1034
UNIT 5
Continuity
27
more GnRH is released, the testes secrete more testosterone, but
the threshold for hypothalamic inhibition keeps rising until the
adult pattern of hormone interaction is achieved, as evidenced
by the presence of mature sperm in the semen.
Mechanism and Effects of Testosterone Activity
Like all steroid hormones, testosterone is synthesized from
cholesterol and exerts its effects by activating specific genes,
which results in enhanced synthesis of certain proteins in the
target cells. (See Chapter 16 for mechanisms of steroid hormone
action.)
In some target cells, testosterone must be transformed into
another steroid to exert its effects. In the prostate, testosterone
is converted to
dihydrotestosterone
(
DHT
) before it can bind in
the nucleus, and in certain neurons of the brain, testosterone is
converted to
estradiol
(es
0
trah-di
9
ol), a female sex hormone, to
bring about its stimulatory effects. Tese transformations oFen
occur in a single enzymatic step because testosterone and the
other gonadal hormones are structurally similar.
As puberty ensues, testosterone not only prompts spermato-
genesis but also has multiple anabolic effects throughout the
body (see ±able 27.1, p. 1052). It targets accessory reproductive
organs—ducts, glands, and the penis—causing them to grow to
adult size and function. In adult males, normal plasma levels
of testosterone maintain these organs. When the hormone is
deficient or absent, all accessory organs atrophy, semen volume
declines markedly, and erection and ejaculation are impaired.
Tus, a man becomes both sterile and unable to carry out sexual
intercourse. ±estosterone replacement therapy can remedy this
situation.
Male
secondary sex characteristics
—that is, features in-
duced in
nonreproductive
organs by the male sex hormones
(mainly testosterone)—develop at puberty. Tese include the
appearance of pubic, axillary, and facial hair, enhanced hair
growth on the chest or other body areas in some men, and a
deepening voice as the larynx enlarges. Te skin thickens and
becomes oilier (which predisposes young men to acne), bones
Once this balance is established during puberty (a process
that takes about three years), the amount of testosterone and
sperm produced remains fairly stable throughout life.
Because input from other brain areas also influences the hy-
pothalamus, the whole axis is under CNS control. In the absence
of GnRH and gonadotropins, the testes atrophy, and sperm and
testosterone production cease.
Development of male reproductive structures (discussed
later in the chapter) depends on prenatal secretion of male hor-
mones, and for a few months before birth, a male infant has
plasma gonadotropin and testosterone levels nearly two-thirds
those of an adult male
(Figure 27.11)
. Soon aFer birth, blood
levels of these hormones recede and they remain low through-
out childhood. As puberty nears, higher levels of testosterone
are required to suppress hypothalamic release of GnRH. So, as
GnRH
Anterior
pituitary
Inhibin
GnRH
Testosterone
Via portal
blood
Interstitial
endocrine
cells
Sustentocyte
Spermatogenic
cells
Seminiferous
tubule
ABP
Somatic and
psychological
effects at other
body sites;
maintenance
of secondary sex
characteristics
LH
FSH
Spermatogenesis
1
2
3
4
5
6
7
Stimulates
Inhibits
Figure 27.10
Hormonal regulation of testicular function,
the hypothalamic-pituitary-gonadal (HPG) axis.
(Only one
sustentocyte is depicted to show its structural relationship to the
spermatogenic cells it encloses. However, it would be flanked by
sustentocytes on each side.)
Plasma testosterone level
Sperm pr
oduction (% of maximal)
Fertilization
Birth
Puberty
Adult
100
50
0
3
6
9
1
10
20
60
Age
Months
Years
Figure 27.11
Plasma testosterone and sperm production
levels versus age in male humans.
previous page 1068 Human Anatomy and Physiology (9th ed ) 2012 read online next page 1070 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off