Figure 2. Two-step mechanism of activation.

The sHG and bulge cells are biochemically and functionally distinct populations (35). During late telogen/anagen onset, sHG cells respond first (orange) to signals from the DP to undergo activation and proliferation, which initiates hair growth during the anagen phase. The bulge cells respond by undergoing activation and proliferation later, during the anagen phase (purple). sHG cells are proposed to jump start hair growth, while bulge cells continue to cycle slowly during the anagen phase to extend the ORS and maintain the matrix to support hair growth.