In mammalian testis, 'stem cells' that are both capable of self-renewal and differentiation supports the continuity of spermatogenesis. Stem cells are also considered to be crucial not only for steady-state spermatogenesis but also for regeneration after damage or transplantation. However, it is still largely to be elucidated what is the cellular identity and how they behave in the testis. We have been addressing this issue by means of live imaging and pulse labeling of particular subsets of spermatogonia, which enable investigation of the cellular behavior in steady-state spermatogenesis in undisturbed testis. These have shown us, slowly but steadily, how the stem cell functions are achieved: Rather than a very limited number of defined 'stem cells', extended population of undifferentiated cells with different degree of self-renewing and differentiating abilities compose the functional stem cell compartment, and how much the subpopulation of cells is recruited to maintain the stem cell pool is largely influenced by the tissue situation (steady-state, regenerating after damage, or post-transplantation). We also discovered that "stem" spermatogonia are highly mobile and showed a biased localization to the vasculature. In this symposium, I would like to summarize our recent progress in the identity of stem cells and the principle of their behaviors, and the nature of microenvironment provided from seminiferous tubules as well.

References: 

Klein A. M. et al., 2010. Cell Stem Cell 7: 214–224; Nakagawa T. et al., 2010. Science 328: 62–67 ;Yoshida S. et al., 2007. Science 317, 1772–1776