Provisional organs of mammals and humans, their functions

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Formed in a certain periodindividual development, the temporal organs of larvae of multicellular animals and embryos were called provisional organs. In humans and mammals, they function only at the embryonic stage and perform both basic functions of the body and specific ones. After maturity of adult organs in the process of metamorphosis, the temporary disappear. These education data accompanying the development of many animals are of interest for evolutionary morphology, physiology and embryology.

For human and mammalian, the following temporary organs are characteristic: amnion, chorion, allantois, yolk sac and placenta.

Amnion

An amniotic fluid, an aqueous membrane, an amniotic bladder, orThe bag is one of the embryonic membranes, characteristic of mammals, birds and reptiles. It arose in the process of evolution when animals adapted to life on land. The main function of the amnion is to protect the embryo from environmental factors and create favorable conditions for its development. It arises from an ectoblastic bubble and forms a cavity filled with a liquid. Serose is developing in close relationship with the amnion.

During mammalian genera, the aqueous membrane bursts, the liquid flows out, and the remains of the bladder remain on the body of the newborn.

Anamnia division and amniotes

The presence or absence of such a provisionalorgan, like the amnion, served as the main principle of dividing all vertebrate organisms into two groups: amniotes and anamnies. From the point of view of evolution, the most ancient are those animals that developed in the aquatic environment (cyclostomes, fish, amphibians). They do not need an additional water shell for the embryo. They belong to anamnies.

Mammals, birds and reptiles arehigher vertebrate organisms having highly efficient and coordinated systems of organs that allow them to exist in a wide variety of conditions of land and water. In fact, they have mastered all habitats. This would be impossible without complex and specific embryonic development.

The general provisional organ of the anamnium and amniot isyolk sac. In addition to it, the first group of animals has nothing else. In the amniot, the temporal organs are also represented by chorion, allantoin, amnion and placenta. In the photo below the scheme of the primordial embryo.

Allantois

In Greek, allantois means"Sausage", which quite accurately reflects its appearance. It is formed as a result of protrusion of the wall of the primary intestine into the space between the yolk sac and amnion. In the human embryo, this occurs to 16 days after fertilization.

Allantois is a provisional organ consisting oftwo sheets: extra-germinal ectoderm and mesoderm. It is most pronounced in animals, the development of which occurs in the egg. They act as a reservoir for the accumulation of metabolic products, mainly urea. In mammals, such a need is completely absent, therefore, the allantois is poorly developed. It performs a different function. In its walls there is the formation of umbilical vessels, branched in the placenta. Thanks to them in the future the placental circulation is formed.

Yolk sac

The yolk sac is a provisional organ (birds,amphibians, reptiles, mammals) of an endoderm origin. As a rule, it is the outgrowth of the intestine, inside which the yolk stock is located. The latter is used by an embryo or a feeding larva. From the point of view of evolution, the primary role of the yolk sac was to digest the yolk and assimilate the digestive products, and then transport them to the bloodstream of the embryo. For this, he has a branched network of blood vessels. However, the yolk reserve is absent in embryonic development of mammals and humans. Preservation of the yolk sac is associated with an important secondary function - hematopoiesis. In the photo it is indicated by a black circle (6th week of embryonic development).

The role of yolk sac in human development

Formation of yolk sac ofan endoblastic vesicle occurs on the 29th-30th day of pregnancy. During the period of human embryonic development, the provisional organ plays an important role. The size of the yolk sac in the early stages of pregnancy (up to six weeks) is significantly larger than the amnion along with the embryonic disc. On 18-19 days after fertilization, foci of erythropoiesis form in its walls, which subsequently form a capillary network. After another ten days, the yolk sac becomes the source of primary sex cells. They migrate from it to the tabs of the gonads.

Until the sixth week after fertilization, the yolk sac continues to produce many proteins (including transferrin, alpha-fetoprotein, alpha-2-microglobulin), acting as a "primary liver".

Like all other provisional bodiesmammal, the yolk sac at a certain point becomes unnecessary. His tissues perform a variety of functions, including excretory, hematopoietic, immunoregulatory, synthetic, metabolic. However, this occurs smoothly until the fetus begins to work the relevant organs. In humans, the yolk sac ceases to function at the end of the first trimester of pregnancy. It is reduced and remains only in the form of a small cystic type of formation located at the base of the umbilical cord.

Exclusively yolk sac is represented by provisional organs in anamniev.

Implantation of the embryo

A characteristic feature of the development of higher mammalsis a relatively dense junction of the embryo with the wall of the uterus, which is established several days after the onset of development. For example, in a mouse this happens on the 6th day, and in a man it takes place on the 7th day. The process is called implantation, it is based on immersion in the wall of the uterus of secondary villi of the chorion. As a result, a special provisional organ is formed - the placenta. It consists of the embryonic part - the villus of the chorion and the maternal one - the relatively altered wall of the uterus. The first also includes the allantoid leg, which plays an important role in the fetal blood supply in lower (marsupial) mammals. At them the parent part of a placenta is not developed.

Chorion

Chorion, or, as it is often called, serosa isthe outer shell of the embryo, it is adjacent to the shell or maternal tissues. It is formed like an amnion from somatopleura and ectoderm in a person on the 7-12th day after fertilization, and its transformation into the placenta part occurs at the end of the first trimester of pregnancy.

The chorion consists of two parts: smooth and branched. The first one does not contain villi and surrounds the fetal egg almost completely. The branch chorion forms in the place of contact of the walls of the uterus with the embryo. It has numerous outgrowths (villi), which penetrate the mucous and submucous layer of the uterus. It is the branchy chorion that later becomes the fetal part of the placenta.

This provisional body performs functions,similar to those for which the functionally mature placenta: fetal breathing and nutrition, the allocation of metabolic products, protection from adverse external factors, including infections.

Placenta

The placenta is an embryonic organ,formed in all placental mammals from the embryonic membranes (chorion, villous, allantois), closely adjacent to the wall of the uterus. With the embryo, it is connected through the umbilical cord (umbilical cord).

The placenta forms the so-calledhematoplacental barrier. Fetal vessels branch out into the smallest capillaries and together with the supporting tissues form the villus of the chorion. In primates (including humans) they are immersed in lacuna filled with maternal blood. This determines the following functions of the provisional authority:

gas exchange - oxygen penetrates into fetal blood from the mother's blood according to the laws of diffusion, and in the opposite direction carbon dioxide moves;
excretory and trophic: removal of metabolites (creatine, creatinine, urea) and the supply of water, minerals and nutrients, electrolytes, vitamins;
hormonal;
protective, because the placenta has immune properties and passes the mother's antibodies to the fetus.

Types of the placenta

Depending on how deep the chorionic embryo vagi are immersed in the uterine mucosa, the following types of placenta are distinguished.

Poluplacenta. It is found in horses, lemurs, cetaceans, hippos, pigs, and camels. The half-placenta is characterized by the fact that the chorionic villi simply sink into the folds of the uterine mucosa, like fingers in a glove, and no penetration into the epithelial layer is observed.
Desmochorial placenta. It is characteristic of ruminants. In this form of the placenta, the chorionic villi destroy the uterine mucosa at the site of contact and are embedded in its connective layer, but do not reach the walls of its blood vessels.
Endotheliochoria placenta. It is characteristic of the highest predatory amniot. The provisional body establishes even closer contact between the mother and fetal vessels. The villi of the chorion penetrate the entire layer of connective tissue of the uterus. Only the endothelial wall separates them from its vessels.
Hemohorial placenta. It provides the closest connection between the vessels of the mother and the fetus, characteristic of primates. The chorionic villi penetrate the endothelium of the maternal blood vessels located in the uterine mucosa and sink into the blood lacunae filled with the mother's blood. In fact, the blood of the fetus and mother is separated only by the thin outer shell of the chorion and the walls of the capillary vessels of the embryo itself.