Hormonal relationships in the brain-testicular axis. (Green arrows and plus signs indicate stimulatory effects; red arrows and bars indicate inhibitory effects.)

1. GnRH from the hypothalamus stimulates the anterior pituitary to secrete FSH and LH.
   
2. FSH stimulates Sertoli (sustentacular) cells to secrete androgen-binding protein (ABP).
   
3. LH stimulates interstitial cells to secrete testosterone.
   
4. In the presence of ABP, testosterone stimulates spermatogenesis.
   
5. Testosterone also stimulates development of secondary sex organs, and secondary sex characteristics, and stimulates libido.
   
6. Testosterone has a negative effect on the hypothalamus and pituitary, reducing GnRH secretion and pituitary sensitivity to GnRH.
   
7. Sertoli cells secrete inhibin which selectively inhibits FSH without reducing testosterone secretion.