CHAPTER 19
Female Reproductive System
379
Figure 19-8.
Events of the female reproductive cycle.
The following sequence of events refer to the numbered events labeled in
red
in the diagram above.
(1)
At the beginning of the
female
reproductive cycle
, there are rising levels of
gonadotropic hormones
from the anterior pituitary, most importantly
FSH
.
(2)
This rise
promotes ovarian recruitment of a cohort of antral follicles to proceed into advanced development and then selection of typically a
single dominant follicle at about day 6.
(3)
These follicles secrete steroid hormones, most prominently estrogens, that
(4a)
promote
rebuilding of the endometrium (
proliferative phase
) and
(4b)
exert a negative feedback on FSH secretion by pituitary gonadotropes.
(5)
In the latter part of the
follicular phase
, the dominant follicle secretes increasing amounts of estrogens (and, to a lesser extent,
progesterone).
(6)
When circulating estrogen reaches a threshold level (about 200 pg/mL) for a duration of about 36 hours, pituitary
gonadotropes are stimulated to sharply increase secretion of gonadotropic hormones––most importantly, LH.
(7)
This
LH surge
from the pituitary brings about ± nal maturation of the dominant follicle culminating in ovulation (about 40 hours after initiation
of the LH surge) and formation of the
corpus luteum
from the remaining components of the follicle.
(8)
The corpus luteum secretes
progesterone as well as estrogens.
(9a)
This induces a change in the endometrium from the proliferative phase to the
secretory phase
.
(9b)
Meanwhile, gonadotropin secretion is greatly reduced, probably because of negative feedback effects of the high progesterone
and estrogen levels coming from the corpus luteum.
(10)
Without LH support, the corpus luteum fails after about 10 days, and ste-
roid hormone levels fall.
(11a)
This loss of steroid hormone support results in degenerative changes in the endometrium culminating
in
menstruation
.
(11b)
The fall in progesterone also releases the pituitary gonadotropes from negative feedback with the result that
FSH secretion starts to rise toward the end of the cycle, and this starts another round of follicle recruitment.
J. Naftel
Day of nominal female reproductive cycle
0
71
42
1
28
Progesterone
Progesterone
Progesterone
Estradiol
Estradiol
Estradiol
LH
LH
LH
FSH
FSH
FSH
Pituitary
gonadotroph
hormones
Ovarian
steroid
hormones
Ovary
Endometrium
Endometrium
Hormone concentration in plasma
(1)
(2)
(3)
(4a)
(4b)
(9b)
(9a)
(5)
(6)
(7)
(8)
(10)
(11a)
(11b)
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