Reproduction
611
Hormonal Control of Male
Reproductive Functions
Control of the Testes
Figure 17–11
summarizes the control of the testes. In a nor-
mal adult man, the GnRH-secreting neuroendocrine cells in
the hypothalamus fi re a brief burst of action potentials approx-
imately every 90 min, secreting GnRH at these times. The
GnRH reaching the anterior pituitary via the hypothalamo-
pituitary portal vessels during each periodic pulse triggers the
release of both LH and FSH from the same cell type, although
not necessarily in equal amounts. Thus, systemic plasma
concentrations of FSH and LH also show pulsatility—rapid
increases followed by slow decreases over the next 90 min or
so as the hormones are slowly removed from the plasma.
There is a clear separation of the actions of FSH and LH
within the testes (see Figure 17–11). FSH acts primarily on
the Sertoli cells to stimulate the secretion of paracrine agents
needed for spermatogenesis. LH, by contrast, acts primarily on
the Leydig cells to stimulate testosterone secretion. In addi-
tion to its many important systemic effects as a hormone, the
testosterone secreted by the Leydig cells also acts locally, as a
paracrine agent, by moving from the interstitial spaces into the
seminiferous tubules. Testosterone enters Sertoli cells, where
it facilitates spermatogenesis. Thus, despite the absence of a
direct
effect on cells in the seminiferous tubules, LH exerts
an essential
indirect
effect because the testosterone secretion
stimulated by LH is required for spermatogenesis.
The last components of the hypothalamo-pituitary con-
trol of male reproduction that remain to be discussed are the
negative feedback effects exerted by testicular hormones. Even
though FSH and LH are produced by the same cell type, their
secretion rates can be altered to different degrees by negative
feedback inputs.
Testosterone inhibits LH secretion in two ways (see Figure
17–11): (1) it acts on the hypothalamus to decrease the amplitude
of GnRH bursts, which results in a decrease in the secretion of
gonadotropins; and (2) it acts directly on the anterior pituitary
to decrease the LH response to any given amount of GnRH.
How do the testes reduce FSH secretion? The major
inhibitory signal, exerted directly on the anterior pituitary, is
the protein hormone inhibin secreted by the Sertoli cells (see
Figure 17–11). This is a logical completion of a negative feed-
back loop such that FSH stimulates Sertoli cells to increase
both spermatogenesis and inhibin production, and inhibin
decreases FSH release.
Despite all these complexities, the total amounts of GnRH,
LH, FSH, testosterone, and inhibin secreted and of sperm pro-
duced are relatively constant from day to day in the adult male.
This is completely different from the large cyclical variations of
activity so characteristic of the female reproductive processes.
Testosterone
In addition to its essential paracrine action within the testes
on spermatogenesis and its negative feedback effects on the
hypothalamus and anterior pituitary, testosterone exerts many
other effects, as summarized in
Table 17–3
.
In Chapter 11 we mentioned that some hormones must
undergo transformation in their target cells in order to be effec-
tive. This is true of testosterone in many (but not all) of its tar-
get cells. In some cells, like in the adult prostate, after its entry
into the cytoplasm, testosterone is converted to dihydrotestos-
terone (DHT), which is more potent than testosterone. This
Begin
Inhibin
Testosterone
FSH
LH
Hypothalamus
Secretes GnRH
Reproductive tract
and other organs
Respond to testosterone
GnRH
(in hypothalamo-pituitary
portal vessels)
(Only FSH)
(Only LH)
Testes
(Local)
Sertoli
cells
Leydig
cells
Anterior pituitary
Secretes FSH and LH
Testosterone
Stimulate
spermatogenesis
Figure 17–11
Summary of hormonal control of male reproductive function. Note
that FSH acts only on the Sertoli cells, whereas LH acts primarily
on the Leydig cells. The secretion of FSH is inhibited mainly by
inhibin, a protein hormone secreted by the Sertoli cells, and the
secretion of LH is inhibited mainly by testosterone, the steroid
hormone secreted by the Leydig cells. Testosterone, acting locally
on Sertoli cells, stimulates spermatogenesis, while FSH stimulates
inhibin release from Sertoli cells.
Figure 17–11
physiological
inquiry
Men with decreased anterior pituitary gland function often have
decreased sperm production as well as low testosterone levels.
Would you expect the administration of testosterone alone to
restore sperm production to normal?
Answer can be found at end of chapter.
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