| placenta | <anatomy, obstetrics> An organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble, but not particulate, blood borne substances through an apposition of uterine and trophoblastic vascularised 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, the intimacy of apposition may vary according to what layers are lost of those originally interposed between maternal and foetal 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) or may separate from the chorion and remain (nondeciduate). The human placenta is discoid, villous, haemochorial, chorioallantoic and deciduate. After birth, it weighs about 600 gm. And is about 16 cm. In diameter and 2 cm. Thick, discounting a principal functional part, the maternal blood in the intervillous space (which leaks out at birth) into which the chorionic villi dip. The villi are grouped into adjoining cotyledons making about 20 velvety bumps on the side of the placenta facing outward to the uterus, the inner side of the placenta facing the foetus is smooth, being 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, which joins foetus and placenta. The cord usually joins the placenta near the centre but may insert at the edge, on the nonplacental chorion or on an accessory placenta. (31 Dec 1997) |
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| placenta, accessory | An extra placenta separate from the main placenta. Also called a succenturiate or supernumerary placenta. Placenta accreta: the abnormal adherence of the chorionic villi to the myometrium. The vascular processes of the chorion (a foetal membrane that enters into the formation of the placenta) grow directly in the myometrium (the muscular portion of the uterus). Normally there is tissue intervening between the chorionic villi and the myometrium. Here there is not. The word accreta comes from the latin accretio from ad meaning to or toward + crescere meaning to grow. Placenta accreta can progress to placenta percreta. (12 Dec 1998) |
| placenta accreta | Abnormal adherence of part or all of the placenta to the uterine wall, with partial or complete absence of the decidua basalis, especially of the spongiosum layer. (12 Dec 1998) |
| placenta accreta vera | The term applied when villi are juxtaposed to the myometrium. (05 Mar 2000) |
| placenta biloba | A placenta duplex in which the two parts are separated by a constriction. Synonym: placenta bipartita. (05 Mar 2000) |
| placenta bipartita | A placenta duplex in which the two parts are separated by a constriction. Synonym: placenta bipartita. (05 Mar 2000) |
| placenta circumvallata | A cup-shaped placenta with raised edges, having a thick, round, white, opaque ring around its periphery; a portion of the decidua separates the margin of the placenta from its chorionic plate; the remainder of the chorionic surface is normal in appearance, but the foetal vessels are limited in their course across the placenta by the ring. See: placenta marginata, placenta reflexa. (05 Mar 2000) |
| placenta diffusa | An abnormally thin placenta covering an unusually large area of the uterine lining. Synonym: placenta diffusa. (05 Mar 2000) |
| placenta dimidiata | A placenta consisting of two parts, almost entirely detached, being united only at the point of attachment of the cord. See: placenta biloba. Synonym: placenta dimidiata. (05 Mar 2000) |
| placenta duplex | A placenta consisting of two parts, almost entirely detached, being united only at the point of attachment of the cord. See: placenta biloba. Synonym: placenta dimidiata. (05 Mar 2000) |
| placenta extrachorales | A placenta in which the chorionic plate is limited by a thin membranous fold at the edge. (05 Mar 2000) |
| placenta fenestrata | A placenta in which there are areas of thinning, sometimes extending to entire absence of placental tissue. (05 Mar 2000) |
| placenta gonadotropin | A glycoprotein with a carbohydrate fraction composed of d-galactose and hexosamine, extracted from the urine of pregnant women and produced by the placental trophoblastic cells; its most important role appears to be stimulation, during the first trimester, of ovarian secretion of the oestrogen and progesterone required for the integrity of conceptus; it appears to play no significant role in the last two trimesters of pregnancy, as the oestrogen and progesterone are then formed by the placenta. Synonym: anterior pituitary-like hormone, choriogonadotropin, chorionic gonadotropic hormone, chorionic gonadotrophic hormone, placenta gonadotropin, placentagonadotropin. (05 Mar 2000) |
| placenta increta | A form of placenta accreta in which the chorionic villi invade the myometrium. (05 Mar 2000) |
| placenta, low | Location of the placenta in the lower part of the uterus (womb) so that the placenta partially or completely covers the cervix. Also known as a placenta previa. (12 Dec 1998) |