Reproduction
637
compared with the marked breast enlargement that occurs
during pregnancy as a result of the stimulatory effects of high
plasma concentrations of estrogen, progesterone, prolactin,
and human placental lactogen. Except for prolactin, which is
secreted by the maternal anterior pituitary, these hormones
are secreted by the placenta. Under the infl uence of these hor-
mones, both the ductal and the alveolar structures become
fully developed.
As described in Chapter 11, other factors infl uence the
anterior pituitary cells that secrete prolactin. They are inhibited
by
dopamine,
which is secreted by the hypothalamus. They
are probably stimulated by at least one
prolactin-releasing
factor (PRF),
also secreted by the hypothalamus (the chemi-
cal identity of PRF is still uncertain). The dopamine and PRF
secreted by the hypothalamus are hypophysiotropic hormones
that reach the anterior pituitary by way of the hypothalamo-
pituitary portal vessels.
Under the dominant inhibitory infl uence of dopamine,
prolactin secretion is low before puberty. It then increases
considerably at puberty in girls but not in boys, stimulated by
the increased plasma estrogen concentration that occurs at this
time. During pregnancy, there is a marked further increase in
prolactin secretion due to stimulation by estrogen.
Prolactin is the major hormone stimulating the pro-
duction of milk. However, despite the fact that prolactin is
elevated and the breasts are markedly enlarged and fully devel-
oped as pregnancy progresses, there is usually no secretion
of milk. This is because estrogen and progesterone, in large
concentrations, prevent milk production by inhibiting this
action of prolactin on the breasts. Thus, although estrogen
causes an increase in the secretion of prolactin and acts with
prolactin in promoting breast growth and differentiation, it,
along with progesterone, is antagonistic to prolactin’s ability
to induce milk production. Delivery removes the source—the
placenta—of the large amounts of estrogen and progesterone
and, thereby, releases milk production from inhibition.
The decrease in estrogen following parturition also
causes
basal
prolactin secretion to decrease from its peak
late-pregnancy levels. After several months, prolactin returns
toward prepregnancy levels even if the mother continues to
nurse. Superimposed upon this basal level, however, are large
secretory bursts of prolactin during each nursing period. The
episodic pulses of prolactin are signals to the breasts to main-
tain milk production. These pulses usually cease several days
after the mother completely stops nursing her infant, but con-
tinue as long as nursing continues.
The refl exes mediating the surges of prolactin (
Figure
17–33
) are initiated by afferent input to the hypothalamus from
nipple receptors stimulated by suckling. This input’s major effect
is to inhibit the hypothalamic neurons that release dopamine.
One other refl ex process is essential for nursing. Milk
is secreted into the lumen of the alveoli, but the infant can-
not suck the milk out of the breast. It must fi rst be moved
into the ducts, from which it can be sucked. This movement
is called the
milk ejection refl
ex
(also called milk letdown)
and is accomplished by contraction of the myoepithelial cells
surrounding the alveoli. The contraction is under the control
of oxytocin, which is refl exly released from posterior pituitary
neurons in response to suckling (see Figure 17–33). Higher
brain centers can also exert an important infl uence over oxy-
tocin release: a nursing mother may actually leak milk when
she hears her baby cry or even thinks about nursing.
Suckling also inhibits the hypothalamo-pituitary-ovarian
axis at a variety of steps, with a resultant block of ovulation.
This is probably due to increased prolactin and direct effects
on the hypothalamic GnRH release. If suckling is continued
at high frequency, ovulation can be delayed for months to
years. This “natural” birth control may help to space out preg-
nancies. When supplements are added to the baby’s diet and
the frequency of suckling is decreased, however, most women
will resume ovulation even though they continue to nurse.
However, ovulation may resume even without a decrease in
nursing. Failure to use adequate birth control may result in an
unplanned pregnancy in nursing women.
Initially after delivery, the breasts secrete a watery, protein-
rich fl uid called
colostrum.
After about 24 to 48 hours, the
secretion of milk itself begins. Milk contains four major nutri-
ents: water, protein, fat, and the carbohydrate lactose (milk
sugar).
Colostrum and milk also contain antibodies and other
messengers of the immune system, all of which are important
5th rib
Pectoralis
major
muscle
1
Prior to pregnancy,
ducts with few
alveoli exist
2
In early
pregnancy,
alveoli grow
3
In midpregnancy,
alveoli enlarge
and acquire lumen
4
During lactation,
alveoli dilate
5
After weaning,
gland regresses
Fat
Nipple
Figure 17–32
Anatomy of the breast. The numbers refer to the sequential changes
that occur over time.
Adapted from Elias et al.
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