Spermatogenesis and the production of the male hor-
mones testosterone and dihydrotestosterone (DHT)
are regulated by the anterior pituitary gland through the
secretions of luteinizing hormone (LH) and follicle-
stimulating hormone (FSH); the latter is also called
interstitial cell-stimulating hormone (ICSH).
Testosterone and dihydrotestosterone both bind to the
same androgen receptors, producing several effects:
P re n a ta l developm ent.
Before birth, testosterone stimu-
lates the male pattern of development of reproductive
system ducts and the descent of the testes. DHT
stimulates the development of the external genitals.
D evelopm ent o f m ale sexual characteristics.
During puberty,
the hormone testosterone stimulates further devel-
opment of the sex organs. In addition, testosterone
stimulates development of secondary sex character-
istics, or changes in nonreproductive organs. These
changes include increased hair growth (facial, axillary,
chest, and pubic), enlargement of the larynx, increased
oil gland secretion, and the skeletal and muscular
development that leads to wide shoulders and narrow
hips.
D evelopm ent o f sexu a l fu n c tio n .
Androgens contribute to
male sexual behavior and spermatogenesis, as well
as to sex drive in both males and females. Recall that
the adrenal cortex is the main source of androgens in
females.
S tim u la tio n o f anabolism .
Androgens are anabolic hor-
mones; that is, they stimulate protein synthesis. This
effect is obvious in the heavier muscle and bone mass
of most men as compared to women.
FSH and testosterone act together to stimulate sper-
matogenesis. Once the degree of spermatogenesis re-
quired for male reproductive function has been achieved,
Sertoli cells release inhibin, a hormone named for its
inhibition of FSH secretion by the anterior pituitary. Tes-
tosterone and inhibin, which are both produced in the
testes, affect the anterior pituitary gland via negative
feedback (Figure 16.5).
As we look back to the couple discussed in the chapter
opener, we can see that we’ve discussed topics that help
explain the possible causes of infertility for the man. As
we’ve seen, counts below 20 million sperm per millimeter
in the semen would be considered low. High testicular
temperature can also harm sperm. Finally, we’ve seen
that sperm production is regulated by hormones, so ab-
normal hormone levels might lead to male infertility.
Now let’s take a look at the female reproductive system.
H o r m o n a l c o n t r o l in t h e m a l e
F i g u r e 1 6 . 5
Hypothalamus
GnRH
FSH
lerone decreases
! of GnRH and LH.
rior pituitary
Inhibin d
release <
Together with
testosterone,
FSH stimulates
spermatogenesis.
I
\
Inhibin
\
FSH
LH
LH stimulates
testosterone
secretion.
Testosterone
Spermatogenic
/
/
D
cells
Sertoli cells
secrete
androgen-binding
protein (ABP).
te
Dihydro-
testosterone
(DHT)
Leydig cells secrete
testosterone.
• Male pattern of development (before birth)
• Enlargement of male sex organs
and expression of male secondary
sex characteristics (starting at puberty)
• Anabolism (protein synthesis)
Key:
LH
1
FSH
O
Testosterone
__ LH receptor U FSH receptor Androgen
receptor
Dashed red lines indicate negative feedback inhibition.
~\
The anterior pituitary secretes_____ , which stimulates
testosterone secretion by the_____ cells.
V___________________________________________
J
The Reproductive Organs Make, Deliver, and Receive the Sex Cells
475
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