The purpose of the male and female reproductive

systemsis to continue the human species by the pro-
duction of offspring. How dry and impersonal that
sounds, until we remember that each of us is a contin-
uation of our species and that many of us in turn will
have our own children. Although some other animals
care for their offspring in organized families or soci-
eties, the human species is unique, because of cultural
influences, in the attention we give to reproduction
and to family life.
Yet like other animals, the actual production and
growth of offspring is a matter of our anatomy and
physiology. The male and female reproductive systems
produce gametes, that is, sperm and egg cells, and
ensure the union of gametes in fertilization following
sexual intercourse. In women, the uterus provides the
site for the developing embryo/fetus until it is suffi-
ciently developed to survive outside the womb.
This chapter will describe the organs of reproduc-
tion and the role of each in the creation of new life or
the functioning of the reproductive system as a whole.
First, however, we will discuss the formation of
gametes.

MEIOSIS

The cell division process of meiosisproduces the
gametes—sperm or egg cells. In meiosis, one cell with
the diploid number of chromosomes (46 for humans)
divides twice to form four cells, each with the haploid
number of chromosomes. Haploidmeans half the usual
diploid number, so for humans the haploid number is

23. Although the process of meiosis is essentially the
    same in men and women, there are some important
    differences.

SPERMATOGENESIS

Spermatogenesisis the process of meiosis as it takes
place in the testes, the site of sperm production.
Within each testis are seminiferous tubulesthat
contain spermatogonia, which are stem cells that gen-
erate sperm. A spermatogonium divides by mitosis to
form two cells, one of which will remain in place as a
stem cell, while the other differentiates (specializes) to
become a primary spermatocyte that will undergo
meiosis (Fig. 20–1). As you may recall from Chapter
10, gamete formation is regulated by hormones.

Follicle-stimulating hormone (FSH) from the anterior

pituitary gland initiates sperm production, and testos-

terone, secreted by the testes when stimulated by

luteinizing hormone (LH) from the anterior pituitary,

promotes the maturation of sperm. Inhibin, also pro-

duced by the testes, decreases the secretion of FSH. As

you can see in Fig. 20–1, for each primary spermato-

cyte that undergoes meiosis, four functional sperm

cells are produced.

Sperm production begins at puberty(10 to 14

years of age), and millions of sperm are formed each

day in the testes. Although sperm production dimin-

ishes with advancing age, there is usually no complete

cessation, as there is of egg production in women at

menopause.

OOGENESIS

Oogenesisis the process of meiosis for egg cell for-

mation; it begins in the ovaries and is also regulated by

hormones. FSH initiates the growth of ovarian folli-

cles, each of which contains an oogonium, a stem cell

for egg cell production (Fig. 20–2). This hormone also

stimulates the follicle cells to secrete estrogen, which

promotes the maturation of the ovum. Notice that for

each primary oocyte that undergoes meiosis, only one

functional egg cell is produced. The other three cells

produced are called polar bodies. They have no func-

tion, and will simply deteriorate. A mature ovarian fol-

licle actually contains the secondary oocyte; the

second meiotic division will take place if and when the

egg is fertilized.

The production of ova begins at puberty (10 to 14

years of age) and continues until menopause(45 to 55

years of age), when the ovaries atrophy and no longer

respond to pituitary hormones. During this 30- to 40-

year span, egg production is cyclical, with a mature

ovum being produced approximately every 28 days

(the menstrual cycle is discussed later in this chapter).

Actually, several follicles usually begin to develop

during each cycle. However, the rupturing (ovulation)

of the first follicle to mature stops the growth of the

others.

The process of meiosis is like other human proces-

ses in that “mistakes” may sometimes occur. One of

these, trisomy, is discussed in Box 20–1: Trisomy and

Down Syndrome.

The haploid egg and sperm cells produced by

meiosis each have 23 chromosomes. When fertiliza-

456 The Reproductive Systems