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Etymology: Gk, diskos, quoit, eidos, form; L, placenta, flat cake
a round placenta.
A placenta with a flat, circular shape.
See also: placenta
pl. placentae, placentas [L.] an organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble bloodborne substances through apposition of uterine and trophoblastic vascularized parts. Called also afterbirth. See also fetal membranes, placentation.
Domestic animals have a chorioallantoic placenta in which the outer layer of the allantois is fused with the chorion and the fetal umbilical vessels are distributed in the connective tissue between the two. Placentae are classified in several ways; based on the tissues of the dam and the fetus that contact each other; based on the proportion of the surface area of the fetal membranes that is in fact placentacious; based on loss of tissue at birth, etc. Thus the bovine placenta is epitheliochorial, cotyledonary and nondeciduate.
The major function of the placenta is to allow diffusion of nutrients from the dam's blood into the fetus's blood and diffusion of waste products from the fetus back to the dam. This two-way exchange takes place across the placental membrane, which is semipermeable. The placenta also produces hormones such as progesterone and estrogen.
a placentation in which the yolk sac becomes involved in the fetal-maternal union.
distribution of the villi on the fetal chorion is localized in multiple circumscribed areas—the cotyledons.
the villi on the fetal chorion is diffuse over the entire placenta as in mares and sows.
a placenta in which the chorionic villi are arranged in a circular plate as in human and rodent placentae.
the maternal vessels in the endometrium are bared to their endothelium and these are in contact with the chorion of the fetal membranes. This occurs in the bitch and queen.
the uterine epithelium of the uterus and the chorion are in contact in this placentation, and there is no erosion of the epithelium. Characteristic of cows, sows and mares. Called also adeciduate placenta.
a type of placenta in which all maternal layers are lost so that fetal tissue is in contact with frank maternal blood, as occurs in insectivores, rodents, rabbits and most primates.
no maternal tissue is lost when the pregnancy terminates.
the placenta has not been passed within 12 hours after the fetus has been delivered. Represents a potential beginning for metritis and infertility. Often difficult to assess in carnivores which rapidly eat the placenta.
a type of placentation characterized by an endometrial attachment to the chorion with a limited amount of destruction of the endometrial epithelium. Formerly thought to be characteristic of the ewe and goat doe, these species are now known to have epitheliochorial placentae.
a placenta in which the chorionic villi are restricted to an equatorial girdle, as in the bitch and queen.