Topic 7. HUMAN EMBRYOLOGY

Progenesis

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

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

1) breeding period - I;

2) growth period - II;

3) ripening period - III;

4) period of formation - IV.

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

The head contains:

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

2) case;

3) acrosome;

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

The tail of the spermatozoon is divided into:

1) liaison department;

2) intermediate department;

3) main department;

4) terminal department.

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

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

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

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

3) maturation period.

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

Shells of the egg:

1) primary (plasmolemma);

2) secondary - shiny shell;

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

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

It is divided into three phases:

1) remote interaction;

2) contact interaction;

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

Three mechanisms underlie the remote interaction:

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

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

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

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

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

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

2) gradual but incomplete destruction of the zona pellucida.

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

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

Conditions necessary for the fertilization of an egg:

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

2) patency of the female genital tract;

3) normal anatomical position of the uterus;

4) normal body temperature;

5) alkaline environment in the female genital tract.

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

Embryogenesis

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

1) crushing period;

2) the period of gastrulation;

3) the period of histo- and organogenesis.

In obstetrics, embryogenesis is divided into other periods:

1) initial period - 1st week;

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

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

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

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

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

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

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

The developed blastocyst consists of the following structural formations:

1) embryoblasts;

2) trophoblasts;

3) blastocele filled with liquid.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

1) the formation of a smooth chorion;

2) the formation of the villous chorion.

The placenta is subsequently formed from the villous chorion.

Chorionic functions:

1) protective;

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

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

The amnion wall consists of:

1) extra-embryonic ectoderm;

2) extra-embryonic mesenchyme (visceral layer).

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

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

1) extra-embryonic (yolk) endoderm;

2) extra-embryonic mesenchyme.

Functions of the yolk sac:

1) hematopoiesis (formation of blood stem cells);

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

3) trophic (in birds and fish).

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

1) germinal endoderm;

2) extra-embryonic mesenchyme.

Functional role of allantois:

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

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

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

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

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

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

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

1) chords;

2) neural tube;

3) intestinal tube.

The structure and functions of the placenta

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

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

Functions of the placenta:

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

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

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

4) detoxification. The placenta helps detoxify some drugs;

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

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

The structure and functions of the decidua

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

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

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

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

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

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

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

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

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

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

1) dorsal part (somites);

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

3) ventral part (splanchiotoma).

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

1) peripheral zone (dermatome);

2) central zone (myotoma);

3) medial part (sclerotoma).

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

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

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

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

1) visceral leaf;

2) parental sheet;

3) cavities - coelom.

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

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

2) skin ectoderm (skin epidermis develops);

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

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

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

1) intestinal (or germinal), endoderm;

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

From the intestinal endoderm develop:

1) epithelium and glands of the stomach and intestines;

2) liver;

3) pancreas.

Organogenesis

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

Critical periods in human development

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

The critical periods in human development are as follows:

1) gametogenesis (spermato- and ovogenesis);

2) fertilization;

3) implantation (7 - 8 days);

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

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

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

7) the birth of a child;

8) neonatal period (up to 1 year);

9) puberty (11 - 16 years).

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

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

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

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