placentation (redirected from Epitheliochorial)

placentation [plas·ən′tā·shən]

(botany)

The attachment of ovules along the inner ovarian wall by means of the placenta.

(embryology)

The formation and fusion of the placenta to the uterine wall.

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Placentation

The intimate association or fusion of a tissue or organ of the embryonic stage of an animal to its parent for physiological exchange to promote the growth and development of the young. It enables the young, retained within the body or tissues of the mother, to respire, acquire nourishment, and eliminate wastes by bringing the bloodstreams of mother and young into close association but never into direct connection. Placentation characterizes the early development of all mammals except the egg-laying duckbill platypus and spiny anteater. It occurs in some species of all other orders of vertebrates except the birds. In fact, in certain sharks and reptiles it is almost as well developed as in mammals. A few examples are also known among invertebrates (Peripatus, certain tunicates, and insects). See Fetal membrane

Block removed from center of human placenta

Efficient interchange depends on close proximity of large areas of fetal tissues to maternal blood and glandular areas. This is provided in mammals by a remarkable regulatory cooperation between the developing outer layer (trophoblast) of the chorion, together with the vascular yolk sac or allantois or both, and the mother's uterine lining (endometrium). In the typical mammalian placenta, which is always formed by the chorion and the allantoic vessels, the fetal and maternal bloodstreams are as close as a few thousandths of a millimeter from each other (see illustration). The surface area of the fetal villi which contain the functional fetal capillaries is probably several times larger than the body surface of the female. In humans this ratio is known to be about 8:1.