hemochorial placenta

placenta /pla·cen·ta/ (plah-sen´tah) pl. placentas, placen´tae   [L.] an organ characteristic of true mammals during pregnancy, joining mother and fetus, providing endocrine secretion and selective exchange of soluble bloodborne substances through apposition of uterine and trophoblastic vascularized parts.placen´tal

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placenta accre´ta  one abnormally adherent to the myometrium, with partial or complete absence of the decidua basalis.

circumvallate placenta  one in which a dense peripheral ring is raised from the surface and the attached membranes are doubled back over the placental edge.

deciduate placenta , deciduous placenta a placenta or type of placentation in which the decidua or maternal parts of the placenta separate from the uterus and are cast off together with the trophoblastic parts.

fetal placenta  the part of the placenta derived from the chorionic sac that encloses the embryo, consisting of a chorionic plate and villi.

hemochorial placenta  one in which maternal blood comes in direct contact with the chorion, as in humans.

placenta incre´ta  placenta accreta with penetration of the myometrium.

maternal placenta  the maternally contributed part of the placenta, derived from the decidua basalis.

placenta membrana´cea  one that is abnormally thin and spread out over an unusually large area of the uterine wall.

placenta percre´ta  placenta accreta with invasion of the myometrium to its peritoneal covering, sometimes causing rupture of the uterus.

placenta pre´via  one located in the lower uterine segment, so that it partially or completely covers or adjoins the internal os.

placenta spu´ria  an accessory portion having no blood vessel attachment to the main placenta.

placenta succenturia´ta , succenturiate placenta an accessory portion attached to the main placenta by an artery or vein.

villous placenta  one characterized by the presence of villi that are outgrowths of the chorion.

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he·mo·cho·ri·al placenta (hm-kôr-l)

n.

A placenta, as in humans, in which maternal blood is in direct contact with the chorion.

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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.

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choriovitelline placenta

a placentation in which the yolk sac becomes involved in the fetal-maternal union.

cotyledonary placenta

distribution of the villi on the fetal chorion is localized in multiple circumscribed areas—the cotyledons.

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

diffuse placenta

the villi on the fetal chorion is diffuse over the entire placenta as in mares and sows.

discoid placenta

a placenta in which the chorionic villi are arranged in a circular plate as in human and rodent placentae.

endotheliochorial placenta

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.

epitheliochorial placenta

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.

hemochorial 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.

nondeciduate placenta

no maternal tissue is lost when the pregnancy terminates.

retained placenta

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.

syndesmochorial 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.

zonary placenta

a placenta in which the chorionic villi are restricted to an equatorial girdle, as in the bitch and queen.