Endocrine Functions of the Placenta

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The placenta secretes both steroid hormones and protein hormones. The protein hormones include chorionic gonadotropin (hCG) and chorionic somatomammotropin (hCS), both of which have actions similar to those of some anterior pituitary hormones (table 20.7). Chorionic gonadotropin has LH-like effects, as previously described; it also has thyroid-stimulating ability, like pituitary TSH. Chorionic somatomammotropin likewise has actions that are similar to two pituitary hormones: growth hormone and prolactin. The placental hormones hCG and hCS thus duplicate the actions of four anterior pituitary hormones.

Pituitary-like Hormones from the Placenta

The importance of chorionic gonadotropin in maintaining the mother's corpus luteum for the first 5/2 weeks of pregnancy has been previously discussed. There is also some evidence that hCG may in some way help to prevent immunological rejection of the implanting embryo. Chorionic somatomammotropin acts together with growth hormone from the mother's pituitary to produce a diabetic-like effect in the pregnant woman. The effects of these two hormones promote (1) lipolysis and increased plasma fatty acid concentration; (2) glucose-sparing by maternal tissues and, therefore, increased blood glucose concentrations;

Intervillous pool of maternal blood

Chorionic Villi And Intervillous Space

■ Figure 20.50 The circulation of blood within the placenta. Maternal blood is delivered to and drained from the spaces between the chorionic villi. Fetal blood is brought to blood vessels within the villi by branches of the umbilical artery and is drained by branches of the umbilical vein.

Amniotic fluid

Amniotic fluid

Chorionic Villi

■ Figure 20.49 Amniocentesis. In this procedure, amniotic fluid containing suspended cells is withdrawn for examination. Various genetic diseases can be detected prenatally by this means.

Table 20.7 Hormones Secreted by the Placenta



Pituitary-like Hormones Chorionic gonadotropin (hCG)

Chorionic somatomammotropin (hCS)

Similar to LH; maintains mother's corpus luteum for first 5M weeks of pregnancy; may be involved in suppressing immunological rejection of embryo; also exhibits TSH-like activity Similar to prolactin and growth hormone; in the mother, hCS acts to promote increased fat breakdown and fatty acid release from adipose tissue and to promote the sparing of glucose for use by the fetus ("diabetic-like" effects)

Sex Steroids


Helps maintain endometrium during pregnancy; helps suppress gonadotropin secretion; stimulates development of alveolar tissue in mammary glands


Help maintain endometrium during pregnancy; help suppress gonadotropin secretion; help stimulate mammary gland development; inhibit prolactin secretion; promote uterine sensitivity to oxytocin; stimulate duct development in mammary glands

and (3) polyuria (excretion of large volumes of urine), thereby producing a degree of dehydration and thirst. This diabetic-like effect in the mother helps to ensure a sufficient supply of glucose for the placenta and fetus, which (like the brain) use glucose as their primary energy source.

Steroid Hormones from the Placenta

After the first 5M weeks of pregnancy, when the corpus luteum regresses, the placenta becomes the major sex-steroid-producing gland. The blood concentration of estrogens, as a result of pla-cental secretion, rises to levels more than 100 times greater than those existing at the beginning of pregnancy. The placenta also secretes large amounts of progesterone, changing the estrogen/progesterone ratio in the blood from 100:1 at the beginning of pregnancy to close to 1:1 toward full-term.

The placenta, however, is an "incomplete endocrine gland" because it cannot produce estrogen and progesterone without the aid of precursors supplied to it by both the mother and the fetus. The placenta, for example, cannot produce cholesterol from acetate, and so it must be supplied with cholesterol from the mother's circulation. Cholesterol, which is a steroid containing twenty-seven carbons, can then be converted by enzymes in the placenta into steroids that contain twenty-one carbons—such as progesterone. The placenta, however, lacks the enzymes needed to convert progesterone into androgens (which have nineteen carbons). Therefore, androgens produced by the fetus are needed as substrates for the placenta to convert into estrogens (fig. 20.51), which have eighteen carbons.

In order for the placenta to produce estrogens, it needs to cooperate with the steroid-producing tissues (principally the adrenal cortex) in the fetus. Fetus and placenta thus form a single functioning system in terms of steroid hormone production. This system has been called the fetal-placental unit (fig. 20.51).

The ability of the placenta to convert androgens into estrogen helps to protect the female embryo from becoming masculinized by the androgens secreted from the mother's adrenal glands. In addition to producing estradiol, the placenta secretes large amounts of a weak estrogen called estriol. The production of estriol increases tenfold during pregnancy, so that by the third trimester estriol accounts for about 90% of the estrogens excreted in the mother's urine. Since almost all of this estriol comes from the placenta (rather than from maternal tissues), measurements of urinary estriol can be used clinically to assess the health of the placenta.

Maternal blood






- Progesterone ■ Androgens " Estrogens



■ Figure 20.51 Interactions between the embryo and placenta produce the steroid hormones. The secretion of progesterone and estrogen from the placenta requires a supply of cholesterol from the mother's blood and the cooperation of fetal enzymes that convert progesterone to androgens.

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