Ganong's Review of Medical Physiology, 23rd Edition

Other Placental Hormones
In addition to hCG, hCS, progesterone, and estrogens, the placenta
secretes other hormones. Human placental fragments
probably produce proopiomelanocortin (POMC). In culture,
they release corticotropin-releasing hormone (CRH), βendorphin,
α-melanocyte-stimulating hormone (MSH), and
dynorphin A, all of which appear to be identical to their hypothalamic
counterparts. They also secrete GnRH and inhibin,
and since GnRH stimulates and inhibin inhibits hCG secretion,
locally produced GnRH and inhibin may act in a paracrine
fashion to regulate hCG secretion. The trophoblast cells and
amnion cells also secrete leptin, and moderate amounts of this
satiety hormone enter the maternal circulation. Some also enters
the amniotic fluid. Its function in pregnancy is unknown.
The placenta also secretes prolactin in a number of forms.
Finally, the placenta secretes the α subunits of hCG, and the
plasma concentration of free α subunits rises throughout
pregnancy. These α subunits acquire a carbohydrate composition
that makes them unable to combine with β subunits, and
their prominence suggests that they have a function of their
own. It is interesting in this regard that the secretion of the
prolactin produced by the endometrium also appears to
increase throughout pregnancy, and it may be that the circulating
α subunits stimulate endometrial prolactin secretion.
The cytotrophoblast of the human chorion contains prorenin
(see Chapter 39). A large amount of prorenin is also
present in amniotic fluid, but its function in this location is
unknown.
Fetoplacental Unit
The fetus and the placenta interact in the formation of steroid
hormones. The placenta synthesizes pregnenolone and progesterone
from cholesterol. Some of the progesterone enters the fetal
circulation and provides the substrate for the formation of
cortisol and corticosterone in the fetal adrenal glands (Figure
25–34). Some of the pregnenolone enters the fetus and, along
with pregnenolone synthesized in the fetal liver, is the substrate
for the formation of dehydroepiandrosterone sulfate (DHEAS)
and 16-hydroxydehydroepiandrosterone sulfate (16-OHDH-
EAS) in the fetal adrenal. Some 16-hydroxylation also occurs in
the fetal liver. DHEAS and 16-OHDHEAS are transported back
to the placenta, where DHEAS forms estradiol and 16-OHDH-
EAS forms estriol. The principal estrogen formed is estriol, and
since fetal 16-OHDHEAS is the principal substrate for the estrogens,
the urinary estriol excretion of the mother can be monitored
as an index of the state of the fetus.
Parturition
The duration of pregnancy in humans averages 270 d from
fertilization (284 d from the first day of the menstrual period
preceding conception). Irregular uterine contractions increase
in frequency in the last month of pregnancy.
The difference between the body of the uterus and the cervix
becomes evident at the time of delivery. The cervix, which
CHAPTER 25 The Gonads: Development & Function of the Reproductive System 425
Placenta
Cholesterol
Pregnenolone DHEAS
Progesterone
Fetal Adrenal
16-OHDHEAS
Cortisol,
corticosterone
Estradiol DHEAS
Estriol 16-OHDHEAS
FIGURE 25–34 Interactions between the placenta and the
fetal adrenal cortex in the production of steroids.
is firm in the nonpregnant state and throughout pregnancy
until near the time of delivery, softens and dilates, while the
body of the uterus contracts and expels the fetus.
There is still considerable uncertainty about the mechanisms
responsible for the onset of labor. One factor is the
increase in circulating estrogens produced by increased circulating
DHEAS. This makes the uterus more excitable, increases
the number of gap junctions between myometrial cells, and
causes production of more prostaglandins, which in turn cause
uterine contractions. In humans, CRH secretion by the fetal
hypothalamus increases and is supplemented by increased placental
production of CRH. This increases circulating adrenocorticotropic
hormone (ACTH) in the fetus, and the resulting
increase in cortisol hastens the maturation of the respiratory
system. Thus, in a sense, the fetus picks the time to be born by
increasing CRH secretion.
The number of oxytocin receptors in the myometrium and
the decidua (the endometrium of pregnancy) increases more
than 100-fold during pregnancy and reaches a peak during
early labor. Estrogens increase the number of oxytocin receptors,
and uterine distention late in pregnancy may also
increase their formation. In early labor, the oxytocin concentration
in maternal plasma is not elevated from the prelabor
value of about 25 pg/mL. It is possible that the marked
increase in oxytocin receptors causes the uterus to respond to
normal plasma oxytocin concentrations. However, at least in
rats, the amount of oxytocin mRNA in the uterus increases,
reaching a peak at term; this suggests that locally produced
oxytocin also participates in the process.
Premature onset of labor is a problem because premature
infants have a high mortality rate and often require intensive,
expensive care. Intramuscular 17α-hydroxyprogesterone causes
a significant decrease in the incidence of premature labor. The
mechanism by which it exerts its effect is uncertain, but it may
be that the steroid provides a stable level of circulating
progesterone. Progesterone relaxes uterine smooth muscle,
inhibits the action of oxytocin on the muscle, and reduces