Essentials of Anatomy and Physiology

(mucosa) is extensively folded and is made of ciliated
epithelial tissue. The sweeping action of the cilia also
moves the ovum toward the uterus.
Fertilization usually takes place in the fallopian
tube. If not fertilized, an ovum dies within 24 to 48
hours and disintegrates, either in the tube or the
uterus. If fertilized, the ovum becomes a zygote and is
swept into the uterus; this takes about 4 to 5 days (see
Box 20–4: In VitroFertilization).
Sometimes the zygote will not reach the uterus but
will still continue to develop. This is called an ectopic
pregnancy; ectopicmeans “in an abnormal site.” The
developing embryo may become implanted in the fal-
lopian tube, the ovary itself, or even elsewhere in the
abdominal cavity. An ectopic pregnancy usually does
not progress very long, because these other sites are
not specialized to provide a placenta or to expand to
accommodate the growth of a fetus, as the uterus is.
The spontaneous termination of an ectopic pregnancy
is usually the result of bleeding in the mother, and sur-
gery may be necessary to prevent maternal death from
circulatory shock. Occasionally an ectopic pregnancy
does go to full term and produces a healthy baby; such
an event is a credit to the adaptability of the human
body and to the advances of medical science.

UTERUS

The uterusis shaped like an upside-down pear, about
3 inches long by 2 inches wide by 1 inch deep (7.5 cm
by 5 cm by 2.5 cm), superior to the urinary bladder
and between the two ovaries in the pelvic cavity
(see Fig. 20–5). The broad ligament also covers the

uterus (see Fig. 20–6). During pregnancy the uterus

increases greatly in size, contains the placenta to nour-

ish the embryo-fetus, and expels the baby at the end of

gestation.

The parts and layers of the uterus are shown in Fig.

20–6. The fundusis the upper portion above the

entry of the fallopian tubes, and the bodyis the large

central portion. The narrow, lower end of the uterus is

the cervix, which opens into the vagina.

The outermost layer of the uterus, the serosa or

epimetrium, is a fold of the peritoneum. The myome-

triumis the smooth muscle layer; during pregnancy

these cells increase in size to accommodate the grow-

ing fetus and contract for labor and delivery at the end

of pregnancy.

The lining of the uterus is the endometrium,

which itself consists of two layers. The basilar layer,

adjacent to the myometrium, is vascular but very

thin and is a permanent layer. The functional layer

is regenerated and lost during each menstrual cycle.

Under the influence of estrogen and progesterone

from the ovaries, the growth of blood vessels thick-

ens the functional layer in preparation for a possible

embryo. If fertilization does not occur, the functional

layer sloughs off in menstruation. During pregnancy,

the endometrium forms the maternal portion of the

placenta.

VAGINA

The vaginais a muscular tube about 4 inches (10 cm)

long that extends from the cervix to the vaginal orifice

in the perineum(pelvic floor). It is posterior to the

The Reproductive Systems 465

BOX20–4 IN VITROFERTILIZATION

woman’s fallopian tube. Development then pro-

ceeds as if the ova had been fertilized naturally.

Since the birth of the first “test tube baby” in

1978, many thousands of babies have been born

followingin vitrofertilization. The techniques are

not always successful, and repeated attempts can

be very expensive.

As an adjunct to IVF, pre-implantation genetic

diagnosis may be done, to test for genetic diseases

and chromosome abnormalities such as the tri-

somies described in Box 20–1. This may be espe-

cially useful for older women trying to conceive and

for couples with a family history of a genetic disease

such as sickle-cell anemia or cystic fibrosis.

In vitrofertilization(IVF) is fertilization outside

the body, usually in a glass dish. A woman who

wishes to conceive by this method is given FSH to

stimulate the simultaneous development of several

ovarian follicles. LH may then be given to stimulate

simultaneous ovulation. The ova are removed by

way of a small incision in the abdominal wall and

are placed in a solution containing the sperm of the

woman’s partner (or an anonymous donor). After

fertilization and the first mitotic divisions of cleav-

age, the very early embryo is placed in the woman’s

uterus.

It is also possible to mix the removed ova with

sperm and return them almost immediately to the