Paracrine control of spermatogenesis.

Spermatogenesis is a complex process that is both remarkable and enigmatic. While circulating hormones clearly play an important role in initiating and regulating the process, the Sertoli cell barrier prevents most substances from entering the seminiferous tubule compartment and directly influencing germ cell development. Therefore, the testis cannot rely solely upon the delivery of circulating hormones, nutrients, and growth factors, but must independently produce its own regulatory substances. A rapidly increasing number of testicular factors that appear to modulate spermatogenesis in a paracrine fashion are now being identified. These discoveries are beginning to contribute to our understanding of the intricate network of testicular cell-cell interactions that control male reproduction.