Ganong's Review of Medical Physiology, 23rd Edition

426 SECTION IVEndocrine & Reproductive Physiology

the formation of gap junctions between the muscle fibers. All
these actions would be expected to inhibit the onset of labor.
Once labor is started, the uterine contractions dilate the
cervix, and this dilation in turn sets up signals in afferent
nerves that increase oxytocin secretion (Figure 25–35). The
plasma oxytocin level rises and more oxytocin becomes avail-
able to act on the uterus. Thus, a positive feedback loop is
established that aids delivery and terminates on expulsion of
the products of conception. Oxytocin increases uterine con-
tractions in two ways: (1) It acts directly on uterine smooth
muscle cells to make them contract, and (2) it stimulates the
formation of prostaglandins in the decidua. The prostaglan-
dins enhance the oxytocin-induced contractions.
During labor, spinal reflexes and voluntary contractions of
the abdominal muscles (“bearing down”) also aid in delivery.
However, delivery can occur without bearing down and with-
out a reflex increase in secretion of oxytocin from the poster-
ior pituitary gland, since paraplegic women can go into labor
and deliver.

LACTATION

Development of the Breasts

Many hormones are necessary for full mammary develop-
ment. In general, estrogens are primarily responsible for pro-
liferation of the mammary ducts and progesterone for the
development of the lobules. In rats, some prolactin is also
needed for development of the glands at puberty, but it is not
known if prolactin is necessary in humans. During pregnancy,
prolactin levels increase steadily until term, and levels of estro-

gens and progesterone are elevated as well, producing full lob-

uloalveolar development.

Secretion & Ejection of Milk

The composition of human and cows’ milk is shown in Table

25–10. In estrogen- and progesterone-primed rodents, injec-

tions of prolactin cause the formation of milk droplets and

their secretion into the ducts. Oxytocin causes contraction of

the myoepithelial cells lining the duct walls, with consequent

ejection of the milk through the nipple.

FIGURE 25–35 Role of oxytocin in parturition.

Increase in oxytocin receptors

Prostaglandins

Uterine contractions

Dilation of cervix and

distention of vagina

Stimuli from cervix

and vagina

Increased secretion

of oxytocin

TABLE 25–10 Composition of colostrum and milk.*

Component

Human

Colostrum

Human

Milk

Cows’

Milk

Water, g... 88 88

Lactose, g 5.3 6.8 5.0

Protein, g 2.7 1.2 3.3

Casein: lactalbumin ratio... 1:2 3:1

Fat, g 2.9 3.8 3.7

Linoleic acid... 8.3% of fat 1.6% of fat

Sodium, mg 92 15 58

Potassium, mg 55 55 138

Chloride, mg 117 43 103

Calcium, mg 31 33 125

Magnesium, mg 4 4 12

Phosphorus, mg 14 15 100

Iron, mg 0.09^2 0.15a 0.10a

Vitamin A, μg 89 53 34

Vitamin D, μg... 0.03a 0.06a

Thiamine, μg 15 16 42

Riboflavin, μg 30 43 157

Nicotinic acid, μg 75 172 85

Ascorbic acid, mg 4.4a 4.3a 1.6a

*Weights per deciliter.

aPoor source.

Reproduced with permission from Findlay ALR: Lactation. Res Reprod (Nov)

1974;6(6).