The development of the human embryo is a complex process. And an important role in the correct formation of all organs and the viability of the future person belongs to extra-embryonic organs, which are also called provisional. What are these organs? When do they form and what role do they play? What is the evolution of human extra-embryonic organs?
Specific subject matter
In the second or third week of the existence of the human embryo, the formation of extra-embryonic organs begins, in other words, the membranes of the embryo.
The embryo has five provisional organs: yolk sac, amnion, chorion, allantois and placenta. All these are temporary formations, which neither a born child nor an adult will have. In addition, extra-embryonic organs are not part of the body of the embryo itself. But their functions are varied. The most important of them is that extra-embryonic human organs play a significant role in providing nutrition and regulating the processes of interaction between the embryo and the mother.
Evolutionary excursion
Extra-embryonic organs appeared on the stage of evolution as an adaptation of vertebrates to living on land. The most ancient shell - the yolk sac appeared in fish. Initially, its main function was to store and store nutrients for the development of the embryo (yolk). Later, the role of provisional authorities expanded.
Following in birds and mammals, an additional shell is formed - the amnion. The extra-embryonic organs, the chorion and the placenta, are the privilege of mammals. They provide a link between the mother's body and the fetus, through which the latter is provided with nutrients.
Provisional human organs
External organs include:
- Yolk sac.
- Amnion.
- Chorion.
- Allantois.
- Placenta.
In general, the functions of extra-embryonic organs are reduced to the creation of an aqueous environment around the embryo - the most favorable for its development. But they also perform protective, respiratory and trophic functions.
Further in the article, the structure and development of extra-embryonic human organs is presented in more detail.
The oldest fetal membrane
The yolk sac appears in humans at 2 weeks and is a rudimentary organ. It is formed from the extra-embryonic epithelium (endoderm and mesoderm) - in fact, it is part of the primary intestine of the embryo, which is taken out of the body. It is thanks to this membrane that the transport of nutrients and oxygen from the uterine cavity is possible. His existencelasts about a week, since from the 3rd week the embryo is introduced into the walls of the uterus and switches to hematotrophic nutrition. But during the period of its existence, it is this fetal membrane that gives rise to embryonic processes of hematopoiesis (blood islands) and primary germ cells (gonoblasts), which later migrate into the body of the embryo. Later, later formed fetal membranes will squeeze this membrane, turning it into a yolk stalk, which will completely disappear by the 3rd month of embryo development.
Water shell - amnion
The water membrane appears in the early stages of gastrulation and is a sac filled with amniotic (amniotic) fluid. It is formed by connective tissue - it is its remains that are called the "shirt" in a newborn. This shell is filled with liquid, and therefore its function is to protect the embryo from concussions and to prevent the growing parts of its body from sticking together. Amniotic fluid is 99% water and 1% organic and inorganic matter.
Allantois
This fetal membrane is formed by the 16th day of embryo development from a sausage-like outgrowth of the posterior wall of the yolk sac. In many ways, it is also a rudimentary organ that performs the functions of nutrition and respiration of the embryo. During 3-5 weeks of development, the blood vessels of the umbilical cord form in the allantois. At week 8, it degenerates and turns into a strand connecting the bladder and umbilical ring. After that, the allantois combines with the serous layers and forms the chorion - a choroid with manylint.
Chorion
Chorion is a sheath with many villi pierced by blood vessels. It is formed in three stages:
- Anterior villous - the membrane destroys the mucous endometrium of the uterus with the formation of gaps filled with maternal blood.
- Formation of villi of primary, secondary and tertiary orders. Tertiary villi with blood vessels mark the period of placentation.
- Stage of cotyledons - structural units of the placenta, which are stem villi with branches. By the 140th day of pregnancy, about 12 large, up to 50 small and 150 rudimentary cotyledons are formed.
Chorion activity persists until the end of pregnancy. In this fetal membrane, gonadotropin, prolactin, prostaglandin and other hormones are synthesized.
Children's seat
An important temporary organ for the development of the fetus is the placenta (from the Latin placenta - "cake") - the place where the blood vessels of the chorion and endometrium of the uterus intertwine (but do not merge). In the places of these plexuses, gas exchange and the penetration of nutrients from the mother's body to the fetus occur. The location of the placenta often does not affect the course of pregnancy and fetal development. Its formation ends by the end of the first trimester, and by the time of delivery, it has a diameter of up to 20 centimeters and a thickness of up to 4 centimeters.
It is difficult to overestimate the importance of the placenta - it provides gas exchange and nutrition,performs hormonal regulation of the course of pregnancy, performs a protective function, passing maternal blood antibodies, and forms the fetal immune system.
The placenta has two parts:
- fetal (from the side of the embryo),
- uterine (from the side of the uterus).
Thus, a stable mother-fetus interaction system is formed.
Linked by the same placenta
The mother and child, together with the placenta, form the mother-fetus system, regulated by neurohumoral mechanisms: receptor, regulatory and executive.
Receptors are located in the uterus, which are the first to receive information about the development of the fetus. They are represented by all types: chemo-, mechano-, thermo- and baroreceptors. When the mother is irritated, the intensity of breathing, blood pressure and other indicators change.
Regulatory functions are provided by calving of the central nervous system - the hypothalamus, the reticular formation, the hypothalamic-endocrine system. These mechanisms ensure the safety of pregnancy and the functional work of all organs and systems, depending on the needs of the fetus.
Receptors of the temporary organs of the fetus respond to changes in the state of the mother, and regulatory mechanisms mature in the process of development. The development of the nerve centers of the fetus is evidenced by motor reactions that appear at 2-3 months.
The weakest link
In the described system, the placenta becomes such a link. It is the pathologies of its development that most oftenlead to abortion. There may be the following problems with the development of the placenta:
- Low location. Placenta previa, when it partially covers the uterine os, is a common pathology (up to 20%). It provokes bleeding and can cause premature labor.
- Increment of the placenta to the muscular layer of the uterus, which leads to its dense attachment. In this case, the placenta does not separate from the uterus during labor.
- Detachment of a child's place. Small detachments may not manifest themselves in any way, but significant ones lead to blood loss. In such cases, a caesarean section is usually recommended.
- Early maturation is an increase or thinning of the placenta. This leads to fetal malnutrition.
- Late maturation - underdevelopment of the placenta, which is often found in Rh-conflict mother and child. In this case, the placenta does not adequately perform its functions, which can lead to stillbirth and various pathologies of fetal development.
- Hyperplasia (enlargement) of the placenta is a no less dangerous pathology. In this case, placental insufficiency develops, which leads to intrauterine growth retardation.
Pathologies of development of membranes
Besides the placenta, the amnion and the chorion also play their role in ensuring the normal course of pregnancy. Particularly dangerous pathologies of the chorion in the first trimester (formation of hematomas - 50% of pathologies, heterogeneous structure - 28% and hypoplasia - 22%), they increasethe probability of spontaneous abortion from 30 to 90%, depending on the pathology.
In conclusion
Maternal and fetal organisms during pregnancy are a system of dynamic connection. And violations in any of its links lead to irreparable consequences. Violations in the work of the mother's body clearly correlate with similar disorders in the functioning of the fetal systems. For example, increased insulin production in a pregnant woman with diabetes leads to various pathologies in the formation of the pancreas in the fetus. That is why it is very important for all pregnant women to monitor their he alth and not neglect preventive examinations, because any deviation from the norm can signal an unfavorable development of the fetus.
