The developing follicles and corpus lutea also secrete
hormones
(estrogens,
progesterone,
relaxin,
inhibin),
which communicate with the hypothalamus and anteri-
or pituitary, as well as evoke changes in the lining of the
uterus in preparation for pregnancy in case fertilization
occurs. The uterine lining thickens and transforms under
the influence of the hormones, and it sloughs off when hor-
mone levels dwindle. These cyclical changes in the uterus
are referred to as the
m e n s t r u a l c y c l e
, or
uterine cycle.
Collectively, the hormonal changes, the ovarian cycle,
and the menstrual cycle are lumped together in a process
called the
f e m a le r e p r o d u c t iv e c y c le
. Let’s take a closer
look at this complex series of events.
The Female Reproductive
Cycle Has Several Phases
In the first half of the female reproductive cycle, the ova-
ries and uterus prepare for ovulation, fertilization, and
subsequent pregnancy through the actions of various hor-
mones (
F ig u r e 1 6 . 1 1
); this portion, which consists of the
menstrual phase and preovulatory phase, is collectively
called the
f o llic u l a r p h a s e
.
The second half of the cycle (the
lu t e a l p h a s e
) consists
of ovulation and the postovulatory phase. Here, the ovaries
and uterus act as though the female is pregnant and secrete
hormones to support the pregnancy. If fertilization and
pregnancy do occur, the luteal phase continues throughout
pregnancy. However, if the egg is
not fertilized, the luteal phase is
programmed to end and give way
to
m e n s tr u a tio n
, also called the
m enstrual phase,
or
menses.
Let’s look at each phase of
the female reproductive cycle in
detail. The entire cycle can take
about 28 days, though there is
some normal variation in length.
M e n s t r u a l
P h a s e ,
D a y s
1 - 5 .
By convention, the
first day of menstruation begins the cycle because it is the
most outwardly visible sign.
In the ovaries:
several ovarian follicles grow and enlarge.
In the uterus:
menstrual flow from the uterus consists of
50 to 150 mL of blood and tissue cells from the endome-
trium. This discharge occurs because the declining level
of progesterones and estrogens causes the uterine arter-
ies to constrict. As a result, the cells they supply become
oxygen-deprived and start to die. Eventually, part of the
endometrium sloughs off. The menstrual flow passes from
the uterine cavity to the cervix and through the vagina to
the exterior.
P r e o v u l a t o r y P h a s e , D a y s 6 - 1 3 .
The length of the
preovulatory phase can vary the most from female to female.
In the ovary:
under the influence of FSH, several fol-
licles continue to grow and begin to secrete estrogens and
inhibin. By about day 6, a single follicle in one of the two
ovaries has outgrown the others and becomes the mature
(graafian) follicle, which enlarges until it is ready for ovu-
lation under the influence of LH.
In the uterus:
estrogens liberated into the blood by
growing ovarian follicles stimulate the repair of the en-
dometrium. The endometrium thickens, the endometrial
glands develop, and the arterioles coil and lengthen.
O v u l a t i o n , D a y 1 4 .
Ovulation, as we have seen, is the
rupture of the mature (graafian) follicle and the release of
the secondary oocyte into the pelvic cavity. The high level
of estrogens at the end of the preovulatory phase stimu-
lates the hypothalamus to release more gonadotropin-
releasing hormone (GnRH) and the anterior pituitary to
produce more LH. GnRH promotes the release of even
more LH, which brings about ovulation. An over-the-coun-
ter home test can detect this LH surge to predict ovulation
a day in advance.
P o s t o v u l a t o r y P h a s e , D a y s 1 5 - 2 8 .
Events in the
postovulatory phase depend on whether the oocyte is fer-
tilized.
In one ovary:
after ovulation, the mature follicle col-
lapses. Stimulated by LH, the remaining follicular cells
enlarge and form the corpus luteum, which secretes pro-
gesterone, estrogens, relaxin, and inhibin.
If the oocyte is not fertilized, the corpus luteum lasts
for two weeks, after which it degenerates. As the hormone
levels decrease, release of GnRH, FSH, and LH rise due to
loss of negative feedback. Follicular growth resumes and a
new ovarian cycle begins.
m e n str u a tio n
(men'-
stroo-A-shun) Periodic
discharge of blood,
tissue fluid, mucus,
and epithelial cells that
usually lasts for 5 days;
caused by a sudden
reduction in estrogens
and progesterone.
(for Figure 16.11) One method of contraception in-
volves administration of hormones to "trick" the body
into thinking that a pregnancy has already occurred.
As a result, the ovary will not produce additional fol-
licles for ovulation. Which of the following hormones
would you need to include in a pill in order to "fool"
the female body into thinking that it is pregnant?
a. LH
b. progesterone
c. GnRH
d. FSH
J
482 CHAPTER 16
The Reproductive Systems
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