Chapter 27
The Reproductive System
1051
27
33.
Which hormone(s) prompt follicle growth? Which hormone
prompts ovulation?
34.
Which gonadal hormone exerts positive feedback on the
anterior pituitary that results in a burstlike release of LH?
For answers, see Appendix H.
Effects of Estrogens and Progesterone
Discuss the physiological effects of estrogens and
progesterone.
With a name meaning “generators of sexual activity,” estrogens
are analogous to testosterone, the male steroid. As estrogen lev-
els rise during puberty, they (1) promote oogenesis and follicle
growth in the ovary and (2) exert anabolic effects on the fe-
male reproductive tract
(Table 27.1)
. Consequently, the uterine
tubes, uterus, and vagina enlarge and become functional—more
ready to support a pregnancy. Te uterine tubes and uterus ex-
hibit enhanced motility; the vaginal mucosa thickens; and the
external genitalia mature.
Estrogens also support the growth spurt at puberty that makes
girls grow much more quickly than boys between the ages of 11
and 12. But this growth is short-lived because rising estrogen levels
also cause the epiphyses of long bones to close sooner, and females
reach their full height between the ages of 13 and 15. In contrast,
the aggressive growth of males continues until ages 15 to 19, at
which point rising estrogen levels cause epiphyseal closure.
Te estrogen-induced secondary sex characteristics of fe-
males include:
Breast development
Increasing deposition of subcutaneous fat, especially in the
hips and breasts
Development of a wider and lighter pelvis (adaptations for
childbirth)
Estrogens also have several metabolic effects, including
maintaining low total blood cholesterol levels (and high HDL
levels) and facilitating calcium uptake, which helps sustain the
density of the skeleton. Tese metabolic effects begin under es-
trogen’s influence during puberty, but they are not true second-
ary sex characteristics.
Progesterone works with estrogen to establish and then help
regulate the uterine cycle and promotes changes in cervical mu-
cus (see ±able 27.1). Its other effects are exhibited largely during
pregnancy, when it inhibits uterine motility and takes up where
estrogen leaves off in preparing the breasts for lactation. Indeed,
progesterone is named for these important roles (
pro
5
for,
gesta-
tion
5
pregnancy). However, the source of progesterone and es-
trogen during most of pregnancy is the placenta, not the ovaries.
Female Sexual Response
Describe the phases of the female sexual response.
Te
female sexual response
is similar to that of males in most
respects. During sexual excitement, the clitoris, vaginal mucosa,
bulbs of the vestibule, and breasts engorge with blood and the
3.
Days 15–28: Secretory (postovulatory) phase.
Tis 14-day
phase is the most constant timewise. During the secre-
tory phase the endometrium prepares for an embryo to
implant. Rising levels of progesterone from the corpus lu-
teum act on the estrogen-primed endometrium, causing
the spiral arteries to elaborate and converting the func-
tional layer to a secretory mucosa. Te endometrial glands
enlarge, coil, and begin secreting nutrients into the uterine
cavity that will sustain the embryo until it has implanted in
the blood-rich endometrial lining.
As progesterone levels rise, the cervical mucus becomes
viscous again, forming the
cervical plug
, which helps to
block entry of sperm and pathogens or other foreign mate-
rials. Progesterone also plays an important role in keeping
the uterus “private” in the event an embryo has begun to
implant. Rising progesterone (and estrogen) levels inhibit
LH release by the anterior pituitary.
As noted earlier, if fertilization has not occurred, the
corpus luteum degenerates toward the end of the secretory
phase as LH blood levels decline. Progesterone levels fall,
depriving the endometrium of hormonal support, and the
spiral arteries kink and go into spasms. Denied oxygen
and nutrients, the ischemic endometrial cells die and the
glands regress, setting the stage for menstruation to begin
on day 28. Te spiral arteries constrict one final time and
then suddenly relax and open wide. As blood gushes into
the weakened capillary beds, they fragment, causing the
functional layer to slough off. Te uterine cycle starts over
again on this first day of menstrual flow.
Figure 27.22b and d also illustrate how the ovarian and uter-
ine cycles fit together. Notice that the menstrual and prolifera-
tive phases overlap the follicular phase and ovulation in the
ovarian cycle, and that the uterine secretory phase corresponds
to the ovarian luteal phase.
Homeostatic Imbalance
27.8
Extremely strenuous physical activity can delay menarche in girls
and disrupt the normal menstrual cycle in adult women, even
causing
amenorrhea
(a-men
0
o-re
9
ah), cessation of menstruation.
Female athletes have little body fat, and adipose cells help con-
vert adrenal androgens to estrogens and are the source of leptin
which, as noted above, plays a critical permissive role in the onset
of puberty in females. Leptin informs the hypothalamus whether
energy stores are sufficient to support the high energy demands
of reproduction. If not, the reproductive cycles are shut down.
Amenorrhea is usually reversible when the woman discon-
tinues athletic training, but it has a worrisome consequence in
young, healthy adult women: dramatic losses in bone mass nor-
mally seen only in osteoporosis of old age. Once estrogen levels
drop and the menstrual cycle stops (regardless of cause), bone
loss begins.
Check Your Understanding
32.
Which hormone plays an important role in “letting the brain
know” that puberty may occur in girls?
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