- A thin membrane surrounding the fetus during pregnancy. The amnion is the inner of the two fetal membranes (the chorion is the outer one). It contains the amniotic fluid.
* * *Innermost of the extraembryonic membranes enveloping the embryo in utero and containing the amniotic fluid; it consists of an internal embryonic layer with its ectodermal component, and an external somatic mesodermal component; in the later stages of pregnancy the a. expands to come in contact with and partially fuse to the inner wall of the chorionic vesicle; derived from the trophoblast cells. SYN: amnionic sac. [G. the membrane around the fetus, fr. amnios, lamb]- a. nodosum nodules in the a. that consist of typical stratified squamous epithelium. SYN: squamous metaplasia of a..
* * *am·ni·on 'am-nē-.än, -ən n, pl amnions or am·nia -nē-ə a thin membrane forming a closed sac about the embryos and fetuses of reptiles, birds, and mammals and containing the amniotic fluid called also amniotic sac
* * *n.the membrane that forms initially over the dorsal part of the embryo but soon expands to enclose it completely within the amniotic cavity; it is connected to the embryo at the umbilical cord. It expands outwards and fuses with the chorion, obliterating virtually all the intervening cavity. The double membrane (amniochorion) normally ruptures at birth.• amniotic adj.
* * *am·ni·on (amґne-on) [Gr. â€œbowlâ€; â€œmembrane enveloping the fetusâ€] the thin but tough extraembryonic membrane of reptiles, birds, and mammals that lines the chorion and contains the embryo and later the fetus, with the amniotic fluid around it; in mammals it is derived from trophoblast by folding or splitting. See also amniotic sac, under sac.
Amnion and other extraembryonic membranes surrounding a human embryo at 10 weeks; the amnion has enveloped the umbilical cord.
Medical dictionary. 2011.