Human embryo development is a complex process. And an important role in the correct formation of all organs and the vitality of the future person belongs to extra-germ 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-germ organs?
Item Specifics
In the second or third week of the existence of the human embryo, the formation of extra-embryonic organs, in other words, the membranes of the embryo begins.
The embryo has five provisional organs: the yolk sac, amnion, chorion, allantois and placenta. All these are temporary formations, which no longer will be in either the born child or the adult. In addition, extra-germ organs are not part of the body of the embryo itself. But their functions are diverse. The most important of them - extra-embryonic organs of a person play a significant role in providing nutrition and regulating the processes of interaction between the embryo and the mother.
Evolutionary excursion
Extra-germ organs appeared on the stage of evolution as an adaptation of vertebrates to living on land. The oldest shell - the yolk sac appeared in fish. Initially, its main function was the storage and storage of nutrients for the development of the embryo (yolk). Later, the role of the provisional authorities expanded.
Following in birds and mammals, an additional shell is formed - the amnion. The extra-germinal organs of the chorion and 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.
Human provisional organs
Extra-germ organs include:
- Yolk Sack.
- Amnion.
- Chorion.
- Allantois.
- Placenta.
In general, the functions of extra-germ organs are reduced to creating an aqueous medium around the embryo - the most favorable for its development. But they also perform protective, respiratory and trophic functions.
Further, the article presents in more detail the structure and development of human extra-germ organs.
The oldest fetal membrane
The yolk sac appears in humans at 2 weeks and is a vestigial organ. It is formed from extra-embryonic epithelium (endoderm and mesoderm) - in fact, this is part of the primary intestine of the embryo, which is carried outside the body. Thanks to this shell, transport of nutrients and oxygen from the uterine cavity is possible. Its existence lasts about a week, since from 3 weeks 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 blood formation processes (blood islands) and primary reproductive cells (gonoblasts), which later migrate to the embryo's body. Later, this membrane is squeezed by the later formed fetal membranes, turning into a yolk stem, 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 bag filled with amniotic (amniotic) fluid. It is formed by connective tissue - it is precisely its remnants that are called the “shirt” of the newborn. This shell is filled with liquid, and therefore its function is to protect the embryo from shaking and to prevent the growing parts of its body from sticking together. Amniotic fluid is 99% water and 1% organic and inorganic substances.
Allantois
This fetal membrane is formed by the 16th day of embryo development from the sausage outgrowth of the posterior wall of the yolk bladder. In many ways, it is also a rudimentary organ that performs the functions of nutrition and respiration of the embryo. Within 3-5 weeks of development, blood vessels of the umbilical cord form in allantois. At 8 weeks, it degenerates and turns into a cord connecting the bladder and the umbilical ring. After that, allantois combines with the serous layers and forms the chorion - the chorion with many villi.
Chorion
Chorion is a shell with many villi penetrated by blood vessels. It is formed in three stages:
- Peredvillate - the membrane destroys the mucous endometrium of the uterus with the formation of lacunae filled with maternal blood.
- The formation of villi of the primary, secondary and tertiary orders. Tertiary villi with blood vessels mark the placentation period.
- Stage cotyledons - structural units of the placenta, which are stem villi with branching. By the 140th day of pregnancy, about 12 large, up to 50 small and 150 rudimentary cotyledons are formed.
Chorionic activity persists until the end of pregnancy. In this fetal membrane, the synthesis of gonadotropin, prolactin, prostaglandin and other hormones occurs.
Baby place
An important temporary organ for the development of the fetus is the placenta (from the Latin placenta - “flat cake”) - the place where the blood vessels of the chorion and endometrium of the uterus intertwine (but do not merge). In 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 at the end of the first trimester, and by the time of birth it has a diameter of up to 20 centimeters and a thickness of up to 4 centimeters.
It is difficult to overestimate the value of the placenta - it provides gas exchange and nutrition, performs hormonal regulation of the course of pregnancy, performs a protective function, passing the antibodies of the mother’s blood, and forms the immune system of the fetus.
The placenta has two parts:
- fetal (from the side of the embryo),
- uterine (from the uterus).
Thus, a stable mother-fetus interaction system is formed.
Connected by one placenta
The body of the mother and child together with the placenta form the mother-fetus system, regulated by neurohumoral mechanisms: receptor, regulatory and executive.
In the uterus are receptors that are the first to receive information about the development of the fetus. They are represented by all types: chemo, mechano, thermo and baroreceptors. The mother, with their irritation, changes in respiratory rate, blood pressure and other indicators.
Regulatory functions are provided by the calving of the central nervous system - the hypothalamus, reticular formation, 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 temporal organs of the fetus respond to changes in the state of the mother, and regulatory mechanisms mature in the development process. The development of 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 pathology of its development that most often leads to abortion. There may be the following problems in the development of the placenta:
- Low location. Placenta previa, when it partially overlaps the uterine pharynx, is a common pathology (up to 20%). It provokes bleeding and can cause premature labor.
- The increment of the placenta to the muscle layer of the uterus, which leads to its tight attachment. In this case, the placenta does not separate from the uterus during childbirth.
- Detachment of a child's place. Small exfoliations may not manifest themselves in any way, but significant ones lead to blood loss. In such cases, caesarean section is usually recommended.
- Early ripening is an increase or thinning of the placenta. This leads to fetal malnutrition.
- Late maturation is the underdevelopment of the placenta, which is often found in the Rh conflict of the mother and child. In this case, the placenta inappropriately performs its functions, which can lead to stillbirth and various pathologies of the fetal development.
- Hyperplasia (increase) of the placenta is an equally dangerous pathology. In this case, placental insufficiency develops, which leads to delays in the intrauterine development of the child.
Pathologies of the development of the membranes
In addition to the placenta, amnion and chorion also play a role in ensuring the normal course of pregnancy. Chorion pathologies in the first trimester are especially dangerous (hematomas - 50% of pathologies, heterogeneous structure - 28% and hypoplasia - 22%), they increase the probability of spontaneous abortion from 30 to 90%, depending on the pathology.
Finally
Organisms of the mother and fetus during pregnancy are a system of dynamic connection. And violations in any of its links lead to irreparable consequences. Disorders in the work of the mother's body clearly correlate with similar disorders in the work of the fetal systems. For example, increased insulin production in a pregnant woman with diabetes leads to various pathologies of the formation of the pancreas in the fetus. That is why it is very important for all pregnant women to monitor their health and not neglect preventive examinations, because any deviation from the norm can signal an unfavorable development of the fetus.