Identification of dynamic undifferentiated cell states within the male germline

The role of stem cells in tissue maintenance is appreciated and hierarchical models of stem cell self-renewal and differentiation often proposed. Stem cell activity in the male germline is restricted to undifferentiated A-type spermatogonia (A_undiff); however, only a fraction of this population act as stem cells in undisturbed testis and A_undiff hierarchy remains contentious. Through newly developed compound reporter mice, here we define molecular signatures of self-renewing and differentiation-primed adult A_undiff fractions and dissect A_undiff heterogeneity by single-cell analysis. We uncover an unappreciated population within the self-renewing A_undiff fraction marked by expression of embryonic patterning genes and homeodomain transcription factor PDX1. Importantly, we find that PDX1 marks a population with potent stem cell capacity unique to mature, homeostatic testis and demonstrate dynamic interconversion between PDX1+ and PDX1- A_undiff states upon transplant and culture. We conclude that A_undiff exist in a series of dynamic cell states with distinct function and provide evidence that stability of such states is dictated by niche-derived cues.