Hormonal control of spermatogenesis.
<P>
   Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates
   the anterior pituitary to secrete two hormones.
<P>
<BR><LI> Follicle-Stimulating  Hormone  (FSH)  acts on Sertoli cells, which
nourish developing sperm.
Sertoli cells also secrete Inhibin, which reduces FSH secretion by
negative feedback.
<BR><LI> Luteinizing  Hormone  (LH)  acts  on  Leydig  cells, which produce
androgens, chiefly testosterone.
Testosterone   regulates  production  of  the  brain  hormones  by
negative feedback.