Sertoli and germ cell development in hypogonadal (hpg) mice expressing transgenic follicle-stimulating hormone alone or in combination with testosterone.

We recently created a novel transgenic (tg) model to examine the specific gonadal actions of FSH, distinct from LH effects, by expressing tg-FSH in gonadotropin-deficient hypogonadal (hpg) mice. Using this unique in vivo paradigm, we now describe the postnatal cellular development in seminiferous tubules selectively stimulated by tg-FSH alone or combined with testosterone (T). In the alphabeta.6 line, tg-FSH stimulated the maturation and proliferation ( approximately 2-fold) of Sertoli cells in hpg testes. Total Sertoli cell numbers were also significantly increased (1.5-fold) independently of FSH effects by T treatment alone. Selective FSH activity in alphabeta.6 hpg testes increased total spermatogonia numbers 3-fold, which established a normal spermatogonia/Sertoli cell ratio. FSH also elevated meiotic spermatocyte numbers 7-fold, notably at pachytene (28-fold), but induced only limited numbers of postmeiotic haploid cells (absent in hpg controls) that arrested during spermatid elongation. In contrast, T treatment alone had little effect on postnatal spermatogonial proliferation but greatly enhanced meiotic progression with total spermatocytes increased 12-fold (pachytene 53-fold) relative to hpg testes, and total spermatid numbers 11-fold higher than tg-FSH hpg testes. Combining tg-FSH and T treatment had no further effect on Sertoli or spermatogonia numbers relative to FSH alone but had marked additive and synergistic effects on meiotic cells, particularly pachytene (107-fold more than hpg), to establish normal meiotic germ cell/Sertoli cell ratios. Furthermore, tg-FSH had a striking synergistic effect with T treatment on total spermatid numbers (19-fold higher than FSH alone), although spermatid to Sertoli cell ratios were not fully restored to normal, indicating elevated Sertoli cell numbers alone are insufficient to establish a maximal postmeiotic germ cell capacity. This unique model has allowed a detailed dissection of FSH in vivo activity alone or with T and provided compelling evidence that FSH effects on spermatogenesis are primarily via Sertoli and spermatogonial proliferation and the stimulation of meiotic and postmeiotic germ cell development in synergy with and dependent on T actions.