Endocrinology of the hypothalamic-pituitary-testicular axis with particular reference to the hormonal control of spermatogenesis.

The normal physiology of the hypothalamic-pituitary-testicular axis in man is reviewed. According to current concepts, LH plays an important role in the initiation and maintenance of spermatogenesis by stimulating Leydig cell production of high concentrations of T within the testes. FSH is thought to be important in spermatid maturation (spermiogenesis) during the initiation of spermatogenesis by stimulation of Sertoli cells. Studies of selective gonadotrophin replacement in experimentally-induced hypogonadotrophic hypogonadal men demonstrate that qualitatively normal sperm production can be achieved by replacement of either LH or FSH alone, but both LH and FSH are necessary to maintain quantitatively normal spermatogenesis. Studies of gonadotrophin replacement in spontaneously-occurring hypogonadotrophic men suggest that the requirement for FSH activity to stimulate sperm production is greatest during the initiation of sperm production at the time of puberty. The initiation of spermatogenesis in postpubertal men with acquired hypogonadotrophic hypogonadism and the maintenance of spermatogenesis after its initiation can often be achieved with LH activity alone.