A temporary organ joining the mother and fetus, the placenta transfers oxygen and nutrients from the mother to the fetus, and permits the release of carbon dioxide and waste products from the fetus. It is roughly disk-shaped, and at full term measures about seven inches in diameter and a bit less than two inches thick. The upper surface of the placenta is smooth, while the under surface is rough. The placenta is rich in blood vessels. The placenta is expelled during the birth process with the fetal membranes; together, these structures form the afterbirth.
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Organ of metabolic interchange between fetus and mother. It has a portion of embryonic origin, derived from a highly developed area of the outermost embryonic membrane (chorion frondosum), and a maternal portion formed by a modification of the part of the uterine mucosa (decidua basalis) in which the chorionic vesicle is implanted. Within the p., the chorionic villi, with their contained capillaries carrying blood of the embryonic circulation, are exposed to maternal blood in the intervillous spaces in which the villi lie; no direct mixing of fetal and maternal blood occurs, but the intervening tissue (the placental membrane) is sufficiently thin to permit the absorption of nutritive materials, oxygen, and some harmful substances, like viruses, into the fetal blood and the release of carbon dioxide and nitrogenous waste from it. At term, the human p. is disk shaped, about 4 cm in thickness and 18 cm in diameter, and averages about 16 to 17 the weight of the fetus; its fetal surface is smooth, being formed by the adherent amnion, with the umbilical cord normally attached near its center; the maternal surface of a detached p. is rough because of the torn decidual tissue adhering to the chorion and shows lobular elevations called cotyledons or lobes. [L. a cake]
- accessory p. a mass of placental tissue distinct from the main p.. SYN: succenturiate p., supernumerary p..
- p. accreta the abnormal adherence of the chorionic villi to the myometrium, associated with partial or complete absence of the decidua basalis and, in particular, the stratum spongiosum. SEE ALSO: p. percreta.
- p. accreta vera the term applied when villi are juxtaposed to the myometrium.
- adherent p. a p. that fails to separate cleanly from the uterus after delivery.
- anular p. a p. in the form of a band encircling the interior of the uterus. SYN: ring-shaped p., zonary p..
- battledore p. a p. in which the umbilical cord is attached at the border; so called because of the fancied resemblance to the racquet (racket) used in battledore, a precursor to badminton.
- bidiscoidal p. a p. with two separate disc-shaped portions attached to opposite walls of the uterus, normal for certain monkeys and shrews, and occasionally found in humans.
- p. biloba a p. duplex in which the two parts are separated by a constriction. SYN: p. bipartita.
- p. bipartita SYN: p. biloba.
- central p. previa SYN: p. previa centralis.
- chorioallantoic p. a p. (such as that of primates) in which the chorion is formed by the fusion of the allantoic mesoderm and vessels to the inner face of the serosa.
- chorioamnionic p. a form of placentation in which the amnion is fused to the inside of the chorion, thus permitting interchange of water and electrolytes between mother and fetus.
- p. circumvallata a cup-shaped p. with raised edges, having a thick, round, white, opaque ring around its periphery; a portion of the decidua separates the margin of the p. from its chorionic plate; the remainder of the chorionic surface is normal in appearance, but the fetal vessels are limited in their course across the p. by the ring. SEE ALSO: p. marginata, p. reflexa.
- cotyledonary p. a p. in which the substance is divided into lobes or cotyledons.
- deciduate p. a p. in which the maternal decidua is cast off with the fetal p..
- dichorionic diamnionic p. twin p..
- p. diffusa SYN: p. membranacea.
- p. dimidiata SYN: p. duplex.
- disperse p. a p. in which the umbilical arteries divide dichotomously before entering the placental substance.
- Duncan p. a separated p. that appears at the vulva with the chorionic surface outward.
- p. duplex a p. consisting of two parts, almost entirely detached, being united only at the point of attachment of the cord. See p. biloba. SYN: p. dimidiata.
- endotheliochorial p. a p. in which the chorionic tissue penetrates to the endothelium of the maternal blood vessel s.
- endothelio-endothelial p. a p. in which the endothelium of the maternal vessels comes in direct contact with the endothelium of the fetal vessels to form the placental barrier.
- epitheliochorial p. a p. in which the chorion is merely in contact with, and does not erode, the endometrium.
- p. extrachorales a p. in which the chorionic plate is limited by a thin membranous fold at the edge.
- p. fenestrata a p. in which there are areas of thinning, sometimes extending to entire absence of placental tissue.
- fetal p., p. fetalis the chorionic portion of the p., containing the fetal blood vessel s, from which the umbilical cord develops; specifically, in humans, it develops from the chorion frondosum. SYN: pars fetalis placentae.
- hemochorial p. the type of p., as in humans and some rodents, in which maternal blood is in direct contact with the chorion.
- hemoendothelial p. the type of p., as in rabbits, in which the trophoblast becomes so attenuated that, by light microscopy, maternal blood appears to be separated from fetal blood only by the endothelium of the chorionic capillaries.
- horseshoe p. an exaggerated p. reniformis curved in the form of a horseshoe; present in some twin pregnancies.
- incarcerated p. SYN: retained p..
- p. increta a form of p. accreta in which the chorionic villi invade the myometrium.
- labyrinthine p. a p. in which maternal blood circulates through channels within the fetal syncytiotrophoblast.
- p. marginata a p. with raised edges, less pronounced than the p. circumvallata. SEE ALSO: p. reflexa.
- maternal p. SYN: pars uterina placentae.
- p. membranacea an abnormally thin p. covering an unusually large area of the uterine lining. SYN: p. diffusa.
- monochorionic diamnionic p. twin p..
- monochorionic monoamnionic p. twin p..
- p. multiloba a p. having more than three lobes separated from each other by simple constrictions, the fetus being single. SYN: p. multipartita.
- p. multipartita SYN: p. multiloba.
- nondeciduous p. a p. in which the fetal p. is cast off, leaving the uterine mucosa intact ( e.g., an epitheliochorial p.).
- p. panduraformis a form of p. dimidiata with the two halves placed side by side in a shape suggestive of a lutelike musical instrument (pandura).
- p. percreta the term applied when the villi have invaded the full thickness of myometrium to or through the serosa of the uterus, causing incomplete or complete uterine rupture, respectively. SEE ALSO: p. accreta.
- p. previa the condition in which the p. is implanted in the lower segment of the uterus, extending to the margin of the internal os of the cervix or partially or completely obstructing the os. SYN: placental presentation.
- p. previa centralis p. previa in which the p. entirely covers the internal os of the cervix. SYN: central p. previa, total p. previa.
- p. previa marginalis p. previa in which the p. comes to the margin of, but does not occlude, the internal os of the cervix.
- p. previa partialis p. previa in which the internal os of the cervix is partially covered by placental tissue.
- p. reflexa an anomaly of the p. in which the margin is thickened so as to appear turned back upon itself. SEE ALSO: p. circumvallata, p. marginata.
- p. reniformis a kidney-shaped p..
- retained p. incomplete separation of the p. and its failure to be expelled at the usual time after delivery of the child. SYN: incarcerated p..
- ring-shaped p. SYN: anular p..
- Schultze p. a p. that appears at the vulva with the glistening fetal surface (amnion) presenting.
- p. spuria a mass of placental tissue that has no vascular connection with the main p..
- succenturiate p. SYN: accessory p..
- supernumerary p. SYN: accessory p..
- total p. previa SYN: p. previa centralis.
- p. triloba SYN: p. tripartita.
- p. tripartita a p. consisting of three parts almost entirely separate, being joined together only by the blood vessel s of the umbilical cord; the fetus is single. SYN: p. triloba, p. triplex.
- p. triplex SYN: p. tripartita.
- twin p. the p.(s) of a twin pregnancy; if dizygotic, the placentas may be separate or fused, the latter retaining two amnionic and two chorionic sacs (dichorionic diamnionic p.); if monozygotic, the p. may be a monochorionic monoamnionic p. or monochorionic diamnionic p., depending on the stage at which twinning took place; if twinning occurs early, there may be a fused p. with two chorionic and two amnionic membranes.
- p. uterina SYN: pars uterina placentae.
- p. velamentosa a p. in which the umbilical cord is attached to the adjoining membranes, with the umbilical vessels spread out and entering the p. independently.
- villous p. a p. in which the chorion forms villi.
- zonary p. SYN: anular p..

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pla·cen·ta plə-'sent-ə n, pl -centas or -cen·tae -'sent-(.)ē the vascular organ in mammals except monotremes and marsupials that unites the fetus to the maternal uterus and mediates its metabolic exchanges through a more or less intimate association of uterine mucosal with chorionic and usu. allantoic tissues permitting exchange of material by diffusion between the maternal and fetal vascular systems but without direct contact between maternal and fetal blood and typically involving the interlocking of fingerlike vascular chorionic villi with corresponding modified areas of the uterine mucosa see ABRUPTIO PLACENTAE

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an organ within the uterus by means of which the embryo is attached to the wall of the uterus. Its primary function is to provide the embryo with nourishment, eliminate its wastes, and exchange respiratory gases. This is accomplished by the close proximity of the maternal and fetal blood systems within the placenta. It also functions as a gland, secreting human chorionic gonadotrophin, progesterone, and oestrogens, which regulate the maintenance of pregnancy. See also afterbirth.
placental adj.

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pla·cen·ta (plə-senґtə) pl. placentas, placenґtae [L. “a flat cake”] a fetomaternal organ characteristic of true mammals during pregnancy, joining mother and fetus, providing endocrine secretion and selective exchange of soluble, bloodborne substances through an apposition of uterine and trophoblastic vascularized parts. According to species, the area of vascular apposition may be diffuse, cotyledonary, zonary, or discoid; the nature of apposition may be labyrinthine or villous; and the intimacy of apposition may vary according to what layers are lost of those originally interposed between maternal and fetal blood (maternal endothelium, uterine connective tissue, uterine epithelium, chorion, extraembryonic mesoderm, and endothelium of villous capillary). The chorion may be joined by and receive blood vessels from either the yolk sac or the allantois, and the uterine lining may be largely shed with the chorion at birth (deciduate p.) or may separate from the chorion and remain (nondeciduate p.). The human placenta is discoid, villous, hemochorial, chorioallantoic, and deciduate. After birth, it weighs about 600 g and is about 16 cm in diameter and 2 cm thick, discounting the decidua basalis and the maternal blood in the intervillous space (a principal functional part into which the chorionic villi dip and which leaks out at birth). The villi are grouped into adjoining cotyledons making about 20 velvety bumps on the side of the placenta facing the uterus; the side facing the fetus is smooth and covered with amnion, a thin avascular layer that continues past the edges of the placenta to line the entire hollow sphere of chorion except where it is reflected to cover the umbilical cord. The umbilical cord joins fetus and placenta and usually joins the placenta near the center, although it sometimes inserts at the edge, on the nonplacental chorion, or on an accessory placenta.

Structural features of the placenta.

Medical dictionary. 2011.


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