For the embryonic period of development of vertebrates , the formation of temporary (provisional) organs, such as the chorion, yolk sac, allantois and amnion, is characteristic. The last of them plays one of the most important roles, as it produces amniotic fluid, providing an environment for the development of the body. Read about what an amnion is, how it is formed, what its structure and purpose are.
What is an amniotic membrane?
The amniotic membrane or amnion is a temporary organ that provides a comfortable aquatic environment for the development of the embryo. It is a continuous membrane, which is involved in the development of amniotic fluid, starting from the seventh week of embryogenesis.
Amnion arises in close relationship with the chorion or, as it is often called, serosa. Their laying occurs 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, the amniotic folds, or rather their lateral sections, grow on both sides of the embryo in the direction from front to back, drawing closer together. In the end, they connect with each other and grow together. The embryo is enclosed in an aqueous membrane (amniotic cavity).
However, it does not fill with liquid immediately, but gradually. Initially, the cavity has the form of a narrow gap between the inner surface of the amniotic fold and the embryo. Then it is filled with amniotic fluid (a product of the vital activity of cells) and stretched. The embryo is connected to extra-fetal parts of the body only through the umbilical cord. In the photo above, the human embryo at 7 weeks of development.
Amniotes and Anamnias
Amnion arose in the process of evolution in connection with the transition of vertebrates to land from water. Initially, its main purpose is to protect the embryos from drying out during development not in the aquatic environment. In this regard, all vertebrates laying eggs (reptiles and birds), as well as mammals, are amniotes or, in other words, animals whose embryos have egg shells.
The preceding classes and superclasses (fish, amphibians, cyclostomes, cephalopods) spawn in the 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).
Amnion development and structural features
The formation of the amnion occurs from the extra-embryonic ectoderm and mesenchyme. In the human embryo, it appears in the second stage of gastrulation in the form of a small vesicle in the epiblast. At the end of the seventh week, the connective tissue of the amnion and chorion come into contact. The epithelium of the amniotic sac passes to the amniotic leg, which later turns into an umbilical cord and closes with the epithelial cover of the skin of the embryo in the umbilical ring. The amniotic membrane forms the wall of a kind of reservoir filled with the liquid in which the embryo is located.
In the early stages of development of the amnion epithelium, it is a single-layer, flat row of large polygonal cells closely adjacent to each other. Many of them share mitosis. In the third month of embryogenesis, the epithelium becomes prismatic, and villi appear on its surface. Vacuoles of various sizes are present in the apical part of cells; their contents are secreted into the amniotic cavity. The amnion epithelium in the area of the placental disc is prismatic and single-layered, only in some places multirow. It performs mainly a secretory function. The epithelium outside the placental amnion mainly conducts amniotic fluid resorption.
The connective stroma of the amniotic membrane has a basement membrane, a layer of fibrous, dense connective tissue and a layer of loose, spongy connective tissue that binds the amnion to the chorion.
Amnion in reptiles
As mentioned above, amniotes are chordate animals in which, in the process of individual development, special embryonic membranes (allantois and amnion) are formed. In mammals, birds and reptiles, embryogenesis has common features. However, at the lowest stage of evolution are reptiles.
The provisional (temporary) organs, which include the amnion, also appear in reptile embryos as in bony and cartilaginous fish. A large amount of yolk leads to the formation of a yolk sac. The first animals, in the embryos of which in the process of evolution a water shell appeared, are reptiles. Their eggs have no protein and the developing embryo is closely adjacent to the shell membranes. Gradually, he plunges into a rarefied yolk, bending a layer of extra-fetal ectoderm, and it forms amniotic folds around his body. The process of their closure is gradual. Ultimately, an amniotic cavity is formed. Folds do not close only at the posterior end of the embryo. There remains a narrow channel connecting the amniotic and serous cavity.
Amnion formation in birds
The formation of provisional organs in birds and reptiles has much in common. The yolk sac in birds is formed in exactly the same way. The formation of the serous and amniotic membranes occurs in a different way. Bird eggs have a thick layer of protein located under the shell membrane. The embryo does not immerse in the yolk, it rises above it, and depressions are formed on both sides, called trunk folds. Growing and deepening, they raise the embryo and contribute to the folding of the intestinal endoderm into the tube. Then the trunk folds continue into amniotic folds that grow together over the embryo and form an amniotic cavity.
The difference in the structure of bird eggs and reptiles did not affect the mechanism of development of allantois. In representatives of these two amniotic groups, it occurs similarly. Allantois birds and reptiles perform identical functions.
Amnion Value
Chorion, allantois and amnion are germinal membranes characteristic of all higher vertebrates and some invertebrates. From the point of view of evolution, these organs can be considered as developed during a long time of adaptation of the embryo. They, together with the yolk sac, protect it from various environmental factors. These embryonic adaptations arose and improved through natural selection, that is, under the influence of the changing conditions of the biotic and abiotic environment.
Figuratively speaking, the amnion is an aquarium in which embryos of vertebrates and some invertebrate animals repeat the aquatic lifestyle of their distant ancestors. The presence of the shell guarantees the development of the fetus in an environment with the most optimal composition of proteins, electrolytes and carbohydrates.
Amniotic fluid contains antibodies that protect the embryo from pathogenic factors. In addition, the aquatic environment performs a shock-absorbing function in case of various strokes, concussions and prophylactic in case of mechanical damage to the fetus.