The niche for spermatogonial stem cells in the mammalian testis.

The theory of the "stem cell niche" was originally proposed for the hematopoietic system, and the existence of the niche as an actual entity was proved in the Drosophila germ cell system. Historically, mammalian spermatogenesis has been studied extensively as a prime example of a stem cell system, and studies have established a stem-progenitor hierarchical order of spermatogonia. In the niche on the basal lamina of seminiferous tubules, spermatogonial stem cells (SSCs) are secluded from the outside world and divide constantly to self-renew and differentiate. During the last 10 years, the development and exploitation of the germ cell transplantation method has expanded our understanding of the nature of SSCs and their niches. The ability to maintain and expand SSCs in vitro, which recently became possible, has further reinforced this research area as a mecca of stem cell biology. Nonetheless, the mammalian germ stem cell and its niche remain to be defined more strictly and precisely. We are still on a journey in search of the real stem cell and its true niche.