The placenta is a mystery organ. Few people have ever seen one and thus it is difficult to visualize its shape and attachment to the uterus. Because of this, my students prefer to talk about the umbilical cord (instead of the placenta) as the main organ of transmission between mother and fetus. This section is an attempt to restore the more complicated placenta to its rightful place. Much of my information comes from Beaconsfield (1980).
The placenta develops from the fertilized egg, therefore half its genes are from the father and it is a foreign entity that the mother’s body should have a natural tendency to reject. But even though the placenta erodes its way into her uterine lining she does not reject it and scientists are not really sure why.
Two current hypotheses are these: The cells of the outer layer of the placenta do not produce antigens that the mother’s lymphocytes would recognize as foreign, and also, the high level of hormones produced by the placenta suppress the activity of lymphocytes.
When mature the placenta is from eighteen to twenty inches in diameter and from two to three inches thick at the center where the cord is attached. Usually weighing one/sixth as much as the fetus at birth, it is smooth and shiny viewed from the fetal side, and rough and divided into many sections on the maternal side (see Beaconsfield, 1980 and Nilsson, 1977 for good pictures.)
The two umbilical arteries run into the placenta subdividing again and again until they end in capillaries inside thousands of little projections called villi (see diagram of placenta in Lesson Plan I at the end of unit.) These villi increase the surface area of the placenta to about ten square meters; a large surface area is very important wherever movement of molecules occurs across a membrane. The villi are bathed in a pool of approximately 150 cc of maternal blood which spurts out of spiral arteries in the uterine lining. There are no capillaries on the maternal side and the pool of blood simply drains back into open uterine veins. The supply of blood replenishes itself about three or four times every minute so that new supplies of food and oxygen are constantly moving into the fetal villi. From here the fetal capillaries join to finally form one large umbilical vein that returns to the fetus.
The placenta has an incredible range of functions: it is an organ of respiration, digestion, excretion and also an endocrine gland. It is important for students to understand its structure and functions, and also to know which substances it will transmit and how they affect the fetus. The following is a brief description of what the placenta can do:
Oxygen diffuses readily into the fetal villi since there is always a lower level in fetal blood. Aiding this movement is the fact that fetal erythrocytes (red blood cells) have a greater affinity for oxygen than maternal erythrocytes. Carbon dioxide diffuses in the opposite direction. (At this point you might ask your students to compare and contrast the placenta and lung. See the section on respiration.)
Glucose is the main energy source for the fetus and again it diffuses easily across to fetal capillaries because the gradient favors it. Amino acids move from mother to fetus also but since the fetal blood contains a higher concentration than the maternal blood there is probably active transport taking place across the membranes of the cells in the villi. (Active transport requires energy.) The placenta is very selective in the amino acids it takes in so that it can meet the specific needs of the growing fetus.
Pregnant teenagers are particularly susceptible to toxemia, a condition characterized by high blood pressure, the accumulation of fluid in the limbs and excess loss of protein in the urine. Because of this loss there may not be enough protein available for the fetus to build all its brain cells.
The placenta does not form a very efficient barrier against many drugs so that the fetus may be affected by alcohol, barbiturates, ether, morphine, heroin and many others (Burnhan, 1972). Both nicotine and carbon monoxide can reduce the size of the baby of a smoking mother. The immature liver of the fetus is unable to clear these drugs very fast and the effects may be profoundly damaging in the developing baby. (See Sherree Kassuba’s unit in this volume).
Very few bacteria can cross the placenta (the spirochete of syphilis is an exception) but viral infections of the fetus do occur, such as rubella, herpes and CMV (cytomegalovirus), all with devastating results in early pregnancy. Luckily, IgG, a gamma globulin or antibody produced by the mother also crosses the placenta to give protection against many diseases. (The section on immunity describes how the mother protects her baby before and after birth if she is nursing.)
Excretion occurs in the opposite direction, from fetus to mother. The main substances are urea and uric acid which are excreted by the mother’s kidneys, and also bilirubin, a breakdown product of red blood cells that can cause brain damage, but which is cleared by the mother’s liver.
(figure available in print form)
The most fascinating function of the placenta is as an endocrine gland and its interaction with both fetus and mother to sustain a healthy pregnancy. The first hormone the placenta makes is HCG ( Human Chorionic Gonadotropin), which can be detected in the mother’s blood only eight days after ovulation (Niswander, 1976). HCG, which is the basis of urine Pregnancy tests, keeps the maternal ovary producing progesterone for the first two months until the placenta takes over. Progesterone is necessary to maintain pregnancy.
The placenta also produces a hormone with the incredible name of Human Chorionic Somatomammotropin (HCS), which as the name implies, makes the mother’s breasts grow.
Another group of hormones are the estrogens,made jointly by the placenta and fetus during pregnancy. First the placenta makes progesterone which reaches the fetal adrenal glands by the umbilical vein. The fetus then makes a precursor of estrogen which is returned to the placenta where the final product of estrogen is finally produced. The fact that the fetus and placenta act together causes scientists to refer to them as the feto-placental unit. Estrogen which is necessary for growth of the uterus, breasts and other maternal organs is excreted in the mother’s urine. Doctors use the levels as an indication of fetal and placental health—the higher the level, the better.