In tissues, niche-derived signals promote stem cell self-renewal and the spatially restricted environment shields stem cells from differentiating signals, thus maintaining the stem cell pool. In an open niche environment, such as the seminiferous tubules, where both self-renewal and differentiating signals are ubiquitously distributed, it is unclear how stem cells are maintained in an undifferentiated state. In this study, Shosei Yoshida and colleagues (p. 1582) use lineage-tracing analysis to show that retinoic acid (RA) induces the differentiation of a subpopulation of germ cells marked by NGN3, while another subpopulation (GFRα1+) is maintained. This differential response to RA is explained by the fact that the expression of RARγ, the RA receptor, is restricted to the NGN3+ cell population, thus conferring its differentiation capacity. Interestingly, forced expression of RARγ in GFRα1+cells provided them with the competence to differentiate. This study reveals that the selective response of different germ stem cell populations to RA preserves an undifferentiated stem cell pool, allowing spermatogenesis to persist throughout life.