846
UNIT SIX
4. Increased muscular growth, broadening shoulders, and
narrowing of the waist.
5. Thickening and strengthening of the bones.
Testosterone also increases the rate of cellular metabo-
lism and production of red blood cells by stimulating release
of erythropoietin. For this reason, the average number of red
blood cells in a microliter of blood is usually greater in males
than in females. Testosterone stimulates sexual activity by
affecting certain parts of the brain.
RECONNECT
To Chapter 14, Red Blood Cell Production and Its Control,
page 527.
Regulation of Male Sex Hormones
The extent to which male secondary sex characteristics
develop is directly related to the amount of testosterone that
the interstitial cells secrete. The hypothalamus regulates tes-
tosterone output through negative feedback (±
g. 22.18).
As the concentration of testosterone in the blood
increases, the hypothalamus becomes inhibited, decreasing
its stimulation of the anterior pituitary gland by GnRH. As
the pituitary’s secretion of LH falls in response, the amount
of testosterone the interstitial cells release decreases.
As the blood testosterone concentration drops, the hypo-
thalamus becomes less inhibited, and it once again stimu-
lates the anterior pituitary gland to release LH. The increasing
secretion of LH causes the interstitial cells to release more
testosterone, and blood testosterone concentration increases.
Testosterone level decreases somewhat during and after the
male climacteric,
which is a decline in sexual function that
accompanies aging. At any given age, the testosterone con-
centration in the male body is regulated to remain relatively
constant.
PRACTICE
26
Which hormone initiates the changes associated with male sexual
maturity?
27
Describe several male secondary sex characteristics.
28
Explain how the secretion of male sex hormones is regulated.
22.4
ORGANS OF THE FEMALE
REPRODUCTIVE SYSTEM
The organs of the female reproductive system are specialized
to produce and maintain the female sex cells, the
egg cells
(or
oocytes); transport these cells to the site of fertilization; provide
a favorable environment for a developing offspring; move the
offspring to the outside; and produce female sex hormones.
The
primary sex organs
(gonads) of this system are the
two ovaries, which produce the female sex cells and sex hor-
mones. The
accessory sex organs
of the female reproductive
system are the internal and external reproductive organs (
f g.
22.19
; reference plates 5 and 6).
amounts are synthesized in the adrenal cortex (see chapter
13, p. 508).
The hormone
testosterone
(tes-tos
te
˘-ro
ˉn) is the most
important androgen. It is secreted and transported in the
blood, loosely attached to plasma proteins. Like other ste-
roid hormones, testosterone binds receptor molecules which
are usually in the nuclei of its target cells (see chapter 13,
p. 485). However, in many target cells, such as those in the
prostate gland, seminal vesicles, and male external accessory
organs, testosterone is ±
rst converted to another androgen
called
dihydrotestosterone
(di-hi
dro-tes-tos
ter-o
¯n), which
stimulates the cells of these organs. Androgen molecules that
do not reach receptors in target cells are usually changed by
the liver into forms that can be excreted in bile or urine.
Testosterone secretion begins during fetal develop-
ment and continues for several weeks following birth; then
it nearly ceases during childhood. Between the ages of thir-
teen and ± fteen, a young man’s androgen production usu-
ally increases rapidly. This phase in development, when an
individual becomes reproductively functional, is
puberty
(pu
ber-te). After puberty, testosterone secretion continues
throughout the life of a male.
In a group of disorders called male pseudohermaphroditism, testes
are usually present, but a block in testosterone synthesis prevents
the genetically male fetus from developing male structures, and as
a result, later, the child appears to be a girl. But at puberty, the adre-
nal glands begin to produce testosterone, as they normally do in any
male. This leads to masculinization. The voice deepens, and muscles
build up into a masculine physique. Breasts do not develop, nor does
menstruation occur. The clitoris may enlarge so greatly under the
adrenal testosterone surge that it looks like a penis. Individuals with a
form of this condition prevalent in the Dominican Republic are called
guevedoces,
which means “penis at age twelve.”
Actions of Testosterone
Cells of the embryonic testes ±
rst produce testosterone after
about eight weeks of development. This hormone stimu-
lates the formation of the male reproductive organs, includ-
ing the penis, scrotum, prostate gland, seminal vesicles, and
ducts. Later in development, testosterone causes the testes
to descend into the scrotum.
During puberty, testosterone stimulates enlargement of
the testes (the primary male sex characteristic) and acces-
sory organs of the reproductive system, as well as develop-
ment of male
secondary sex characteristics,
which are special
features associated with the adult male body. Secondary sex
characteristics in the male include:
1. Increased growth of body hair, particularly on the face,
chest, axillary region, and pubic region. Sometimes
growth of hair on the scalp slows.
2. Enlargement of the larynx and thickening of the vocal
folds, with lowering of the pitch of the voice.
3. Thickening of the skin.
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