Hierarchical differentiation competence in response to retinic acid ensures stem cell maintenance during mouse spermatogenesis.

Stem cells ensure tissue homeostasis through the production of differentiating and self-renewing progeny. In some tissues, this is achieved by the function of a definitive stem cell niche. However, the mechanisms that operate in mouse spermatogenesis are unknown because undifferentiated spermatogonia (A_undiff ) are motile and intermingle with differentiating cells in an 'open' niche environment of seminiferous tubules. A_undiff include glial cell line-derived neurotrophic factor receptor α1 (GFRα1)⁺ and neurogenin 3 (NGN3)⁺ subpopulations, both of which retain the ability to selfrenew. However, whereas GFRα1⁺ cells comprise the homeostatic stem cell pool, NGN3⁺ cells showa higher probability to differentiate into KIT⁺ spermatogonia by as yet unknown mechanisms. In the present study, by combining fate analysis of pulse-labeled cells and a model of vitamin A deficiency, we demonstrate that retinoic acid (RA),whichmay periodically increase in concentration in the tubules during the seminiferous epithelial cycle, induced only NGN3⁺ cells to differentiate. Comparison of gene expression revealed that retinoic acid receptor γ (Rarg) was predominantly expressed in NGN3⁺ cells, but not inGFRα1⁺ cells,whereas the expression levels of many other RA response-related genes were similar in the two populations. Ectopic expression of RAR_γ was sufficient to induce GFRα1⁺ cells to directly differentiate to KIT⁺ cells without transiting the NGN3⁺ state. Therefore, RAR_γ plays key roles in the differentiation competence of NGN3⁺ cells. We propose a novel mechanismof stem cell fate selection in an open niche environment whereby undifferentiated cells show heterogeneous competence to differentiate in response to ubiquitously distributed differentiationinducing signals. [ABSTRACT FROM AUTHOR]