For the embryonic period of vertebrate developmentanimals are characterized by the formation of temporary (provisional) organs, such as chorion, yolk sac, allantois and amnion. The last of them plays one of their most important roles, as it produces amniotic fluid, providing an environment for the development of the body. About what the amnion is, how it is formed, what structure and purpose it has - read on.
What is the amniotic membrane?
Amniotic shell or amnion is a temporarya body that provides a comfortable water environment for embryonic development. It is a continuous shell, which participates in the production of amniotic fluid, beginning with the seventh week of embryogenesis.
The amnion arises in close relationship with the chorionor, as it is often called, serosa. Their bookmark appears at a certain distance from the head end of the embryo in the form of a transverse fold, which subsequently bends over it as it grows and closes like a hood. Further amniotic folds, or rather their lateral areas, grow on both sides of the embryo in the direction from front to back, converging ever closer. In the end, they unite and coalesce. The embryo is enclosed in an aqueous membrane (amniotic cavity).
However, the liquid does not fill up immediately, butgradually. Initially, the cavity has the appearance of a narrow gap between the inner surface of the amniotic fold and the embryo. Then it is filled with an amniotic fluid (the product of the vital activity of cells) and stretched. With the extra-germinal parts of the body, the embryo is connected only through the umbilical cord. In the photo above, the human embryo is at the 7th week of development.
Amniotes and anamnies
Amnion arose in the process of evolution in connection withthe transition of vertebrates to land from water. Initially, its main purpose is to protect embryos from drying out during development not in the aquatic environment. In this connection, all vertebrates, laying eggs (reptiles and birds), as well as mammals are amniotes or, in other words, animals whose embryos have egg shells.
Previous classes and superclasses (fish,amphibians, cyclostomes, cephalothorax) lay eggs in an aquatic environment, and they do not need any additional shell. Therefore this group of animals is called anamnias. Their existence is associated with the aquatic environment in which they spend most of their life, or its initial stages (egg, larval).
Development of the amnion and structural features
Formation of the amnion occurs from the extra-germectoderm and mesenchyme. In the human embryo, it appears in the second stage of gastrulation in the form of a small blister in the epiblast. At the end of the seventh week, the connective tissue of the amnion and the chorion come into contact. Epithelium amniotic sac passes to the amniotic leg, which later turns into an umbilical cord and closes with the epithelial cover of the embryo skin in the region of the umbilical ring. The amniotic shell forms the wall of a peculiar reservoir filled with a liquid in which the embryo is located.
In the early stages of development, the epithelium of the amnion issingle-layered, flat row of closely adjacent large polygonal cells. Many of them share mitosis. In the third month of embryogenesis, the epithelium becomes prismatic, and villi appear on its surface. In the apical part of the cells there are vacuoles of different sizes, their contents are released into the amniotic cavity. The epithelium of the amnion in the region of the placental disk is prismatic and single-layered, only in several rows. It performs mainly secretory function. Epithelium outside the placental amnion, mainly, conducts resorption of amniotic fluid.
The connecting stroma of the amniotic membrane has a basal membrane, a layer of fibrous, dense connective tissue and a layer of loose, spongy connective tissue connecting the amnion with the chorion.
Amnion in reptiles
As already mentioned above, amniotes are chordatesanimals in which in the course of individual development special embryonic membranes are formed (allantois and amnion). In mammals, birds and reptiles, embryogenesis has common features. However, at the lowest stage of evolution there are reptiles.
Provisional (temporary) organs, to whichincluding the amnion, reptile buds also appear, as in bony and cartilaginous fishes. A large amount of yolk leads to the formation of a yolk sac. The first animals, in embryos of which a water shell appeared in the course of evolution, are reptiles. Their eggs do not have protein and the developing embryo closely adjoins the sub shell shells. Gradually, he immerses himself in a thin yolk, bending through a layer of extra-embryonic ectoderm, and it forms amniotic folds around his body. The process of their joining proceeds gradually. Eventually, an amniotic cavity is formed. The folds do not close only at the posterior end of the embryo. There remains a narrow channel connecting the amniotic and serous cavity.
Formation of avian amnion in birds
The process of the formation of provisional organs in birds andreptile has much in common. The yolk sac in the birds is formed in exactly the same way. Formation of serous and amniotic membranes occurs in a different way. Eggs of birds have a thick layer of protein, located under the sub shell shell. Immersion of the embryo in the yolk does not occur, it rises above it, and on both sides are formed grooves, called trunk folds. Growing and deepening, they raise the embryo and facilitate the folding of the intestinal endoderm into the tube. Then the trunk folds continue into the amniotic folds that fuse over the embryo and form an amniotic cavity.
The difference in the structure of the bird's egg and reptile is notinfluenced the mechanism of development of allantois. In representatives of these two groups of amniotes, it occurs in a similar way. Allantois of birds and reptiles perform identical functions.
The value of the amnion
Chorion, allantois and amnion are germinalshells, characteristic of all higher vertebrates and some invertebrates. From the point of view of evolution, these organs can be regarded as the embryo adaptation that has been developed for a long time. They, along with the yolk sac, protect it from a variety of environmental factors. These embryonic adaptations arose and improved through natural selection, that is, under the influence of changing conditions of the biotic and abiotic environment.
If to be expressed figuratively, the amnion isan aquarium in which the embryos of vertebrates and some invertebrates repeat the aquatic life of their distant ancestors. The presence of the envelope guarantees the development of the fetus in an environment with the most optimal composition of proteins, electrolytes and carbohydrates.
In the amniotic fluid there are antibodies,providing protection of the embryo from disease-causing factors. In addition, the water environment performs a cushioning function for various shocks, shocks and preventive - with mechanical damages to the fetus.
