Reproduction
623
this by acting on the hypothalamus to
suppress
the secretion of
GnRH. (The progesterone also prevents any LH surges during
the fi rst half of the luteal phase despite the high concentrations
of estrogen at this time.) The increase in plasma inhibin con-
centration in the luteal phase also contributes to the suppres-
sion of FSH secretion. Consequently, during the luteal phase
of the cycle, plasma concentrations of the gonadotropins are
very low
13
. The feedback suppression of gonadotropins in the
luteal phase is summarized in
Figure 17–22
.
The corpus luteum has a fi nite life in the absence of
an increase in gonadotropin secretion. Thus, it degenerates
within two weeks if pregnancy does not occur
14
.
With degeneration of the corpus luteum, plasma pro-
gesterone and estrogen concentrations decrease
15
.The secre-
tion of FSH and LH (and probably GnRH, as well) increase
(
16
and
1
) as a result of being freed from the inhibiting effects
of high concentrations of ovarian hormones. The cycle then
begins anew.
This completes the description of the control of ovarian
function. It should be emphasized that, although the hypothala-
mus and anterior pituitary are essential controllers, events within
the
ovary
are the real sources of timing for the cycle. When the
ovary secretes enough estrogen, the LH surge is induced, which
in turn causes ovulation. When the corpus luteum degenerates,
the decrease in hormone secretion allows the gonadotropin lev-
els to increase enough to promote the growth of another group
of follicles. Thus, ovarian events, via hormonal feedbacks, con-
trol the hypothalamus and anterior pituitary.
Uterine Changes
in the Menstrual Cycle
The phases of the menstrual cycle can also be described in
terms of uterine events (
Figure 17–23
). Day 1 is, as noted
earlier, the fi rst day of menstrual fl ow, and the entire duration
Anterior pituitary
secretes FSH + LH
FSH
LH
Hypothalamus
secretes GnRH
GnRH
(in hypothalamo–pituitary
portal vessels)
––
Progesterone
and estrogen
Inhibin
(Primarily FSH)
Corpus Iuteum
FSH + LH
Begin
Ovary
Figure 17–22
Suppression of FSH and LH during luteal phase. If implantation of
a developing conceptus does not occur and hCG does not appear
in the blood, the corpus luteum dies, progesterone and estrogen
decrease, menstruation occurs, and the next menstrual cycle begins.
Table 17–5
Effects of the LH Surge
on Ovarian Function
1. The primary oocyte completes its fi rst meiotic division and
undergoes cytoplasmic changes that prepare the ovum for
implantation should fertilization occur. These LH effects
on the oocyte are mediated by messengers released from the
granulosa cells in response to LH.
2. Antrum size (fl
uid volume) and blood fl ow to the follicle
increase markedly.
3. The granulosa cells begin releasing progesterone and
decreasing the release of estrogen, which accounts for the
midcycle decrease in plasma estrogen concentration and the
small rise in plasma progesterone just before ovulation.
4. Enzymes and prostaglandins, synthesized by the granulosa
cells, break down the follicular-ovarian membranes. These
weakened membranes rupture, allowing the oocyte and
its surrounding granulosa cells to be carried out onto the
surface of the ovary.
5. The remaining granulosa cells of the ruptured follicle
(along with the theca cells of that follicle) are transformed
into the corpus luteum, which begins to release
progesterone and estrogen.
Table 17–6
Functions of Granulosa Cells
1. Nourish oocyte
2. Secrete chemical messengers that infl uence the oocyte and
the theca cells
3. Secrete antral fl
uid
4. Are the site of action for estrogen and FSH in the control of
follicle development during early and middle follicular phases
5. Express aromatase, which converts androgen (from theca
cells) to estrogen
6. Secrete inhibin, which inhibits FSH secretion via an action
on the pituitary
7. Are the site of action for LH induction of changes in the
oocyte and follicle culminating in ovulation and formation
of the corpus luteum
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