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

The placenta is a disk-shaped organ that serves as the interface for maternal and fetal exchange of materials. It is formed early in pregnancy when the outer cell layer that envelops the developing embryo, the chorion, fuses with the uterine wall forming fingerlike projections called chorionic villi. Each villus, surrounded by a pool of maternal blood, contains a network of fetal capillaries through which nutrients and waste products are transferred (although there is no actual exchange of blood). Products harmful to fetal development, such as nicotine, cocaine, alcohol, some medications, and environmental pollutants, may also be transferred to the fetus. Upon completion of the pregnancy, the placenta is expelled from the uterus. For the purposes of diagnosis in cases of complications, physicians may examine the placenta. Complications involving the placenta itself include postpartum hemorrhage, placenta previa, preeclampsia, and intrauterine growth restriction.

See also:BIRTH; PREGNANCY; PRENATAL DEVELOPMENT

Bibliography

Begley, David J., Anthony Firth, and Robin Hoult. Human Reproduction and Developmental Biology. New York: Macmillan, 1980.

Faber, J. J., and Kent Thornburg. Placental Physiology: Structure and Function of Fetomaternal Exchange. New York: Raven Press, 1983.

Jansson, Thomas, and Theresa Powell. "Placental Nutrient Transfer and Fetal Growth." Nutrition 16 (7/8):500-502.

Vander, Arthur, Jane Sherman, and Dorothy Luciano. Human Physiology: The Mechanisms of Body Function, 5th edition. New York: McGraw-Hill, 1990.

Patricia CraneEllerson

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 (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.

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.