744 Chapter 20
(2) glucose-sparing by maternal tissues and, therefore, increased
blood glucose concentrations; and (3) polyuria (excretion of large
volumes of urine), thereby producing a degree of dehydration and
thirst. This diabetic-like effect in the mother helps to ensure a suf-
ficient supply of glucose for the placenta and fetus, which (like the
brain) use glucose as their primary energy source. Meanwhile, the
beta cells of the mother’s pancreatic islets proliferate during preg-
nancy to supply increased insulin, so that the development of ges-
tational diabetes (chapter 11, section 11.6) is normally prevented.
Steroid Hormones from the Placenta
After the first 5½ weeks of pregnancy, when the corpus luteum
regresses, the placenta becomes the major sex-steroid-producing
gland. As a result of placental secretion, the blood concentration
of estrogens rises to levels more than 100 times greater than those
existing at the beginning of pregnancy. The placenta also secretes
large amounts of progesterone, changing the estrogen/progesterone
ratio in the blood from 100:1 at the beginning of pregnancy to close
to 1:1 toward full-term. Progesterone “calms” the uterus during
pregnancy by inhibiting genes that code for labor-promoting uterine
proteins: connexin proteins for gap junctions between myometrial
cells and oxytocin receptor proteins.
The placenta, however, is an “incomplete endocrine gland”
because it cannot produce estrogen and progesterone without
the aid of precursors supplied to it by both the mother and the
fetus. For example, the placenta cannot produce cholesterol from
acetate, and so it must be supplied with cholesterol from the
mother’s circulation. Cholesterol, which is a steroid containing
27 carbons, can then be converted by enzymes in the placenta
into steroids that contain 21 carbons—such as progesterone. The
placenta, however, lacks the enzymes needed to convert proges-
terone into androgens (which have 19 carbons). Because of this,
androgens produced by the fetus (principally by the fetal adre-
nal cortex) are needed for the placenta to convert into estrogens
( fig. 20.49 ),which have 18 carbons. Fetus and placenta thereby
function together in the production of steroid hormones, an asso-
ciation termed the fetal-placental unit.
In addition to producing estradiol, the placenta secretes
large amounts of a weak estrogen called estriol. The produc-
tion of estriol increases tenfold during pregnancy, so that by
the third trimester estriol accounts for about 90% of the estro-
gens excreted in the mother’s urine. Because almost all of this
estriol comes from the placenta (rather than from maternal tis-
sues), measurements of urinary estriol can be used clinically to
assess the health of the placenta.Labor and Parturition
Powerful contractions of the uterus are needed to expel the fetus
in the sequence of events called labor. These uterine contractions
are known to be stimulated by two agents: (1) oxytocin, a poly-
peptide hormone produced in the hypothalamus and released
by the posterior pituitary (and also produced by the uterus
itself), and (2) prostaglandins, a class of cyclic fatty acids with
paracrine functions produced within the uterus. The particularFigure 20.49 Interactions between the embryo and
placenta produce the steroid hormones. The secretion of
progesterone and estrogen from the placenta requires a supply
of cholesterol from the mother’s blood and the cooperation of
fetal enzymes that convert progesterone to androgens.Maternal blood Placenta FetusCholesterolProgesteroneAndrogensProgesteroneAndrogensEstrogensProgesteroneEstrogensCholesterolTable 20.7 | Hormones Secreted
by the Placenta
Hormones Effects
Pituitary-like Hormones
Chorionic
gonadotropin (hCG)Similar to LH; maintains mother’s
corpus luteum for first 5½ weeks
of pregnancy; may be involved in
suppressing immunological rejection
of embryo; also exhibits TSH-like
activity
Chorionic
somatomammotropin
(hCS)Also called placental lactogen (hPL), it
has growth-hormone-like effects that
supplement the action of placental
growth hormone (PGH), or growth
hormone-variant (hGH-V), discussed
below.
Placental growth
hormone (PGH)Also called growth hormone-variant
(hGH-V), it promotes anabolism,
lipolysis, gluconeogenesis, and the
sparing of glucose for use by the
fetus (“diabetic-like” effects)
Sex Steroids
Progesterone Helps maintain endometrium during
pregnancy; helps suppress
gonadotropin secretion; stimulates
development of alveolar tissue in
mammary glands
Estrogens Help maintain endometrium during
pregnancy; help suppress
gonadotropin secretion; help
stimulate mammary gland
development; inhibit prolactin
secretion; promote uterine sensitivity
to oxytocin; stimulate duct
development in mammary glands