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placenta (pla-sen´ta) [NA]

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 1 / 6 to 1 / 7 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.succenturiate p., supernumerary p;
p. accre´ta 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. accre´ta ve´ra the term applied when villi are juxtaposed to the myometrium.
adherent p. a p. that fails to separate cleanly from the uterus after delivery.
annular p. a p. in the form of a band encircling the interior of the uterus.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. bilo´ba a p. duplex in which the two parts are separated by a constriction.p. bipartita;
p. biparti´ta p. biloba
central p. pre´via 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.
choriovitelline p. a p. (seen in some lower animals) in which the chorion is formed by the fusion of yolk-sac mesoderm and vessels to the inner face of the serosa.
p. circumvalla´ta 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 diamniotic p. See twin p.
p. diffu´sa p. membranacea
p. dimidia´ta p. duplex
disperse p. a p. in which the umbilical arteries divide dichotomously before entering the placental substance.
p. du´plex a p. consisting of two parts, almost entirely detached, being united only at the point of attachment of the cord. See p. biloba.p. dimidiata;
endotheliochorial p. a p. in which the chorionic tissue penetrates to the endothelium of the maternal blood vessels.
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. extrachora´les a p. in which the chorionic plate is limited by a thin membranous fold at the edge.
p. fenestra´ta a p. in which there are areas of thinning, sometimes extending to entire absence of placental tissue.
fetal p. , p. feta´lis the chorionic portion of the placenta, containing the fetal blood vessels, from which the funis develops; specifically, in humans, it develops from the chorion fetalis placentae [NA];
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. retained p
p. incre´ta 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. margina´ta a p. with raised edges, less pronounced than the p. circumvallata. See also p. reflexa.
maternal p. pars uterina placentae
p. membrana´cea an abnormally thin p. covering an unusually large area of the uterine lining.p. diffusa;
monochorionic diamniotic p. See twin p.
monochorionic monoamniotic p. See twin p.
p. multilo´ba a p. having more than three lobes separated from each other by simple constrictions, the fetus being single.placenta multipartita;
nondeciduous p. a p. in which the fetal p. is cast off, leaving the uterine mucosa intact (e.g., an epitheliochorial p.).
p. pandurafor´mis a form of p. dimidiata with the two halves placed side by side in a shape suggestive of a lutelike musical instrument (pandura).
p. percre´ta 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. pre´via 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.placental presentation;
p. pre´via centra´lis p. previa in which the p. entirely covers the internal os of the cervix.central p. previa, total p. previa;
p. pre´via margina´lis p. previa in which the p. comes to the margin of, but does not occlude, the internal os of the cervix.
p. pre´via partia´lis p. previa in which the internal os of the cervix is partially covered by placental tissue.
p. reflex´a 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. renifor´mis 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.incarcerated p;
Schultze's p. a p. that appears at the vulva with the glistening fetal surface (amnion) presenting.
p. spu´ria a mass of placental tissue which has no vascular connection with the main p.
succenturiate p. accessory p
supernumerary p. accessory p
syndesmochorial p. in ruminant animals, a type of p. in which the chorion is attached to maternal connective tissue.
total p. pre´via p. previa centralis
p. tri´loba p. tripartita
p. triparti´ta a p. consisting of three parts almost entirely separate, being joined together only by the blood vessels of the umbilical cord; the fetus is single.p. triloba, p. triplex;
p. tri´plex p. tripartita
twin p. the placenta(s) of a twin pregnancy; if dizygotic, the p.'s may be separate or fused, the latter retaining two amniotic and two chorionic sacs (dichorionic diamniotic p.); if monozygotic, the p. may be a monochorionic monoamniotic p. or monochorionic diamniotic 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 amniotic membranes.
p. uteri´na pars uterina placentae
p. velamento´sa 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. annular p


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