622
Chapter 17
increasing plasma concentrations of estrogen for one to two
days, as occurs during the estrogen peak of the late follicular
phase (
7
in Figure 17–18), act upon the pituitary to enhance
the sensitivity of LH-releasing mechanisms to GnRH (Table
17–4 and
Figure 17–21
) and may also stimulate GnRH release
from the hypothalamus. These are the positive-feedback effects
of estrogen.
The net result is that rapidly increasing estrogen leads to
the LH surge (
8
in Figure 17–18). As shown in Figure 17–18
9
,
an increase in FSH and progesterone also occurs at the time
of the LH surge, but it is not known whether this has a physi-
ological role in the regulation of the cycle.
The midcycle surge of LH is the primary event that
induces ovulation. The high plasma concentration of LH
acts upon the granulosa cells to cause the events, presented
in
Table 17–5
, that culminate in ovulation
10
, as indicated by
the dashed vertical line in Figure 17–18.
The function of the granulosa cells in mediating the
effects of the LH surge is the last in the series of these cells’
functions described in this chapter. They are all summarized
in
Table 17–6
.
The Luteal Phase
The LH surge not only induces ovulation by the mature follicle
but also stimulates the reactions that transform the remaining
granulosa and theca cells of that follicle into a corpus luteum
(
11
in Figure 17–18). A low but adequate LH concentration
maintains the function of the corpus luteum for about 14 days.
During its short life in the nonpregnant woman, the cor-
pus luteum secretes large quantities of progesterone and estro-
gen
12
, as well as inhibin. In the presence of estrogen, the high
plasma concentration of progesterone causes a decrease in the
secretion of the gonadotropins by the pituitary. It probably does
Estrogen
Reproductive tract
and other organs
Respond to estrogen
Inhibin
Granulosa
cells
Influence
oocytes
FSH
LH
(Primarily FSH)
Ovaries
Theca cells
Androgens
Estrogen
Anterior pituitary
Secretes FSH and LH
Hypothalamus
Secretes GnRH
GnRH
(in hypothalamo–pituitary
portal vessels)
+
Begin
Figure 17–20
Summary of hormonal control of ovarian function during the early
and middle follicular phases. Compare with the analogous pattern of
the male (see Figure 17–11). Inhibin is a protein hormone that inhibits
FSH secretion. The wavy broken arrows in the granulosa cells denote
the conversion of androgens to estrogen in these cells, as shown
in Figure 17–19. The dotted line within the ovaries indicates that
estrogen increases granulosa cell function (local positive feedback).
Figure 17–20
physiological
inquiry
A 30-year-old woman has failed to have menstrual cycles for the
past few months; her pregnancy test is negative. Her plasma FSH
and LH levels are elevated whereas her plasma estrogen levels are
low. What is the likely cause of her failure to menstruate?
Answer can be found at end of chapter.
Begin
Anterior pituitary
Secretes LH
LH surge
Hypothalamus
Secretes GnRH
GnRH
(in hypothalamo-pituitary
portal vessels)
+
+
Large amounts
of estrogen
Progesterone
and estrogen
Corpus
Iuteum
Ovary
Figure 17–21
In the late follicular phase, the dominant follicle secretes large
amounts of estrogen, which act on the anterior pituitary and,
possibly, the hypothalamus to cause an LH surge. The increased
plasma LH then triggers both ovulation and formation of the corpus
luteum. These actions of LH are mediated via the granulosa cells.
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