2002b) Germ line stem cell competition in postnatal mouse testes

Niche is believed to affect stem cell behavior. In self-renew-ing systems for which functional transplantation assays are avail-able, it has long been assumed that stem cells are fixed in the niche and that ablative treatments to remove endogenous stem cells are required for successful donor engraftment. Our results demonstrate that enriched populations of donor stem cells can produce long-lasting spermatogenic colonies in testes of imma-ture and mature, nonablated mice, albeit at a lower frequency than in ablated mice. Colonization of nonablated recipient tes-tes by neonate, pup, and cryptorchid adult donor spermatogo-nial stem cells demonstrates that competition for niche begins soon after birth and that endogenous stem cells influence the degree and pattern of donor cell colonization. Thus, a dynamic relationship between stem cell and niche exists in the testis, as has been suggested for hematopoiesis. Therefore, similar com-petitive properties of donor stem cells may be characteristic of all self-renewing systems. developmental biology, gamete biology, male reproductive tract, spermatogenesis, testi