Reproductive System
The beginning of the menstrual fl ow marks the end of a
reproductive cycle and the beginning of a new cycle. This cycle
is summarized in
table 22.4
and diagrammed in
f gure 22.33
Low blood concentrations of estrogens and progester-
one at the beginning of the reproductive cycle mean that the
hypothalamus and anterior pituitary gland are no longer inhib-
ited. Consequently, the concentrations of FSH and LH soon
increase, and a new follicle is stimulated to mature. As this fol-
licle secretes estrogens, the uterine lining undergoes repair, and
the endometrium begins to thicken again. Clinical Application
22.3 addresses some causes of infertility in the female.
Follicular cells secrete some progesterone during the ±
part of the reproductive cycle. However, corpus luteum cells
secrete abundant progesterone and estrogens during the sec-
ond half of the cycle. Consequently, as a corpus luteum is
established, the blood concentration of progesterone sharply
Progesterone causes the endometrium to become more
vascular and glandular. It also stimulates the uterine glands
to secrete more glycogen and lipids (secretory phase). The
endometrial tissues ± ll with fl uids containing nutrients and
electrolytes, which provide a favorable environment for
embryo development.
The endometrium contains stem cells that enable this tissue to pro-
liferate each month. A company sells a device to collect a menstrual
sample and send it in to their laboratory, where the stem cells are
extracted and stored. These menstrual stem cells in a lab dish give
rise to a variety of specialized cell types. Storing them could provide
a resource for future medical treaments.
High levels of estrogens and progesterone inhibit the
release of LH and FSH from the anterior pituitary gland.
Consequently, no other follicles are stimulated to develop
when the corpus luteum is active. However, if the second-
ary oocyte released at ovulation is not fertilized, the cor-
pus luteum begins to degenerate (regress) on about the
twenty-fourth day of the cycle. Eventually, connective tissue
replaces it. The remnant of such a corpus luteum is called a
corpus albicans
(see ±
g. 22.26).
When the corpus luteum ceases to function, concentra-
tions of estrogens and progesterone rapidly decline, and in
response, blood vessels in the endometrium constrict. This
reduces the supply of oxygen and nutrients to the thickened
endometrium, and these lining tissues (decidua) soon disin-
tegrate and slough off. At the same time, blood leaves dam-
aged capillaries, creating a fl
ow of blood and cellular debris,
which passes through the vagina as the
menstrual flow
(menses). This fl ow usually begins about the twenty-eighth
day of the cycle and continues for three to ± ve days, while
the concentrations of estrogens are relatively low.
Major Events in a
Reproductive Cycle
1. The anterior pituitary gland secretes FSH and LH.
2. FSH stimulates maturation of a follicle.
3. Granulosa cells of the follicle produce and secrete estrogens.
a. Estrogens maintain secondary sex traits.
b. Estrogens cause the endometrium to thicken.
4. The anterior pituitary gland releases a surge of LH, which stimulates
5. Follicular and thecal cells become corpus luteum cells, which secrete
estrogens and progesterone.
a. Estrogens continue to stimulate uterine wall development.
b. Progesterone stimulates the endometrium to become more
glandular and vascular.
Estrogens and progesterone inhibit secretion of FSH and LH from
the anterior pituitary gland.
6. If the secondary oocyte is not fertilized, the corpus luteum
degenerates and no longer secretes estrogens and progesterone.
7. As the concentrations of luteal hormones decline, blood vessels in
the endometrium constrict.
8. The uterine lining disintegrates and sloughs o±
, producing a
menstrual ²
9. The anterior pituitary gland is no longer inhibited and again secretes
FSH and LH.
10. The reproductive cycle repeats.
Elite female athletes may have disturbed reproductive cycles, rang-
ing from diminished menstrual ²
ow (oligomenorrhea) to complete
stoppage (amenorrhea). The more active an athlete, the more likely
it is that she will have menstrual irregularities, and this may impair
her ability to conceive. The culprit in infertility appears to be too little
body fat. The diminished fat reserves results in decreased secretion of
the hormone leptin, which lowers secretion of gonadotropin releas-
ing hormone from the hypothalamus, which in turn lowers estro-
gen levels. The infertility apparently results from too little estrogen.
Adipose tissue also contains some estrogen, a small supply made
even smaller in the trim elite athlete.
About 30% of women su±
er from premenstrual syndrome (PMS), and
3 to 8% of them have severe enough symptoms for the condition to
be considered premenstrual dysphoric disorder (PMDD). PMS is asso-
ciated with water retention (bloating), fatigue, headache, depression,
crying jags, and di³
culty concentrating. For women with PMDD, the
symptoms are much more pronounced, and interfere with daily func-
tioning. Some women with PMDD may sink into such despair that they
have suicidal thoughts. Diagnosis of either disorder is based mostly on
timing—the symptoms appear only during the days leading up to the
start of menstrual ²
ow and perhaps a day or two after. Selective sero-
tonin reuptake inhibitors, a class of anti-depressant, and certain oral
contraceptives, improve symptoms in some women with PMDD.
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