Final answer:
Follicle-stimulating hormone (FSH) directly stimulates Sertoli cells to secrete androgen-binding protein (ABP), which is essential for spermatogenesis by maintaining high testosterone concentration in the testes.
Step-by-step explanation:
The hormone which directly stimulates sustentacular (Sertoli) cells to secrete androgen-binding protein (ABP) is follicle-stimulating hormone (FSH). The regulation of hormone production for reproductive function involves a coordinated effort between the hypothalamus, pituitary gland, and the testes. When the gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, it triggers the anterior pituitary gland to release FSH and luteinizing hormone (LH). FSH then binds to Sertoli cells in the seminiferous tubules of the testes, encouraging them to secrete ABP, which binds to testosterone, maintaining its high concentration needed for spermatogenesis. LH, on the other hand, stimulates Leydig cells in the testes to produce testosterone.