FIG. 1.

Phenotypic and functional heterogeneity among rodent SSCs. Within the pool of A_single spermatogonia, some express only ID4 (red nuclei), while other A_singles also express PLZF (yellow nuclei), suggesting subpopulations of A_singles and SSCs. It is not known whether some A_singles express PLZF only (green nuclei) or neither marker (not shown). It is also not known whether all or some A_single spermatogonia (or cells beyond A_singles) make up the SSC pool. Based on results of in vitro knockdown and in vivo knockout studies that suggest that ID4 participates in SSC self-renewal, Oatley et al. propose that ID4 delineates different functional subpopulations of SSCs (i.e., quiescent and proliferating).