Ganong's Review of Medical Physiology, 23rd Edition

TABLE 25–7 Twenty-four-hour production
rates of sex steroids in women at different
stages of the menstrual cycle.
Sex Steroids
Secretion
The concentration of estradiol in the plasma during the menstrual
cycle is shown in Figure 25–25. Almost all of this estrogen
comes from the ovary, and two peaks of secretion occur:
one just before ovulation and one during the midluteal phase.
The estradiol secretion rate is 36 μg/d (133 nmol/d) in the early
follicular phase, 380 μg/d just before ovulation, and 250 μg/d
during the midluteal phase (Table 25–7). After menopause,
estrogen secretion declines to low levels.
As noted previously, the estradiol production rate in men is
about 50 μg/d (184 nmol/d).
Effects on the Female Genitalia
Estrogens facilitate the growth of the ovarian follicles and increase
the motility of the uterine tubes. Their role in the cyclic
changes in the endometrium, cervix, and vagina has been discussed
previously. They increase uterine blood flow and have
important effects on the smooth muscle of the uterus. In immature
and castrated females, the uterus is small and the myometrium
atrophic and inactive. Estrogens increase the
amount of uterine muscle and its content of contractile proteins.
Under the influence of estrogens, the muscle becomes
more active and excitable, and action potentials in the individual
fibers become more frequent. The “estrogen-dominated”
uterus is also more sensitive to oxytocin.
Chronic treatment with estrogens causes the endometrium
to hypertrophy. When estrogen therapy is discontinued,
sloughing takes place with withdrawal bleeding. Some “breakthrough”
bleeding may occur during treatment when estrogens
are given for long periods.
CHAPTER 25 The Gonads: Development & Function of the Reproductive System 417
Early
Follicular Preovulatory Midluteal
Progesterone (mg) 1.0 4.0 25.0
17-hydroxyprogesterone
(mg)
Dehydroepiandrosterone
(mg)
0.5 4.0 4.0
7.0 7.0 7.0
Androstenedione (mg) 2.6 4.7 3.4
Testosterone (μg) 144.0 171.0 126.0
Estrone (μg) 50.0 350.0 250.0
Estradiol (μg) 36.0 380.0 250.0
Modified and reproduced, with permission, from Yen SSC, Jaffe RB, Barbieri RL:
Reproductive Endocrinology, 4th ed. Saunders, 1999.
Effects on Endocrine Organs
Estrogens decrease FSH secretion. Under some circumstances,
they inhibit LH secretion (negative feedback); in other circumstances,
they increase LH secretion (positive feedback).
Women are sometimes given large doses of estrogens for 4 to
6 d to prevent conception after coitus during the fertile period
(postcoital or “morning-after” contraception). However, in
this instance, pregnancy is probably prevented by interference
with implantation of the ovum rather than changes in gonadotropin
secretion.
Estrogens cause increased secretion of angiotensinogen and
thyroid-binding globulin. They exert an important protein
anabolic effect in chickens and cattle, possibly by stimulating
the secretion of androgens from the adrenal, and estrogen
treatment has been used commercially to increase the weight
of domestic animals. They cause epiphysial closure in humans
(see Chapter 23).
Effects on the Central Nervous System
The estrogens are responsible for estrous behavior in animals,
and they increase libido in humans. They apparently exert
this action by a direct effect on certain neurons in the hypothalamus
(Figure 25–28). Estrogens also increase the proliferation
of dendrites on neurons and the number of synaptic
knobs in rats.
Effects on the Breasts
Estrogens produce duct growth in the breasts and are largely responsible
for breast enlargement at puberty in girls; they have
been called the growth hormones of the breast. They are responsible
for the pigmentation of the areolas, although pigmentation
usually becomes more intense during the first pregnancy
than it does at puberty. The role of the estrogens in the overall
control of breast growth and lactation is discussed below.
Optic
chiasm
Third
ventricle
Massa
intermedia
Midbrain
FIGURE 25–28 Loci where implantations of estrogen in the
hypothalamus affect ovarian weight and sexual behavior in rats,
projected on a sagittal section of the hypothalamus. The implants
that stimulate sex behavior are located in the suprachiasmatic area
above the optic chiasm (blue area), whereas ovarian atrophy is produced
by implants in the arcuate nucleus and surrounding ventral hypothalamus
(red). MB, mamillary body.
MB
Pituitary