Testicular Architecture Is Critical for Mediation of Retinoic Acid Responsiveness by Undifferentiated Spermatogonial Subtypes in the Mouse

Abstract

SummarySpermatogenesis requires retinoic acid (RA) induction of the undifferentiated to differentiating transition in transit amplifying (TA) progenitor spermatogonia, whereas continuity of the spermatogenic lineage relies on the RA response being suppressed in spermatogonial stem cells (SSCs). Here, we discovered that, in mouse testes, both spermatogonial populations possess intrinsic RA-response machinery and exhibit hallmarks of the differentiating transition following direct exposure to RA, including loss of SSC regenerative capacity. We determined that SSCs are only resistant to RA-driven differentiation when situated in the normal topological organization of the testis. Furthermore, we show that the soma is instrumental in “priming” TA progenitors for RA-induced differentiation through elevated RA receptor expression. Collectively, these findings indicate that SSCs and TA progenitor spermatogonia inhabit disparate niche microenvironments within seminiferous tubules that are critical for mediating extrinsic cues that drive fate decisions.

Revealing cellular and molecular transitions in neonatal germ cell differentiation using single cell RNA sequencing

“…Our study also showed that higher Oct4-GFP was associated with a more differentiated state and that the gradient of Oct4 induction was paralleled by the gradual loss of self-renewal genes and gain of differentiation-promoting genes. Functionally, we showed that Oct4-GFP-high cells displayed stronger RA responsiveness, which shared the functional property of Id4-GFP-dim progenitors as reported previously (Lord et al, 2018).…”

Fsh stimulates Leydig cell Wnt5a production, enriching zebrafish type A spermatogonia

“…Studies in mice have shown that a diet deficient in the retinoic acid precursor vitamin A resulted in the accumulation of undifferentiated spermatogonia (Van Pelt & de Rooij 1990, Li et al 2011. Interestingly, it was found recently that somatic cells protect SSC from exogenous retinoic acid while allowing further differentiated cells to respond to retinoic acid in mice (Lord et al 2018). In zebrafish, exposure to a diet low in vitamin D and also to an inhibitor of Aldh1a2 resulted in lower sperm count and fecundity after 3 weeks (Pradhan & Olsson 2015).…”

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

“…The clear implication is that SSCs remain undifferentiated because they lack RARG. However, a recent report challenged this concept by reporting that RARG was detectable in ID4-EGFP+ spermatogonia in the developing testis (Lord et al, 2018). As previously shown in the adult and developing testis (Helsel et al, 2017), the spermatogonial population labeled by the Id4-Egfp transgene is a mixture of those with the highest regenerative capacity (ID4-EGFP bright ; thus, enriched for SSCs) as well as progenitor spermatogonia (ID4-EGFP dim ).…”

“…We next addressed whether unequal availability of RA to germ cells following subcutaneous injection at a site distant from the testis in vivo underlies the differential ability of spermatogonia to respond to RA. This could be mediated by several factors, including differential RA concentrations throughout the testis or the effect of distinct spermatogonial niche microenvironments, as suggested recently (Lord et al, 2018). We tested the intrinsic, or germ cellautonomous, responsiveness of neonatal germ cells in vitro by culturing freshly isolated testicular single cell suspensions ±RA for 24 h to maximize the uniformity of RA exposure.…”