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The epiblast forms the floor of the amniotic cavity and is continuous peripherally with the amnion. The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic membrane. This membrane along with hypoblast forms the primary yolk sac. The embryonic disc now lies between the amniotic cavity and the primary ...
The trophoblast layer differentiates into amnion and the chorion, which then comprise the fetal membranes. [4] 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 amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish lack the amnion and thus are anamniotes (non-amniotes). The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side. [1]
Shortly after, the human epiblast will assume a disc shape while the amniotic cavity forms. The epiblast cells adjacent to the trophoblast are specified to become amnion cells. The mouse epiblast transitions from a rosette structure to a cup. A pro-amniotic cavity forms, surrounded by the epiblast cup fused to extraembryonic ectoderm.
Amniotic fluid is removed from the mother by an amniocentesis procedure, where a long needle is inserted through the abdomen into the amniotic sac, using ultrasound guidance such that the fetus is not harmed. Amniocentesis is a low risk procedure, with risk of pregnancy loss between 1 in 1,500 – 1 in 700 procedures.
The chorion and the amnion together form the amniotic sac. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes; [ 1 ] the chorionic villi emerge from the chorion, invade the endometrium , and allow the transfer of nutrients from maternal blood to fetal blood.
The fluid then collects into a single lumen in a process akin to Ostwald ripening to form the blastocoel, which determines the first axis of symmetry of the mammalian embryo. [19] The side of the blastocyst where the inner cell mass forms is called the embryonic pole, and the opposite side is the abembryonic pole.
In this procedure, a thin needle is inserted into the abdomen of the pregnant woman. [4] The needle punctures the amnion, which is the membrane that surrounds the developing fetus. [4] The fluid within the amnion is called amniotic fluid, and because this fluid surrounds the developing fetus, it contains fetal cells. [4]