1048
UNIT 5
Continuity
27
causing them to release estrogens, whereas LH (at least ini-
tially) prods the thecal cells to release androgens (actually
the steroid hormone androstenedione). Tese hormones
diffuse through the basement membrane, where the granu-
losa cells convert them to estrogens. Only tiny amounts of
ovarian androgens enter the blood, because they are almost
completely converted to estrogens within the ovaries.
3
Negative feedback inhibits gonadotropin release.
As es-
trogen levels in the plasma rise, they exert
negative feedback
on the hypothalamus and anterior pituitary, inhibiting re-
lease of FSH and LH. Within the ovary, estrogen enhances
output of estrogens by intensifying the effect of FSH on
Hormonal Interactions During the Ovarian Cycle
Next, let’s look at how the waxing and waning of anterior pitui-
tary gonadotropins (FSH and LH) and ovarian hormones and
the negative and positive feedback interactions regulate ovarian
function.
Figure 27.21
shows these events in a 28-day cycle.
1
GnRH stimulates FSH and LH secretion.
GnRH secreted by
the hypothalamus stimulates the anterior pituitary to pro-
duce and release follicle-stimulating hormone (FSH) and
luteinizing hormone (LH).
2
FSH and LH stimulate follicles to grow, mature, and se-
crete sex hormones.
FSH exerts its main effects on the
granulosa cells of late secondary and vesicular follicles,
Hypothalamus
GnRH
Hypothalamus
Early and midfollicular phases
Late follicular and luteal phases
1
1
2
2
2
2
2
3
4
4
4
5
5
6
6
Slightly
elevated
estrogen and
rising inhibin
levels inhibit
FSH
secretion.
Positive
feedback exerted
by large
in
estrogen output by
maturing follicle.
Travels via
portal blood
Granulosa
cells
Inhibin
Androgens
Convert androgens
to estrogens
Thecal cells
Anterior pituitary
Mature vesicular follicle
Corpus luteum
Ovulated
secondary
oocyte
Ruptured
follicle
LH surge
Progesterone
Estrogens
Inhibin
Estrogens
GnRH
FSH
LH
Stimulates
Inhibits
Figure 27.21
Regulation of the ovarian cycle.
Numbers refer to events listed in the text.
Note that all feedback signals exerted by ovarian hormones are negative except one—that
exerted by estrogens immediately before ovulation. Events that follow step
6
(negative
feedback inhibition of the hypothalamus and anterior pituitary by progesterone and estrogens)
are not depicted, but involve a gradual deterioration of the corpus luteum and, therefore,
a decline in ovarian hormone production. Ovarian hormones reach their lowest blood levels
around day 28.
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