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.
Negative feedback inhibits gonadotropin release.
trogen levels in the plasma rise, they exert
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
shows these events in a 28-day cycle.
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).
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,
Early and midfollicular phases
Late follicular and luteal phases
estrogen output by
Mature vesicular follicle
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
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.