placenta

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

See also antenatal development; labour; ovary; uterus; sex hormones.