Final answer:
Sustentacular cells, known as Sertoli cells, release the hormone inhibin when sperm count is high, which regulates sperm production through a negative feedback system.
Step-by-step explanation:
When the sperm count is high, sustentacular cells in the testes, also known as Sertoli cells, release the hormone inhibin. The regulation of sperm production involves a negative feedback system, where rising levels of testosterone act on the hypothalamus and anterior pituitary to inhibit the release of gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
When sperm count exceeds a certain threshold, Sertoli cells secrete inhibin which specifically inhibits the release of GnRH and FSH, slowing down spermatogenesis. If the sperm count decreases to below 20 million/mL, Sertoli cells cease the release of inhibin, allowing the sperm count to increase.