624
Chapter 17
of menstruation is known as the
menstrual phase
(generally
about 3 to 5 days in a typical 28-day cycle). During this time,
the epithelial lining of the uterus—the
endometrium
degenerates, resulting in the menstrual fl ow. The menstrual
fl ow then ceases, and the endometrium begins to thicken as
it regenerates under the infl uence of estrogen. This period
of growth, the
proliferative phase,
lasts for the 10 days or
so between cessation of menstruation and the occurrence of
ovulation. Soon after ovulation, the endometrium increases
secretory activity under the infl uence of progesterone and
estrogen. Thus, the part of the menstrual cycle between ovu-
lation and the onset of the next menstruation is called the
secretory phase.
As shown in Figure 17–23, the ovarian follicular phase
includes the uterine menstrual and proliferative phases, whereas
the ovarian luteal phase is the same as the uterine secretory
phase.
The uterine changes during a menstrual cycle are caused
by changes in the plasma concentrations of estrogen and pro-
gesterone (see Figure 17–23). During the proliferative phase,
an increasing plasma estrogen level stimulates growth of both
the endometrium and the underlying uterine smooth muscle
(
myometrium
). In addition, it induces the synthesis of recep-
tors for progesterone in endometrial cells. Then, following
ovulation and formation of the corpus luteum (during the
secretory phase), progesterone acts upon this estrogen-primed
endometrium to convert it to an actively secreting tissue. The
endometrial glands become coiled and fi lled with glycogen,
the blood vessels become more numerous, and enzymes accu-
mulate in the glands and connective tissue. These changes are
essential to make the endometrium a hospitable environment
for implantation and nourishment of the developing embryo.
Progesterone also inhibits myometrial contractions, in
large part by opposing the stimulatory actions of estrogen and
locally generated prostaglandins. This is very important to
ensure that a fertilized egg, once it arrives in the uterus, will
not be swept out by uterine contractions before it can implant
in the wall. Uterine quiescence is maintained by progesterone
throughout pregnancy and is essential to prevent premature
delivery.
Estrogen and progesterone also have important effects
on the secretion of mucus by the cervix. Under the infl uence
of estrogen alone, this mucus is abundant, clear, and watery.
All of these characteristics are most pronounced at the time
of ovulation and allow sperm deposited in the vagina to move
easily through the mucus on their way to the uterus and fal-
lopian tubes. In contrast, progesterone, present in signifi -
cant concentrations only after ovulation, causes the mucus to
become thick and sticky—in essence a “plug” that prevents
bacteria from entering the uterus from the vagina. The anti-
bacterial blockage protects the uterus and the fetus if concep-
tion has occurred.
The decrease in plasma progesterone and estrogen levels
that results from degeneration of the corpus luteum deprives
the highly developed endometrium of its hormonal support
and causes menstruation. The fi rst event is profound con-
striction of the uterine blood vessels, which leads to a dimin-
ished supply of oxygen and nutrients to the endometrial cells.
Disintegration starts in the entire lining, except for a thin,
underlying layer that will regenerate the endometrium in the
next cycle. Also, the uterine smooth muscle begins to undergo
rhythmical contractions.
Both the vasoconstriction and uterine contractions are
mediated by prostaglandins produced by the endometrium
in response to the decrease in plasma estrogen and progester-
one. The major cause of menstrual cramps,
dysmenorrhea
,
is
overproduction of these prostaglandins, leading to excessive
uterine contractions. The prostaglandins also affect smooth
Follicle
Ovum
Corpus luteum
Follicular
Follicular
Luteal
Menstrual
Day
Ovarian
phase
Uterine
phase
Endometrial
thickness
Ovarian
event
Menstrual
Secretory
Proliferative
Ovulation
Estrogen
Progesterone
Estrogen
1
5
10
15
20
25
28
5
Figure 17–23
Relationships between ovarian and uterine changes during the menstrual cycle. Refer to Figure 17–18 for specifi c hormonal changes.
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