833
CHAPTER TWENTY-TWO
Reproductive System
male sex hormones are formed. The other structures of the
male reproductive system are termed
accessory sex organs
(secondary sex organs). They include the internal reproduc-
tive organs and the external reproductive organs (
f
g. 22.4
;
reference plates 3 and 4).
Testes
The
testes
(tes
te
ˉz; sing.,
testis
) are ovoid structures about 5
centimeters in length and 3 centimeters in diameter. Both tes-
tes, each suspended by a spermatic cord, are within the cavity
of the saclike
scrotum
(F g. 22.4 and reference plate 12).
Descent of the Testes
In a male fetus, the testes originate from masses of tissue
posterior to the parietal peritoneum, near the developing kid-
neys. Usually a month or two before birth, the testes descend
to the lower abdominal cavity and pass through the abdomi-
nal wall into the scrotum.
The male sex hormone
testosterone,
which the developing
testes secrete, stimulates the testes to descend. A F
bromus-
cular cord called the
gubernaculum
(goo
ber
-nak
u-lum)
aids movement of the testes. This cord is attached to each
developing testis and extends into the inguinal region of the
abdominal cavity. It passes through the abdominal wall and
is fastened to the skin on the outside of the scrotum. The tes-
tis descends, guided by the gubernaculum, passing through
the
inguinal canal
(ing
gwı˘-nal kah-nal
) of the abdominal
Anaphase I.
Homologous chromosome pairs separate,
and each replicated member moves to one end of the spin-
dle. Each new, or daughter, cell receives only one replicated
member of a homologous pair of chromosomes, overall halv-
ing the chromosome number.
Telophase I.
The original cell divides in two. Nuclear
membranes form around the chromosomes, nucleoli reap-
pear, and the spindle F
bers disassemble into their constitu-
ent microtubules.
Second Meiotic Division
After telophase I, the second meiotic division begins. Meiosis
II is similar to a mitotic division (see F
g. 22.1). During
pro-
phase II,
chromosomes condense and reappear, still repli-
cated. They move into positions midway between the poles
of the developing spindle. In
metaphase II,
the replicated
chromosomes attach to spindle F bers. In
anaphase II,
cen-
tromeres separate, freeing the chromatids to move to oppo-
site poles of the spindles. The former chromatids are now
considered to be chromosomes. In
telophase II,
each of the
two cells resulting from meiosis I divides to form two cells.
Therefore, each cell undergoing meiosis has the potential to
produce four gametes. In males, the gametes mature into four
sperm cells. In females, three of the products of meiosis are
“cast aside” as polar bodies, and one cell becomes the egg.
Meiosis generates astounding genetic variety. Any one of
a person’s more than 8 million possible combinations of 23
chromosomes can combine with any one of the more than 8
million combinations of his or her mate, raising the potential
variability to more than 70 trillion genetically unique indi-
viduals! Crossing over contributes even more genetic vari-
ability.
Figure 22.3
illustrates in a simpliF ed manner how
maternal and paternal traits reassort during meiosis.
Sex cells have one set of genetic instructions, carried on
23 chromosomes, compared to two sets on 46 chromosomes
in other cells. When sex cells join at fertilization, the amount
of genetic information held in 46 chromosomes is restored.
PRACTICE
1
What are male and female sex cells called?
2
Describe the major events of meiosis.
3
How does meiosis provide genetic variability?
22.2
ORGANS OF THE MALE
REPRODUCTIVE SYSTEM
Organs of the male reproductive system are specialized to
produce and maintain the male sex cells, or
sperm cells;
transport these cells and supporting fl
uids to the outside;
and secrete male sex hormones.
The
primary sex organs
(gonads) of this system are the
two testes in which the sperm cells (spermatozoa) and the
Parent cell
Paternal
chromatids
Maternal
chromatids
Gene for
eye color
Gene for
hair color
Result of
crossing
over
Gene for
blood type
FIGURE 22.3
As a result of crossing over, the genetic information
in sex cells varies from cell to cell. Colors represent parent of origin.
Although only one eye color gene pair is illustrated, eye color is
polygenic (involving more than one gene pair).
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