Human Physiology, 14th edition (2016)

748 Chapter 20

There is also indirect evidence that the stimulus of suckling
may cause the secretion of a prolactin-releasing hormone, but
this is controversial. Suckling thus results in the reflex secre-
tion of high levels of prolactin that promotes the secretion of
milk from the alveoli into the ducts. In order for the baby to get
the milk, however, the action of another hormone is needed.
The stimulus of suckling also results in the reflex secretion
of oxytocin from the posterior pituitary. This hormone is pro-
duced in the hypothalamus and stored in the posterior pituitary;
its release results in the milk-ejection reflex, or milk letdown.
The milk-ejection reflex occurs because oxytocin stimulates
contraction of the myoepithelial cells surrounding the lactifer-
ous ducts as it also stimulates contraction of the uterus.
Milk letdown can become a conditioned reflex made in
response to visual or auditory cues. The crying of a baby, for
example, can elicit oxytocin secretion and the milk-ejection
reflex. On the other hand, the adrenergic effects of the fight-or-
flight response can suppress this reflex if the mother becomes
anxious while breast-feeding. In this case, she will still pro-
duce milk but it will not flow.
Breast-feeding supplements the immune protection
given to the infant by its mother. While the fetus is in utero,

immunoglobin G (IgG—chapter 15) antibodies cross the pla-

centa from the maternal to the fetal blood. These anti bodies

provide passive immune protection to the baby for the first

three to twelve months after birth ( fig. 20.54 ). Infants that are

breast-fed also receive IgA antibodies from the mother’s milk,

which provides additional passive immune protection within

the baby’s intestine. In addition, the mother’s milk contains

cytokines, lymphocytes, and antibodies that may promote the

development of the baby’s system of active immunity. The

ability of the baby to produce its own antibodies is not well

developed for several months after birth ( fig.  20.54 ), so the

passive immunity provided by maternal antibodies in breast

milk may be significant in protecting the baby from a variety

of infections.

Breast-feeding, acting through reflex inhibition of GnRH

secretion, can also inhibit the secretion of gonadotropins from

the mother’s anterior pituitary and thus inhibit ovulation. Breast-

feeding is thus a natural contraceptive mechanism that helps to

space births. This mechanism appears to be most effective in

women with limited caloric intake and in those who breast-feed

their babies at frequent intervals throughout the day and night.

In the traditional societies of the less industrialized nations,

therefore, breast-feeding is an effective contraceptive. Breast-

feeding has much less of a contraceptive effect in women who

are well nourished and who breast-feed their babies at more

widely spaced intervals.

Figure 20.54 Maternal antibodies that protect
the baby. Circulating IgG antibodies cross the placenta and
protect the baby for 3 months to 1 year after birth. This passive
immunity is supplemented by IgA antibodies in the baby’s
intestine obtained from the mother’s milk. This protection lasts
longer for babies weaned at a later age. Notice the inability of
the baby to produce a large amount of its own antibodies until it
is several months of age. Adapted from R. M. Zinkernagel, “Advances
in immunology: Maternal antibodies, childhood infections, and autoimmune
diseases.” New England Journal of Medicine, 345:18, pp. 1331–1335.
Copyright © 2001 Massachusetts Medical Society. All rights reserved.

Circulating

IgG antibody

from mother

Before birth Birth 3 Mo 1 Yr 4 Yr

Intestinal IgA

antibody from

mother's milk

Early

weaning

Late

weaning

Child’s own

antibodies

(active immunity)

| CHECKPOINT

12a. Describe the changes that occur in the sperm cell

and ovum during fertilization.

12b. Identify the source of hCG and explain why this

hormone is needed to maintain pregnancy for the

first 10 weeks.

13a. List the fetal and maternal components of the

placenta and describe the circulation in these two

components. Explain how fetal and maternal gas

exchange occurs.

13b. List the protein hormones and sex steroids secreted

by the placenta and describe their functions.

14. Identify the two agents that stimulate uterine
    contraction during labor and describe the proposed
    mechanisms that may initiate labor in humans.


15. Describe the hormonal interactions required for
    breast development during pregnancy and for
    lactation after delivery.