Mammalian spermatogenesis is a classic adult stem cell system. Identification of a crucialself renewal factor, glial cell line derived neurotrophic factor (GDNF), has provided new opportunities in cell culture systems of testicular cells. Spermatogonia can be maintained for years in culture with GDNF. Transplantation experiments of cultured spermatogonia have shown that they are able to reconstitute all the germ cells with proper programming in the testes, suggesting that the stem cell population of spermatogonia is maintained in vitro for at least, a couple years. Recently, it has alsobeen reported that cultured spermatogonia have the potency to convert to multi-potent stem cells. The rapid progress of in vitro culture systems of spermatogonia makes it possible to apply the culture systems not only to the study of male infertility but also to regenerative medicine.