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The amnion (pl.: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid , which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo.
The yolk sac, amnion, chorion, and allantois are the four extraembryonic membranes that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo. [5] They form from the inner cell mass; the first to form is the yolk sac followed by the amnion which grows over the developing embryo. The amnion ...
The amnion is the innermost layer and, therefore, contacts the amniotic fluid, the fetus and the umbilical cord. [5] The internal pressure of the amniotic fluid causes the amnion to be passively attached to the chorion. [4] The chorion functions to separate the amnion from the maternal decidua and uterus. [4]
This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later. [22]
The extraembryonic membranes in insects include a serous membrane (serosa) originating from blastoderm cells, an amnion or amniotic cavity whose expression is controlled by the Zerknüllt gene, and a yolk sac. [3]
Amniotic embryo. a=embryo, b=yolk, c=allantois, d=amnion, e=chorion. In reptiles, birds, and monotremes, the chorion is one of the four extraembryonic membranes that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival. It is located inside the albumen, which is the white of the egg.
When fetal kidneys begin to function around week 16, fetal urine also contributes to the fluid. [1] In earlier times, it was believed that the amniotic fluid was composed entirely of excreted fetal urine. The fluid is absorbed through the fetal tissue and skin. [2] After 22 to 25 week of pregnancy, keratinization of an embryo's skin occurs.
The chorioallantoic membrane (CAM), also known as the chorioallantois, is a highly vascularized membrane found in the eggs of certain amniotes like birds and reptiles.It is formed by the fusion of the mesodermal layers of two extra-embryonic membranes – the chorion and the allantois. [1]