Overview Overview hypothalamic-pituitary axis controls 2 functions of testes spermatogenesis synthesis and secretion of male sex steroid hormones testosterone Hypothalamic-pituitary axis hypothalamus GnRH aka gonadotropin-releasing hormone pulsatile GnRH secretion via preoptic nucleus of hypothalamus GnRH travels in hypothalamic-hypophyseal portal blood system GnRH acts on gonadotrophs in anterior lobe of pituitary GnRH → ↑ FSH and LH (pulsatile) pituitary LH aka luteinizing horome LH → (+) Leydig (interstitial) cells → ↑ testosterone LH acts on Leydig cells increases activity of cholesterol desmolase enzyme increases synthesis and secretion of testosterone via cholesterol testosterone has local (paracrine) and distal (endocrine) effects locally, testosterone acts on Sertoli cells reinforces pro-spermatogenesis action of Sertoli cells FSH aka follicle-stimulating hormone FSH → (+) Sertoli (sustentacular) cells → (+) spermatogenesis FSH acts on Sertoli cells increases synthesis and secretion of androgen-binding protein (ABP) Sertoli cells secrete ABP into luminal space of seminiferous tubule, near developing spermatogonia ABP binds testosterone and maintains high local testosterone levels increases synthesis of growth factors and other products that supports developing spermatogonia and supports spermatogenesis increases synthesis and secretion of inhibin inhibin provides negative feedback of hypothalamic-pituitary axis low inhibin levels are asociated with high levels of FSH Negative feedback testosterone (via Leydig cells) testosterone acts on hypothalamus inhibits GnRH secretion testosterone acts on anterior lobe of pituitary inhibits LH and FSH secretion inhibin (via Sertoli cells) inhibin acts on anterior lobe of pituitary inhibits FSH secretion