620
Chapter 17
This is followed by a second increase, due to secretion by the
corpus luteum, and fi nally, a rapid decrease during the last
days of the cycle. Very small amounts of progesterone are
released by the ovaries during the follicular phase until just
before ovulation. Very soon after ovulation, the developing
corpus luteum begins to release large amounts of progester-
one, and from this point the progesterone pattern is similar to
that for estrogen.
Not shown in Figure 17–18 is the plasma concentra-
tion of inhibin. Its pattern is similar to that of estrogen: it
increases during the late follicular phase, remains high dur-
ing the luteal phase, and then decreases as the corpus luteum
degenerates.
The following discussion will explain how these hor-
monal changes are interrelated to produce a self-cycling pat-
tern. The numbers in Figure 17–18 are keyed to the text. The
feedback effects of the ovarian hormones to be described in
the text are summarized for reference in
Table 17–4
.
Follicle Development and Estrogen
Synthesis During the Early
and Middle Follicular Phases
There are always a number of preantral and early antral follicles
in the ovary between puberty and menopause. Further devel-
opment of the follicle beyond these stages requires stimulation
by FSH. Prior to puberty, the plasma concentration of FSH
is too low to induce such development. This changes during
puberty, and menstrual cycles commence. The increase in FSH
secretion that occurs as one cycle ends and the next begins
(numbers
16
to
1
in Figure 17–18) provides this stimulation,
and a group of preantral and early antral follicles enlarge
2
.
The increase in FSH at the end of the cycle (
16
to
1
) is due
to decreased progesterone, estrogen, and inhibin (removal of
negative feedback).
During the next week or so, there is a division of labor
between the actions of FSH and LH on the follicles: FSH
acts on the granulosa cells, and LH acts on the theca cells.
Follicular
Luteal
Day
Ovarian
phase
1
5
10
15
20
25
28
Ovulation
5
10
15
0
10
20
30
40
0
LH
FSH
Estrogen
Progesterone
Ovarian
Follicle
Plasma gonadotropins
(mlu/ml)
Plasma progesterone
(ng/ml)
50
100
150
0
Plasma estrogen (pg/ml)
15
11
14
4
2
12
7
5
9
10
3
6
13
16
8
1
Figure 17–18
Summary of systemic plasma hormone concentrations
and ovarian events during the menstrual cycle. The
events marked by the circled numbers are described
later in the text and are listed here to provide a
summary. The arrows in this legend denote causality.
1
FSH and LH secretion increase (because plasma
estrogen concentration is low and exerting little
negative feedback).
2
Multiple antral follicles
begin to enlarge and secrete estrogen.
3
Plasma
estrogen concentration begins to rise.
4
One follicle
becomes dominant (probably by chance) and secretes
very large amounts of estrogen.
5
Plasma estrogen
level increases markedly.
6
FSH secretion and
plasma FSH concentration decrease, causing atresia of
nondominant follicles, but then
7
increasing plasma
estrogen exerts a “positive” feedback on gonadotropin
secretion.
8
An LH surge is triggered.
9
The
egg completes its fi rst meiotic division and cytoplasmic
maturation while the follicle secretes less estrogen
accompanied by some progesterone,
10
ovulation
occurs, and
11
the corpus luteum forms and begins
to secrete large amounts of both estrogen and
progesterone.
12
Plasma estrogen and progesterone
increase.
13
FSH and LH secretion are inhibited and
their plasma concentrations decrease.
14
The corpus
luteum begins to degenerate (cause unknown) and
decrease its hormone secretion.
15
Plasma estrogen
and progesterone concentrations decrease.
16
FSH
and LH secretion begin to increase, and a new cycle
begins (back to
1
).
Figure 17–18
physiological
inquiry
1. Why do blood FSH levels increase at the end of
the luteal phase
16
? 2. What naturally occurring
event could rescue the corpus luteum and prevent
its degeneration starting in the middle of the
luteal phase?
Answers can be found at end of chapter.
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