The Mesenchymal Niche of the Hair Follicle Induces Regeneration by Releasing Primed Progenitors from Inhibitory Effects of Quiescent Stem Cells.

The mechanisms by which stem cell (SC) quiescence is regulated to allow normal regeneration are poorly understood. Here, we show that the mesenchymal niche of the hair follicle, the dermal papilla (DP), governs the properties of quiescent SCs in the bulge despite its relatively distant location. The DP induces regeneration by downregulating bulge-dependent inhibitory effects that restrain the intrinsic proliferation features of primed progenitors. Once regeneration initiates, the DP orchestrates Shh expression in primed-progenitor descendants by an autoregulatory circuit to restrict Shh expression to the DP vicinity and to confine Shh levels to act only on nearby cells. As the DP moves away from the bulge, quiescent SCs are exposed to Shh transiently. This ensures a short period of quiescent SC activation required for normal regeneration. Furthermore, our findings show that Shh signaling in the DP fine-tunes Wnt signaling activity and reveal the importance of signaling cross talk in coordinating regeneration pace.