Estradiol enhances the stimulatory effect of FSH on testicular maturation and contributes to precocious initiation of spermatogenesis.

Male rats were daily injected with human FSH (hFSH) or estradiol benzoate (EB) or hFSH+EB between day 5 and 15 of life and autopsied on day 16. hFSH accelerated testicular growth, increased number of spermatogonia and serum level of testosterone. hFSH stimulated also spermatogonia differentiation, which resulted in 5-fold increase of the number of spermatocytes. EB given alone induced adverse, inhibitory effects on spermatogenesis and serum testosterone, did not influence serum FSH and LH but increased 14-fold the level of prolactin. Except from testosterone, EB given with hFSH not only overcame inhibitions, but multiplied hFSH stimulatory effects on spermatogenesis up to 30-times of control values. In addition, after FSH+EB premeiotic germ cell ratio reached adult type value precociously. Estradiol may play regulatory roles in testicular maturation (1) inhibitory, direct one or resulting from decrease in testosterone secretion; (2) stimulatory, by enhancement of FSH action, with a possible involvement of prolactin that may act in concert with FSH.