - core; 3 - transparent area; 4 - follicular epithelium; 5 - sperm; 6 - reduction bodies; 7 - completion of mitotic division of the oocyte; 8 - a tubercle of fertilization; 9 - fertilization shell; 10 - female pronucleus; 11 - male pronucleus; 12 - syncarion; 13 - the first mitotic division of the zygote; 14 - blastomeres

"Recognizes" the receptor of the female reproductive cell. Plasma membranes at the point of contact of germ cells merge, and plasmogamy occurs - the union of the cytoplasms of both gametes.

In mammals, during fertilization, only one sperm penetrates into the egg. This phenomenon is called monospermia.Fertilization is facilitated by hundreds of other sperm involved in insemination. Enzymes secreted from acrosomes - spermolysins (trypsin, hyaluronidase) - destroy the radiant crown, cleave the transparent zone of the egg with glycosides-noglycans. Detached follicular epithelial cells stick together into a conglomerate, which, following the egg cell, moves along the fallopian tube due to the flickering of the cilia of the epithelial cells of the mucous membrane.

Figure: 21.7.Ovum and human zygote (according to B.P. Khvatov):

and- human egg cell after ovulation: 1 - cytoplasm; 2 - core; 3 - transparent area; 4 - follicular epithelial cells forming a radiant crown; b- human zygote at the stage of convergence of the male and female nuclei (pronuclei): 1 - female nucleus; 2 - male nucleus

Third phase.The head and the intermediate part of the tail section penetrate into the ovoplasm. After the entry of the sperm into the ovum at the periphery of the ovoplasm, it is compacted (zone reaction) and fertilization shell.

Cortical reaction- the fusion of the ovum plasmolemma with the membranes of the cortical granules, as a result of which the contents from the granules exit into the perivitelline space and affect the glycoprotein molecules of the transparent zone (Fig. 21.5).

As a result of this zone reaction, Zp3 molecules are modified and lose their ability to act as sperm receptors. A fertilization shell with a thickness of 50 nm is formed, which prevents polyspermy - the penetration of other sperm.

The mechanism of the cortical reaction includes the influx of sodium ions through the site of the sperm plasmolemma, built into the plasmolemma of the egg after the acrosomal reaction is completed. As a result, the negative membrane potential of the cell becomes weakly positive. The influx of sodium ions causes the release of calcium ions from intracellular depots and an increase in its content in the hyaloplasm of the egg. This is followed by exocytosis of cortical granules. The proteolytic enzymes released from them break the bonds between the transparent zone and the plasmolemma of the egg, as well as between the sperm and the transparent zone. In addition, a glycoprotein is released that binds water and attracts it into the space between the plasmolemma and the transparent zone. As a result, the perivitelline space is formed. Finally,

a factor is highlighted that contributes to the hardening of the transparent zone and the formation of a fertilization shell from it. Thanks to the mechanisms for preventing polyspermia, only one haploid nucleus of the sperm is able to merge with one haploid nucleus of the egg, which leads to the restoration of the diploid set characteristic of all cells. The penetration of the sperm into the egg after a few minutes significantly enhances the processes of intracellular metabolism, which is associated with the activation of its enzymatic systems. The interaction of sperm with the egg can be blocked by antibodies against substances entering the transparent zone. On this basis, methods of immunological contraception are being sought.

After the rapprochement of the female and male pronuclei, which lasts about 12 hours in mammals, a zygote is formed - a unicellular embryo (Fig. 21.6, 21.7). At the zygote stage, presumptive zones(lat. presumptio- probability, hypothesis) as sources of development of the corresponding areas of the blastula, from which the germ layers are subsequently formed.

21.2.2. Crushing and formation of blastula

Splitting up (fissio)- sequential mitotic division of the zygote into cells (blastomeres) without the growth of daughter cells to the size of the maternal one.

The resulting blastomeres remain united into a single organism of the embryo. In the zygote, a mitotic spindle is formed between the distant

Figure: 21.8.The human embryo in the early stages of development (according to Gertig and Rock):

and- stage of two blastomeres; b- blastocyst: 1 - embryoblast; 2 - trophoblast;

3 - blastocyst cavity

Figure: 21.9.Crushing, gastrulation and implantation of the human embryo (diagram): 1 - crushing; 2 - morula; 3 - blastocyst; 4 - blastocyst cavity; 5 - embryo-blast; 6 - trophoblast; 7 - embryonic nodule: and -epiblast; b- hypoblast; 8 - fertilization shell; 9 - amniotic (ectodermal) vesicle; 10 - extraembryonic mesenchyme; 11 - ectoderm; 12 - endoderm; 13 - cytotrophoblast; 14 - symplastotrophoblast; 15 - embryonic disc; 16 - gaps with maternal blood; 17 - chorion; 18 - amniotic leg; 19 - yolk vesicle; 20 - the mucous membrane of the uterus; 21 - oviduct

to the poles by centrioles introduced by the sperm. Pronuclei enter the prophase stage with the formation of a combined diploid set of egg and sperm chromosomes.

After going through all the other phases of mitotic division, the zygote is divided into two daughter cells - blastomeres(from the Greek. blastos- the rudiment, meros- part). Due to the actual absence of the G 1 -period, during which the growth of cells formed as a result of division occurs, the cells are much smaller than the maternal, therefore, the size of the embryo as a whole during this period, regardless of the number of its constituent cells, does not exceed the size of the original cell - the zygote. All this allowed us to call the described process crushing(i.e., by grinding), and the cells formed in the process of cleavage - blastomeres.

The fragmentation of the human zygote begins by the end of the first day and is characterized as full uneven asynchronous.During the first day, it occurs

walks slowly. The first cleavage (division) of the zygote is completed in 30 hours, resulting in the formation of two blastomeres, covered with a fertilization membrane. The two-blastomere stage is followed by the three-blastomere stage.

From the very first cleavages of the zygote, two types of blastomeres are formed - "dark" and "light". "Light", smaller, blastomeres split faster and are located in one layer around large "dark" ones, which are in the middle of the embryo. From the surface "light" blastomeres, later trophoblast,connecting the embryo with the mother's body and providing it with nutrition. Internal, "dark" blastomeres form embryoblast,from which the body of the embryo and extraembryonic organs (amnion, yolk sac, allantois) are formed.

Starting from the 3rd day, cleavage proceeds faster, and on the 4th day the embryo consists of 7-12 blastomeres. After 50-60 hours, a dense accumulation of cells forms - morula,and on the 3-4th day, the formation of blastocysts- a hollow bubble filled with liquid (see Fig. 21.8; Fig. 21.9).

The blastocyst moves through the fallopian tube to the uterus for 3 days and enters the uterine cavity after 4 days. The blastocyst is in the uterine cavity in a free form (free blastocyst)within 2 days (5th and 6th days). By this time, the blastocyst increases in size due to an increase in the number of blastomeres - embryoblast and trophoblast cells - up to 100 and due to increased absorption of uterine secretion by trophoblast and active production of fluid by trophoblast cells (see Fig. 21.9). The trophoblast in the first 2 weeks of development provides nutrition to the embryo due to the decay products of maternal tissues (histiotrophic type of nutrition),

The embryoblast is located in the form of a nodule of germ cells ("embryonic nodule"), which is attached from the inside to the trophoblast at one of the poles of the blastocyst.

21.2.4. Implantation

Implantation (lat. implantatio- ingrowth, rooting) - the introduction of the embryo into the lining of the uterus.

There are two stages of implantation: adhesion(adhesion) when the fetus is attached to the inner surface of the uterus, and invasion(immersion) - the introduction of the embryo into the tissue of the uterine mucosa. On the 7th day, changes occur in the trophoblast and embryoblast associated with the preparation for implantation. The blastocyst retains the fertilization membrane. In the trophoblast, the number of lysosomes with enzymes increases, which ensure the destruction (lysis) of the tissues of the uterine wall and thereby contribute to the introduction of the embryo into the thickness of its mucous membrane. Microvilli appearing in the trophoblast gradually destroy the fertilization membrane. The embryonic nodule flattens and turns

in germinal scutellum,in which preparation for the first stage of gastrulation begins.

Implantation lasts about 40 hours (see Fig.21.9; Fig.21.10). Simultaneously with the implantation, gastrulation begins (the formation of germ layers). it first critical perioddevelopment.

In the first stagetrophoblast attaches to the epithelium of the uterine mucosa, and two layers are formed in it - cytotrophoblastand symplastotro-phoblast. In the second stagesymplastotrophoblast, producing proteolytic enzymes, destroys the mucous membrane of the uterus. At the same time, the villitrophoblast, penetrating into the uterus, sequentially destroy its epithelium, then the underlying connective tissue and vascular walls, and the trophoblast comes into direct contact with the blood of the maternal vessels. Formed implantation fossa,in which areas of hemorrhage appear around the embryo. The embryo is fed directly from the mother's blood (hematotrophic type of nutrition). From the mother's blood, the embryo receives not only all the nutrients, but also the oxygen necessary for breathing. At the same time, in the mucous membrane of the uterus, from connective tissue cells rich in glycogen, the formation of decidualcells. After the embryo is completely immersed in the implantation fossa, the hole formed in the uterine mucosa is filled with blood and products of destruction of the tissues of the uterine mucosa. Subsequently, the defect of the mucous membrane disappears, the epithelium is restored by cellular regeneration.

The hematotrophic type of nutrition, replacing the histiotrophic one, is accompanied by a transition to a qualitatively new stage of embryogenesis - the second phase of gastrulation and the establishment of extraembryonic organs.

21.3. GASTRULATION AND ORGANOGENESIS

Gastrulation (from lat. gaster- stomach) - a complex process of chemical and morphogenetic changes, accompanied by reproduction, growth, directed movement and differentiation of cells, as a result of which germ layers are formed: external (ectoderm), middle (mesoderm) and internal (endoderm) - sources of development of a complex of axial organs and embryonic tissue rudiments.

Human gastrulation proceeds in two stages. Stage one(deeds-nation) falls on the 7th day, and second stage(immigration) - on the 14-15th day of intrauterine development.

When delamination(from lat. lamina- plate), or splitting,from the material of the embryonic nodule (embryoblast), two sheets are formed: the outer sheet - epiblastand internal - hypoblast,facing the blastocyst cavity. Epiblast cells have the appearance of pseudostratified prismatic epithelium. Hypoblast cells are small cubic, with foamy cyto-

Figure: 21.10. Human embryos 7.5 and 11 days of development in the process of implantation into the uterine mucosa (according to Gertig and Rock):

and- 7.5 days of development; b- 11 days of development. 1 - ectoderm of the embryo; 2 - embryo endoderm; 3 - amniotic vesicle; 4 - extraembryonic mesenchyme; 5 - cyto-trophoblast; 6 - symplastotrophoblast; 7 - uterine gland; 8 - gaps with maternal blood; 9 - epithelium of the uterine mucosa; 10 - own plate of the uterine mucosa; 11 - primary villi

plasma, form a thin layer under the epiblast. Some of the epiblast cells later form a wall amniotic vesicle,which begins to form on the 8th day. In the area of \u200b\u200bthe bottom of the amniotic vesicle, a small group of epiblast cells remains - the material that will go to the development of the body of the embryo and extraembryonic organs.

Following delamination, cells are evicted from the outer and inner sheets into the blastocyst cavity, which marks the formation extraembryonic mesenchyme.By the 11th day, the mesenchyme grows to the trophoblast and the chorion is formed - the villous membrane of the embryo with primary chorial villi (see Fig. 21.10).

Stage twogastrulation occurs by immigration (movement) of cells (Fig. 21.11). Cell movement occurs in the area of \u200b\u200bthe bottom of the amniotic vesicle. Cell flows arise from front to back, to the center and inward as a result of cell proliferation (see Fig. 21.10). This leads to the formation of a primary streak. At the head end, the primary strip thickens, forming primary,or head, nodule(Fig. 21.12), where the head process originates. The cephalic process grows in the cranial direction between the epi- and hypoblast and later gives rise to the development of the notochord of the embryo, which determines the axis of the embryo, is the basis for the development of the bones of the axial skeleton. A spinal column will form around the choir in the future.

The cellular material, which moves from the primary stripe into the space between the epiblast and the hypoblast, is located in the form of mesodermal wings parachordally. Some of the epiblast cells are incorporated into the hypoblast, participating in the formation of the intestinal endoderm. As a result, the embryo acquires a three-layer structure in the form of a flat disc, consisting of three germ layers: ectoderm, mesodermand endoderm.

Factors affecting the mechanisms of gastrulation.The methods and rate of gastrulation are determined by a number of factors: dorsoventral metabolic gradient, which determines the asynchrony of reproduction, differentiation and movement of cells; surface tension of cells and intercellular contacts, contributing to the displacement of cell groups. Inductive factors play an important role here. According to the theory of organizational centers proposed by G. Spemann, inductors (organizing factors) appear in certain parts of the embryo, which have an inducing effect on other parts of the embryo, causing their development in a certain direction. There are inductors (organizers) of several orders, acting sequentially. For example, it has been proven that the organizer of the first order induces the development of the neural plate from the ectoderm. In the neural plate, a second-order organizer arises, contributing to the transformation of a portion of the neural plate into an optic cup, etc.

At present, the chemical nature of many inducers (proteins, nucleotides, steroids, etc.) has been clarified. The role of gap junctions in intercellular interactions has been established. Under the influence of inductors emanating from one cell, the inducible cell, which has the ability of a specific response, changes the path of development. A cell that is not exposed to induction will retain its previous potencies.

Differentiation of germ layers and mesenchyme begins at the end of the 2nd - beginning of the 3rd week. One part of the cells is converted into the rudiments of tissues and organs of the embryo, the other into extraembryonic organs (see Chapter 5, Figure 5.3).

Figure: 21.11.The structure of a 2-week-old human embryo. The second stage of gastrulation (scheme):

and- cross section of the embryo; b- embryonic disc (view from the amniotic vesicle). 1 - chorial epithelium; 2 - chorionic mesenchyme; 3 - gaps filled with maternal blood; 4 - the base of the secondary villi; 5 - amniotic leg; 6 - amniotic vesicle; 7 - yolk vesicle; 8 - embryonic scutellum during gastrulation; 9 - primary strip; 10 - rudiment of intestinal endoderm; 11 - vitelline epithelium; 12 - the epithelium of the amniotic membrane; 13 - primary nodule; 14 - prechordal process; 15 - extraembryonic mesoderm; 16 - extraembryonic ectoderm; 17 - extraembryonic endoderm; 18 - embryonic ectoderm; 19 - embryonic endoderm

Figure: 21.12.Human embryo 17 days ("Crimea"). Graphic reconstruction: and- embryonic disc (top view) with a projection of axial anlages and a definitive cardiovascular system; b- sagittal (middle) slice through the axial tabs. 1 - projection of bilateral endocardial anlages; 2 - projection of bilateral anlages of the pericardial coelom; 3 - projection of bilateral anlages of corporal blood vessels; 4 - amniotic leg; 5 - blood vessels in the amniotic leg; 6 - blood islands in the wall of the yolk bladder; 7 - allantois bay; 8 - the cavity of the amniotic vesicle; 9 - cavity of the yolk sac; 10 - trophoblast; 11 - chordal process; 12 - head knot. Legend: primary stripe - vertical shading; the primary head node is indicated by crosses; ectoderm - without shading; endoderm - lines; extraembryonic mesoderm - points (according to N.P.Barsukov and Yu.N. Shapovalov)

Differentiation of germ layers and mesenchyme, leading to the appearance of tissue and organ primordia, occurs not simultaneously (heterochronously), but interrelated (integratively), resulting in the formation of tissue primordia.

21.3.1. Differentiation of ectoderm

With differentiation, ectoderm is formed embryonic parts -cutaneous ectoderm, neuroectoderm, placodes, prechordal plate, and extra-embryonic ectoderm,which is the source of the formation of the epithelial lining of the amnion. The smaller part of the ectoderm located above the notochord (neuroectoderm),gives rise to differentiation neural tubeand neural crest. Cutaneous ectodermgives rise to stratified squamous epithelium of the skin (epidermis)and its derivatives, the epithelium of the cornea and conjunctiva of the eye, the epithelium of the oral cavity organs, enamel and cuticle of the teeth, the epithelium of the anal rectum, the epithelial lining of the vagina.

Neurulation- the process of formation of a neural tube - takes place differently in time in different parts of the embryo. The closure of the neural tube begins in the cervical region, and then spreads posteriorly and somewhat more slowly in the cranial direction, where the vesicles form. Approximately on the 25th day, the neural tube is completely closed, only two open openings at the anterior and posterior ends communicate with the external environment - anterior and posterior neuropores(fig.21.13). Posterior neuropore corresponds neurointestinal canal.After 5-6 days, both neuropores are overgrown. From the neural tube, neurons and neuroglia of the brain and spinal cord, retina and the organ of smell are formed.

When the lateral walls of the nerve ridges close and the neural tube is formed, a group of neuroectodermal cells appears, which are formed in the area of \u200b\u200bjunction of the neural and the rest (skin) ectoderm. These cells, initially located in longitudinal rows on either side between the neural tube and ectoderm, form neural crest.Neural crest cells are capable of migration. In the trunk, some cells migrate in the superficial layer of the dermis, others - in the ventral direction, forming neurons and neuroglia of parasympathetic and sympathetic nodes, chromaffin tissue and adrenal medulla. Part of the cells differentiate into neurons and neuroglia of the spinal nodes.

Cells are released from the epiblast prechordal plate,which is included in the head of the intestinal tube. From the material of the prechordal plate, the multilayer epithelium of the anterior part of the digestive tube and its derivatives subsequently develops. In addition, the epithelium of the trachea, lungs and bronchi, as well as the epithelial lining of the pharynx and esophagus, derivatives of the branchial pockets - thymus, etc., are formed from the prechordal plate.

According to A. N. Bazhanov, the source of the formation of the lining of the esophagus and respiratory tract is the endoderm of the head intestine.

Figure: 21.13.Neurulation in the human embryo:

and- view from the back; b- cross sections. 1 - anterior neuropore; 2 - posterior neuropore; 3 - ectoderm; 4 - neural plate; 5 - nerve groove; 6 - mesoderm; 7 - chord; 8 - endoderm; 9 - neural tube; 10 - neural crest; 11 - brain; 12 - spinal cord; 13 - spinal canal

Figure: 21.14.The human embryo at the stage of formation of the trunk fold and extra-respiratory organs (according to P. Petkov):

1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - extraembryonic mesenchyme; 4 - the place of the amniotic leg; 5 - primary intestine; 6 - amnion cavity; 7 - amnion ectoderm; 8 - extraembryonic mesenchyme of the amnion; 9 - cavity of the yolk bubble; 10 - endoderm of the yolk vesicle; 11 - extraembryonic mesenchyme of the yolk vesicle; 12 - allantois. Arrows indicate the direction of formation of the trunk fold

As part of the embryonic ectoderm, placodes are laid, which are the source of the development of the epithelial structures of the inner ear. The epithelium of the amnion and the umbilical cord is formed from the extra-respiratory ectoderm.

21.3.2. Endoderm differentiation

Differentiation of the endoderm leads to the formation of the endoderm of the intestinal tube in the body of the embryo and the formation of an extraembryonic endoderm, which forms the lining of the yolk vesicle and allantois (Fig. 21.14).

Isolation of the intestinal tube begins with the appearance of the trunk fold. The latter, deepening, separates the intestinal endoderm of the future intestine from the extraembryonic endoderm of the yolk vesicle. In the posterior part of the embryo, the part of the endoderm from which the endodermal outgrowth of allantois arises is also part of the formed intestine.

A single-layer integumentary epithelium of the stomach, intestines and their glands develops from the endoderm of the intestinal tube. In addition, from ento-

the dermis develops epithelial structures of the liver and pancreas.

The extraembryonic endoderm gives rise to the epithelium of the yolk sac and allantois.

21.3.3. Differentiation of the mesoderm

This process begins on the 3rd week of embryogenesis. The dorsal sections of the mesoderm are divided into dense segments lying on the sides of the notochord - somites. The segmentation of the dorsal mesoderm and the formation of somites begins in the head of the embryo and spreads rapidly in the caudal direction.

On the 22nd day of development, the embryo has 7 pairs of segments, on the 25th - 14, on the 30th - 30 and on the 35th - 43-44 pairs. In contrast to somites, the ventral mesoderm (splanchnotome) is not segmented, but is split into two sheets - visceral and parietal. A small area of \u200b\u200bthe mesoderm that connects the somites with the splanchnotome is divided into segments - segmental pedicles (nephrogonotome). At the posterior end of the embryo, segmentation of these departments does not occur. Here, instead of segmental legs, there is an unsegmented nephrogenic primordium (nephrogenic cord). The paramesonephral canal also develops from the mesoderm of the embryo.

Somites are differentiated into three parts: the myotome, which gives rise to the striated skeletal muscle tissue, the sclerotome, which is the source of the development of bone and cartilaginous tissues, and the dermatome, which forms the connective tissue basis of the skin - the dermis.

From the segmental legs (nephrogonotomes), the epithelium of the kidneys, gonads and vas deferens develop, and from the paramesonephral canal - the epithelium of the uterus, fallopian tubes (oviducts) and the epithelium of the primary lining of the vagina.

The parietal and visceral sheets of the splanchnotome form the epithelial lining of the serous membranes - the mesothelium. From part of the visceral layer of the mesoderm (myoepicardial plate), the middle and outer membranes of the heart - the myocardium and epicardium, as well as the adrenal cortex develop.

The mesenchyme in the body of the embryo is the source of the formation of many structures - blood cells and hematopoietic organs, connective tissue, blood vessels, smooth muscle tissue, microglia (see Chapter 5). From the extraembryonic mesoderm, the mesenchyme develops, giving rise to the connective tissue of the extraembryonic organs - amnion, allantois, chorion, yolk vesicle.

The connective tissue of the embryo and its provisional organs is characterized by high hydrophilicity of the intercellular substance, the richness of glycosaminoglycans in the amorphous substance. The connective tissue of the provisional organs differentiates faster than in organ rudiments, which is due to the need to establish a connection between the embryo and the maternal organism and

ensuring their development (for example, the placenta). Chorionic mesenchyme differentiation occurs early, but does not occur simultaneously over the entire surface. The most active process is in the development of the placenta. Here, the first fibrous structures appear, which play an important role in the formation and strengthening of the placenta in the uterus. With the development of the fibrous structures of the stroma of the villi, argyrophilic pre-collagen fibers are sequentially formed first, and then collagen fibers.

At the 2nd month of development in the human embryo, differentiation of the skeletogenic and cutaneous mesenchyme, as well as the mesenchyme of the heart wall and large blood vessels, begins first of all.

The arteries of the muscular and elastic type of human embryos, as well as the arteries of the stem (anchor) villi of the placenta and their ramifications contain desmin-negative smooth myocytes, which have the property of faster contraction.

At the 7th week of the development of the human embryo, small lipid inclusions appear in the skin mesenchyme and mesenchyme of internal organs, and later (8-9 weeks) the formation of fat cells occurs. Following the development of the connective tissue of the cardiovascular system, the connective tissue of the lungs and the digestive tube differentiates. Differentiation of the mesenchyme in human embryos (11-12 mm long) at the 2nd month of development begins with an increase in the amount of glycogen in the cells. In the same areas, the activity of phosphatases increases, and later, in the course of differentiation, glycoproteins accumulate, RNA and protein are synthesized.

Fruit period.The fetal period begins from the 9th week and is characterized by significant morphogenetic processes in the body of both the fetus and the mother (Table 21.1).

Table 21.1.A short calendar of intrauterine human development (with additions according to R.K.Danilov, T.G. Borovoy, 2003)

Continuation of table. 21.1

Continuation of table. 21.1

Continuation of table. 21.1

Continuation of table. 21.1

Continuation of table. 21.1

Continuation of table. 21.1

Continuation of table. 21.1

The end of the table. 21.1

21.4. EXTRAORDINARY ORGANS

The extraembryonic organs that develop in the process of embryogenesis outside the body of the embryo perform diverse functions that ensure the growth and development of the embryo itself. Some of these organs surrounding the embryo are also called embryonic membranes.These organs include the amnion, yolk sac, allantois, chorion, placenta (Fig.21.15).

The sources of development of tissues of extraembryonic organs are the troph-ectoderm and all three germ layers (Scheme 21.1). General properties of fabric

Figure: 21.15.Development of extraembryonic organs in the human embryo (diagram): 1 - amniotic vesicle; 1a - amnion cavity; 2 - the body of the embryo; 3 - yolk sac; 4 - extraembryonic whole; 5 - primary chorionic villi; 6 - secondary chorionic villi; 7 - allantois stalk; 8 - tertiary chorionic villi; 9 - allan-tois; 10 - umbilical cord; 11 - smooth chorion; 12 - cotyledons

Scheme 21.1.Classification of tissues of extraembryonic organs (according to V.D. Novikov, G.V. Pravotorov, Yu.I. Sklyanov)

her extraembryonic organs and their differences from the definitive ones are as follows: 1) the development of tissues is reduced and accelerated; 2) connective tissue contains few cellular forms, but a lot of amorphous substance rich in glycosaminoglycans; 3) the aging of the tissues of the extrapartum organs occurs very quickly - towards the end of intrauterine development.

21.4.1. Amnion

Amnion- a temporary organ that provides an aquatic environment for the development of the embryo. It arose in evolution in connection with the release of vertebrates from water to land. In human embryogenesis, it appears at the second stage of gastrulation, first as a small vesicle in the epiblast.

The wall of the amniotic vesicle consists of a layer of cells of the extraembryonic ectoderm and of the extraembryonic mesenchyme, which forms its connective tissue.

The amnion rapidly enlarges, and by the end of the 7th week its connective tissue comes into contact with the connective tissue of the chorion. In this case, the epithelium of the amnion passes to the amniotic leg, which later turns into the umbilical cord, and in the region of the umbilical ring it closes with the epithelial cover of the skin of the embryo.

The amniotic membrane forms the wall of a reservoir filled with amniotic fluid, in which the fetus is located (Fig. 21.16). The main function of the amniotic membrane is the production of amniotic fluid, which provides an environment for the developing organism and protects it from mechanical damage. The epithelium of the amnion, facing its cavity, not only secretes amniotic fluid, but also takes part in their reabsorption. The required composition and concentration of salts are maintained in the amniotic fluid until the end of pregnancy. Amnion also performs a protective function, preventing harmful agents from entering the fetus.

The epithelium of the amnion in the early stages is monolayer flat, formed by large polygonal cells closely adjacent to each other, among which there are many mitotically dividing. At the 3rd month of embryogenesis, the epithelium is transformed into prismatic. There are microvilli on the surface of the epithelium. The cytoplasm always contains small drops of lipids and glycogen granules. In the apical parts of the cells there are vacuoles of various sizes, the contents of which are released into the amnion cavity. The epithelium of the amnion in the area of \u200b\u200bthe placental disc is single-layered, prismatic, in places multi-row, performs mainly a secretory function, while the epithelium of the extra-placental amnion is mainly responsible for the resorption of amniotic fluid.

In the connective tissue stroma of the amniotic membrane, a basement membrane, a layer of dense fibrous connective tissue and a spongy layer of loose fibrous connective tissue are distinguished,

Figure: 21.16.Dynamics of the relationship between the embryo, extraembryonic organs and uterine membranes:

and- human embryo at 9.5 weeks of development (micrograph): 1 - amnion; 2 - chorion; 3 - the forming placenta; 4 - umbilical cord

chorionic amnion. In the layer of dense connective tissue, the acellular part and the cellular part lying under the basement membrane can be distinguished. The latter consists of several layers of fibroblasts, between which there is a dense network of thin bundles of collagen and reticular fibers closely adjacent to each other, forming an irregular lattice oriented parallel to the surface of the shell.

The spongy layer is formed by loose mucous connective tissue with rare bundles of collagen fibers, which are a continuation of those that lie in the layer of dense connective tissue, connecting the amnion with the chorion. This bond is very fragile, and therefore both shells can be easily separated from each other. There are many glycosaminoglycans in the main substance of the connective tissue.

21.4.2. Yolk sac

Yolk sac- the most ancient extraembryonic organ in evolution, which arose as an organ that stores nutrients (yolk) necessary for the development of the embryo. In humans, this is a rudimentary formation (yolk vesicle). It is formed by extraembryonic endoderm and extraembryonic mesoderm (mesenchyme). Appearing on the 2nd week of human development, the yolk vesicle in the nutrition of the embryo takes

Figure: 21.16.Continuation

b- scheme: 1 - muscular membrane of the uterus; 2 - decidua basalis;3 - amnion cavity; 4 - the cavity of the yolk sac; 5 - extraembryonic whole (chorionic cavity); 6 - decidua capsularis;7 - decidua parietalis;8 - uterine cavity; 9 - cervix; 10 - embryo; 11 - tertiary chorionic villi; 12 - allantois; 13 - mesenchyme of the umbilical cord: and- Chorionic villus blood vessels; b- gaps with maternal blood (according to Hamilton, Boyd and Mossman)

participation is very short-lived, since from the 3rd week of development, a connection is established between the fetus and the maternal organism, i.e., hematotrophic nutrition. The yolk sac of vertebrates is the first organ in the wall of which blood islands develop, forming the first blood cells and the first blood vessels that provide oxygen and nutrients to the fetus.

With the formation of the trunk fold, which raises the embryo above the yolk bladder, an intestinal tube is formed, while the yolk bladder is separated from the body of the embryo. The connection of the embryo with the yolk bladder remains in the form of a hollow cord called the yolk stalk. As a hematopoietic organ, the yolk sac functions until 7-8 weeks, and then undergoes reverse development and remains in the umbilical cord in the form of a narrow tube that serves as a conductor of blood vessels to the placenta.

21.4.3. Allantois

Allantois is a small finger-like process in the caudal part of the embryo that grows into the amniotic pedicle. It is a derivative of the yolk sac and consists of an extraembryonic endoderm and a visceral mesoderm. In humans, allantois does not reach significant development, but its role in providing nutrition and respiration of the embryo is still great, since vessels located in the umbilical cord grow along it to the chorion. The proximal part of the allantois is located along the yolk stalk, and the distal, growing, grows into the gap between the amnion and the chorion. This is the organ of gas exchange and excretion. Oxygen is delivered through the vessels of the allantois, and metabolic products of the embryo are released into the allantois. At the 2nd month of embryogenesis, allantois is reduced and turns into a cell cord, which, together with the reduced yolk vesicle, is part of the umbilical cord.

21.4.4. Umbilical cord

The umbilical cord, or umbilical cord, is an elastic cord that connects the embryo (fetus) to the placenta. It is covered with an amniotic membrane surrounding the mucous connective tissue with blood vessels (two umbilical arteries and one vein) and rudiments of the yolk bladder and allantois.

The mucous connective tissue, called "Warton's jelly", provides the elasticity of the cord, protects the umbilical vessels from compression, thereby providing a continuous supply of nutrients and oxygen to the embryo. Along with this, it prevents the penetration of harmful agents from the placenta to the embryo by the extravascular route and thus performs a protective function.

It was established by immunocytochemical methods that heterogeneous smooth muscle cells (SMCs) exist in the blood vessels of the umbilical cord, placenta and embryo. In the veins, unlike arteries, desmin-positive SMCs were found. The latter provide slow tonic contractions of the veins.

21.4.5. Chorion

Chorion,or villous sheath,appears for the first time in mammals, develops from trophoblast and extraembryonic mesoderm. Initially, the trophoblast is represented by a layer of cells that form the primary villi. They secrete proteolytic enzymes, with the help of which the lining of the uterus is destroyed and implantation is carried out. At the 2nd week, the trophoblast acquires a two-layer structure due to the formation of an inner cell layer (cytotrophoblast) and a symplastic outer layer (symplastotrophoblast), which is a derivative of the cell layer. The extraembryonic mesenchyme appearing on the periphery of the embryoblast (in humans at the 2nd or 3rd week of development) grows to the trophoblast and forms, together with it, secondary epitheliomesenchymal villi. From this time on, the trophoblast turns into a chorion, or villous membrane (see Fig. 21.16).

At the beginning of the 3rd week, blood capillaries grow into the chorionic villi and tertiary villi form. This coincides with the onset of hematotrophic nutrition of the embryo. The further development of the chorion is associated with two processes - the destruction of the uterine mucosa due to the proteolytic activity of the outer (symplastic) layer and the development of the placenta.

21.4.6. Placenta

Placenta (baby seat)a person belongs to the type of discoidal hemochorial villous placentas (see Fig. 21.16; Fig. 21.17). It is an important temporary organ with multiple functions that ensure the connection of the fetus with the mother's body. However, the placenta creates a barrier between the blood of the mother and the fetus.

The placenta consists of two parts: the embryonic, or fetal (pars fetalis),and maternal (pars materna).The fetal part is represented by a branchy chorion and an amniotic membrane adhered to the chorion from the inside, and the maternal part is represented by a modified mucous membrane of the uterus, which is rejected during childbirth (decidua basalis).

The development of the placenta begins in the 3rd week, when vessels begin to grow into the secondary villi and tertiary villi form, and ends by the end of the 3rd month of pregnancy. At 6-8 weeks around the vessels

Figure: 21.17.Placenta of the hemochorial type. Chorionic villi development dynamics: and- the structure of the placenta (arrows indicate the blood circulation in the vessels and in one of the lacunae, where the villus is removed): 1 - amnion epithelium; 2 - chorionic plate; 3 - villus; 4 - fibrinoid; 5 - yolk vesicle; 6 - umbilical cord; 7 - placenta septum; 8 - lacuna; 9 - spiral artery; 10 - the basal layer of the endometrium; 11 - myometrium; b- the structure of the primary villi of the trophoblast (1st week); in- the structure of the secondary epithelial-mesenchymal chorionic villus (2nd week); r- the structure of the tertiary chorionic villi - epithelial-mesenchymal with blood vessels (3rd week); d- the structure of the chorionic villi (3rd month); e- the structure of chorionic villi (9th month): 1 - intervillous space; 2 - microvilli; 3 - symplastotrophoblast; 4 - nucleus of symplastotrophoblast; 5 - cyto-trophoblast; 6 - cytotrophoblast nucleus; 7 - basement membrane; 8 - intercellular space; 9 - fibroblast; 10 - macrophages (Kashchenko-Gofbauer cells); 11 - endotheliocyte; 12 - lumen of a blood vessel; 13 - erythrocyte; 14 - basement membrane of the capillary (according to E. M. Shvirst)

elements of connective tissue are differentiated. In the differentiation of fibroblasts and their synthesis of collagen, vitamins A and C play an important role, without sufficient supply of which to the body of a pregnant woman, the strength of the connection between the embryo and the mother's body is disturbed and a threat of spontaneous abortion is created.

The main substance of the chorionic connective tissue contains a significant amount of hyaluronic and chondroitinsulfuric acids, which are associated with the regulation of placental permeability.

With the development of the placenta, the destruction of the uterine mucosa occurs, due to the proteolytic activity of the chorion, and the change of histiotrophic nutrition to hematotrophic one. This means that the chorionic villi are washed by the mother's blood, which poured out from the destroyed endometrial vessels into the lacunae. However, the blood of the mother and fetus under normal conditions never mixes.

Hematochorionic barrier,separating both blood flows, it consists of the endothelium of the fetal vessels, the connective tissue surrounding the vessels, the epithelium of the chorionic villi (cytotrophoblast and symplastotrophoblast), and, in addition, of the fibrinoid, which in places covers the villi outside.

Embryonic,or fetal, partby the end of the 3rd month, the placenta is represented by a branching chorionic plate, consisting of fibrous (collagen) connective tissue covered with cyto- and symplastotrophoblast (multinucleated structure covering a reducing cytotrophoblast). The branching villi of the chorion (stem, anchor) are well developed only from the side facing the myometrium. Here they pass through the entire thickness of the placenta and with their tops plunge into the basal part of the destroyed endometrium.

The chorionic epithelium, or cytotrophoblast, in the early stages of development is represented by a single-layer epithelium with oval nuclei. These cells multiply mitotic. Symplastotrophoblast develops from them.

Symplastotrophoblast contains a large number of various proteolytic and oxidative enzymes (ATP-ases, alkaline and acidic

Figure: 21.18.Chorionic villus section of a 17-day-old human embryo ("Crimea"). Micrograph:

1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - chorionic mesenchyme (according to N.P.Barsukov)

phosphatase, 5-nucleotidase, DPN-diaphorase, glucose-6-phosphate dehydrogenase, alpha-GPDH, succinate dehydrogenase - SDH, cytochrome oxidase - CO, monoamine oxidase - MAO, nonspecific esterases, LDP and other - diaphorases - total about 60), which is associated with its role in metabolic processes between the body of the mother and the fetus. In the cytotrophoblast and in the symplast, pinocytic vesicles, lysosomes, and other organelles are detected. Starting from the 2nd month, the chorionic epithelium becomes thinner and is gradually replaced by symplastotrophoblast. During this period, the symplastotrophoblast is thicker than the cytotrophoblast. At 9-10 weeks, the symplast becomes thinner, and the number of nuclei in it increases. On the surface of the symplast, facing the gaps, numerous microvilli appear in the form of a brush border (see Fig. 21.17; Fig. 21.18, 21.19).

Between symplastotrophoblast and cellular trophoblast, there are slit-like submicroscopic spaces, reaching in places to the basal membrane of the trophoblast, which creates conditions for the bilateral penetration of trophic substances, hormones, etc.

In the second half of pregnancy and, especially, at the end of it, the trophoblast becomes very thin and the villi are covered with a fibrin-like oxyphilic mass, which is a product of plasma coagulation and decay of the trophoblast (“Langhans fibrinoid”).

With increasing gestational age, the number of macrophages and collagen-producing differentiated fibroblasts decreases,

Figure: 21.19.Placental barrier at 28 weeks gestation. Electron micrograph, magnification 45,000 (according to U. Yu. Yatsozhinskaya):

1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - basement membrane of trophoblast; 4 - basement membrane of the endothelium; 5 - endotheliocyte; 6 - erythrocyte in the capillary

there are fibrocytes. The number of collagen fibers, although increasing, remains insignificant until the end of pregnancy in most villi. Most of the stromal cells (myofibroblasts) are characterized by an increased content of cytoskeletal contractile proteins (vimentin, desmin, actin and myosin).

The structural and functional unit of the formed placenta is the cotyledon, formed by the stem ("anchor") villi and its

secondary and tertiary (final) ramifications. The total number of cotyledons in the placenta reaches 200.

Mother partthe placenta is represented by the basal plate and connective tissue septa separating the cotyledons from each other, as well as lacunae filled with maternal blood. Trophoblastic cells (peripheral trophoblast) are also found in the places of contact of the stem villi with the decaying membrane.

In the early stages of pregnancy, the chorionic villi destroy the layers of the main falling-off uterine membrane closest to the fetus, and in their place are formed lacunas filled with maternal blood, into which the chorionic villi hang freely.

The deep, undisturbed parts of the decaying membrane, together with the trophoblast, form the basal plate.

Basal layer of the endometrium (lamina basalis)- connective tissue of the mucous membrane of the uterus, containing decidualcells. These large, glycogen-rich connective tissue cells are located deep in the lining of the uterus. They have clear boundaries, rounded nuclei and oxyphilic cytoplasm. During the 2nd month of pregnancy, the decidual cells enlarge significantly. In their cytoplasm, in addition to glycogen, lipids, glucose, vitamin C, iron, nonspecific esterases, and succinic and lactic acid dehydrogenase are detected. In the basal lamina, more often at the place of attachment of the villi to the maternal part of the placenta, there are accumulations of peripheral cytotrophoblast cells. They resemble decidual cells, but differ in a more intense baseline of the cytoplasm. An amorphous substance (Rohr's fibrinoid) is located on the surface of the basal plate facing the chorionic villi. Fibrinoid plays an essential role in providing immunological homeostasis in the mother-fetus system.

Part of the main falling away shell, located on the border of the branched and smooth chorion, that is, along the edge of the placental disc, is not destroyed during the development of the placenta. Adhering tightly to the chorion, it forms closing plate,preventing the flow of blood from the lacunae of the placenta.

The blood in the gaps is continuously circulating. It comes from the uterine arteries entering here from the muscular membrane of the uterus. These arteries run along the placental septa and open into lacunae. The maternal blood flows from the placenta through the veins originating from the lacunae in large holes.

Placenta formation ends at the end of the 3rd month of pregnancy. The placenta provides nutrition, tissue respiration, growth, regulation of the rudiments of the fetal organs formed by this time, as well as its protection.

Placenta functions.The main functions of the placenta: 1) respiratory; 2) transport of nutrients; water; electrolytes and immunoglobulins; 3) excretory; 4) endocrine; 5) participation in the regulation of myometrium contraction.

Breaththe fetus is provided with oxygen attached to the hemoglobin of the maternal blood, which, by diffusion, enters the fetal blood through the placenta, where it combines with fetal hemoglobin

(HbF). CO 2 associated with fetal hemoglobin in the fetal blood also diffuses through the placenta, enters the mother's blood, where it combines with maternal hemoglobin.

Transportof all nutrients necessary for the development of the fetus (glucose, amino acids, fatty acids, nucleotides, vitamins, minerals) originate from the mother's blood through the placenta into the fetal blood, and, conversely, metabolic products that are excreted from his body (excretory function). Electrolytes and water pass through the placenta by diffusion and by pinocytosis.

Pinocytic vesicles of symplastotrophoblast are involved in the transport of immunoglobulins. Immunoglobulin that has entered the bloodstream of the fetus passively immunizes it from the possible action of bacterial antigens that can enter the mother's diseases. After birth, the maternal immunoglobulin is destroyed and replaced by the newly synthesized in the child's body when it is exposed to bacterial antigens. IgG, IgA penetrate the placenta into the amniotic fluid.

Endocrine functionis one of the most important, since the placenta has the ability to synthesize and secrete a number of hormones that ensure the interaction of the embryo and the mother's body throughout pregnancy. The placental hormone production site is cytotrophoblast and especially symplastotrophoblast, as well as decidual cells.

One of the first placenta synthesizes chorionic gonadotropin,the concentration of which rapidly increases in the 2-3rd week of pregnancy, reaching a maximum in the 8-10th week, and in the blood of the fetus it is 10-20 times higher than in the blood of the mother. The hormone stimulates the formation of adrenocorticotropic hormone (ACTH) of the pituitary gland, increases the secretion of corticosteroids.

Plays an important role in the development of pregnancy placental lactogen,which has the activity of prolactin and pituitary luteotropic hormone. It supports steroidogenesis in the corpus luteum of the ovary in the first 3 months of pregnancy, and also takes part in the metabolism of carbohydrates and proteins. Its concentration in the mother's blood progressively increases at 3-4 months of pregnancy and further continues to increase, reaching a maximum by the 9th month. This hormone, together with the prolactin of the pituitary gland of the mother and the fetus, plays a role in the production of pulmonary surfactant and fetoplacental osmoregulation. Its high concentration is found in amniotic fluid (10-100 times more than in the mother's blood).

In the chorion, as well as in the decidua, progesterone and pregnandiol are synthesized.

Progesterone (produced first by the corpus luteum in the ovary, and from 5-6 weeks in the placenta) suppresses uterine contractions, stimulates its growth, has an immunosuppressive effect, suppressing the rejection of the fetus. About 3/4 of progesterone in the mother's body is metabolized and transformed into estrogens, and part is excreted in the urine.

Estrogens (estradiol, estrone, estriol) are produced in the symplasto-trophoblast of the villi of the placenta (chorion) in the middle of pregnancy, and by the end

pregnancy, their activity increases 10 times. They cause hyperplasia and hypertrophy of the uterus.

In addition, melanocyte-stimulating and adrenocorticotropic hormones, somatostatin, etc. are synthesized in the placenta.

The placenta contains polyamines (spermine, spermidine), which increase the synthesis of RNA in smooth muscle cells of the myometrium, as well as oxidases that destroy them. Amino oxidases (histaminase, monoamine oxidase) play an important role, destroying biogenic amines - histamine, serotonin, tyramine. During pregnancy, their activity increases, which contributes to the destruction of biogenic amines and a drop in the concentration of the latter in the placenta, myometrium and mother's blood.

During childbirth, histamine and serotonin are, along with catecholamines (norepinephrine, adrenaline), stimulators of the contractile activity of smooth muscle cells (SMC) of the uterus, and by the end of pregnancy, their concentration significantly increases due to a sharp decrease (by 2 times) in the activity of amino oxidases (histaminase, etc. .).

With weak labor, there is an increase in the activity of aminooxidases, for example, histaminase (5 times).

A normal placenta is not an absolute protein barrier. In particular, fetoprotein at the end of the 3rd month of pregnancy penetrates in a small amount (about 10%) from the fetus into the mother's blood, but the maternal organism does not respond to this antigen with rejection, since the cytotoxicity of maternal lymphocytes decreases during pregnancy.

The placenta prevents the passage of a number of maternal cells and cytotoxic antibodies to the fetus. The main role in this is played by the fibrinoid covering the trophoblast when it is partially damaged. This prevents the entry of placental and fetal antigens into the intervillous space, and also weakens the humoral and cellular "attack" of the mother against the embryo.

In conclusion, let us note the main features of the early stages of development of the human embryo: 1) the asynchronous type of complete cleavage and the formation of "light" and "dark" blastomeres; 2) early isolation and formation of extraembryonic organs; 3) early formation of the amniotic vesicle and the absence of amniotic folds; 4) the presence of two mechanisms at the stage of gastrulation - delamination and immigration, during which the development of provisional organs also occurs; 5) interstitial type of implantation; 6) strong development of amnion, chorion, placenta and poor development of the yolk sac and allantois.

21.5. MOTHER-FRUIT SYSTEM

The mother-fetus system arises during pregnancy and includes two subsystems - the mother's organism and the fetus's organism, as well as the placenta, which is the connecting link between them.

The interaction between the mother's body and the fetus's body is provided primarily by neurohumoral mechanisms. At the same time, the following mechanisms are distinguished in both subsystems: receptor, perceiving information, regulatory, carrying out its processing, and executive.

The receptor mechanisms of the mother's body are located in the uterus in the form of sensitive nerve endings, which are the first to perceive information about the state of the developing fetus. The endometrium contains chemo-, mechano- and thermoreceptors, and the blood vessels contain baroreceptors. Free-type receptor nerve endings are especially numerous in the walls of the uterine vein and in the decidua in the region of attachment of the placenta. Irritation of uterine receptors causes changes in the intensity of respiration, blood pressure in the mother's body, which provides normal conditions for the developing fetus.

The regulatory mechanisms of the mother's body include parts of the central nervous system (temporal lobe of the brain, hypothalamus, mesencephalic part of the reticular formation), as well as the hypothalamic-endocrine system. An important regulatory function is performed by hormones: sex hormones, thyroxine, corticosteroids, insulin, etc. Thus, during pregnancy there is an increase in the activity of the adrenal cortex of the mother and an increase in the production of corticosteroids, which are involved in the regulation of fetal metabolism. Chorionic gonadotropin is produced in the placenta, stimulating the formation of ACTH of the pituitary gland, which activates the activity of the adrenal cortex and enhances the secretion of corticosteroids.

Regulatory neuroendocrine apparatus of the mother ensure the preservation of pregnancy, the required level of functioning of the heart, blood vessels, hematopoietic organs, liver and the optimal level of metabolism, gases, depending on the needs of the fetus.

The receptor mechanisms of the fetus' body perceive signals about changes in the mother's body or its own homeostasis. They are found in the walls of the umbilical arteries and veins, in the orifices of the hepatic veins, in the skin and intestines of the fetus. Irritation of these receptors leads to a change in the fetal heart rate, blood flow rate in its vessels, affects the blood sugar content, etc.

The regulatory neurohumoral mechanisms of the fetus are formed during development. The first motor reactions in the fetus appear at 2–3 months of development, which indicates the maturation of the nerve centers. The mechanisms regulating gas homeostasis are formed at the end of the second trimester of embryogenesis. The beginning of the functioning of the central endocrine gland - the pituitary gland - is noted at the 3rd month of development. The synthesis of corticosteroids in the adrenal glands of the fetus begins in the second half of pregnancy and increases with its growth. The fetus has increased insulin synthesis, which is necessary to ensure its growth associated with carbohydrate and energy metabolism.

The action of the neurohumoral regulatory systems of the fetus is aimed at the executive mechanisms - the organs of the fetus, which provide a change in the intensity of respiration, cardiovascular activity, muscle activity, etc., and at the mechanisms that determine the change in the level of gas exchange, metabolism, thermoregulation and other functions.

In providing connections in the mother-fetus system, a particularly important role is played placenta,which is able not only to accumulate, but also to synthesize the substances necessary for the development of the fetus. The placenta performs endocrine functions, producing a number of hormones: progesterone, estrogen, chorionic gonadotropin (hCG), placental lactogen, etc. Humoral and nerve connections are made between the mother and the fetus through the placenta.

There are also extraplacental humoral connections through the membranes and amniotic fluid.

The humoral communication channel is the most extensive and informative. Through it, oxygen and carbon dioxide, proteins, carbohydrates, vitamins, electrolytes, hormones, antibodies, etc. are supplied (Fig.21.20). Normally, foreign substances do not penetrate from the mother's body through the placenta. They can begin to penetrate only under pathological conditions, when the barrier function of the placenta is impaired. An important component of humoral connections are immunological connections that ensure the maintenance of immune homeostasis in the mother-fetus system.

Despite the fact that the organisms of the mother and the fetus are genetically foreign in the composition of proteins, an immunological conflict usually does not occur. This is provided by a number of mechanisms, among which the following are of significant importance: 1) proteins synthesized by symplastotrophoblast, which inhibit the immune response of the maternal organism; 2) chorionic gonadotropin and placental lactogen, which are in high concentration on the surface of the symplastotrophoblast; 3) the immunomasking effect of glycoproteins of the pericellular fibrinoid of the placenta, charged in the same way as the lymphocytes of the washing blood, is negative; 4) the proteolytic properties of trophoblast also contribute to the inactivation of foreign proteins.

The amniotic waters, containing antibodies that block antigens A and B, characteristic of the blood of a pregnant woman, and do not allow them into the blood of the fetus, also take part in the immune defense.

Organisms of the mother and fetus are a dynamic system of homologous organs. The defeat of any organ of the mother leads to a violation of the development of the fetal organ of the same name. So, if a pregnant woman suffers from diabetes, in which the production of insulin is reduced, then the fetus has an increase in body weight and an increase in insulin production in the islets of the pancreas.

In an experiment on animals, it was found that the blood serum of an animal from which a part of an organ was removed stimulates proliferation in the organ of the same name. However, the mechanisms of this phenomenon have not been sufficiently studied.

Nerve connections include the placental and extraplacental canals: the placental - irritation of the baro- and chemoreceptors in the vessels of the placenta and umbilical cord, and extraplacental - the receipt of irritations associated with the growth of the fetus in the mother's central nervous system, etc.

The presence of nerve connections in the mother-fetus system is confirmed by data on the innervation of the placenta, a high content of acetylcholine in it,

Figure: 21.20.Transport of substances across the placental barrier

development of the fetus in the denervated uterine horn of experimental animals, etc.

In the process of formation of the mother-fetus system, there are a number of critical periods, the most important for the establishment of interaction between the two systems, aimed at creating optimal conditions for the development of the fetus.

21.6. CRITICAL PERIODS OF DEVELOPMENT

During ontogenesis, especially embryogenesis, there are periods of higher sensitivity of developing germ cells (during progenesis) and the embryo (during embryogenesis). The Australian physician Norman Gregg (1944) was the first to notice this. The Russian embryologist P.G. Svetlov (1960) formulated the theory of critical periods of development and tested it experimentally. The essence of this theory

consists in the statement of the general position that each stage of development of the embryo as a whole and of its individual organs begins with a relatively short period of qualitatively new restructuring, accompanied by the determination, proliferation and differentiation of cells. At this time, the embryo is most susceptible to damaging influences of various natures (X-ray irradiation, drugs, etc.). Such periods in progenesis are sperm and ovogenesis (meiosis), and in embryogenesis - fertilization, implantation (during which gastrulation occurs), differentiation of germ layers and organ laying, the period of placentation (final maturation and formation of the placenta), the formation of many functional systems, birth.

Among the developing organs and systems of man, a special place belongs to the brain, which in the early stages acts as the primary organizer of the differentiation of the surrounding tissue and organ primordia (in particular, the sensory organs), and later is distinguished by an intensive multiplication of cells (about 20,000 per minute), which requires optimal trophic conditions.

In critical periods, damaging exogenous factors can be chemical substances, including many drugs, ionizing radiation (for example, X-rays in diagnostic doses), hypoxia, starvation, drugs, nicotine, viruses, etc.

Chemicals and drugs that penetrate the placental barrier are especially dangerous for the embryo in the first 3 months of pregnancy, since they are not metabolized and accumulate in high concentrations in its tissues and organs. Drugs disrupt the development of the brain. Starvation, viruses cause malformations and even intrauterine death (Table 21.2).

So, in human ontogeny, several critical periods of development are distinguished: in progenesis, embryogenesis and postnatal life. These include: 1) development of sex cells - ovogenesis and spermatogenesis; 2) fertilization; 3) implantation (7-8 days of embryogenesis); 4) the development of axial organ rudiments and the formation of the placenta (3-8 weeks of development); 5) the stage of increased brain growth (15-20 weeks); 6) the formation of the main functional systems of the body and the differentiation of the reproductive apparatus (20-24 weeks); 7) birth; 8) neonatal period (up to 1 year); 9) puberty (11-16 years old).

Diagnostic methods and preventive measures for human developmental anomalies.In order to identify anomalies in human development, modern medicine has a number of methods (non-invasive and invasive). So, all pregnant women twice (at 16-24 and 32-36 weeks) spend ultrasound procedure,which allows you to detect a number of anomalies in the development of the fetus and its organs. At 16-18 weeks of gestation using the content determination method alpha-fetoproteinin the mother's blood serum, malformations of the central nervous system (in the case of an increase in its level by more than 2 times) or chromosomal abnormalities, for example, Down's syndrome - trisomy of chromosome 21 or

Table 21.2.The timing of some anomalies in the development of human embryos and fetuses

other trisomies (this is evidenced by a decrease in the level of the test substance by more than 2 times).

Amniocentesis- an invasive method of examination, in which amniotic fluid is taken through the abdominal wall of the mother (usually at the 16th week of pregnancy). Subsequently, chromosomal analysis of amniotic fluid cells and other studies are performed.

Visual control of fetal development is also used with laparoscope,introduced through the abdominal wall of the mother into the uterine cavity (fetoscopy).

There are other ways to diagnose fetal anomalies. However, the main task of medical embryology is to prevent their development. For this purpose, methods of genetic counseling and selection of married couples are being developed.

Artificial insemination methodssex cells from known healthy donors allow you to avoid the inheritance of a number of unfavorable traits. The development of genetic engineering makes it possible to correct local damage to the genetic apparatus of the cell. So, there is a method, the essence of which is to obtain a testicular biopsy from

men with a genetically determined disease. Introducing normal DNA into spermatogonia, and then transplanting spermatogonia into a pre-irradiated testicle (to destroy genetically defective germ cells), the subsequent multiplication of the transplanted spermatogonia leads to the fact that the newly formed spermatozoa are freed from a genetically determined defect. Consequently, such cells can produce normal offspring when the female reproductive cell is fertilized.

Sperm cryopreservation methodallows you to preserve the fertilizing ability of sperm for a long time. This is used to preserve the sex cells of men associated with the risk of radiation, injury, etc.

Method of artificial insemination and embryo transfer(in vitro fertilization) is used to treat both male and female infertility. Laparoscopy is used to obtain female germ cells. A special needle is used to pierce the ovarian membrane in the area of \u200b\u200bthe vesicular follicle, an oocyte is aspirated, which is further fertilized by sperm. Subsequent cultivation, as a rule, to the stage of 2-4-8 blastomeres and transfer of the embryo to the uterus or fallopian tube ensures its development in the conditions of the maternal organism. In this case, the transplantation of the embryo into the uterus of the "surrogate" mother is possible.

Improving methods of treating infertility and preventing anomalies in human development are closely intertwined with moral, ethical, legal, social problems, the solution of which largely depends on the established traditions of a particular people. This is the subject of special research and discussion in the literature. At the same time, advances in clinical embryology and reproductive medicine cannot significantly affect population growth due to the high cost of treatment and methodological difficulties in working with germ cells. That is why the basis of activities aimed at improving the health and numerical growth of the population is the preventive work of a doctor, based on knowledge of the processes of embryogenesis. For the birth of healthy offspring, it is important to lead a healthy lifestyle and give up bad habits, as well as to carry out a set of those activities that are in the competence of medical, public and educational institutions.

Thus, as a result of studying the embryogenesis of humans and other vertebrates, the main mechanisms of the formation of germ cells and their fusion with the emergence of a unicellular stage of development, the zygote, have been established. The subsequent development of the embryo, implantation, the formation of germ layers and embryonic rudiments of tissues, extraembryonic organs show a close evolutionary connection and succession of development of representatives of different classes of the animal world. It is important to know that there are critical periods in the development of the embryo when the risk of intrauterine death or development of pathological

paths. Knowledge of the basic natural processes of embryogenesis makes it possible to solve a number of problems in medical embryology (prevention of fetal malformations, treatment of infertility), to carry out a set of measures to prevent the death of fetuses and newborns.

test questions

1. Tissue composition of the child and maternal parts of the placenta.

2. Critical periods of human development.

3. Similarities and differences in embryogenesis of vertebrates and humans.

4. Sources of development of tissues of provisional organs.

Histology, embryology, cytology: textbook / Yu. I. Afanasyev, N. A. Yurina, EF Kotovsky et al .. - 6th ed., Revised. and add. - 2012 .-- 800 p. : ill.

Selection on the base: The doctrine of the disease, general etiology, general pathogenesis Methodological m, Question of the copies General part.docx, 1 Introductory General nosology.doc, 2017-18 g General entomology.doc, examination tests for the course histology, cytology, embryologists, Official general VMT.doc, Tutorial GENERAL AND MILITARY HYGIENA.doc, practical general energetics.doc, lecture 2 General etiology and pathogenesis In E.ppt, Song why general.docx.

Blastula types

1. 
   Celloblastula formed with complete uniform crushing. It has a single-layer blastoderm and a cavity (blastocoel) located in the center. Lancelet.
2. 
   Amphiblastula formed with complete uneven crushing. Has a multilayer blastoderm and an eccentrically located blastocoel. Amphibians.
3. 
   Discoblastula formed by partial meroblastic cleavage and the embryonic disc spread over the yolk. It is formed by blastomeres and corresponds to the roof and marginal zone. The yolk corresponds to the bottom of the blastula and the narrow gap between them is the blastocoel. Birds, reptiles.

In placental mammals and humans, as a result of complete subequal asynchronous cleavage, a morula is initially formed, which consists of small light blastomeres located outside. Large dark blastomeres are located in the middle. The light ones form the trophoblast, and the dark ones form the embryoblast. At this stage of development, the human embryo corresponds to the stage of the blastula of other animals, but is not homologous to it, since the blastocyst wall does not participate in the construction of the body of the embryo.

Gastrulation - the period of embryogenesis, which is accompanied by the formation of germ layers: ectoderm, endoderm, mesoderm, and the embryo itself acquires a three-layer structure.

The germ layers are arranged in layers: a) ectoderm - the outer layer; b) endoderm - inner leaf; c) chord and mesoderm between them;

Types of gastrulation

The gastrulation process takes place in four main ways:

1. 
   Imigration - the movement of a part of the wall blastomeres into the middle of the embryo and the formation of the inner layer - endoderm;
2. 
   Intussusception is the process of pressing (invagination) of a part of the wall (bottom) of the blastula into the middle;
3. 
   Epibolia is the process of overgrowing by rapidly dividing cells of one section of the blastula wall of cells of another section, the division of which occurs more slowly. This occurs when the blastomeres of the vegetative pole have a large amount of yolk and divide slowly (amphibians).
4. 
   Delamination is a process that is accompanied by tangential division of the blastula wall and leads to the formation of two layers: primary ectoderm - outdoor; primary endoderm - ininternal. Typical for birds and mammals.

The types of gastrulations depend on the previous stages of development and on the degree of yolk accumulation in the ovum. In vertebrates, a combination of two or three types of gastrulation is observed.

Gastrulation in humans occurs during the period from 7 to 17 days of prenatal ontogenesis and consists of 2 consecutive phases:

Phase I lasts from 7 to 14 days and consists in the formation of the outer (ectoderm) and inner (endoderm) germ layers. As a result delaminationa layer of cells is split off from the embryonic nodule, facing the blastocyst cavity - primary endoderm (hypoblast). At the same time, among the cells of the embryonic nodule under the hypoblast, a process occurs cavitation - due to the accumulation of fluid in the center of the nodule, a cavity appears, and the cells surrounding it acquire an epithelium-like shape (an amniotic vesicle is formed). The opposite edges of the primary endoderm are tucked downward and, growing together, form yolk vesicle... The adjacent parts of both vesicles (the bottom of the amniotic and the roof of the yolk) form the embryonic navel or embryonic disc (from this formation the body of the embryo is formed).

In parallel with the formation of these vesicles from the embryonic nodule, starting from the 8th day, the cells are evicted extraembryonic mesoderm, which forms the chorion and amniotic stem, which is the basis of the future umbilical cord.

Phase II occurs from 15 to 17 days, and consists in the formation of the embryonic mesoderm. It is carried out by immigration of cells of the primary ectoderm in the space between the two germ layers. Its formation occurs by the formation of an average thickening of the anterior strip and a primary nodule. The space between the germ layers is filled with embryonic connective tissue - mesenchyme.

The source of the formation of the mesenchyme is the mesoderm and, to a greater extent, the ecto- and endoderm. Therefore, they distinguish - endomesenchyme, which develops from endomesoderm and ectomesenchyme - ectodermal origin.

There are no morphological differences between them, but they give rise to different structures:

a) entomesenchyme - to the tissues of the internal environment;

b) ectomesenchyme - to the auditory bones, connective tissues of the head;

The expulsion of cells from the zone of the primary nodule leads to the formation of an axial string of the embryo - chords... On the wall of the yolk sac at the end of the second week, blood islands and rudiments of primary blood vessels are formed. A digital process grows into the amniotic leg of the posterior part of the intestinal endoderm - alantois. The vessels of the yolk sac grow into the wall of the alantois and the chorionic villi, which are washed by the maternal blood. Formed as a result of these processes alantochorionprovides nutrition and respiration of the fetus at this stage.

Histo-organogenesis

Histo-organogenesis - the process of laying and forming tissues of organs and organ systems in the embryonic period as a result of a number of successive stages: induction, determination, reproduction, migration, cell growth, intercellular interactions and cell death.

Induction - the influence of organizing factors of some parts of the embryo on others, as a result of which the further development of organs and tissues is determined.

An organizing factor (inductor) is a specific area (point) of the embryo that affects other areas of the embryo and determines the further direction of its development. Such inducers can be proteins, nucleoproteins, steroids. Organizing factors can be of I and II order.

For example: the organizer, which is present in the dorsal lip of the blastopore, induces a section of the ectoderm and causes its differentiation into the neural plate. This is the first order organizer. In turn, a second-order organizer appears in the neural plate, which contributes to the transformation of a section of the neural tube into an optic cup.

Determination - determination of the further path of development of cells on a genetic basis due to blocking of individual components of the genome.

Determination is the basis of differentiation processes and there are 4 main types of differentiation: 1) ootypic; 2) blastomeric; 3) rudimentary; 4) histogenetic;

Ootypic -when the source material is represented by the presumptive regions of the zygote cytoplasm.

Blastomeric -during the blastula period.

Embryonic- characterized by the appearance of separate sections of germ layers (stage of early gastrulation).

Histogenetic - characterized by the appearance of rudiments of different tissues within the boundaries of one germ layer.

Reproduction cells. Cell multiplication is based on different programs of the metabolic apparatus of the cell - autosynthetic activity and heterosynthetic.When autosynthetic activity of cell metabolism is aimed at enhancing the processes of reproduction - increasing the number of cells. Heterosynthetic - is aimed at the formation of specific structures, or the synthesis and isolation of specific products.

The process of restoring the structure of a biological object after its destruction is called regeneration ... Depending on the level of organization, regeneration is: 1) cellular; 2) fabric; 3) organ.

Depending on the state of tissues, organs, regeneration is divided into:

1. 
   physiological, which occurs constantly in a healthy body;

1. 
   reparative, which occurs after trauma.

Migration - this is an active mass morphogenetic movement of cells from one part of the embryo to another, the result of which is the formation of tissues and organs.

Growth - the process of formation, development and organization of animal or human cells as a result of a number of complex transformations that occur from the moment of division to the next division.

Interaction of cells - the processes of interaction between cells of different primordia and their cells of the same type of the same primordium are of leading importance for the progressive differentiation of cells of embryonic primordia.

It has been established that for the normal development and existence of the skin epithelium and other epitheliums, constant or temporary contact with the developing connective tissue is necessary.

Cell death is the process of irreversible stopping of all functions of the cell and its connection with the environment.

The concept of provisional organs. Structure.

Provisional organs - temporary organs that develop during embryogenesis, outside the body of the embryo, and perform functions that ensure the growth and development of the embryo itself.

These include: 1) the yolk sac; 2) amnion; 3) serous membrane; 4) allantois; 5) chorion; 6) placenta.

Yolk sac - is a vesicle associated with the intestinal tube, the wall of which is covered with epithelium inside, and outside is formed by connective tissue.

Its formation occurs at the stage of early gastrula, when the embryonic (intestinal) endoderm and the extraembryonic yolk ectoderm located along the periphery of the disc can be distinguished on the inner embryonic layer. After the formation of the trunk fold, the yolk sac remains connected to the intestinal tube by the yolk stalk.

¨Functions: a) trophic; b) hematopoietic (7-8 weeks);

Amnion - a solid shell that is located around the fetus and takes part in the production of amniotic fluid and consists of two parts amnioticfacing the embryo, and external - serous.

The formation of the amnion occurs at the expense of the ectoderm and the parietal layer of the mesoderm, which first form amniotic folds that grow towards the dorsal surface of the embryo. The folds that grow on the embryo are connected, and both sheets - the ectoderm and the adjacent parietal mesoderm - grow together with the sheets of the same name on the opposite side. Two sheets of folds form two membranes - amniotic (water) and serous - outer.

The fluid produced by the cells of the ectoderm of the amniotic membrane contains proteins, carbohydrates and ensures the free development of the embryo, its amortization from possible damage.

¨Functions: ensuring the free development of the embryo, protection from possible mechanical damage and the influence of gravity.

Serous membrane is formed simultaneously with the amniotic one, participates in the supply of oxygen to the embryo, which makes it possible to consider it as a provisional respiratory organ.

Allantois begins its development in the caudal part of the embryo itself in the form of an outgrowth of the ventral wall of the posterior intestine, formed by the endoderm and the visceral layer of the mesoderm. The proximal part of the allantois is located along the yolk stalk, and the distal, growing, grows into the gap between the amnion and the serous membrane. it oran gas exchange and release:oxygen is delivered through the vessels formed in the allantois mesoderm; metabolic products are released into allantois. Recently, it has been established that at the early stages of human ontogenesis, allantois performs the function of an analogue of the bag of Fabricius, i.e. performs the function of the central organ of b-lymphocytopoiesis. After 2 months of embryogenesis, allantois is reduced.

Chorion- the villous membrane, develops from the trophoblast and extraembryonic mesoderm. Initially, the trophoblast is represented by a shell with primary villi, through which, after implantation of the embryo, a connection with the mother's body is established. At 2-3 weeks of development, an extraembryonic mesoderm appears, which germinates to the trophoblast and together with it forms secondary epitheliomesenchymal villi. After that, the trophoblast turns into a chorion, or villous membrane.

Penetrating into the mucous membrane of the uterus, the chorion forms with it placenta.

Placenta - it is the organ that provides a constant connection between the fetus and the mother's body.

¨ Development of the placenta begins at the 3rd week, when vessels begin to grow into the secondary epitheliomesenchymal villi and tertiary villi form. Later (6-8 weeks) macrophages, fibroblasts, collagen fibers differentiate around the vessels. The formation of collagen fibers in the villi coincides with an increase in the proteolytic activity of the trophoblastic epithelium (cytotrophoblast) and its derivative (syncytiotrophoblast).

With the development of the placenta, the mucous membrane of the uterus is destroyed and the histiotrophic nutrition changes to hepatotrophic. This means that the chorionic villi are washed by the mother's blood, which poured out from the destroyed endometrial vessels into the lacunae.

The placenta consists of 2 parts: 1) maternal; 2) fetal (embryonic).

The maternal part of the placenta -formed by the mucous membrane of the uterus in the area where the villi of the fetal chorion grow into it, this is the so-called main peel off (decidual). In addition to this shell, in the composition of the endometrium of the uterus of pregnant women, one distinguishes free from ingrowths of chorionic villi detached parietal shell, and bag falling off, which delimits the embryo from the uterine cavity.

Fetal part of the placenta formed by the villous chorion.

There are two types of chorion: a) branched chorion; b) smooth chorion;

Branched chorion, the villi of which grow into the endometrium at the site of the decidua.

Smooth chorion - the place of contact of the trophoblast with the bursa falling off the shell.

The process of formation of the human placenta most intensively occurs from 3 to 6 weeks of embryogenesis.

According to the structure of the mammalian placenta, 4 types are distinguished:

1) epitheliochorial; 2) desmochorial; 3) endotheliochorial; 4) hemochorial;

The human placenta refers to the discoid hemochorial villous placenta.

The structural unit of the placenta is cotyledon - these correspond to the branching of one stem villi. The stem or anchor villus is a large outgrowth of the chorionic plate, tightly fused with the decidua, from the lateral surface of which numerous branches of small chorionic plates extend.

The human placenta has about 200 cotyledons, which are delimited from each other by connective tissue septa - septa through which arterial vessels pass, which carry oxygen-rich and nutrient-rich blood to the lacunae of the placenta. In the lacunae, lacunar veins open with wide openings, through which maternal blood flows from the placenta.

The wall of the lacuna is formed by the connective tissue of the endometrium, which is covered with layers of amorphous substance - rohr's phobrinoid.

That part of the decidual membrane, which is located between the branched and smooth chorion, tightly fused with it forms the so-called closing plate, which prevents blood from flowing into the uterine cavity.

Hemoplacental barrier it is a barrier that separates the mother's blood from that of the fetus. It consists of epithelial cells and the basement membrane of the hemocapillaries of the chorionic villi, enriched with macrophages and fibroblasts, the connective tissue that surrounds the microcircular bed, the basement membrane of the chorionic villi, the syncytiotrophoblast layer, and also the Langhans fibrinoid located on the surface of the last fibrinoid.

Placenta functions: 1. trophic; 2. excretory; 3.depositing; 4. endocrine; 5. respiratory; 6.protective.

Umbilical cord- the cord formed by the connective tissue, in which the great vessels pass - two arteries and one vein, which provide blood circulation between the fetus and the placenta. The umbilical cord also includes the remains of the yolk stalk, allantois.

At the heart of the umbilical cord is mucous connective tissue - the so-called B arton jellywhich contains large amounts of hyaluronic acid. The latter provides a turgor of the umbilical cord. Of the cellular elements in the umbilical cord, tissue basophils and Kashchenko-Hofbauer cells were found, which protect the fetus from intrauterine infection.

CRITICAL PERIODS OF DEVELOPMENT

Critical periods of development - these are periods of the highest sensitivity of developing germ cells (progenesis) and the embryo (embryogenesis) to the action of unfavorable factors.

Domestic embryologist P.G. Svetlov in 1960. the theory of critical periods of development was formulated and experimentally proved. The essence of this theory is that each stage in the development of the embryo as a whole and its individual organs begins with a relatively short period of qualitatively new restructuring, accompanied by determination, proliferation and differentiation of cells. During this period, the embryo is most sensitive to the action of damaging agents.

Such periods are in: progenesis- spermatogenesis and ovogenesis; embryogenesis - fertilization, implantation (6-8 days), placentation and development of axial primordia (3rd, 8th weeks), the period of enhanced development of the brain (15-20 weeks), the period of formation of the main functional systems of the body (20-24 weeks ), the process of childbirth. postnatal ontogenesis - the period of newborns (up to 1 year), the period of puberty (11-16 years).

SKELETAL TISSUE.
Skeletal tissue (textus sceletales) are cartilage and bone tissues that take part in water-salt metabolism and perform supporting, protective and mechanical functions.

Cartilage tissue

Cartilage tissue(tekhtus cartilaginei) are the only type of tissue in which there are no vessels and are part of a number of organs of the respiratory system, joints, intervertebral discs. They consist of cells (chondrocytes and chondroblasts) and a large amount of extracellular substance

Cartilage contains about 75% water, 10-15% organic matter and 4-7% inorganic salts. The dry matter contains 50 to 70% collagen.

Cartilage classification

Depending on the structure and structural and functional features of the intercellular substance, there are 3 types of cartilaginous tissue:

1. Hyaline cartilage tissue.

2. Elastic cartilage tissue.

3. Fibrous cartilage tissue.

¨Functions. Cartilage tissue in the body performs supporting and shaping functions.

Cartilage histogenesis

The development of cartilage tissue is carried out in the embryonic period and in the postembryonic period during regeneration.

Embryonic histogenesis

The source of the development of cartilage tissue in ontogenesis is the mesenchyme - the embryonic connective tissue.

Cartilage histogenesis occurs in 3 stages:

¨ First stage -the formation of a chondrogenic primordium or chondrogenic islet. In those places where cartilage is formed, at the beginning there is a compaction of the mesenchyme, its cells lose their processes, their reproduction intensifies and they, adhering tightly to each other, form a certain tension - turgor. Such sites are called chondrogenic budsor chondrogenic islets... Mesenchymal cells, which are part of the chondrogenic islet, differentiate into chondroblasts, cells that give rise to the formation of cartilage tissue. In the cytoplasm of such cells, the number of free ribosomes increases, and areas of the granular endoplasmic reticulum appear.

¨ Stage two- formation of primary cartilage tissue... The cells of the central region are rounded, their size increases, a granular endoplasmic reticulum develops in the cytoplasm, with the help of which fibrillar proteins (type III collagen) are synthesized and secreted. As a result of such changes, chondroblasts turn into primary chondrocytes... The intercellular substance formed in this way is distinguished by its oxyphilia.

¨ Third stage- cartilage differentiation ... At this stage, primary chondrocytes are converted into secondary chondrocytes and acquire the ability to synthesize, in addition to the substances listed above, sulfated glycosaminoglycans (chondroitin sulfates) associated with collagen proteins (proteoglycans).

On the periphery of the cartilaginous anlage at the border with the mesenchyme, a perichondrium - the shell covering the outside of the developing cartilage, consisting of the outer fibrousand internal chondrogenic(cambial) layers.

Cartilage growth can occur in two ways:

¨ By overlay (appositional growth) while in the chondrogenic zone, cells are intensively dividing, differentiating chondroblasts in the chondrocytes, which produce the intercellular substance, the newly formed cells are layered onto the cartilage already present on the periphery. In the process of secretion of the intercellular substance, these cells walled themselves up in it.

¨ By internal (interstitial) growth, cartilage cells located in the center of the young developing cartilage retain the ability to divide mitotically for some time, remaining in one lacuna (isogenic groups of cells). By increasing the number of these cells, there is an increase in the mass of cartilage from the inside. Such growth is observed in embryogenesis, as well as during the regeneration of cartilage tissue.

Physiological regeneration - occurs due to the activity of chondrocytes, their production of the substance chondromucoid, collagen and elastin, which contributes to the formation of chondrin fibers.

As the cartilage grows and develops, its central parts become more and more separated from the vessels and experience difficulty in nutrition, which is carried out diffusely from the side of the perichondrium. Therefore, chondrocytes lose their ability to reproduce, some are destroyed, and proteoglycans are converted into a simpler protein - albumoid.

Cartilage cells

Chondroblasts - poorly differentiated young cells, capable of proliferation and synthesis of intercellular substance.

The form - irregular, elongated, flattened.

Development - from semi-stem cells (perechondroblast), which originate from stem cells. Stem cells, semi-stem cells, chondroblasts and chondrocytes form a diferon (histogenetic series).

Cytoplasm - contains a well-developed endoplasmic reticulum (granular and agranular) and elements of the Golgi complex, a lot of RNA. It is stained basophilically.

In the process of cartilage development, chondroblasts turn into chondrocytes. Chondroblasts carry out peripheral (appositional) growth of cartilage.

Chondrocytes - the main cells of cartilage tissue.

The form - oval, rounded or polygonal.

Localization - located in special cavities of the intercellular substance (lacunae). These groups of cells are called (isogenic).

Occur - due to the division of one cell. There are three types of chondrocytes in the isogenic group:

I the type of cells predominates in the young developing cartilage; division is often observed in these cells, which allows us to consider them as a source of reproduction of isogenic groups.

Characteristic for these cells is the presence of a high nuclear - cytoplasmic index.

Cytoplasm - has well-developed vacuolar elements, lamellar complex, mitochondria and free ribosomes.

IItype of cells - characterized by a decrease in the nuclear-cytoplasmic index, a weakening of DNA synthesis, but RNA synthesis is increased, the granular endoplasmic reticulum, the Golgi complex, which provides the formation and secretion of glycosaminoglycans and proteoglycans into the intercellular substance is intensively developed. Cytolemma and karyolemma are usually tortuous.

III type of chondrocytes. These cells are characterized by a low nuclear - cytoplasmic index, strong development and an ordered arrangement of the granular endoplasmic reticulum. This type of cells retains the ability to form and secrete protein, while the synthesis of glycosaminoglycans decreases.

Cartilage intercellular substancerepresented by an organic component - proteins, lipids, glycosaminoglycans and proteoglycans. The concentration of proteoglycans in this tissue is the highest. Fibrillar proteins are abundant, mainly type II collagen.

Fiber orientation determined by the direction of the lines of force.

The layer of intercellular substance adjacent to the cell cavity and forming its wall is distinguished by high light refraction and contains a felt-forming network of fibrils. It is sometimes called the cartilage cell capsule.

Hyaline cartilage tissue.

Localization- in the wall of the trachea, bronchi, at the junction of the ribs and sternum, articular surfaces and in the metaepiphyseal plates.

Structure... The hyaline cartilage is covered on the outside perichondrium (perichondrium).
The perichondrium consists of two layers: 1) outer; 2) internal;
* Outer - formed by fibrous connective tissue with blood vessels.

* Internal - formed mainly by cells prechondroblasts and chondroblasts.

Under the perichondrium in the surface layer of the cartilage itself, young chondrocytes spindle-shaped, the long axis of which is directed along the surface of the cartilage.

In deeper layers, chondrocytes acquire an oval and rounded shape, arranged in several groups, forming isogenic groups. Young chondrocytes and isogenic groups are surrounded by chondromucoid and collagen fibers (type II collagen).

· However, not all hyaline cartilage has the same structure.

The hyaline cartilage of the articular surface does not have perichondrium on the inward-facing surface of the joint. Articular cartilage consists of three areas, which are not clearly defined: a) external; b) medium; in the deep;

In the outdoor area small, flattened, poorly specialized cells are located.

In the middle - cells are larger, oval, rounded, arranged in the form of columns perpendicular to the surface.

Deep zone consists of calcified cartilage; only in this area are blood vessels found.

Elastic cartilage tissue

This tissue is localized in those structures that are subject to bending - these are: the auricle, carob and wedge-shaped cartilages of the larynx.

*Structure. - According to the general plan of the structure, elastic cartilage is similar to hyaline. Outside, it is covered with perichondrium. Cartilage cells are located in capsules singly or form isogenic groups.

In contrast to hyaline cartilage, the intercellular substance contains, along with collagen fibers, elastic fibers consisting of the protein elastin. They penetrate the intercellular substance in all directions. In the layers adjacent to the perichondrium, the elastic fibers pass without interruption into the elastic fibers of the perichondrium.

There are fewer lipids, glycogen and chondroetin sulfates in elastic cartilage than in hyaline. In addition, calcification never occurs in the elastic cartilage.

Fibrous cartilage

Localization between the vertebrae of the discs, semi-movable joints, in places where the transition of fibrous connective tissue (tendons, ligaments) to hyaline cartilage occurs and where the restriction of movement is accompanied by strong tension.

Structure - the intercellular substance contains parallel collagen bundles, which gradually loosen up and pass into hyaline cartilage. Chondrocytes in fibrous cartilage are arranged in a kind of rows - columns.

Cytoplasm cells are often vacuolated. From the hyaline cartilage to the tendon, the fibrous cartilage becomes more tendon-like. On the border of cartilage and tendon, instead of columns of tendon cells, between collagen bundles soldered into the base substance, there are columns of compressed cartilage cells, which without any border pass into real tendon cells located in dense connective tissue.

Bone tissue, general characteristics.
Bone (textus osseus) is a specialized type of connective tissue that has a high degree of mineralization of the intercellular substance.

Bone tissue consists of cellular elements (osteoblasts, osteocytes and osteoclasts) and intercellular substance (ossein and osseomucoid).

The intercellular substance contains about 70% inorganic compounds, mainly calcium phosphates. Organic compounds are represented mainly by proteins and lipids that make up the matrix. Organic and inorganic compounds combine to provide a very strong supporting fabric.
Functions

1. 
   support-mechanical- due to the significant strength of the bone tissue, it ensures the movement of the body in space and its support.

2. protective- bone tissue protects vital organs from damage;

3. depotcalcium and phosphorus in the body;

Bone classification

Depending on the structure and physical properties, two types of bone tissue are distinguished:

1. Reticulofibrous (coarse-fibrous)

2. Lamellar

Reticularly fibrous bone tissue- has a multidirectional arrangement of bundles of ossein fibers (type I collagen) surrounded by a calcified osseomucoid. Osteocytes lie between the bundles of ossein fibers in the lacunae of the osteomucoid. This tissue is characteristic of the skeleton of the embryo; in adults, it is found only in the areas of the seams of the skull and in the places where the tendons attach to the bones.

Lamellar bone tissue - strictly parallel arrangement of collagen fiber bundles and the formation of bone plates are characteristic.

Depending on the orientation of these plates in space, this tissue is divided into: 1) compact; 2) spongy;

Compact- characterized by the absence of cavities. The diaphysis of tubular bones is built from it.

Spongy - characterized by the fact that the bone plates form trabeculae located at an angle to one another. As a result, a spongy structure is formed. Spongy bone tissue forms the flat bones of the epiphyses of the tubular bones.

Bone tissue histogenesis

The source of bone tissue development is the mesenchyme. With the development of bone tissue, two differentions of cells (histogenetic series) are formed.

¨ First row- osteogenic stem cells, semi-stem stromal cells, osteoblasts, osteocytes.

¨ Second row - hematogenous origin - hematopoietic stem cell, half-stem hematopoietic cell (precursor of myeloid cells and macrophages), unipotent colony-forming monocytic cell (monoblast), promonocyte, monocyte, osteoclast (macrophages).

Distinguish between embryonic and postembryonic development of bone tissue.

Embryonic bone development can occur in two ways:

1. Directly from the mesenchyme - direct osteohistogenesis.

2. Indirect osteohistogenesis from the mesenchyme at the site of the previously developed cartilaginous bone model.

Postembryonic development bone is carried out during regeneration and ectopic osteohistogenesis.

Gastrulation (from Latin gaster - stomach) is the process of transformation of blastula into gastrula, accompanied by increased proliferation and directed movement of cells with simultaneous chemical and morphological restructuring (differentiation) in them. The morphogenetic movements of cells vary considerably in different classes of vertebrates. In the process of gastrulation, from a single-layer blastula, first two germ layers are formed - (epi - and hypoblast), and then, due to the continuing movements of cells, a third embryonic layer is formed - mesoderm and axial primordia (Fig. 3).

Figure: 3. A chicken embryo at the stage of late gastrula and the formation of axial primordia. 1 - ectoderm; 2 - endoderm; 3 - mesoderm; 4 - neural tube; 5 - chord; 6 - whole; 7 - aorta.

After that, the epiblast is called the ectoderm, and the hypoblast is called the endoderm. There are 4 ways of converting blastula into gastrula. They closely depend on the types of oocytes, and, therefore, the types of cleavage and blastula:

1. Intussusception (invagination) - typical for animals with oligolecitic primary isocytal oocytes (lancegum). In this case, the vegetative pole of the celloblastula invades the blastocoel, as a result of which a two-layer embryo is formed first, Consisting of the ecto - and endoderm, and then, due to the same invagination transformations, the third leaf, the mesoderm, is formed from the material of the internal germ layer (endoderm).

2. Epibolia (fouling) - occurs in animals with mesolecital eggs. Small blastomeres of the animal pole of the amphiblastula, cleaving more rapidly, overgrow the macroblastomeres of the vegetative pole, resulting in the formation of ecto - and endoderm.

3. Immigration (moving). Populations of blastomeres move into the blastula cavity and germ layers are formed.

4. Delamination (bundle). The embryonic layers are formed as a result of the splitting of the cells of the embryonic disc into 2 layers.

In animals with mesolecital and polyilecital eggs, a combination of several types of gastrulation is usually observed, which can proceed either simultaneously or replace each other.

Let us briefly dwell on the characteristics of gastrulation in birds and mammals. Their gastrulation proceeds in two stages.

During the first stage, the formed embryonic disc (germinal scutellum) is divided by delamination into 2 sheets - epiblast (primary ectoderm) and hypoblast (primary endoderm). The epiblast contains material from the secondary ectoderm, embryonic and extraembryonic mesoderm, and notochord. The hypoblast includes material from the intestinal and yolk endoderm. The epiblast is the bottom of the forming amniotic vesicle, and the hypoblast is the roof of the forming yolk sac.

Immediately after the formation of the epiblast and hypoblast, cells of the extraembryonic mesoderm are evicted from their composition into the cavity bounded by the trophoblast, which are further transformed into mesenchymocytes, which, together with the trophoblast, form the chorionic wall - the villous membrane of the embryo.

The second stage of gastrulation is characterized by the directed movement of rapidly multiplying epiblast cells from front to back, to the center and inward. As a result of such migration, in combination with invagination of cells, a primary stripe (with a primary groove) is formed, which is an analogue of the lateral lips of the blastopore, and a primary (head) nodule (with a primary fossa) is an analogue of the dorsal lip of the blastopore.

Through the edges of the primary groove, epiblast cells migrate into the depth of the embryonic disc and, spreading laterally between the epi- and hypoblast, form the mesoderm, and the head nodule, whose cells move through the anterior edge of the primary fossa and spread between the epi- and hypoblast in the form of a strand in the cranial direction, gives the beginning of the chord (cephalic process).

Thus, in birds (similarly to reptiles) and mammals, in the process of formation of germ layers, the leading methods are delamination and migration, and the auxiliary ones are invagination and epiboly.

Histo-, organ - and system genesis

As a result of the differentiation of the cells of the embryonic layers, some of them go to the construction of tissue and organ rudiments of the body of the embryo, and the other to extraembryonic organs. The formation of these primordia occurs on the basis of determination and committing.

Determination is a genetically programmed way of development of cells and tissues, and committing is a limitation of the possibilities of ways of their development. These processes are accompanied by further cell differentiation, leading to the formation of tissue rudiments.

Differentiation is understood as changes in the structure of cells associated with their functional specialization. At the same time, there are 4 main stages of differentiation: 1st - ootypic differentiation, when the material of future primordia is presumpted in certain areas of the ooplasm or zygote; 2nd — blastomeric differentiation; 3rd - rudimentary differentiation and 4th stage - histogenetic differentiation.

The chord is an inducer of the development of neuroectoderm, due to which the material of the neural plate begins to invaginate, sequentially forming a neural groove and a neural tube. The intestinal tube is formed from the material of the endoderm located under the neural tube.

The neural tube and intestinal tube are the axial organs of the embryo.

Thus, embryonic histogenesis and organogenesis is a process of transformation of poorly differentiated cellular material of embryonic rudiments into specialized tissues and organs. In parallel with this, system genesis is carried out.

General and Comparative Embryology

Plan

1. Morphological and functional characteristics of male germ cells.

2. Oocyte types according to the number and location of the yolk. The structure and function of the egg.

3. Fertilization, the concept of its distant and contact phases.

4. Definition of crushing and its types.

5. Gastrulation, methods of early and late gastrulation.

6. Extraembryonic organs of vertebrates (amnion, yolk sac, chorion, allantois, umbilical cord, placenta).

7. Placenta, types of placentas according to their structure, shape and way of feeding the fetus.

8. .The concept of in vitro fertilization and its meaning.

9. The human placenta, its morphological features and values.

10. Placenta structure.

11. Structural components of the hemochorial (placental) barrier.

12. The mother-fetus system.

13. The concept of critical periods of development.

Embryology occupies one of the prominent places in the complex of medical sciences. Knowledge of embryology is necessary to understand the main patterns of intrauterine development and its species characteristics in different representatives of the animal kingdom in connection with different conditions of their life and specific origin. Knowledge of the basics of comparative embryology helps to understand the general biological laws of the evolution of vertebrates, the phylogenetic conditioning of the processes of the formation of the human body, as well as to understand the foundations of genetic engineering. At the same time, it is importantabout understanding, by consequence I the influence of various unfavorable environmental factors on the embryogenesis of representatives of different species.

Knowledge of embryology is necessary for the future doctor for the rational prevention of anomalies and malformations, as well as for the prevention of adverse effects of damaging environmental and everyday factors on the course of pregnancy. The study of human embryology is the scientific basis for such disciplines as obstetrics, gynecology, pediatrics. Knowledge of the early stages of human embryogenesis makes it possible to correct the processes of formation and development of primary germ cells, to determine the causes of gametopathies, to prevent infertility, as well as to determine the stages of cleavage of the embryo, the causes of the occurrence of identical twins, to determine the timing and stages of implantation, which are necessary in the case of extracorporeal development of the embryo.

Embryology - the science of the formation and development of the embryo.

General embryology - studies the most general patterns of formation and development of the embryo.

Special embryology - studies the characteristics of the individual development of representatives of certain groups or species.

Embryology , a science that studies the development of an organism at the earliest stages, preceding metamorphosis, hatching, or birth. The fusion of gametes - an egg and a sperm - with the formation of a zygote gives rise to a new individual, but before becoming the same creature as the parents, it has to go through certain stages of development: cell division, the formation of primary germ layers and cavities, the emergence of embryo axes and axes of symmetry, the development of coelomic cavities and their derivatives, the formation of extraembryonic membranes and, finally, the emergence of organ systems that are functionally integrated and form one or another recognizable organism. All this constitutes the subject of study of embryology.

Processes and stages embryogenesis

1. Fertilization

2. Crushing

3. Gastrulation

4. Neurulation

5. Histogenesis

6. Organogenesis

7. Systemogenesis

Development is preceded by gametogenesis, i.e. formation and maturation of sperm and eggs. The development process of all eggs of a given species is generally the same.

Gametogenesis. Mature spermatozoon and egg differ in their structure, only their nuclei are similar; however, both gametes are formed from similar-looking primary germ cells. In all sexually reproducing organisms, these primary germ cells separate at the early stages of development from other cells and develop in a special way, preparing to perform their function - the production of sex, or embryonic, cells. Therefore, they are called germplasm - unlike all other cells that make up the somatoplasm. It is quite obvious, however, that both germplasm and somatoplasm originate from a fertilized egg - a zygote that gave rise to a new organism. So they are basically the same. The factors that determine which cells become reproductive and which ones become somatic have not yet been established. However, in the end, the sex cells acquire quite clear differences. These differences arise during gametogenesis.

Primary germ cells, being in the gonads, divide to form small cells - spermatogonia in the testes and oogonia in the ovaries. Spermatogonia and oogonia continue to divide repeatedly, forming cells of the same size, which indicates a compensatory growth of both the cytoplasm and the nucleus. Spermatogonia and oogonia divide mitotically, and, therefore, they retain the original diploid number of chromosomes.

After a while, these cells stop dividing and enter a growth period, during which very important changes take place in their nuclei. Chromosomes originally obtained from two parents are connected in pairs (conjugated), entering into very close contact. This makes possible subsequent crossing over (crossing), during which homologous chromosomes are broken and joined in a new order, exchanging equivalent sections; as a result of crossing-over, new combinations of genes appear in the chromosomes of oogonia and spermatogonia.

When the nucleus is rearranged and a sufficient amount of cytoplasm has accumulated in the cell, the process of division resumes; the entire cell and nucleus undergo two different types of divisions, which determine the actual process of maturation of germ cells. One of them - mitosis - leads to the formation of cells similar to the original; as a result of the other - meiosis, or reduction division, during which cells divide twice - cells are formed, each of which contains only half (haploid) number of chromosomes compared to the original, namely one from each pair. In some species, these cell divisions occur in reverse order. After the growth and reorganization of nuclei in oogonia and spermatogonia and immediately before the first division of meiosis, these cells are named oocytes and spermatocytes of the first order, and after the first division of meiosis - oocytes and spermatocytes of the second order. Finally, after the second division of meiosis, the cells in the ovary are called eggs (ova), and those in the testis are called spermatids. Now the egg is finally ripe, and the spermatid still has to undergo metamorphosis and turn into a sperm.

The biological role of sperm in the fertilization process

1. Provides a meeting with the oocyte.

2. Provides 23 parental chromosomes.

3. Determines the gender of the child.

4. Introduces centril into the oocyte.

5. Provides mitochondrial DNA.

6. Provokes the completion of meiosis by the egg.

7. Introduces cleavage signaling protein.

One important difference between oogenesis and spermatogenesis must be emphasized here. From one oocyte of the first order, as a result of maturation, only one mature egg is obtained; the other three nuclei and a small amount of cytoplasm turn into polar bodies, which do not function as germ cells and subsequently degenerate. All cytoplasm and yolk, which could be distributed over four cells, are concentrated in one - in a mature egg. In contrast, one first-order spermatocyte gives rise to four spermatids and the same number of mature spermatozoa, without losing a single nucleus. During fertilization, the diploid, or normal, number of chromosomes is restored.

Egg. The ovum is inert and usually larger than the somatic cells of the organism. The egg cell of a mouse is about 0.06 mm in diameter, while the diameter of an ostrich egg is more than 15 cm. Egg cells are usually spherical or oval in shape, but are also oblong. The size and other signs of the egg depends on the amount and distribution of nutritious yolk in it, which accumulates in the form of granules or, less often, in the form of a solid mass. Therefore, eggs are divided into different types depending on the content of the yolk in them. In homolecital eggs, also called isocytal or oligolecital, the yolk is very small and it is evenly distributed in the cytoplasm.

Sperm. Unlike a large and inert egg, spermatozoa are small, from 0.02 to 2.0 mm in length, they are active and can travel a long distance to reach the egg. There is little cytoplasm in them, and there is no yolk at all.

The form of spermatozoa is diverse, but among them two main types can be distinguished - flagellate and flagellate. Flagellate forms are relatively rare. In most animals, the sperm plays an active role in fertilization.

Fertilization- fusion of germ cells. Biological significance: renewal of diplomaand one set of chromosomes; determining the sex of the child; crushing initiation... Phases: d Istant I (capacitats and i, taxis); contact (acrosomali reaction, denudatsand I, penetrat and i, cortical reaction)

Fertilization. Fertilization is a complex process during which the sperm enters the egg and their nuclei fuse. As a result of the fusion of gametes, a zygote is formed - in fact, a new one, capable of developing under the conditions necessary for this. Fertilization activates the egg cell, stimulating it to successive changes, leading to the development of a formed organism.

When the sperm comes into contact with the surface of the egg, the vitelline membrane of the egg changes, turning into a fertilization membrane. This change is considered proof that egg activation has occurred. At the same time, on the surface of oocytes containing little or no yolk, the so-called. a cortical reaction that prevents other sperm from entering the egg. In oocytes containing a lot of yolk, the cortical reaction occurs later, so that several spermatozoa usually enter them. But even in such cases, fertilization is performed only by one sperm, the first to reach the nucleus of the egg.

In some oocytes, a protrusion of the membrane is formed at the point of contact of the sperm with the plasma membrane of the oocyte. fertilization tubercle; it facilitates the penetration of the sperm. Usually the head of the sperm and centrioles, which are in its middle part, penetrate into the egg, and the tail remains outside. Centrioles promote spindle formation during the first division of a fertilized egg. The fertilization process can be considered complete when two haploid nuclei - an egg and a sperm - fuse and their chromosomes are conjugated in preparation for the first cleavage of a fertilized egg.

Splitting up- formation of a multicellular embryo of blastulas.Characteristics: a) full, partial; b) uniform, uneven; c) synchronous, asynchronous.

Splitting up. If the appearance of the fertilization membrane is considered an indicator of the activation of the egg, division (cleavage) is the first sign of the actual activity of the fertilized egg. The nature of cleavage depends on the amount and distribution of the yolk in the egg, as well as on the hereditary properties of the zygote nucleus and the characteristics of the cytoplasm of the egg (the latter are entirely determined by the genotype of the maternal organism). There are three types of cleavage of a fertilized egg.

Crushing rules. It was found that fragmentation obeys certain rules named after the researchers who first formulated them. Pfluger's rule: the spindle always stretches in the direction of least resistance. Balfour's rule: the rate of holoblastic cleavage is inversely proportional to the amount of yolk (yolk makes it difficult to divide both the nucleus and the cytoplasm). Sachs rule: cells are usually divided into equal parts, and the plane of each new division intersects the plane of the previous division at a right angle. Hertwig's Rule: The nucleus and the spindle are usually located in the center of active protoplasm. The axis of each fission spindle is located along the long axis of the protoplasm mass. The division planes usually intersect the protoplasm mass at right angles to its axes.

{!LANG-4e74d52f0375c361f808be2a4831bf4c!}

{!LANG-d250e615dda724cf813eeb72e192c166!} {!LANG-5bfa14d69b03ea4875e71432fa5acb72!}

{!LANG-8b4286c7e546b69eae31b5915939be50!}

When {!LANG-43613e14f485737cb1e195d9347150ea!}{!LANG-9e27bf1d2e62ce15f3b3ca5a8b4a8f52!}

{!LANG-08c4f5522f42ad07fddc2131c5959f4e!}
{!LANG-56bbff797329f2f7814c87bb335e3508!}
{!LANG-9d9f40b09bc2585dfc9375101d40d7a3!}{!LANG-d432d62f80dcf1c20e90cdd4c10a20fe!} .
{!LANG-d2ec1fb73790853db1b8e6be7c24dd70!}{!LANG-3c2716a3ec8e875c0c596439445308ce!}{!LANG-6db772bd20557342f8aa227dcaedc98a!}{!LANG-d27e0293a682d68ad21081b0c84abd5c!}.
{!LANG-63ac19e086fc834464147fa31449abbd!}{!LANG-c8bb6a1870572f12a3fb2d51027a3c2d!}

{!LANG-7b5355f1e00088db71ae4b9f75d21b44!} {!LANG-3c2716a3ec8e875c0c596439445308ce!} {!LANG-c48d879b119deff23c2e8206a5f9ae34!}

{!LANG-bc10f4359fc8de9d0f02b671db3db870!} {!LANG-9a943d3414aa1643a7e6c37013bb9a74!}{!LANG-f732ad9ef95cff118a934b65e7bb3edc!}

{!LANG-20fcc7eb22d85ed0100d974d73e0d3c3!} {!LANG-556d0c2142c6b4ddfdc71e4ea32235d5!}{!LANG-5fdbf304676a621f0ea1166732f806ed!}

{!LANG-b1648747b500370a3ca726876306e700!}

{!LANG-decc6be1586a336072538813b2827d22!}

Induction

{!LANG-54fbc4ae8d4434b4432f34c1f52b0c11!} {!LANG-f01f8e2ab43af340244e875dd481f922!}

{!LANG-cd93d05fca212d7e1c3cf5e2589e188d!}

{!LANG-ed9dcfc59f8e82d175b52e8d1dbedbdd!}

{!LANG-0db0e317d18c144191d845ebff0fcf2d!}{!LANG-4f8a95e5c9afdd43e3d1d70b074f77d8!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}{!LANG-2cb5f1bdbc1fff6b2831a1333bb6b0a9!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}

{!LANG-eedbe9529b49c86b4131dc77cdca0639!} {!LANG-3c2716a3ec8e875c0c596439445308ce!}	{!LANG-8790f119009cb0e59d0dd3da278191c6!}	{!LANG-4a9bbebc9cbd341fc7836dc9915082d9!} {!LANG-a9e45b8daac82ba9c4bd333d98e04862!}	{!LANG-d3cf52e99066daeeb6fed8fb0b402ec3!}	{!LANG-4a9bbebc9cbd341fc7836dc9915082d9!} {!LANG-7e5a752f302c8bfcca96c8a263e6a598!}
Intussusception	{!LANG-57fa5d9beef9d1edae31a0db1284b650!}	{!LANG-bfb0642048520607b32a9a0299074b22!}{!LANG-ee4fcdcfdd688ecc1435644f4b977720!}	{!LANG-c2cb6e7465995657f06a60d5dd509bac!}	Celloblastula
{!LANG-ddcc2d21a971a5cc1c53fe0ebec6e0f9!}	{!LANG-eda093beb75dcd1ebc3d3e9059c0feb6!}	{!LANG-c953026cc4dc0b8ed7286965a4c2e14a!}	{!LANG-3e663e1ab5db4324253013d2cdc569d9!}	Amphiblastula
{!LANG-ec0250ba42988289f5797320eeab6cef!}	{!LANG-14facf68f74f735c845df804aefc2c22!}	{!LANG-334f33526c57c9bb5742770d4fe57b64!}	{!LANG-ec743ebd2c38a896cc919a4bf629e078!}	{!LANG-26d6c0ec56bf5ddb6f31add82d3e6c4b!}
{!LANG-644e149720128ca859a10ec6a19e7d49!}	{!LANG-ed2c6f5d61b56d3026a6f3dccdfde3d5!}	{!LANG-334f33526c57c9bb5742770d4fe57b64!}	{!LANG-e4348b8bcc9d9eae325ec94b0e88e330!}

{!LANG-1a13306bc79853e6dddb904ea7dc2173!}{!LANG-38b451399ca885610b44314c5c7326d3!}	{!LANG-a5c7c0094fa9f4ce119327656fb6ee66!}	{!LANG-e2df1d2e4061d85003827dac705a60ae!}{!LANG-d27e0293a682d68ad21081b0c84abd5c!}	{!LANG-25ede63eb2e7f35a823465bffd9688cc!}
{!LANG-b4388ba596a3a73ca9291e3dd27c5aee!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}{!LANG-cfaa9d22d14f1cd485972efc8f371fd1!}	Intussusception	{!LANG-9e72c9ddf37104c011b9ce5178e0791d!}	{!LANG-6cd21974ca3f7b97b4c5f200e32ca189!}
{!LANG-9986efb4a303fa7213b6048be8189664!} {!LANG-902e172d9b4e8244a1752886b82ec5a6!}{!LANG-766d2ac402125866103f72ac14665762!}	{!LANG-ddcc2d21a971a5cc1c53fe0ebec6e0f9!}	{!LANG-7b2d0fbea9407d405754cf246d4c9f28!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}{!LANG-329859cc30a527540f6496080aef174f!}	{!LANG-05f56d07dabb07184689d863425e526e!}
{!LANG-3fd2836dd1296a34ebb278aa869b4014!}	{!LANG-0ab5e213063c0ce497e0e7b6a6a224c5!}{!LANG-605236a8375befe6990d06c73470d815!}	{!LANG-6e590449f83bde09680b248b71f73000!}	{!LANG-05f56d07dabb07184689d863425e526e!}

Provisional organs

{!LANG-7b19194baefbf86b5d5b8135b01cce4c!}

{!LANG-2fba3ce86acf5c462f94f0c3b801aa7a!}

{!LANG-5e30154165b6855d28eecf46af0f67f3!}

{!LANG-0d7ba0c99fefab5d2fbcaa8d293614af!}

{!LANG-dac60e390143f2189269da33240b9871!}

{!LANG-648bed11837f22ec91f3b1e7f7ce4f1b!}

{!LANG-1a5fc4978ffd5702b17af8e338b7f83b!}

{!LANG-4dffecfbe8f1fc6c4bc6b4153f938d73!} {!LANG-c034e8bffa7a954e695880af00f9365e!}

{!LANG-5dac5a65cf3d62d4c97464afe56937e5!} {!LANG-86346faa6f0222897dbe6cbe414f412e!}

{!LANG-2d9b99ad84e29914ab87da1523b75da9!} {!LANG-bcd01266c0950f9998b7ca09d2478558!}

{!LANG-714797073020b38be133268b454185fe!}{!LANG-a52b0ed61e19a4ebbb21434faa8d555e!}

{!LANG-948ebfd5a71bfb8bac1dcd7cc6189c82!} {!LANG-76e97b97f8b276fcdeaa6d79f8add2b7!} {!LANG-d06753693c282025f391771d82f7d511!}{!LANG-2228433a258faa14b72325d1622d549a!}

{!LANG-e675f8b18875dbcee4be63d518aeb5dd!} {!LANG-6e5f658688057f26d7968c9ce520515c!} {!LANG-a5e12604381f069a275a916ead1c609f!}{!LANG-7e96c860fbbfff5ad2fea73bd77b5c5e!} {!LANG-42db10bd95164786c01748c39ff3c176!}{!LANG-1b698a372018bf61096844ed8bd1b3ee!}

{!LANG-842924f97f729c269e5ae7940080ba63!} . {!LANG-7070adef458d02220c69ca8621b2b6ec!}

{!LANG-d1eb1eff5458a80e74dd2b95c0b4595a!} {!LANG-582b36a302fd0e2327ed56457bd646e8!}

{!LANG-8ef7dcb58f49fcecc781b6d13ea3d916!}

{!LANG-8884fa7083a02d100069af2716d15b38!} {!LANG-b5906dfd8be4421ca246fe407da11431!} {!LANG-632734e94696ad41969883fdf6265692!} {!LANG-89d7ca8b1e823d30968bb4c1241475e2!} {!LANG-556d0c2142c6b4ddfdc71e4ea32235d5!} {!LANG-a514afe5cec1cf998cba75705d2f1230!}

{!LANG-be5357467c8ab7b63335c4001b7032c3!} {!LANG-a412b623c6945b65a40e241329e735cd!}{!LANG-c87a617640d4121c7a563ff233b8aa2c!} {!LANG-4eb385f339372d80e57f1050082a516f!}{!LANG-412371306ce78931413273d66eccd2f9!}

{!LANG-6a52c860391445adeded782098dac0e0!} {!LANG-902e172d9b4e8244a1752886b82ec5a6!}{!LANG-9b9a54ce6db375b91f3b2a5f8cfbf7d0!}

{!LANG-ecf812f5b290fa90342763389e9947ea!} {!LANG-66d18cbd489694938b92dd747804ec09!}{!LANG-0c5ac48b18420d994bf823c1da14906e!}

{!LANG-ea4675b8398fce1c2effd87917e0b1f2!}{!LANG-cad54bb342a909f0dcc06b3ed26a55ad!}

{!LANG-5e0b976398fc3c22785fee5ece7585ca!}{!LANG-5ef2c0857844fd95b5e30401a37e1246!} {!LANG-902e172d9b4e8244a1752886b82ec5a6!}{!LANG-4ab332cc93cc5c3c8b6f0c2e8dd3c30c!} {!LANG-88f0edec3265f2f087ccbd48db4ff30d!}{!LANG-ef77ccd698bf0658b52c81ee459899c8!}

{!LANG-f280caf434e094c2dd42cc3f115b4224!}{!LANG-5ef2c0857844fd95b5e30401a37e1246!} {!LANG-902e172d9b4e8244a1752886b82ec5a6!}{!LANG-159c93331766ff5a83204f6670ea9dd4!}

{!LANG-1a13306bc79853e6dddb904ea7dc2173!} {!LANG-b5906dfd8be4421ca246fe407da11431!} {!LANG-93d10a8d06b08b4910f952942e44f9b0!} {!LANG-3e8266521a292af00286850dc934b8c2!} {!LANG-556d0c2142c6b4ddfdc71e4ea32235d5!} {!LANG-5dcf9067cfc62de972bcdb548c28686d!} {!LANG-1c72047c677c50144ecc6c5e2e2b4048!} .

{!LANG-59f4d24f262e02bae9e12468a098b10c!}

{!LANG-ba1ef0470794a08f07a54ad533eb7673!} {!LANG-224392aa6e297795aa8b82d6da9f0523!}{!LANG-b97b7e96370405270f747effee1ef1ad!} {!LANG-a3cf81ac412decd52647e3f1d837d18c!}{!LANG-c22fe5a0fe3af0750c4bf8a56867cb37!}

{!LANG-ff9ebe950d972f2b7985da2155d4adcb!}{!LANG-4f8a95e5c9afdd43e3d1d70b074f77d8!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}.

{!LANG-8370505640c93db5439d6d11c51736a6!}{!LANG-3c2716a3ec8e875c0c596439445308ce!}{!LANG-34d052cd131de81f712a514763623554!}

{!LANG-6ff8698ce56994f5f84c322c09ab3dd2!} {!LANG-a59d3f129668e633ec79adeecc6b7aaf!}{!LANG-d27e0293a682d68ad21081b0c84abd5c!}.

{!LANG-c2c514445211542159eb830d19fe624e!}{!LANG-854fb3f31c663569bc73c34d984fef22!} {!LANG-224392aa6e297795aa8b82d6da9f0523!}in {!LANG-fa8f0684889dccdc23eb6e5d4891ac77!}{!LANG-6bd6f9eced54aced70f6aa3da61bc670!}

{!LANG-2d9082d09e954615c4c915aa6d3c298b!}{!LANG-89b05a698efb784e152d19058c02b3c5!}

{!LANG-8479a97b3e2b8ac6f6b06839b3bb1180!}{!LANG-4f8a95e5c9afdd43e3d1d70b074f77d8!} {!LANG-d27e0293a682d68ad21081b0c84abd5c!}.

{!LANG-7867c2a137bc36e23da078ebf4f650cd!} {!LANG-897351b9ef15bd715f819d0f58af7aaa!}

{!LANG-53b9fc646dfec94a849025cd73e51cba!}

{!LANG-6a2b232970244fbb883754ca5895c1c2!}

{!LANG-dac906a18b88e971fbe2453d7128510c!} {!LANG-28d91c128ab1ea31a5f4a27622592a67!}

{!LANG-3aa2a66a300f04528a017ec0184e261f!}

{!LANG-01b4714fe76bca5f2dee59d746e8727e!}

{!LANG-9192cf0c85ddeb5960d76a3ba94d8f81!} {!LANG-31f9c0b4c08919aa20d0e4dfc2066ab5!}{!LANG-614b855c50bb5f49c94d39d34b0a9b6c!}

{!LANG-457f9fd57d12760bb45597e5079cf20a!} {!LANG-44b6bf5a763f257fc2abc85b96809dc8!}

{!LANG-a0793447926b37cef496151ec96b55cd!}

{!LANG-febe9fca94dca73bf5be662fbf47a94f!}

{!LANG-bbd9592366c190b6c4f518d14ed4d8ac!}

{!LANG-561192267a2c96ed69b7452b2c19e22f!}
{!LANG-63da9de530dea79d530aa0b69cd9814b!}
{!LANG-eeabfdd497d858018f2db82972bae100!}
{!LANG-bf085f5efe06164a2865402e83e433b3!}
{!LANG-730f49c652f35ef70e68daaec6e568e2!}

{!LANG-b9751673f935a7826f19bab8a0357c2a!}{!LANG-688a19ff12b369dcf87b3547d0380135!} {!LANG-38eea7941b5199c2f1ea36a1ba0b5ad0!}

{!LANG-acb7d1d2ad9726c2a85d336fea80bd3b!}

{!LANG-cc0922dba7676b95fc817f7e76fa767f!}

{!LANG-4e234d690abda9941e080b0c00b4a738!}{!LANG-bb7cc5a3c1d8ed21928e5ff5a7c71fb1!}

{!LANG-007f3e961c50fc1f0bc0affd2bbcdbf5!}

{!LANG-f1a8df11ebe97465e1cf2550ce5534b8!}

{!LANG-8dc4b00e3c40b6a5d2761513c7a89549!}

{!LANG-dce4a3e6828ca44de652295c7ba8e3f6!} {!LANG-93c929e00029c99062e2ef80a7d6d545!}

{!LANG-71a4369480c261adca51919fd6de129d!} {!LANG-23a99468d3587aef3488343c394c4f6d!}{!LANG-525746de6b69c3ef6661f0941d39b9e3!} {!LANG-b5906dfd8be4421ca246fe407da11431!}{!LANG-a3cef276182801aaf7fb201589218976!}.

{!LANG-e05ec678cf2732ec7adaafabcc89422c!}

{!LANG-cc3c1259b78b7cb2f916fc5e526e4fba!}{!LANG-f9740a5a49e7d9e1bf54e3f1d870487e!}

{!LANG-09aa7be361368c3c82ace33ea81d6799!}{!LANG-2ce69c98d7fa7e9347e7481b4e48388c!}{!LANG-c8d6ad6fab77fd4f1bc83dfe235195d6!}

{!LANG-f2c1612a0c057c360746088381edb019!}

{!LANG-1541ff01f00ffdee1fd1d2e6ee84f97d!} {!LANG-2bd80992747570cc2cb02af2a16a9245!}

{!LANG-9b37569d50acd9039897c5bbeb69ec2c!}

{!LANG-46354b8d6b26c3e9a6e224a555bb86fe!}

{!LANG-fe5b0c56ffe76225b034f0a6c39b3e49!}

{!LANG-60cd501d0bdfd81d5ee69cecc8c9002d!}

{!LANG-01bf6bda3fec8982d98f01dcdd73727e!}{!LANG-992d5ba5fde110c2bfd9436efde4d9ee!}-24 {!LANG-992d5ba5fde110c2bfd9436efde4d9ee!}{!LANG-03b8eca10cae649d4c7516d0d7900bda!}

{!LANG-b400eec925b727d1cbdd1ed463245133!}

{!LANG-d9363f9f83a3e1971b219c9cfc8c4d11!}

{!LANG-3b605d96e74986d804b967013467e1c2!}

{!LANG-04e40042de216ac47cda519a79acab0e!}

{!LANG-b0cab532d2131e381e09d6bc37eaa1b5!}

{!LANG-4dd7bdb5e1de931daec97b274b3678ad!}

{!LANG-abcd4a92daa24d5f9c289c758152f0de!}

{!LANG-9e964a49dc200b155e71fc395e81d7da!}
{!LANG-1f0bf46136931f4184bddd09c85fa183!}
{!LANG-032486e5aff4e2720e79724bef7a2ef9!}
{!LANG-aaaf39e3bd001e0a8e053e7847bd4e6c!}
{!LANG-d08ce6c08d7d8ba785b8c3fbb5d072de!}
{!LANG-2e4fc328d1b238798daf8a72ec1f00b3!}
{!LANG-8a79d0154fff5a5491d0e92b3fa2703d!}

{!LANG-fceb6c9ad7c9c3cb1a7cd30406e9fe63!}{!LANG-5686e1385870020be7c5d40c04026046!} {!LANG-d513e24970257a5069f59910a9197b70!}{!LANG-04d63bad25f0c4dadd2f307f3b65e0ae!}{!LANG-faff4c95634b870ae229159c499294b7!}{!LANG-876b259d938fbb18bc9d4865ab4194c6!}

{!LANG-740a9cb3d2e397c06fcf6c41eb03ef41!}

{!LANG-5ed53446ad21150dc42abdc72a299c01!}

{!LANG-b032a7b871de04e7bcd0dd62e80973b5!}

{!LANG-3a6cba1b4d3d507c92c6b1c14b2dd50e!}

{!LANG-731a6f17396a2bc5363a118618b8cc66!}

{!LANG-2b569dfe6aee6915386fc7902aedd2a9!}

{!LANG-4d4ad35e951b7af7ee8611c0657075d9!}{!LANG-0c00e78ebc9223bf1218e2af694bd05a!}

{!LANG-2d3272154c072a58061a853bdde28cec!}{!LANG-c2177d214d1176df969d15c618dc723c!}” {!LANG-f80cf35b0a00e2bc3560b215ec9e5470!}

{!LANG-f6c3d0683f8525b7a66a965dc35cfa47!}{!LANG-281ea569e4ae7ff550f81a5657d7ce1d!}

{!LANG-da93e9d77cd293f18e0c5da6c8e4a8d9!} {!LANG-49d990110fb83695778b4ccdf18e26c0!}{!LANG-8d9df93033c71cee6356c67db6a2e99b!} {!LANG-b7b1e47300826afc0730153c724f5bac!}] ; {!LANG-09403a4bfd648b1568f8b88f00ff19e4!} {!LANG-ade6b1863b932f8cff9f7e9428cf96e0!}{!LANG-14d34ab65aecafa91ef20192753014e1!}{!LANG-6e786bb80d139fda3d0e891327e90597!} {!LANG-882791489818f5267ca60326fc53f8e6!}. 93 –107 .

{!LANG-45501f9f1a6eafab01e17c36868074e6!} {!LANG-4ffc7b6540c9994e74e292e5f9f3da7a!}{!LANG-40db467f6fc2fbcd825f790f68e62dbb!} {!LANG-131576db5928850f5d36fce528e01d5d!}{!LANG-01fff9491d9c818cc1e5343894f69bff!}

6. {!LANG-00213991f2665e3040b2ad1a6dd564fc!}/ {!LANG-a250625ef5ddae73cc216f021f9d7770!}{!LANG-dc80dbf08880dc03a488c82e252ddbe4!}

{!LANG-7bf72fc731ac80b1c2ebfc59d3c9a816!}

{!LANG-8b46621ba833fcb1f8fab13c89926e1d!}

2. {!LANG-041d3314a27bd62c7cac25e91b9367ee!}

{!LANG-84fbf2d8df73f615121409fb2f34606f!} {!LANG-0220f37d9cd3d228a09c3b7b7d39603d!}{!LANG-ceffe15d85dd194970e70b59c3df1040!} {!LANG-52b40d200373455f6556f47617c0d3a6!}/ {!LANG-eeb17cbb56e285ca39ce597b91f73ba9!}{!LANG-f8c9fbccc19a2a642841883c29be230f!} {!LANG-23e8d71e9cb44e3a0800b0e0f0c1de93!}

{!LANG-bc8ae17b9b388e08716cb906089ce05a!}

{!LANG-6c6415439d718328fba6e0e4e065e245!}

{!LANG-3187152be7def4add339972fac82c7e4!}

{!LANG-509822f6a6e90eeea1f6502bf46c0ad9!}

{!LANG-738fceedb54df0eb9cca24c434c34db6!}

{!LANG-cff452d721a21f6f53a8d50a2e117893!}

{!LANG-8ac8d83536e10e68e5be7c0964006582!}

{!LANG-20e55cc4c4a3b49e3d6cca60c018ad2f!}

{!LANG-f96098011f75dc778ac314cc02a67f1d!}

{!LANG-8af5c6e6b5e75f771ddbc1b502d4dd90!}

{!LANG-057f616f0b0f0f36cdca9678752f425e!}

{!LANG-aa516eb709f5be514e3bfab079dfcbce!}

{!LANG-07378f8fe182597796ecdbe1dbb6be80!}

{!LANG-22ac2f557074f8f291708ebafc903e80!}

{!LANG-febce8db5b3398766f6014280e89ff6e!}

{!LANG-3553974ef16940c721bf6c10e48b4ddc!}

{!LANG-9e93849b42113a967a489258beffc832!}

{!LANG-cba7b471b5f08ca8d7d3b42ac6cf1775!}

{!LANG-5342e700ece398a39dfbf10e04381b3b!}

{!LANG-2fd338bc870954d1b42ff1c885568857!}

{!LANG-21270c0fa598cf3f12b932653a628242!}

{!LANG-46bb8e733a7168039e22b315b89608c7!}

{!LANG-4a2c9bb1eb02de5263714ca79f98014d!}