Reproduction
619
Occasionally, two or more follicles reach maturity, and
more than one egg may be ovulated. This is the most com-
mon cause of multiple births. In such cases the siblings are
fraternal,
not identical, because the eggs carry different sets
of genes. We will describe later how identical twins form.
Formation of the Corpus Luteum
After the mature follicle discharges its antral fl
uid and egg,
it collapses around the antrum and undergoes a rapid trans-
formation. The granulosa cells enlarge greatly, and the entire
glandlike structure formed is called the
corpus luteum,
which secretes estrogen, progesterone, and inhibin. If the
discharged egg, now in a fallopian tube, is not fertilized,
the corpus luteum reaches its maximum development within
approximately 10 days. It then rapidly degenerates by apop-
tosis. As we will see, it is the loss of corpus luteum function
that leads to menstruation and the beginning of a new men-
strual cycle.
In terms of ovarian function, therefore, the menstrual
cycle may be divided into two phases approximately equal in
length and separated by ovulation (
Figure 17–17
): (1) the
follicular phase,
during which a mature follicle and second-
ary oocyte develop; and (2) the
luteal phase,
beginning after
ovulation and lasting until the death of the corpus luteum.
Sites of Synthesis of Ovarian Hormones
The sites of ovarian hormone syntheses can be summarized
as fo
llows: estrogen is synthes
ized and re
leased into the
blood during the follicular phase mainly by the granulosa
cells. After ovulation, estrogen is synthesized and released
by the corpus luteum. Progesterone, the other major ovarian
steroid hormone, is synthesized and released in very small
amounts by the granulosa and theca cells just before ovula-
tion, but its major source is the corpus luteum. Inhibin, a
peptide hormone, is secreted by both the granulosa cells and
the corpus luteum.
Control of Ovarian Function
The major factors controlling ovarian function are analogous to
the controls described for testicular function. They constitute
a hormonal system made up of GnRH, the anterior pituitary
gonadotropins FSH and LH, and gonadal sex hormones—
estrogen and progesterone.
As in the male, the entire sequence of controls depends
upon the pulsatile secretion of GnRH from hypothalamic
neuroendocrine cells. In the female, however, the frequency
and amplitude of these pulses during a 24-hour period change
over the course of the menstrual cycle. So does the respon-
siveness both of the anterior pituitary to GnRH and of the
ovaries to FSH and LH.
Let us look fi rst at the patterns of hormone concentra-
tions in systemic plasma during a normal menstrual cycle
(
Figure 17–18
). (GnRH is not shown because its impor-
tant concentration is not in systemic plasma, but rather in the
plasma within the hypothalamo-pituitary portal vessels.) In
Figure 17–18, the lines are plots of average daily concentra-
tions; that is, the increases and decreases during a single day
stemming from episodic secretion have been averaged. For
now, ignore both the legend and circled numbers in this fi g-
ure because we are concerned here only with hormonal pat-
terns and not the explanations of these patterns.
FSH increases in the early part of the follicular phase
and then steadily decreases throughout the remainder of the
cycle except for a small midcycle peak. LH is constant during
most of the follicular phase but then shows a very large mid-
cycle increase—the
LH surge
—peaking approximately 18 h
before
ovulation. This is followed by a rapid decrease and then
a further slow decline during the luteal phase.
After remaining fairly low and stable for the fi rst week,
estrogen increases rapidly during the second week as the
dominant ovarian follicle grows and secretes more estrogen.
Estrogen then starts decreasing shortly before LH has peaked.
28
25
14
7
1
Bleeding
starts
Multiple
follicles
develop
Dominant
follicle
matures
Corpus luteum
functions
Corpus luteum
degenerates
One follicle
becomes
dominant
Ovulation
occurs
Follicular phase
Uterine
bleeding
Luteal phase
Day
Ovarian
events
Figure 17–17
Summary of ovarian events during a menstrual cycle (if fertilization does not occur). The fi rst day of the cycle is named for a uterine event—the
onset of bleeding—even though ovarian events are used to denote the cycle phases.
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