Differential RA responsiveness directs formation of functionally-distinct spermatogonial populations at the initiation of spermatogenesis in the mouse

In the mammalian testis, sustained spermatogenesis relies on spermatogonial stem cells (SSCs); their progeny remain as stem cells (self-renewal) or proliferate and differentiate to enter meiosis in response to retinoic acid (RA). Here, we sought to uncover elusive mechanisms regulating a key switch fundamental to spermatogonial fate, the capacity of spermatogonia to respond to RA. Using the developing mouse testis as a model, we found that spermatogonia and precursor prospermatogonia exhibit a heterogeneous capacity to respond to RA with at least two underlying causes. First, progenitor spermatogonia are prevented from responding to RA by catabolic activity of ‘cytochrome P450 family 26’ (CYP26) enzymes. Second, a smaller subset of undifferentiated spermatogonia enriched for SSCs exhibit catabolism-independent RA-insensitivity. Moreover, for the first time, we observed that precursor prospermatogonia are heterogeneous and comprised of subpopulations which exhibit the same differential RA responsiveness found in neonatal spermatogonia. We propose a novel model by which mammalian prospermatogonial and spermatogonial fates are regulated by intrinsic capacity to respond (or not) to the differentiation signal provided by RA prior to and concurrent with the initiation of spermatogenesis.