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Concept
Amniotic sac
Summary
The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and via the umbilical cord, the placenta.
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. 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 remains an important extraembryonic membrane throughout prenatal development. The third membrane is the allantois, and the fourth is the chorion which surrounds the embryo after about a month and eventually fuses with the amnion.
Amniocentesis is a medical procedure where fluid from the sac is sampled during fetal development, between 15 and 20 weeks of pregnancy, to be used in prenatal diagnosis of chromosomal abnormalities and fetal infections.
The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid. The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the cephalic extremity and subsequently at the caudal end and sides of the embryo. As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity is formed.
At the beginning of the second week, a cavity appears within the inner cell mass, and when it enlarges, it becomes the amniotic cavity. The floor of the amniotic cavity is formed by the epiblast. Epiblast migrates between the epiblastic disc and trophoblast.
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