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These cells are also derived from epiblast which is transformed into ectoderm. [citation needed] The amniotic cavity is surrounded by an extraembryonic membrane, called the amnion. As the implantation of the blastocyst progresses, a small space appears in the embryoblast, which is the primordium of the amniotic cavity.
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]
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 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.
Fluid collects between the epiblast and the hypoblast, which splits the epiblast into two portions. The layer at the embryonic pole grows around the amniotic sac, creating a barrier from the cytotrophoblast. This becomes known as the amnion, which is one of the four fetal membranes and the cells it comprises are referred to as amnioblasts. [6]
The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus.By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer ...
The inner cell mass will go on to become the actual embryo. The external, surrounding cells develop into trophoblast cells, which only contribute to extra-embryonic tissues. At this stage there is no lumen within the embryo. In a process called cavitation, trophectoderm cells transport fluid into the embryo to create a blastocoel, the fluid ...