placenta The placenta forms from both embryonic and maternal tissues, and hosts an astonishing array of hormonal, nutritional, respiratory, excretory, and immunological functions. It is expelled after the baby as the ‘afterbirth’.

When the developing, fertilized egg at the ‘blastocyst’ stage becomes implanted in the lining of the uterus, it develops ‘villi’ — fine, frond-like cellular projections from its outermost layer, the trophoblast. It is initially through these villi that nutrients are absorbed. Then, as the embryonic circulatory system develops, blood vessels grow into the villi on the implanted side of the embryo; this becomes the fetal component of the placenta. The nutritional functions of the placenta become concentrated in the intervillous space, which is bathed by the mother's blood from the spiral arteries, which are branches of the arteries to the uterus. The spiral arteries are converted in early to mid pregnancy, by trophoblast (placental) cell invasion, to become blood vessels that more resemble veins than arteries. (If this process does not occur, then the pregnancy may become complicated by pre-eclampsia, a condition characterized by high blood pressure and protein in the urine.) Normal, converted spiral arteries ensure steady supply of blood in a low-resistance circulation. Glucose and amino acids in the mother's blood pass to capillary blood vessels in the fetal villi that dangle in the intervillous space, covered only by a thin membrane, and from them pass to the fetus, through the umbilical vein in the umbilical cord, to be used as building blocks for intrauterine growth.

At this same interface between mother and fetus, gas exchange occurs, with passage of oxygen to the fetus, and carbon dioxide to the mother. Thus, the placenta fulfils in intrauterine life the functions of the lungs after birth. A low concentration of oxygen in fetal blood encourages this direction of transfer, together with the particular nature of fetal haemoglobin.

Similarly, the placenta has equivalent functions to the kidney after birth in permitting the excretion of the biochemical waste products of metabolism. There are fetuses that develop without kidneys (a condition known as renal agenesis). Because of the function of the placenta they often survive until birth, although they cannot survive long thereafter.

Although one might expect the placenta to be rejected by the mother's immune system, because the fetal component is ‘foreign’, this does not happen, because of the presence of unique antigens on the cell surfaces.

In addition to these functions of exchange between the two individual blood streams, the placenta also produces an extensive array of hormones. These include human chorionic gonadotropin (HCG) produced by embryonic tissue right from the time of implantation: this promptly protects the embryo from rejection, by acting on the ovaries, causing them to sustain the hormone production that supports pregnancy. The presence of HCG also acts as the basis of pregnancy testing. After the third month, hormone production by the placenta takes over the pregnancy-supporting role from the ovary, by virtue of progressively increasing secretion of oestrogens and progesterone.

Growth of the fetus may be impaired if the placenta malfunctions. If the degree is severe, oxygenation may also become impaired, ultimately with death of the fetus and stillbirth. Other clinical problems associated with the placenta are placenta praevia, in which the placenta is located below the fetus, and placental abruption, in which the placenta separates prematurely from the wall of the uterus. Both of these conditions may be associated with brisk haemorrhage.

The placenta is ejected during the third stage of labour.

Jim Neilson

placenta (pləsĕn´tə) or afterbirth, organ that develops in the uterus during pregnancy. It is a unique characteristic of the higher (or placental) mammals. In humans it is a thick mass, about 7 in. (18 cm) in diameter, liberally supplied with blood vessels. Composed mainly of tissue that develops from the embryo beginning early in pregnancy, the placenta is attached to the uterus, and the fetus is connected to the placenta by the umbilical cord. The placenta acts as an interface between the mother and fetus, drawing nourishment and oxygen, which it supplies to the fetus, from the maternal circulation. In turn, the placenta receives the wastes of fetal metabolism and discharges them into the maternal circulation for disposal. It also acts as an endocrine gland, producing estrogen, progesterone, gonadotrophin, and serotonin, and works to prevent the mother's immune system from rejecting the fetus. Shortly after delivery of the fetus the placenta is forced out by contractions of the uterus. Severe hemorrhage may occur if the placenta does not emerge in its entirety or if the uterus fails to contract properly.

placenta (plă-sent-ă) n. 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. It also functions as a gland, secreting chorionic gonadotrophin, progesterone, and oestrogens. See also afterbirth. p. praevia a condition in which the placenta is situated wholly or partially in the lower and noncontractile part of the uterus. When this becomes elongated and stretched during the last few weeks of pregnancy, and the cervix becomes stretched either before or during labour, placental separation and haemorrhage will occur. If the placenta is situated entirely before the presenting part of the fetus, delivery must be by Caesarean section.
—placental (plă-sen-t'l) adj.

placenta
1. The organ in mammals and other viviparous animals by means of which the embryo is attached to the wall of the uterus. It is composed of embyronic and maternal tissues: extensions of the chorion and allantois grow into the uterine wall so that materials (e.g. oxygen, nutrients) can pass between the blood of the embryo and its mother (there is, however, no direct connection between the maternal and embryonic blood). The placenta is eventually expelled as part of the afterbirth.

2. A ridge of tissue on the ovary wall of flowering plants to which the ovules are attached. The arrangement of ovules on the placenta (placentation) is variable, depending on the number of carpels and whether they are free (see apocarpy) or fused (see syncarpy).

placenta Organ in mammals (except monotremes and marsupials) that connects the fetus to the uterus of the mother. Part of the placenta contains tiny blood vessels through which oxygen and food are carried from the mother to the embryo via the umbilical cord and wastes are carried from the embryo to the mother's bloodstream to be excreted. The placenta secretes hormones that maintain pregnancy and is discharged from the mother's body as the afterbirth, immediately after delivery.

pla·cen·ta / pləˈsentə/ • n. (pl. -tae / -tē/ or -tas ) 1. a flattened circular organ in the uterus of pregnant eutherian mammals, nourishing and maintaining the fetus through the umbilical cord. 2. Bot. (in flowers) part of the ovary wall to which the ovules are attached.

placenta (anat.) afterbirth; (bot.) part of carpel to which scales are attached. XVII. — L. — Gr. plakóenta (-oûnta), accus. of plakóeis (-oûs) flat cake, sb. use of adj. f. plak-, in pláx flat surface.

placenta In animals, the organ by which embryos of viviparous species are nourished and waste products removed, formed by the fusion of embryonic and maternal tissues.

placenta In flowers, the part of the ovary wall formed from the fused margins of the carpel or carpels, on which are carried the ovules.

placenta •abetter, begetter, better, bettor, biretta, bruschetta, carburettor (US carburetor), debtor, feta, fetter, forgetter, getter, go-getter, Greta, Henrietta, letter, Loretta, mantelletta, operetta, petter, Quetta, setter, sinfonietta, sweater, upsetter, Valletta, vendetta, whetter •bisector, collector, connector, convector, corrector, defector, deflector, detector, director, ejector, elector, erector, hector, injector, inspector, nectar, objector, perfecter, projector, prospector, protector, rector, reflector, rejector, respecter, sector, selector, Spector, spectre (US specter), vector •belter, delta, helter-skelter, melter, pelta, Shelta, shelter, swelter, welter •pre-emptor, tempter •assenter, cementer, centre (US center), concentre (US concenter), dissenter, enter, eventer, fermenter (US fermentor), fomenter, frequenter, inventor, lamenter, magenta, placenta, polenta, precentor, presenter, preventer, renter, repenter, tenter, tormentor •inceptor, preceptor, receptor, sceptre (US scepter) •arrester, Avesta, Chester, contester, ester, Esther, fester, fiesta, Hester, investor, jester, Leicester, Lester, molester, Nestor, pester, polyester, protester, quester, semester, sequester, siesta, sou'wester, suggester, tester, trimester, vesta, zester •Webster • dexter • Leinster •Dorchester • Poindexter • newsletter •genuflector • implementer •experimenter • trendsetter •epicentre (US epicenter) •typesetter • jobcentre • photosetter •Cirencester • interceptor • Sylvester