Amnion

AMNION (from the Greek amnion—sacrificial bowl), the water membrane, one of the membranes in the embryos of reptiles, birds, and mammals. The embryos of fish and amphibians, which develop in water, are protected by the environment itself from various kinds of mechanical shocks that could have a harmful effect on the delicate tissues of the embryo; in reptiles, birds, and mammals, the embryo, developing inside the hard shell of an egg or inside the uterus, receives a special protective membrane filled with fluid (amniotic fluid). The presence or absence of the amnion makes it possible to divide all vertebrates into two large groups: Amniota and Anamnia. The amnion is formed in most vertebrates in the form of a fold of the extraembryonic part of the blastoderm (see Blastula), the formation of which begins at the front and then moves to the lateral and caudal parts of the embryo. When the neural tube has already been laid down, and its anterior end still appears straight, a semilunar fold consisting of the outer and inner germ layers—the proamnion—rises in front of the neural tube; this fold then arises on the lateral and posterior parts of the embryo, and thus, the latter is surrounded by a circular fold, the edges of which grow toward each other and fuse along the midline, starting from the anterior end of the embryo to the posterior. Initially, the fold consists of the outer and inner layers, but soon elements of the middle germ layer grow between them and displace the inner layer, so that subsequently only the outer and middle layers are included in the composition of the fold. Due to the fusion of the edges of the fold, a closed amniotic sac is obtained around the embryo, and the outer layer of the fold forms another fetal membrane—the serosa.

3 Development of the amnion and allantois in a chick: 1—on the third day of incubation, cross-section; 2—longitudinal section; 3—at the beginning of the fifth day; am—amnion, s—serous membrane, ds—yolk sac, al—allantois. As is clear from the method of formation of the amniotic sac, the ectodermal epithelium lines the surface in it that faces the embryo, while the outer surface, consisting of embryonic connective tissue, comes into contact with the same surface of the serous membrane and fuses with it more or less tightly. The amniotic sac is filled with fluid, which transudes from the vessels and serves as a protective environment for the embryo. The mesodermal part of the wall of the amnion undergoes further development, and in it, one can see the formation of smooth muscle, collagenous, and elastic fibers (Gardner, in pig embryos). Some authors have described nerve branches in the wall of the amnion, but this question appears to be controversial. The formation of the amnion proceeds completely differently in some rodents, hedgehogs, lemurs, and in humans. The embryonic node does not form a superficially lying blastoderm but sinks into the trophoblast, and within it, by means of splitting, a cavity is created, the floor of which constitutes the outer germ layer, and the walls directly transform into the amniotic vesicle. Subsequently, the vesicle is overgrown by mesenchyme. This form of development in humans is explained by the implantation of the embryo into the uterine mucosa, during which its free surface disappears.