994
U N I T 1 1
Genitourinary and Reproductive Function
Testicular Function
The two primary functions of the testes or male gonads
are
steroidogenesis,
or synthesis of androgens or male
sex hormones, and
spermatogenesis,
or sperm produc-
tion.
1–3
The testes are paired ovoid organs that lie within
the scrotum, which hangs outside the abdominal cavity.
The testes are surrounded by two tunics. The outer tunic
is the two-layered tunica vaginalis derived from the
peritoneum. The inner tunic is the
tunica albuginea
, the
dense connective tissue capsule of the testes (Fig. 39-2).
Septal extensions of the tunica albuginea divide the
testes into 200 to 300 wedge-shaped compartments or
lobules. Each lobule contains one to four tightly coiled
seminiferous tubules,
in which sperm are produced,
and a connective tissue stroma, in which the androgen-producing
Leydig
or
interstitial cells
are located.
Hormone Production
The testes produce several male sex hormones, includ-
ing
testosterone, dihydrotestosterone,
and
androstene-
dione.
1–3
Testosterone, which is the most abundant of
these hormones, is considered the main testicular hor-
mone. The adrenal cortex also produces androgens,
albeit in much smaller quantities (<5% of the total
male androgens) than the testes. Larger amounts of
testosterone are formed from dihydrotestosterone and
androstenedione in other tissues of the body, especially
the liver.
1
In addition to testosterone, small amounts of
estrogens are formed in the male. Some of this estrogen,
thought to be formed from testosterone, plays an impor-
tant role in spermatogenesis.
Testosterone is metabolized in the liver and excreted
by the kidneys. In the bloodstream, testosterone exists
in an unbound (free) or a bound form. The bound form
is attached to plasma proteins, including albumin and
the sex hormone–binding protein produced by the liver.
Only approximately 3% of circulating testosterone is
unbound and therefore able to enter the cell and exert
its metabolic effects.
1
Much of the testosterone that
becomes fixed to the tissues is converted to dihydrotes-
tosterone, especially in certain target tissues such as
the prostate gland. Some of the actions of testosterone
depend on this conversion, whereas others do not.
Ureter
Bladder
Ductus deferens
(vas deferens)
Glans penis
Prepuce
Scrotum
Testis
Epididymis
Bulbourethral
gland
Prostate
Rectum
Seminal
vesicle
FIGURE 39-1.
Structures of the male reproductive
system, including the testes, scrotum, and excretory
ducts.
Tunica vaginalis
Rete testis
Epididymis
Ductus deferens
Septum
Tunica albuginea
Seminiferous tubules
Efferent
ductules
Lobules
FIGURE 39-2.
The parts of the testes and epididymis.
