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Overview
hypo pit axis
  • 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
  • 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 
 

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