The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone. Estrogen has been found to be essential for spermatogenesis in animals.   However, a man with estrogen insensitivity syndrome (a defective ERα ) was found produce sperm with a normal sperm count , albeit abnormally low sperm viability ; whether he was sterile or not is unclear.  Levels of estrogen that are too high can be detrimental to spermatogenesis due to suppression of gonadotropin secretion and by extension intratesticular testosterone production.  Prolactin also appears to be important for spermatogenesis.