902
UNIT SIX
mary glands can secrete milk, none is produced. The micro-
graphs in
f
gure 23.29
compare the mammary gland tissues
of a nonpregnant woman with those of a lactating woman.
Following childbirth and the expulsion of the placenta,
the maternal blood concentrations of placental hormones
rapidly decline. The action of prolactin is no longer inhib-
ited. Prolactin stimulates the mammary glands to secrete
abundant milk. This hormonal effect does not occur until
two or three days following birth. In the meantime, the
glands secrete a thin, watery fl
uid called
colostrum.
It is
rich in proteins, particularly antibodies from the mother’s
immune system that protect the newborn from certain
infections, but has lower concentrations of carbohydrates
and fats than milk.
Milk does not fl
ow readily through the ductile system
of the mammary gland but must be actively ejected as spe-
cialized
myoepithelial cells
surrounding the alveolar glands
contract
(f
g. 23.30)
. A refl
ex action controls this process
and is elicited when the breast is suckled or the nipple
or areola is otherwise mechanically stimulated. Then,
impulses from sensory receptors in the breasts travel to the
hypothalamus, which signals the posterior pituitary gland
to release oxytocin. The oxytocin reaches the breasts by
means of the blood and stimulates the myoepithelial cells
in both breasts to contract
(f
g. 23.31)
. Within about thirty
seconds, milk squirts into a suckling infant’s mouth.
Sensory impulses triggered by mechanical stimulation
of the nipples also signal the hypothalamus to continue
secreting prolactin. Thus, prolactin is released as long as
breastfeeding continues. However, if stimulation of the nipple
does not occur regularly, the hypothalamus inhibits secretion
of prolactin, and within about one week, the mammary
glands lose their capacity to produce milk.
For several weeks following childbirth, the uterus
shrinks by a process called
involution.
Its endometrium
sloughs off and is discharged through the vagina. The new
mother passes a bloody and then yellowish discharge from
the vagina for a few weeks. An epithelial lining characteristic
of a nonpregnant female then returns.
PRACTICE
28
Describe the role of progesterone in initiating labor.
29
Explain how dilation of the cervix aF
ects labor.
30
Explain how bleeding is naturally controlled after the placenta is
expelled.
Milk Production and Secretion
During pregnancy, placental estrogens and progesterone
stimulate further development of the mammary glands.
Estrogens cause the ductile systems to grow and branch, and
deposit abundant fat around them. Progesterone stimulates
the development of the alveolar glands at the ends of the
ducts. Placental lactogen also promotes these changes.
The breasts may double in size during pregnancy
because of hormonal activity. At the same time, glandular
tissue replaces the adipose tissue of the breasts. Beginning
about the fifth week of pregnancy, the anterior pituitary
gland releases increasing amounts of
prolactin.
Prolactin
is synthesized from early pregnancy throughout gestation,
peaking at the time of birth. However, milk secretion does
not begin until after birth. This is because during preg-
nancy, placental progesterone inhibits milk production,
and placental lactogen blocks the action of prolactin (see
chapter 13, p. 495). Consequently, even though the mam-
(a)
Glandular
tissue
Connective
tissue
(b)
Glandular tissue
with secretions
FIGURE 23.29
Mammary glands. (
a
) Light micrograph of a mammary gland in a nonpregnant woman (160×). (
b
) Light micrograph of an active
(lactating) mammary gland (160×).
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