Absence of catagen/telogen phase and loss of cytokeratin 15 expression in hair follicles in lichen planopilaris

Damaged follicles that have lost their CK15+ stem cells disappear when they enter catagen phase. CK15+ stem cell loss explains the clinical observation that LPP progresses despite immunosuppressive therapies. Finally, the absence of catagen/telogen hair follicles is a helpful diagnostic clue for LPP.