The development of the human embryo

To understand the individual features of the human body structure, it is necessary to become familiar with the development of the human body in the prenatal period. Each person has individual features of external appearance and internal structure, the presence of which is determined by two factors. First of all, this is heredity - features inherited from parents, as well as the result of the influence of the external environment in which a person grows, develops, studies, works.

Individual development, or development in ontogenesis, occurs in all periods of life - from conception to death. In human ontogenesis, two periods are distinguished: before birth (intrauterine, prenatal; from the Greek natos - born) and after birth (extrauterine, postnatal). In the intrauterine period, from conception to birth, the fetus (embryo) is located in the mother's body. During the first 8 weeks, the main processes of formation of organs and body parts occur. This period is called embryonic, and the organism of the future person is an embryo (embryo). Starting from the 9th week, when the main external human features have already begun to appear, the organism is called a fetus, and the period is called fetal.

After fertilization (fusion of sperm and egg), which usually occurs in the fallopian tube, the fused sex cells form a single-celled embryo - a zygote, which has all the properties of both sex cells. From this moment, the development of a new (daughter) organism begins.

The first week of embryonic development

This is the period of division of the zygote into daughter cells. During the first 3-4 days, the zygote divides and simultaneously moves along the fallopian tube towards the uterine cavity. As a result of the division of the zygote, a multicellular vesicle is formed - a blastula with a cavity inside (from the Greek blastos - sprout). The walls of this vesicle consist of two types of cells: large and small. The walls of the vesicle - the trophoblast - are formed from the outer layer of small light cells. Later, the trophoblast cells form the outer layer of the embryo membranes. Larger dark cells (blastomeres) form a cluster - an embryoblast (embryonic nodule, embryo rudiment), which is located medially from the trophoblast. The embryo and adjacent extraembryonic structures (except for the trophoblast) develop from this cluster of cells (embryoblast). A small amount of fluid accumulates between the superficial layer (trophoblast) and the embryonic node.

By the end of the first week of development (6-7 days of pregnancy), the embryo is implanted into the uterine mucosa. The embryo's surface cells, which form a vesicle - trophoblast (from the Greek trophe - nutrition, trophicus - trophic, nourishing), secrete an enzyme that loosens the surface layer of the uterine mucosa. The latter is already prepared for the embryo to be implanted into it. By the time of ovulation (the release of an egg from the ovary), the uterine mucosa becomes thicker (up to 8 mm). Uterine glands and blood vessels grow in it. Numerous outgrowths - villi - appear on the trophoblast, which increases the surface of its contact with the tissues of the uterine mucosa. The trophoblast turns into a nutritious membrane of the embryo, which is called the villous membrane, or chorion. At first, the chorion has villi on all sides, then these villi are preserved only on the side facing the wall of the uterus. In this place, a new organ, the placenta (baby's place), develops from the chorion and the adjacent mucous membrane of the uterus. The placenta is an organ that connects the mother's body with the fetus and provides it with nutrition.

The second week of embryonic development

This is the stage when the embryoblast cells are divided into two layers (two plates), from which two vesicles are formed. The ectoblastic (amniotic) vesicle filled with amniotic fluid is formed from the outer layer of cells adjacent to the trophoblast. The endoblastic (yolk) vesicle is formed from the inner layer of cells of the embryoblast germinal node. The embryo's rudiment ("body") is located where the amniotic vesicle comes into contact with the yolk vesicle. During this period, the embryo is a two-layer shield consisting of two germ layers: the outer one - the ectoderm (from the Greek ektos - outside, derma - skin) and the inner one - the endoderm (from the Greek ёntos - inside). The ectoderm faces the amniotic vesicle, and the endoderm is adjacent to the yolk vesicle. At this stage, the surfaces of the embryo can be determined. The dorsal surface is adjacent to the amniotic sac, and the ventral surface is adjacent to the yolk sac. The trophoblast cavity around the amniotic and yolk sacs is loosely filled with strands of extraembryonic mesenchyme cells. By the end of the 2nd week, the embryo is only 1.5 mm long. During this period, the embryonic shield thickens in its posterior (caudal) part. Here, the axial organs (chord, neural tube) begin to develop.

Third week of embryonic development

The period of formation of the three-layer shield. The cells of the outer, ectodermal, layer of the embryonic shield shift to its posterior end, resulting in the formation of a ridge extended in the direction of the embryo's axis. This cellular strand is called the primary streak. In the head (front) part of the primary streak, the cells grow and multiply faster, resulting in the formation of a small elevation - the primary node (Hensen's node). The primary streak determines the bilateral symmetry of the embryo's body, i.e. its right and left sides. The location of the primary node indicates the cranial (head) end of the embryo's body.

As a result of the rapid growth of the primary streak and the primary node, the cells of which grow to the sides between the ectoderm and endoderm, the middle germinal layer, the mesoderm, is formed. The mesoderm cells located between the scutellum layers are called the intraembryonic mesoderm, and those that migrate beyond its limits are called the extraembryonic mesoderm.

Some of the mesoderm cells within the primary node grow forward especially actively, forming the head (chordal) process. This process penetrates between the outer and inner layers from the head to the tail end of the embryo and forms a cellular strand - the dorsal string (chord). The head (cranial) part of the embryo grows faster than the tail (caudal), which, together with the area of the primary tubercle, seems to retreat backward. At the end of the 3rd week, a longitudinal strip of actively growing cells - the neural plate - stands out in front of the primary tubercle in the outer germinal layer. This plate soon bends, forming a longitudinal groove - the neural groove. As the groove deepens, its edges thicken, come closer together and grow together, closing the neural groove into a neural tube. Subsequently, the entire nervous system develops from the neural tube. The ectoderm closes over the formed neural tube and loses connection with it.

During this same period, a finger-like outgrowth, the allantois, penetrates from the back of the inner (endodermal) layer of the embryonic shield into the extraembryonic mesenchyme (the so-called amniotic stalk), but does not perform any specific functions in humans. Along the allantois, blood umbilical (placental) vessels grow from the embryo through the amniotic stalk to the chorionic villi. The strand containing blood vessels, connecting the embryo to the extraembryonic membranes (placenta), forms the abdominal stalk. Thus, by the end of the 3rd week, the human embryo has the appearance of a three-layered shield. In the area of the outer embryonic layer, the neural tube is visible, and deeper - the dorsal string, i.e. the axial organs of the human embryo appear.

The fourth week of embryonic development

It is the period when the embryo, which looks like a three-layer shield, begins to bend in the transverse and longitudinal directions. The embryonic shield becomes convex, and its edges are delimited from the amnion by a deep groove - the trunk fold. The body of the embryo turns from a flat shield into a three-dimensional one, the exoderm covers the body of the embryo from all sides.

The endoderm, which is inside the body of the embryo, curls up into a tube and forms the embryonic rudiment of the future intestine. The narrow opening through which the embryonic intestine communicates with the yolk sac later turns into the umbilical ring. The endoderm forms the epithelium and glands of the digestive tract and respiratory tract. The ectoderm forms the nervous system, the epidermis of the skin and its derivatives, the epithelial lining of the oral cavity, the anal section of the rectum, and the vagina. The mesoderm gives rise to internal organs (except for endoderm derivatives), the cardiovascular system, the organs of the musculoskeletal system (bones, joints, muscles), and the skin itself.

The embryonic (primary) intestine is initially closed in front and behind. In the anterior and posterior ends of the embryo's body, invaginations of the ectoderm appear - the oral pit (the future oral cavity) and the anal (anal) pit. Between the cavity of the primary intestine and the oral pit there is a two-layer (ectoderm and endoderm) anterior (oropharyngeal) plate (membrane), between the intestine and the anal pit - the cloacal (anal) plate (membrane), also two-layer. The anterior (oropharyngeal) membrane breaks through in the 4th week of development. In the 3rd month, the posterior (anal) membrane breaks through.

As a result of the bending, the body of the embryo is surrounded by the contents of the amnion - amniotic fluid, which acts as a protective environment, protecting the embryo from damage, primarily mechanical (shaking). The yolk sac lags in growth and in the 2nd month of intrauterine development looks like a small sac, and then completely reduces. The abdominal stalk lengthens, becomes relatively thin and later receives the name of the umbilical cord.

During the 4th week, the differentiation of the mesoderm, which began at the end of the 3rd week of embryonic development, continues. The dorsal part of the mesoderm, located on the sides of the notochord, forms paired protrusions - somites. The somites are segmented, i.e. divided into metamerically located sections. Therefore, the dorsal part of the mesoderm is called segmented. Segmentation of the somites occurs gradually in the direction from front to back. On the 20th day, the 3rd pair of somites is formed, by the 30th day there are already 30, and on the 35th day - 43-44 pairs. The ventral part of the mesoderm is not divided into segments, but is represented on each side by two plates (the non-segmented part of the mesoderm). The medial (visceral) plate is adjacent to the endoderm (primary intestine) and is called the splanchnopleura, the lateral (outer) plate is adjacent to the wall of the embryo's body, to the ectoderm, and is called the somatopleura. The epithelial covering of the serous membranes (mesothelium), as well as the proper plate of the serous membranes and the subserous base, develop from the splanchnopleura and somatopleura. The mesenchyme of the splanchnopleura also goes to the construction of all layers of the digestive tract, except for the epithelium and glands, which are formed from the endoderm. The endoderm gives rise to the glands of the esophagus, stomach, liver with bile ducts, glandular tissue of the pancreas, epithelial covering and glands of the respiratory organs. The space between the plates of the unsegmented part of the mesoderm turns into the cavity of the embryo's body, which is subdivided into the abdominal, pleural and pericardial cavities.

The mesoderm at the border between the somites and the splanchnopleura forms nephrotomes (segmental legs), from which the tubules of the primary kidney develop. Three rudiments are formed from the dorsal part of the mesoderm - the somites. The ventromedial part of the somites - the sclerotome - is used to build skeletogenic tissue, which gives rise to the bones and cartilages of the axial skeleton - the spine. Lateral to it is the myotome, from which the striated skeletal muscles develop. In the dorsolateral part of the somite is the dermatome, from whose tissue the connective tissue base of the skin - the dermis - is formed.

In the 4th week, the rudiments of the inner ear (first the auditory pits, then the auditory vesicles) and the future lens of the eye, which is located above the lateral protrusion of the brain - the optic vesicle - are formed in the head section on each side of the embryo from the ectoderm. At the same time, the visceral sections of the head are transformed, grouped around the oral bay in the form of the frontal and maxillary processes. Caudal to these processes, the contours of the mandibular and sublingual (hyoid) visceral arches are visible.

On the anterior surface of the embryo's body, the cardiac tubercle stands out, followed by the hepatic tubercle. The depression between these tubercles indicates the place of formation of the transverse septum - one of the rudiments of the diaphragm.

Caudal to the hepatic tubercle is the ventral stalk, which contains large blood vessels and connects the embryo to the placenta (umbilical cord).

The period from the 5th to the 8th week of embryo development

The period of development of organs (organogenesis) and tissues (histogenesis). This is the period of early development of the heart, lungs, complication of the intestinal tube structure, formation of visceral and branchial arches, formation of capsules of the sense organs. The neural tube is completely closed and expands in the head section (the future brain). At the age of about 31-32 days (5th week, the length of the embryo is 7.5 cm), fin-like rudiments (buds) of the arms appear at the level of the lower cervical and 1st thoracic segments of the body. By the 40th day, rudiments of the legs are formed (at the level of the lower lumbar and upper sacral segments).

At the 6th week, the rudiments of the outer ear are noticeable, and from the end of the 6th to 7th week - the fingers, and then the toes.

By the end of the 7th week, the eyelids begin to form. Thanks to this, the eyes are outlined more clearly. In the 8th week, the laying of the embryo's organs is completed. From the 9th week, i.e. from the beginning of the 3rd month, the embryo takes on the appearance of a human being and is called a fetus.

The period of embryo development from 3 to 9 months

Beginning with the third month and throughout the entire fetal period, the growth and further development of the formed organs and body parts occurs. At the same time, differentiation of the external genitalia begins. Fingernails are laid. From the end of the fifth month, eyebrows and eyelashes become noticeable. In the seventh month, the eyelids open, and fat begins to accumulate in the subcutaneous tissue. In the ninth month, the fetus is born. Age-related features of the development of individual organs and organ systems are described in the relevant sections of the textbook.