The proliferation and differentiation of a stem cell are regulated intrinsically by the stem cell and extrinsically by the stem cell niche. Elucidation of regulatory mechanisms of spermatogonial stem cells (SSCs), the stem cell of the postnatal male germ line, would be facilitated by in vitro studies that provide a defined microenvironment reconstituted ex vivo. We analyzed the effect of in vitro environment on the maintenance of adult and immature SSCs in a 7-day culture system. Allthough the number of adult and immature SSCs decreased in a time-dependent manner, nearly one in four stem cells (24%) could be maintained in vitro for 7 days. Stem cell maintenance was enhanced by coculture with OP9 bone marrow stroma or L fibroblast cell lines, addition of glial cell line-derived neurotrophic factor, or utilization of specific culture medium. In contrast, coculture with TM4 or SF7 Sertoli cell lines and addition of activin A or bone morphogenetic protein 4 (BMP4) reduced stem cell maintenance in vitro. Only 4% of the stem cells remained when cultured with TM4 cells or activin A, and 6% remained when cultured with SF7 cells or BMP4. These results lead to the hypothesis that suppression of germ cell differentiation improves in vitro maintenance of SSCs by interrupting the unidirectional cascade of spermatogenesis and blocking stem cell differentiation.