FIG. 3. The three levels of regulation of gene expression in germ cells. An intrinsic genetic program is responsible for regulating gene expression in male germ cells. Germ cells develop in overlapping waves, with cohorts of germ cells developing in synchrony. This requires coordination of the intrinsic programs for the cohorts of germ cells. The Sertoli cells serve a crucial role in this process. The interactive level of influence over germ cell development, in turn, is responsive to extrinsic cues that influence gene expression in Sertoli and Leydig cells, thereby regulating the milieu within which germ cells develop and indirectly influencing gene expression in germ cells. The extrinsic and interactive programs are essential for supporting germ cell development but gene expression is regulated by the germ cells themselves.
