Ganong's Review of Medical Physiology, 23rd Edition

414 SECTION IVEndocrine & Reproductive Physiology

however, and the proliferative endometrium becomes thick
enough to break down and begins to slough. The time it takes
for bleeding to occur is variable, but it usually occurs in less
than 28 d from the last menstrual period. The flow is also vari-
able and ranges from scanty to relatively profuse.

Cyclical Changes in the Uterine Cervix

Although it is continuous with the body of the uterus, the cer-
vix of the uterus is different in a number of ways. The mucosa
of the uterine cervix does not undergo cyclical desquamation,
but there are regular changes in the cervical mucus. Estrogen
makes the mucus thinner and more alkaline, changes that pro-
mote the survival and transport of sperms. Progesterone makes
it thick, tenacious, and cellular. The mucus is thinnest at the
time of ovulation, and its elasticity, or spinnbarkeit, increases
so that by midcycle, a drop can be stretched into a long, thin
thread that may be 8 to 12 cm or more in length. In addition, it
dries in an arborizing, fern-like pattern (Figure 25–24) when a
thin layer is spread on a slide. After ovulation and during preg-
nancy, it becomes thick and fails to form the fern pattern.

Vaginal Cycle

Under the influence of estrogens, the vaginal epithelium be-
comes cornified, and cornified epithelial cells can be identified
in the vaginal smear. Under the influence of progesterone, a
thick mucus is secreted, and the epithelium proliferates and
becomes infiltrated with leukocytes. The cyclical changes in

the vaginal smear in rats are relatively marked. The changes in

humans and other species are similar but not so clear-cut.

Cyclical Changes in the Breasts

Although lactation normally does not occur until the end of

pregnancy, cyclical changes take place in the breasts during

the menstrual cycle. Estrogens cause proliferation of mamma-

ry ducts, whereas progesterone causes growth of lobules and

alveoli. The breast swelling, tenderness, and pain experienced

by many women during the 10 d preceding menstruation are

probably due to distention of the ducts, hyperemia, and edema

of the interstitial tissue of the breast. All these changes regress,

along with the symptoms, during menstruation.

Changes During Intercourse

During sexual excitement in women, fluid is secreted onto the

vaginal walls, probably because of release of VIP from vaginal

nerves. A lubricating mucus is also secreted by the vestibular

glands. The upper part of the vagina is sensitive to stretch,

while tactile stimulation from the labia minora and clitoris

adds to the sexual excitement. These stimuli are reinforced by

tactile stimuli from the breasts and, as in men, by visual, audi-

tory, and olfactory stimuli, which may build to the crescendo

known as orgasm. During orgasm, autonomically mediated

rhythmic contractions occur in the vaginal walls. Impulses

also travel via the pudendal nerves and produce rhythmic con-

traction of the bulbocavernosus and ischiocavernosus mus-

cles. The vaginal contractions may aid sperm transport but are

not essential for it, since fertilization of the ovum is not depen-

dent on orgasm.

Indicators of Ovulation

Knowing when during the menstrual cycle ovulation occurs is

important in increasing fertility or, conversely, in family plan-

ning. A convenient and reasonably reliable indicator of the

time of ovulation is a change—usually a rise—in the basal

body temperature (Figure 25–25). The rise starts 1 to 2 d after

ovulation. Women interested in obtaining an accurate tem-

perature chart should use a digital thermometer and take their

temperatures (oral or rectal) in the morning before getting out

of bed. The cause of the temperature change at the time of

ovulation is probably the increase in progesterone secretion,

since progesterone is thermogenic.

A surge in LH secretion triggers ovulation, and ovulation

normally occurs about 9 h after the peak of the LH surge at

midcycle (Figure 25–25). The ovum lives for approximately

72 h after it is extruded from the follicle, but it is fertilizable

for a much shorter time than this. In a study of the relation of

isolated intercourse to pregnancy, 36% of women had a

detected pregnancy following intercourse on the day of ovula-

tion, but with intercourse on days after ovulation, the percent-

age was zero. Isolated intercourse on the first and second day

before ovulation also led to pregnancy in about 36% of

FIGURE 25–24 Patterns formed when cervical mucus is
smeared on a slide, permitted to dry, and examined under the
microscope. Progesterone makes the mucus thick and cellular. In the
smear from a patient who failed to ovulate (bottom), no progesterone
is present to inhibit the estrogen-induced fern pattern.

Normal cycle, 14th day

Midluteal phase, normal cycle

Anovulatory cycle with estrogen present