Chorion

The formation of the placenta is characteristic of viviparous vertebrates. Among amniotes, such a breeding method does not occur in birds , sometimes it is observed in reptiles - only in the squamous order (where it is known among the scinkled ones - in representatives of the family of night lizards and some species of the skink family - and snakes : in a number of species from the families of vipers , aspids , ) and is typical of mammals (among living mammals, the placenta is absent only in single passages ) ^{[6] [7]} . The placenta acts as a specific organ of communication between the fetus and the mother's body; it consists of chorion ( germinal part of the placenta) and friable uterine epithelium ( maternal part of the placenta) fused with each other and serves to supply the embryo with oxygen and nutrients and remove carbon dioxide and metabolic products ^{[3] [8]} .

The fusion of the chorion and uterine epithelium is accompanied by the plexus (but not fusion) of the blood vessels of the embryo and the mother's body, which makes gas exchange and the penetration of nutrients through the walls of blood vessels possible. The problem is the tissue incompatibility of the embryo and the mother (the genes of both parents are combined in the embryo genotype , and therefore its proteins are perceived by the immune system of the mother as foreign). The prevention of fetal rejection in different groups of amniotes is provided by various means ^{[8] [9]} .

In marsupial mammals, there is a primitive - chorio - witellin - placenta (also called the “yolk placenta”), which binds the embryo and the mother's body through the blood vessels of the yolk sac (in bandicoots , however, the placenta is chorioallantoic, but the chorion participating in its formation is devoid of villi). This type of placenta is much less effective in the transfer of oxygen from the mother to the embryo; Avoiding fetal rejection is possible only due to a sharp reduction in the period of intrauterine development (therefore, the marsupial cub is born very prematurely and is not even able to suck milk ^on its own ^[10] .

Viviparous fish also have a “vitelline placenta” - most cartilaginous and some species of ray - finned fish. However, they do not have chorion, and such a placenta is similar to the marsupial placenta only functionally, but not anatomically ^[11] .

Chorion at the Euterium

The device of the chorion and placenta in mammals from the infraclass of euteria is much more perfect. Firstly, their placenta is chorioallantoic (provides the connection of the embryo with the mother's body through the blood vessels of allantois). Secondly, a unique formation — trophoblast — is involved in its formation (paleontologists cannot determine the time of its appearance in eutherias, but all representatives of living eutherias — placental mammals — have trophoblast). In the embryonic development of placental there is a special stage - the blastocyst stage (follows the morula phase), in which the blastomeres differentiate into two layers: the outer ( trophoblast ) and inner ( embryoblast ; the bulk of the cells later gives rise to the embryo body, and the remaining cells to the vitelline bag and germinal membranes) ^{[10] [12]} .

When the crushing embryo passes from the fallopian tube to the uterus, it is released from the egg shell , and the trophoblast takes over the function of the outer shell of the embryo. In contact with the uterine wall, trophoblast cells exert a biochemical effect on it, making it possible to implant - attach the embryo. The trophoblast, whose cells form outgrowths - the villi of the prochorion, also participates in the fusion of the sites of serosa and the outer wall of allantois (in which the prochorion forms). a little later, the blood vessels of the growing allantois grow into these villi, turning it into a chorion , which in the area of ​​the emerging placenta takes the form of a massive spongy formation (thus, the eutherium chorion is a complex formation: its outer part is ectodermal and the inner part is of mesodermal origin). Chorionic villi deeply penetrate the uterine epithelium, as a result of which this type of chorioallantoic placenta is distinguished by a closer connection between the blood channels of the embryo and the mother, providing a much more efficient way of supplying the fetus with oxygen and nutrients ^{[10] [13]} .

In addition, trophoblast cells form an active barrier between the tissues of the mother and the embryo, preventing fetal rejection and significantly prolonging the period of intrauterine development. Therefore, the placental cubs at birth are much more developed, able to independently suckle milk, and in many species - to move independently and take care of themselves within a few hours after birth ^[14] .

The nature of the distribution of chorionic villi along its surface is different in different groups of placental. On this basis, three types of chorioallantoic placenta are distinguished ^[4] :

• diffuse - with a uniform distribution of villi over the chorion ( cetaceans , many ungulates , lower primates );
• lobed - with the union of the villi into groups distributed over the surface of the chorion ( ruminants );
• discoidal - with the placement of villi in a limited area of ​​the chorion, which has the appearance of a hoop ( insectivorous , predatory , rodents , higher primates ).

In particular, it is characteristic of the embryonic development of higher primates (including humans ) that the chorion is initially completely covered with villi. Then, along a long stretch, the villi stop growing, so that this part of the chorion seems smooth and gets the name Chorion laeve , while on the side of the chorion that faces the maternal part of the placenta, the villi grow extremely, and it gets the name Chorion frondosum ^[5] .

The presence of a real chorioallantoic placenta with villi, which is fundamentally indistinguishable from the placenta of eutherias, is also characteristic of the aforementioned scaly species with a placenta ^{[15] [16]} .

"Chorion" is also called the cuticular membrane present in the eggs of many multicellular animals. It is a product of the isolation of follicular cells surrounding the egg in the ovary . This shell can be gelatinous or, conversely, very hard, because it is impregnated with a substance close to chitin - chorionin. Sometimes the chorion is penetrated by numerous pores, sometimes it has one hole ( micropile ), which serves to enter the sperm . Through the same hole, gas exchange is also possible if the chorion is not porous. The shell of the egg of mammals (and humans), which was called corona (s. Zona) radiata for its striation by the tubules, is probably a chorion (in this sense) ^[5] .

• Chorion // Encyclopedic Dictionary of Brockhaus and Efron : in 86 volumes (82 volumes and 4 additional). - SPb. , 1890-1907.
• Barabash-Nikiforov I.I. , Formozov A.N. Theriology. - M .: Higher school, 1963. - 396 p.
• Life of animals. Encyclopedia in 6 vols. T. 4. Book. 2: Amphibians. Reptiles / Common. ed. L. A. Zenkevich . - M .: Enlightenment, 1969 .-- 487 p.
• Carroll R. Paleontology and the evolution of vertebrates: In 3 volumes T. 3. - M .: Mir, 1993. - 312 p. - ISBN 5-03-001819-0 . .
• Naumov S.P. Zoology of vertebrates. - M .: Enlightenment, 1982.- 464 p.