858
UNIT SIX
follicle produce increasing amounts of estrogens and some
progesterone. LH stimulates certain ovarian cells (theca
interna) to secrete precursor molecules (such as testoster-
one), also used to produce estrogens.
In a young female, estrogens stimulate development of
various secondary sex characteristics. Estrogens secreted dur-
ing subsequent reproductive cycles continue development of
these traits and maintain them.
Table 22.3
summarizes the
hormonal control of female secondary sex characteristics.
Increasing concentration of estrogens during the first
week or so of a reproductive cycle changes the uterine lin-
ing, thickening the glandular endometrium (proliferative
phase). Meanwhile, the developing follicle fully matures,
and by around the fourteenth day of the cycle, the follicle
appears on the surface of the ovary as a blisterlike bulge.
Within the follicle, the granulosa cells, which surround the
secondary oocyte and connect it to the inner wall, loosen.
Follicular fl
uid rapidly accumulates.
While the follicle matures, it secretes estrogens that
inhibit the release of LH from the anterior pituitary gland but
allow LH to be stored in the gland. Estrogens also make the
anterior pituitary cells more sensitive to the action of GnRH,
which is released from the hypothalamus in rhythmic pulses
about ninety minutes apart.
Near the fourteenth day of follicular development, the
anterior pituitary cells ± nally respond to the pulses of GnRH
FIGURE 22.32
Control of female secondary sex development. Estrogens inhibit LH and FSH during most of the reproductive cycle except during
egg cell production and ovulation.
Bloodstream
Hypothalamus
Pituitary
gland
Ovaries
Estrogens
Increased deposition
of adipose tissue
in breasts, thighs,
and buttocks
Estrogens inhibit
oversecretion of
gonadotropins
FSH, LH
(gonadotropins)
Accessory
reproductive
organs enlarge
Increased
vascularization
of the skin
Stimulates
endometrium
of uterus to
thicken
Breasts develop
Gonadotropins
GnRH
+
+
Release into
bloodstream
Stimulation
Inhibition
TABLE
22.3
|
Hormonal Control of Female
Secondary Sex Characteristics
1. The hypothalamus releases GnRH, which stimulates the anterior
pituitary gland.
2. The anterior pituitary gland secretes FSH and LH.
3. FSH stimulates the maturation of a follicle.
4. Granulosa cells of the follicle produce and secrete estrogens; LH
stimulates certain cells to secrete estrogen precursor molecules.
5. Estrogens are responsible for the development and maintenance of
most of the female secondary sexcharacteristics.
6. Concentrations of androgen a±
ect other secondary sex
characteristics, including skeletal growth and growth of hair.
7. Progesterone, secreted by the ovaries, a±
ects cyclical changes in the
uterus and mammary glands.
and release stored LH. The resulting surge in LH concentration,
which lasts for about thirty-six hours, weakens and ruptures
the bulging follicular wall. This event sends the secondary
oocyte and follicular fl uid out of the ovary (ovulation).
Following ovulation, the remnants of the follicle and the
theca interna within the ovary rapidly change. The space con-
taining the follicular fl uid ± lls with blood, which soon clots.
Under the influence of LH, the follicular and thecal cells
expand, forming a temporary glandular structure in the ovary
called a
corpus luteum
(“yellow body”) (see ± g. 22.26).
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