The embryonic period of development of vertebrates is characterized by the formation of temporary (provisional) organs, such as chorion, yolk sac, allantois and amnion. The last of them plays one of the most important roles, as it produces amniotic fluid, which provides an environment for the development of the body. About what amnion is, how it is formed, what structure and purpose it has - read on.
What is the amniotic sac?
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 that is involved in the production of amniotic fluid, starting from the seventh week of embryogenesis.
The amnion occurs in close relationship with the chorion or, as it is often called, the serosa. Their anlage appears at a certain distance from the head end of the embryo in the form of a transverse fold, which subsequently, as it grows, bends over it and closes like a hood. Further, the amniotic folds, or rather their lateral sections, grow alongboth sides of the embryo in the direction from front to back, approaching more and more. In the end, they connect with each other and grow together. The fetus is enclosed in a water shell (amniotic cavity).
However, it is not filled with liquid immediately, but gradually. Initially, the cavity looks like a narrow gap between the inner surface of the amniotic fold and the embryo. Then it is filled with amniotic fluid (a waste product of cells) and stretched. The embryo is connected with the extra-embryonic parts of the body only through the umbilical cord. Pictured above is a 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 an aquatic environment. In this regard, all vertebrates that lay eggs (reptiles and birds), as well as mammals, are amniotes, or, in other words, animals whose embryos have egg shells.
Preceding classes and superclasses (fish, amphibians, cyclostomes, cephalochords) lay their eggs in the aquatic environment, and they do not need any additional shell. Therefore, this group of animals is called anamniya. 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
The amnion is formed from the extra-embryonic ectoderm and mesenchyme. In the human fetusit appears at the second stage of gastrulation in the form of a small vesicle as part of 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 stalk, which later turns into the umbilical cord and merges 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 liquid in which the embryo is located.
In the early stages of development, the amnion epithelium is a single-layer, flat row of large polygonal cells closely adjacent to each other. Many of them divide by mitosis. In the third month of embryogenesis, the epithelium becomes prismatic, with villi appearing on its surface. In the apical part of the cells there are vacuoles of various sizes, their contents are released into the amniotic cavity. The epithelium of the amnion in the region of the placental disc is prismatic and single-layered, only in places multi-rowed. 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 connects the amnion to the chorion.
Amnion in reptiles
As mentioned above, amniotes are chordate animals in which special embryonic membranes (allantois and amnion) are formed in the process of individual development. in mammals,birds and reptiles embryogenesis has common features. However, the reptiles are at the very bottom of evolution.
Provisional (temporary) organs, which include the amnion, in reptile embryos arise in the same way as in bony and cartilaginous fish. A large amount of yolk leads to the formation of a yolk sac. The first animals whose embryos developed an aquatic shell in the process of evolution are reptiles. Their eggs do not have protein and the developing embryo is closely adjacent to the shell membranes. Gradually, it sinks into the rarefied yolk, bending the layer of extra-embryonic ectoderm, and it forms amniotic folds around its body. The process of their closing is gradual. Ultimately, the 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 the amnion in birds
The process of 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 differently. Bird eggs have a thick layer of protein located under the shell membrane. The immersion of the embryo into the yolk does not occur, it rises above it, and depressions are formed on both sides, called body folds. Growing and deepening, they raise the embryo and contribute to the folding of the intestinal endoderm into a tube. Then the trunk folds continue into the amniotic folds, which fuse over the embryoand form the amniotic cavity.
The difference in the structure of the eggs of birds and reptiles did not affect the mechanism of development of allantois. In representatives of these two groups of amniotes, it occurs similarly. The allantois of birds and reptiles performs identical functions.
Meaning of amnion
Chorion, allantois and amnion are embryonic membranes characteristic of all higher vertebrates and some invertebrates. From the point of view of evolution, these organs can be considered as developed over a long period of adaptation of the embryo. Together with the yolk sac, they protect it from various 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.
To put it figuratively, an amnion is an aquarium in which the embryos of vertebrates and some invertebrates 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 shocks, concussions and a preventive function in case of mechanical damage to the fetus.
