746 Chapter 20
Cortisol secreted from the fetal adrenal cortex also stimulates
the maturation of the fetal lungs and their production of pulmo-
nary surfactant, needed for lung function after the baby is born
(chapter 16, section 16.2).
The fetal adrenal glands lack a medulla, but the cortex itself
is composed of two parts. The outer part secretes cortisol, as does
the adult adrenal cortex. The inner part, called the fetal adrenal
zone, secretes an androgen called dehydroepiandrosterone
sulfate (DHEAS). When ACTH secretion rises in response to
placental CRH, it stimulates the fetal adrenal cortex to secrete
both cortisol and DHEAS. The DHEAS travels from the fetal
adrenal cortex to the placenta, where it is converted into estro-
gens (principally estriol). Estriol from the placenta then trav-
els in the blood to the mother’s uterus, where it increases the
sensitivity of the myometrium to oxytocin and prostaglandins
( fig. 20.50 ). This occurs because estriol stimulates the myome-
trium to produce (1) more receptors for oxytocin; (2) more recep-
tors for prostaglandin F 2 a ; and (3) more gap junctions between
myometrial cells. During labor, oxytocin and prostaglandin F 2 a (^)
stimulate the opening of Ca^2 1 channels in the plasma membrane
for muscle contraction, and the gap junctions help to coordinate
and synchronize uterine contractions.
Parturition in animals such as pigs, rats, and guinea pigs is
aided by a hormone called relaxin, which causes softening of the
pubic symphysis and relaxation of the cervix. In humans, how-
ever, this hormone does not seem to be required for parturition.
Rather, relaxin secreted by the human ovary together with pro-
gesterone seem to be required during the first trimester for the
decidual reaction (the formation of a decidua basalis in the endo-
metrium). Also, relaxin promotes the growth of blood vessels into
the decidua basalis, thus helping to nourish the growing embryo.
Following delivery of the baby, oxytocin is needed to main-
tain the muscle tone of the myometrium and to reduce hemor-
rhaging from uterine arteries. Oxytocin may also play a role
in promoting the involution (reduction in size) of the uterus
following delivery; the uterus weighs about 1 kg (2.2 lb) at term
but only about 60 g (2 oz) by the sixth week following delivery.
Lactation
Each mammary gland is composed of 15 to 20 lobes divided
by adipose tissue. The amount of adipose tissue determines the
size and shape of the breast but has nothing to do with the abil-
ity of a woman to nurse. Each lobe is subdivided into lobules,
which contain the glandular alveoli ( fig. 20.51 ) that secrete the
milk of a lactating female.
The clustered alveoli secrete milk into a series of second-
ary tubules. These tubules converge to form a series of mam-
mary ducts, which in turn converge to form a lactiferous duct
that drains at the tip of the nipple. The lumen of each lactifer-
ous duct expands just beneath the surface of the nipple to form
an ampulla, where milk accumulates during nursing. Epithelial
cells that line the tubules and those that form the alveoli secrete
water and nutrients for the lactating mammary glands. There
are also specialized myoepithelial cells in the mammary glands
that can contract to propel milk through its system of ducts.
The changes in the mammary glands during pregnancy
and the regulation of lactation provide excellent examples of
hormonal interactions and neuroendocrine regulation. Growth
and development of the mammary glands during pregnancy
requires the permissive actions of insulin, cortisol, and thyroid
hormones. In the presence of adequate amounts of these hor-
mones, high levels of progesterone stimulate the development
of the mammary alveoli and estrogen stimulates proliferation
of the tubules and ducts ( fig. 20.52 ).
Prolactin, secreted by the anterior pituitary, stimulates
the mammary glands after parturition to produce milk proteins,
including casein and lactalbumin. Prolactin secretion is controlled
by prolactin-inhibiting hormone (PIH), identified as dopamine,
which is released by the hypothalamus into the hypothalamo-
hypophyseal portal system of vessels. The secretion of PIH is
stimulated by estrogen, and so during pregnancy—when estro-
gen levels remain high—the secretion of prolactin from the ante-
rior pituitary is tonically inhibited. As a result, the high levels of
estrogen during pregnancy helps prepare the mammary glands
for lactation but, by inhibiting prolactin secretion, prevents milk
production.
After parturition, when the placenta is expelled as the
afterbirth, declining levels of estrogen are accompanied by
an increase in the secretion of prolactin. Milk production is
thereby stimulated. If a woman does not wish to breast-feed her
baby she may take oral estrogens to inhibit prolactin secretion.
A different drug commonly given in these circumstances, and
in other conditions in which it is desirable to inhibit prolactin
secretion, is bromocriptine. This drug binds to dopamine recep-
tors and thus promotes the action of dopamine as the prolactin-
inhibiting hormone (PIH).
The act of nursing helps to maintain high levels of prolactin
secretion via a neuroendocrine reflex ( fig. 20.53 ). Sensory end-
ings in the breast, activated by the stimulus of suckling, relay
impulses to the hypothalamus and inhibit the secretion of PIH.CLINICAL APPLICATION
Cord blood banking is something that parents may elect to
do when their baby is born. Blood from the umbilical cord
contains stem cells that can be saved for later in life, when
the child might need them to reconstitute a hematopoietic
system if the bone marrow has been destroyed—for exam-
ple, by chemotherapy for leukemia. Cord blood banking is
a risk-free procedure, but it is an expensive one with a low
probability that it will someday be needed.
Genetic screening of neonates (newborns) is done
on blood obtained by a heel prick when the baby is 24 to
48 hours old. This is a required procedure, and different states
specify that 20 to more than 30 tests be performed on this
blood. These include genetic tests for phenylketonuria (PKU),
cystic fibrosis, lysosomal storage diseases, and many others,
as well as tests for endocrine disorders (such as hypothyroid-
ism) and hemoglobin disorders (such as sickle-cell anemia).