To understand the individual features of the structure of the human body, it is necessary to get acquainted with the development of the human body in the intrauterine period. Each person has individual features of the external appearance and internal structure, the presence of which is determined by two factors. First of all, this is heredity - the features inherited from parents, as well as the result of the influence of the external environment in which a person grows, develops, learns, works.
Individual development, or development in ontogenesis, occurs during all periods of life - from conception to death. In human ontogeny, two periods are distinguished: before birth (intrauterine, prenatal, from Greek natos - born) and after birth (extrauterine, postnatal). In the prenatal period, from conception to birth, the embryo (embryo) is located in the mother's body. During the first 8 weeks, the main processes of the formation of organs, parts of the body. This period is called embryonic, and the human body is an embryo (embryo). Starting with the 9th week, when the main external human features have already begun to be designated, the organism is called a fruit, and the period is fruitful.
After fertilization (fusion of the sperm and egg), which usually occurs in the fallopian tube, the fused germ cells form a unicellular embryo - a zygote that possesses all the properties of both sex cells. From this moment the development of a new (subsidiary) organism begins.
The first week of embryo development
This is the period of fragmentation (division) of the zygote into daughter cells. During the first 3-4 days the zygote is divided 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 - a sprout). The walls of this bubble consist of cells of two types: large and small. From the outer layer of small light cells, the walls of the bubble - the trophoblast - form. Subsequently, trophoblast cells form the outer layer of the embryo shells. Larger dark cells (blastomeres) form a cluster - the embryoblast (embryonic nodule, germ embryo), which is located inside the trophoblast. From this accumulation of cells (embryoblast), the embryo develops and extragenous structures adjacent to it (except the trophoblast). Between the surface layer (trophoblast) and the embryonic nodule, a small amount of liquid accumulates.
By the end of the first week of development (6-7th day of pregnancy), the embryo is implanted (implanted) in the mucous membrane of the uterus. Surface cells of the embryo forming a vesicle - trophoblast (from the Greek trophe - food, trophicus - trophic, feeding), vselchelayut enzyme, loosening the surface layer of the mucous membrane of the uterus. The latter has already been prepared for the introduction of the embryo into it. By the time of ovulation (isolation of the ovum from the ovary), the mucous membrane of the uterus becomes thicker (up to 8 mm). In it, the uterine glands and blood vessels grow. On the trophoblast there are numerous outgrowths - villi, which increases the surface of its contact with the tissues of the mucous membrane of the uterus. The trophoblast turns into a nourishing shell of the embryo, which was called the villous shell, or chorion. Initially, the chorion has villi from all sides, then these villi are preserved only on the side facing the wall of the uterus. In this place a new organ, a placenta (a child's place), develops from the chorion and the adjacent mucous membrane of the uterus. The placenta is the organ that connects the mother's body to the embryo and provides its nourishment.
The second week of embryo development
This is the stage when the cells of the embryoblast are divided into two layers (two plates), from which two bubbles form. From the outer layer of cells adjacent to the trophoblast, an ectoblastic (amniotic) vesicle is formed, filled with an amniotic fluid. An endoblastic (yolk) vesicle is formed from the inner cell layer of the embryonic nodule of the embryoblast. The embryo's "body" is located where the amniotic vesicle comes in contact with the yolk sac. During this period the embryo is a two-layered scutellum consisting of two embryonic leaves: the outer one - ectoderm (from the Greek ektos - outside, derma - skin) and internal - the endoderm (from the Greek ёntos - inside). The ectoderm is directed towards the amniotic vesicle, and the endoderm is attached to the vitelline vesicle. At this stage, it is possible to determine the surface of the embryo. The dorsal surface is attached to the amniotic vesicle, and the ventral surface to the yolk sac. The trophoblast cavity around the amniotic and vitelline vesicles loosely filled with the cells of the extraembryonic mesenchyme. By the end of the second week, the length of the embryo is only 1.5 mm. During this period, the embryonic scute in its posterior (caudal) part thickens. Here, in the future, the axial organs (chord, neural tube) begin to develop.
Third week of embryo development
Period of formation of a three-layered flap. The cells of the outer, ectodermal, embryonic shield sheet are displaced to the posterior end of the embryo, resulting in the formation of a shaft extended in the direction of the axis of the embryo. This cell cord is called the primary band. In the head (front) part of the primary band, the cells grow and multiply faster, resulting in a slight elevation - the primary nodule (Hansen's bundle). The primary strip determines the bilateral symmetry of the embryo's body, i.e. Its right and left sides. The site of the primary nodule points to the cranial (head) end of the embryo's body.
As a result of the rapid growth of the primary stripe and the primary nodule, the cells of which germinate between the ectoderm and the endoderm, an average embryonic leaf, the mesoderm, is formed. Mesoderm cells located between the scutes of the scutellum are called the intra-embryonic mesoderm, and the out-of-embryo mesoderm that have migrated beyond it.
Part of the mesoderm cells within the primary nodule especially actively grows forward, forming the head (chordal) process. This process penetrates between the outer and inner leaves from the head to the tail end of the embryo and forms a cellular strand - the dorsal string (chorda). The head (cranial) part of the embryo grows faster than the caudal part, which, together with the region of the primary tubercle, retreats backward. At the end of the third week, anteriorly from the primary tubercle in the outer embryonic leaf, a longitudinal strip of actively growing cells is isolated - the nerve plate. This plate soon bends, forming a longitudinal furrow - a nerve groove. As the groove becomes deeper, its edges thicken, approach and coalesce with each other, closing the nerve groove into the neural tube. In the future, the entire nervous system develops from the neural tube. The ectoderm is closed over the formed neural tube and loses its connection with it.
At the same time, from the posterior part of the inner (endodermal) leaf of the embryonic scute into the extra-germinal mesenchyme (into the so-called amniotic stem), finger-like growth - the allantois, which does not fulfill certain functions in humans. In the course of the allantois from the embryo, the blood umbilical (placental) vessels grow through the amniotic leg to the villi of the chorion. The blood vessel containing the blood vessels, which connects the embryo to the extraembryonic membranes (placenta), forms the abdominal stem. Thus, by the end of the third week the human embryo has the appearance of a three-layered shield. In the region of the external embryonic leaf, the neural tube is visible, and deeper is the spinal cord, i. There are axial organs of the human embryo.
Fourth week of embryo development
It is a period when the embryo, having the appearance of a three-layered scute, begins to flex in the transverse and longitudinal directions. The embryonic scutellum becomes convex, and its edges are delimited from the amnion by a deep furrow - trunk fold. The body of the embryo from the flat shield turns into a bulk one, the exoderm covers the fetal body from all sides.
The endoderm, which is inside the fetus's body, coagulates into the tube and forms the embryonic rudiment of the future intestine. A narrow opening, through which the embryonic intestine communicates with the yolk sac, later becomes an umbilical ring. From the endoderm, the epithelium and glands of the digestive tract and respiratory tract are formed. From the ectoderm, the nervous system, the epidermis of the skin and its derivatives, the epithelial lining of the oral cavity, the anal section of the rectum, the vagina are formed. The mesoderm gives rise to internal organs (other than endodermal derivatives), the cardiovascular system, organs of the musculoskeletal system (bones, joints, muscles), actually the skin.
The embryonic (primary) gut is initially closed in front and behind. In the anterior and posterior ends of the embryo's body appear ectoderm invaginations - the oral cavity (the future oral cavity) and the anal (anal) fossa. Between the cavity of the primary intestine and the oral cavity there is a two-layer (ectoderm and endoderm) anterior (oropharyngeal) plate (membrane), between the gut and an anal cavity - a cloacal (anal) plate (membrane), also two-layered. The anterior (oropharyngeal) membrane breaks through the 4th week of development. At the 3rd month, a back (anal) membrane breaks through.
As a result of bending, the body of the embryo is surrounded by the contents of the amnion, the amniotic fluid, which acts as a protective environment, protecting the embryo from damage, primarily mechanical shocks. The yolk sac lags behind in growth and on the second month of intrauterine development it looks like a small sac and then completely reduced. The abdominal stem lengthens, becomes relatively thin and later gets the name of an umbilical cord.
During the 4th week, the differentiation of its mesoderm begins at the end of the third week of embryo development. The dorsal part of the mesoderm, located on each side of the chord, forms paired protuberances - somites. Somites are segmented, i.e. Are divided into metamerally located areas. Therefore, the dorsal part of the mesoderm is called segmented. Segmentation of the somites occurs gradually in the front-to-back direction. On the 20th day, the third pair of somites is formed, by the 30th day they are already 30, and on the 35th day - 43-44 pairs. The ventral part of the mesoderm is not subdivided into segments, but is represented on each side by two plates (not a segmented part of the mesoderm). The medial (visceral) plate belongs to the endoderm (the primary gut) and is called the splanchnopleurovous, the lateral (external) plate is to the wall of the embryo's body, to the ectoderm, and is called the somatopleura. Plane and somatopleura develop epithelial cover of serous membranes (mesothelium), as well as its own plate of serous membranes and a subserous base. Mesenchyme splanchnopleura is also used to construct all layers of the digestive tube, except 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 pancreatic tissue, epithelial cover and glands of the respiratory system. The space between the plates of the non-segmented part of the mesoderm is transformed into the body cavity of the embryo, which is subdivided into the abdominal, pleural and pericardial cavities.
Mesoderm on the border between somites and splanchnoplevroi forms nephrotomes (segmental legs), from which the tubules of the primary kidney develop. From the dorsal part of the mesoderm - somite - three rudiments are formed. Ventromedial portion of somites - sclerotome - goes to the construction of skeletal tissue, which gives rise to the bones and cartilages of the axial skeleton - the spine. Lateral to it lies the myotome, from which the developed skeletal muscle develops. In the dorsolateral part of the somite is dermatome, a connective tissue base of the skin, the dermis, is formed from its tissue.
At the 4th week in the head section, on each side of the embryo from the ectoderm, the beginnings of the inner ear (first the auditory fossa, then the auditory vesicles) and the future lens of the eye, which is located above the lateral protrusion of the brain - the eye bladder. At the same time, the visceral parts of the head are transformed, grouped around the oral bay in the form of the frontal and maxillary processes. The contours of the mandibular and sublingual (hyoid) visceral arches are seen more caudal than these processes.
On the anterior surface of the torso of the embryo, cardiac, and behind it the hepatic hillocks. The deepening between these hillocks indicates the place of formation of the transverse septum - one of the rudiments of the diaphragm.
Caudal than the hepatic hillock is the abdominal stem, which includes large blood vessels and connects the embryo with 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 period of early development of the heart, lungs, complicating the structure of the intestinal tube, the formation of visceral and gill arches, the formation of capsules of the senses. The nerve tube completely closes and expands in the head (the future brain). At the age of about 31-32 days (5th week, embryo length 7.5 cm) at the level of the lower cervical and 1st thoracic segments of the body appear fin-like buds (kidneys) of the hands. By the 40th day, the rudiments of the legs are formed (at the level of the lower lumbar and upper sacral segments).
On the 6th week, the ear buds are visible, from the end of the 6th to the 7th week - the fingers, and then the legs.
By the end of the 7th week, the eyelids begin to form. Thanks to this, the eyes are outlined more clearly. At the 8th week, the embryo organs are being laid. From the 9th week, i.е. From the beginning of the third month, the embryo takes the form of a person and is called a fruit.
The period of embryo development from 3 to 9 months
Beginning with the third month and during the entire fetal period, growth and further development of the organs and parts of the body are occurring. At the same time, the differentiation of the external genitalia begins. Fingernails are put on fingers. From the end of the fifth month, eyebrows and eyelashes become noticeable. At the 7th month eyelids open, fat begins to accumulate in the subcutaneous fat. At 9 months the fetus is born. Age features of the development of individual organs and organ systems are set out in the relevant sections of the textbook.