placenta defined in 1951 yearplacenta - placenta;
placenta - (Bot.). Part of ovary wall (fused margins of carpel(s)) on which ovules are borne. (Zool.). Organ, consisting of embryonic and maternal tissues in close union, by which embryo of viviparous animal is nourished. In placenta! mammals the embryonic tissue concerned is the chorion, with blood vessels supplied via allantois or occasionally via yolk-sac. In ungulates the trophoblast of the chorion merely fits closely against endometrium of uterus; but in most mammals the endometrium becomes much altered (decidua) and in parts eroded, so that the trophoblast is in contact with maternal blood-vessels (carnivores) or maternal blood (other mammals including man). There is exchange of small molecules through the trophoblast between maternal and embryonic blood in the mammalian placenta; oxygen and food-substances enter the embryo, urea and carbon dioxide leave it. Only minute amounts of proteins are exchanged. The placenta produces hormones (gonado-tropic, oestrogen, progesterone) which keeps uterus adapted to the pregnancy. At birth the umbilical cord, which connects placenta to embryo, is broken, and the placenta is discharged from the uterus after the embryo.
near placenta in Knolik
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