Extra-Embryonic Membranes and the Placenta

Four extra-embryonic membranes support and protect the developing embryo throughout pregnancy. The amnion is a double-layered, translucent membrane enclosing a fluid-filled cavity, the amniotic fluid; it cushions the embryo from mechanical injury, helps regulate its temperature, and gives it room to move as it grows. The yolk sac becomes part of the developing gut and is the earliest source of the embryo's blood cells and blood vessels. The allantois is a small out-pocketing of tissue at the tail (caudal) end of the yolk sac; it forms the structural base of the umbilical cord, the link between the embryo and the placenta, and its tissue later becomes part of the urinary bladder. The chorion is the outermost membrane, enclosing the embryo and all the other membranes together, and it plays a direct role in building the placenta: trophoblast cells in the blastocyst send out finger-like projections called chorionic villi, carrying foetal blood, which are bathed in maternal-blood-filled sinuses; these villi combine with the uterine tissue to form the disc-shaped placenta. The placenta is the organ that actually meets the embryo's (and later the foetus's) nutritive, respiratory and excretory needs, l …