placentation

1. 1. Formation of a placenta in the uterus.
   2. The type or structure of a placenta.
2. Botany. Arrangement of placentas within the ovary.

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 also Fetal membrane.

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.

Block removed from center of human placenta.

1. Formation of a placenta in the uterus.
2. The type or structure of a placenta.

1. the series of events following implantation of the embryo and leading to development of the placenta.
2. the nature of the implantation. See also placenta.

• adventitial p. — developmental of additional areas outside the normal areas, e.g. intercotyledonary in cattle.
• cotyledonary p. — the areas of placentation are limited to caruncles, approximately circular masses scattered over the placenta. Characteristic of the cow, ewe and goat doe.
• diffuse p. — the whole of the surface of the chorion is placental.
• intercotyledonary p. — abnormal placental attachments between the cotyledons, in cows.
• zonal p. — only some zones of the placenta have a vascular area involved in placentation. Characteristic of the bitch and the queen.

  Zonary placenta of carnivores. By permission from Sack W, Wensing CJG, Dyce KM, Textbook of Veterinary Anatomy, Saunders, 2002

The noun has 2 meanings:

Meaning #1: the formation of the placenta in the uterus

Meaning #2: arrangement of the ovules in the placenta and of the placentas in the ovary