Ganong's Review of Medical Physiology, 23rd Edition

Initiation of Lactation after Delivery
The breasts enlarge during pregnancy in response to high circulating
levels of estrogens, progesterone, prolactin, and possibly
hCG. Some milk is secreted into the ducts as early as the
fifth month, but the amounts are small compared with the
surge of milk secretion that follows delivery. In most animals,
milk is secreted within an hour after delivery, but in women it
takes 1 to 3 d for the milk to “come in.”
After expulsion of the placenta at parturition, the levels of
circulating estrogens and progesterone abruptly decline. The
drop in circulating estrogen initiates lactation. Prolactin and
estrogen synergize in producing breast growth, but estrogen
antagonizes the milk-producing effect of prolactin on the
breast. Indeed, in women who do not wish to nurse their
babies, estrogens may be administered to stop lactation.
Suckling not only evokes reflex oxytocin release and milk
ejection, it also maintains and augments the secretion of milk
because of the stimulation of prolactin secretion produced by
suckling.
Effect of Lactation on Menstrual Cycles
Women who do not nurse their infants usually have their first
menstrual period 6 wk after delivery. However, women who
nurse regularly have amenorrhea for 25 to 30 wk. Nursing
stimulates prolactin secretion, and evidence suggests that prolactin
inhibits GnRH secretion, inhibits the action of GnRH
on the pituitary, and antagonizes the action of gonadotropins
on the ovaries. Ovulation is inhibited, and the ovaries are inactive,
so estrogen and progesterone output falls to low levels.
Consequently, only 5–10% of women become pregnant again
during the suckling period, and nursing has long been known
to be an important, if only partly effective, method of birth
control. Furthermore, almost 50% of the cycles in the first 6
mo after resumption of menses are anovulatory (see Clinical
Box 25–6).
Gynecomastia
Breast development in the male is called gynecomastia. It may
be unilateral but is more commonly bilateral. It is common,
occurring in about 75% of newborns because of transplacental
passage of maternal estrogens. It also occurs in mild, transient
form in 70% of normal boys at the time of puberty and in
many men over the age of 50. It occurs in androgen resistance.
It is a complication of estrogen therapy and is seen in patients
with estrogen-secreting tumors. It is found in a wide variety of
seemingly unrelated conditions, including eunuchoidism, hyperthyroidism,
and cirrhosis of the liver. Digitalis can produce
it, apparently because cardiac glycosides are weakly estrogenic.
It can also be caused by many other drugs. It has been seen
in malnourished prisoners of war, but only after they were liberated
and eating an adequate diet. A feature common to
many and perhaps all cases of gynecomastia is an increase in
the plasma estrogen:androgen ratio due to either increased
circulating estrogens or decreased circulating androgens.
CHAPTER 25 The Gonads: Development & Function of the Reproductive System 427
CLINICAL BOX 25–6
Chiari–Frommel Syndrome
An interesting, although rare, condition is persistence of lactation
(galactorrhea) and amenorrhea in women who do
not nurse after delivery. This condition, called the Chiari–
Frommel syndrome, may be associated with some genital
atrophy and is due to persistent prolactin secretion without
the secretion of the FSH and LH necessary to produce maturation
of new follicles and ovulation. A similar pattern of galactorrhea
and amenorrhea with high circulating prolactin
levels is seen in nonpregnant women with chromophobe
pituitary tumors and in women in whom the pituitary stalk
has been sectioned during treatment of cancer.
HORMONES & CANCER
About 35% of carcinomas of the breast in women of childbearing
age are estrogen-dependent; their continued growth depends
on the presence of estrogens in the circulation. The
tumors are not cured by decreasing estrogen secretion, but
symptoms are dramatically relieved, and the tumor regresses
for months or years before recurring. Women with estrogendependent
tumors often have a remission when their ovaries
are removed. Inhibition of the action of estrogens with tamoxifen
also produces remissions, and inhibition of estrogen formation
with drugs that inhibit aromatase (Figure 25–26) is
even more effective.
Some carcinomas of the prostate are androgen-dependent
and regress temporarily after the removal of the testes or
treatment with GnRH agonists in doses that are sufficient to
produce down-regulation of the GnRH receptors on gonadotropes
and decrease LH secretion.
CHAPTER SUMMARY
■ Differences between males and females depend primarily on a
single chromosome (the Y chromosome) and a single pair of
endocrine structures (the gonads); testes in the male and ovaries
in the female.
■ The gonads have a dual function: the production of germ cells
(gametogenesis) and the secretion of sex hormones. The testes
secrete large amounts of androgens, principally testosterone, but
they also secrete small amounts of estrogens. The ovaries secrete
large amounts of estrogens and small amounts of androgens.
■ Spermatogonia develop into mature spermatozoa that start in
the seminiferous tubules in a process called spermatogenesis.
This is a multistep process that includes maturation of
spermatogonia into primary spermatocytes, which undergo
meiotic division, resulting in haploid secondary spermatocytes
and several further divisions result in spermatids. Each
cell division from a spermatogonium to a spermatid is incomplete
with cells remaining connected via cytoplasmic bridges.
Spermatids eventually mature into motile spermatozoa to