■ Figure 20.55 Milk production and the milk-ejection reflex.
Lactation occurs in two stages: milk production (stimulated by prolactin) and milk ejection (stimulated by oxytocin). The stimulus of sucking triggers a neuroendocrine reflex that results in increased secretion of oxytocin and prolactin.
Intestinal IgA antibody from
Child's own antibodies (active immunity)
Circulating IgG antibody from mother
Birth 3 Mo
■ Figure 20.56 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.
Beast-feeding supplements the immune protection given to the infant by its mother. While the fetus is in utero, im-munoglobin G (IgG—chapter 15) antibodies cross the placenta from the maternal to the fetal blood. These antibodies provide passive immune protection to the baby for the first three to twelve months after birth (fig. 20.56). 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. Since the ability of the baby to produce its own antibodies is not well developed for several months after birth (fig. 20.56), 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. Breastfeeding 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. Breastfeeding has much less of a contraceptive effect in women who are well nourished and who breast-feed their babies at more widely spaced intervals.
Milk letdown can become a conditioned reflex made in response to visual or auditory cues; the crying of a baby can elicit oxytocin secretion and the milk-ejection reflex. On the other hand, this reflex can be suppressed by the adrenergic effects produced in the fight-or-flight reaction. Thus, if a woman becomes nervous and anxious while breast-feeding, she will produce milk but it will not flow (there will be no milk letdown). This can cause increased pressure, intensifying her anxiety and frustration and further inhibiting the milk-ejection reflex. It is therefore important for mothers to nurse their babies in a quiet and calm environment. If needed, synthetic oxytocin can be given as a nasal spray to promote milk letdown.
Test Yourself Before You Continue
1. Describe the changes that occur in the sperm cell and ovum during fertilization.
2. Identify the source of hCG and explain why this hormone is needed to maintain pregnancy for the first i0 weeks.
3. 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.
4. List the protein hormones and sex steroids secreted by the placenta and describe their functions.
5. Identify the two agents that stimulate uterine contraction during labor and describe the proposed mechanisms that may initiate labor in humans.
6. Describe the hormonal interactions required for breast development during pregnancy and for lactation after delivery.