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
Placenta
placenta
placenta (pl
sen´t),
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.
pla·cen·ta (pl -s n t)n. pl. pla·cen·tas or pla·cen·tae (-t  ) The membranous vascular organ in female mammals that permits metabolic interchange between fetus and mother. It develops during pregnancy from the chorion of the embryo and the decidua basalis of the maternal uterus and permits the absorption of oxygen and nutritive materials into the fetal blood and the release of carbon dioxide and nitrogenous waste from it, without the direct mixing of maternal and fetal blood. It is expelled following birth. pla·cen tal adj. |   Top: full-term placenta seen from fetal side Bottom: full-term placenta seen from maternal side |
Placenta
The organ that allows interchange between the fetus and the mother. Blood from the fetus and the mother do not directly mix, but the thin placental membrane allows the fetus to absorb nutrients and oxygen from the mother. Waste products from the fetus can exit through the placenta.
Mentioned in: Childbirth, Cytomegalovirus Infection, Erythroblastosis Fetalis, Fetal Alcohol Syndrome, Gestational Diabetes, Human Chorionic Gonadotropin Pregnancy Test, Hydatidiform Mole, Maternal to Fetal Infections, Pelvic Ultrasound, Placenta Previa, Placental Abruption, Polyhydramnios and Oligohydramnios, Preeclampsia and Eclampsia, Pregnancy, Premature Rupture of Membranes, Prenatal Surgery, Sickle Cell Disease, Thalassemia, Triple Screen
placenta
[pləsen′tə]
Etymology: L, flat cake
a highly vascular fetal organ that exchanges with the maternal circulation, mainly by diffusion of oxygen, carbon dioxide, and other substances. It begins to form on approximately the eighth day of gestation when the blastocyst touches the wall of the uterus and adheres to it. Placentation begins as the trophoblast is able to digest cells of the endometrium, causing a small erosion on the uterine wall in which an embryo nidates. Human chorionic gonadotropin (HCG), which is chemically identical to luteinizing hormone, is secreted by the developing placenta and promotes survival and hormone production of the corpus luteum. The presence of HCG in the maternal blood and urine is an indicator of early pregnancy. The trophoblastic layer continues to infiltrate the maternal tissues with fingerlike projections, called chorionic villi. By the third month of pregnancy the placenta is able to secrete large amounts of progesterone, enough to relieve the corpus luteum of that function. At term the normal placenta is one seventh to one fifth of the weight of the infant. The maternal surface is lobulated and has a dark red rough, liverlike appearance. The fetal surface is smooth and shiny, covered with the fetal membranes, and marked by large white blood vessels beneath the membranes that fan out from the centrally inserted umbilical cord. The time between the infant's birth and the expulsion of the placenta is the third and last stage of labor.
placenta (pl
sen´t),n the organ of metabolic interchange between the fetus and the mother.
placenta abruptio
n a condition in which a typically positioned placenta detaches from the uterine wall prior to delivery, which may threaten both the viability of baby and the life of the mother.
placenta previa
n atypical placental positioning and attachment within the inferior third of uterus, which may cover the cervix in part or fully.
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.
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.
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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.
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