602
P A R T 7
Drugs acting on the reproductive system
The Sertoli cells also produce a substance called
inhibin
, an oestrogen-like molecule. Upon the sensing
of inhibin by the hypothalamus and anterior pituitary, a
negative feedback response occurs, decreasing the circu-
lating level of FSH. When the FSH level falls low enough,
the hypothalamus is stimulated to again release GnRH
to stimulate FSH release. This feedback system prevents
overproduction of sperm in the testes (Figure 39.5).
Inhibin has been investigated for many years as a possible
male birth control drug because it is thought to affect
only sperm production.
The LH stimulates the interstitial (Leydig) cells to
produce testosterone. The concentration of testoster-
one acts in a similar negative feedback system with the
hypothalamus. When the concentration is high enough,
the hypothalamus decreases GnRH release, leading to a
subsequent decrease in FSH and LH release. The levels
of testosterone are thought to remain within a fairly
well-defined range of normal.
With age, the seminiferous tubules and intersti-
tial cells atrophy and the male climacteric, a period of
lessened sexual activity and loss of testosterone effects,
Seminal
vesical
Prostate
Epididymis
Scrotum
Rectum
Ureter
Urinary bladder
Urethra
Corpus
cavernosum
Corpus
spongiosum
Glans
penis
Epididymis
(body)
Vas deferens
Epididymis
(body)
Epididymis
(body)
Seminiferous
tubules
Interstitial or Leydig cells
Testosterone
Testis
Sperm
Testis
FIGURE 39.4
The male reproductive system.
