Reproductive System
released, and the reproductive cycle is activated. If a new
mother does not wish to have another child soon, she or her
partner should practice contraception, because she will be
fertile approximately two weeks prior to the return of her
menstrual period.
Table 23.6
summarizes the hormonal control of milk
production, and
table 23.7
lists some agents that adversely
affect lactation or harm the child. Clinical Application 23.3
explains the beneF
ts of breastfeeding.
How does pregnancy af
ect the mammary glands?
What stimulates the mammary glands to produce milk?
What causes milk ejection?
What happens to milk production iF milk is not regularly removed
From the breast?
A woman who is breastFeeding Feels her milk “let down,” or Flood
her breasts, when her inFant suckles. IF the baby nurses on a regular
schedule, the mother may Feel the letdown shortly beFore the baby
is due to nurse. The connection between mind and hormonal control
oF lactation is so strong that iF a nursing mother hears a baby cry, her
milk may ±
ow. IF this occurs in public, she can keep From wetting her
shirt by pressing her arms strongly against her chest.
To wean a nursing child, it is best to stop breastfeeding
gradually, by eliminating one feeding per day each week, for
example. If a woman stops nursing abruptly, her breasts will
become painfully engorged for several days.
A woman who is breastfeeding usually does not ovu-
late for several months. This may be because prolactin
suppresses release of gonadotropins from the anterior pitu-
itary gland. When a woman discontinues breastfeeding, the
anterior pituitary no longer secretes prolactin. Then, ±SH is
Myoepithelial cells
FIGURE 23.30
Myoepithelial cells contract to release milk From an
alveolar gland.
Nipple or areola of breast
is stimulated
Nerve impulses travel
to hypothalamus
Hypothalamus signals posterior
lobe of pituitary gland to
release oxytocin
Oxytocin causes myoepithelial
cells surrounding alveolar
glands to contract
Milk is released from ductile
system through nipple
FIGURE 23.31
Mechanism that releases milk From the breasts.
Hormonal Control of the Mammary Glands
Before Pregnancy (Beginning of Puberty)
Ovarian hormones secreted during reproductive cycles stimulate alveolar
glands and ducts oF mammary glands to develop.
During Pregnancy
1. Estrogens cause the ductile system to grow and branch.
2. Progesterone stimulates development oF alveolar glands.
3. Placental lactogen promotes development oF the breasts.
4. Prolactin is secreted throughout pregnancy, but placental
progesterone inhibits milk production and placental lactogen blocks
the action oF prolactin.
Following Childbirth
1. Placental hormonal concentrations decline, so the action oF prolactin is
no longer inhibited.
2. The breasts begin producing milk.
3. Mechanical stimulation oF the breasts releases oxytocin From the
posterior pituitary gland.
4. Oxytocin stimulates release oF milk From ducts.
5. As long as breastFeeding continues, more prolactin is released; iF the
nipple is not stimulated regularly, milk production ceases.
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