Lower proximal cup and outer root sheath cells regenerate hair bulbs during anagen hair follicle repair after chemotherapeutic injury.

The cell dynamics and cell origin for anagen hair follicle (HF) repair following chemotherapeutic injury are unclear. We first mapped the HF response to cyclophosphamide (CYP) at natural anagen VI in mice. We found that 30‐60 mg/kg of CYP leads to dose‐dependent HF dystrophy that was spontaneously repaired with anagen resumption, while 120 mg/kg of CYP prematurely induced catagen/telogen entry. To explore how anagen HF repair is achieved in the dystrophic anagen pathway, we analysed the cell dynamics at 30 mg/kg of CYP. Hair bulbs first shrunk due to matrix cell apoptosis associated with DNA double‐strand breaks. DNA damage was repaired, and ordered hair bulb structures were restored within 96 hours. Bulge stem cells did not undergo apoptosis nor proliferation. K5+ basal lower proximal cup cells and outer root sheath cells quickly replenished the cells in the germinative zone and regenerated the concentric layered structures of the lower HF segment. Therefore, anagen HFs are able to summon extra‐bulge progenitor cells in close proximity to the damaged matrix for quick repair after CYP injury. [ABSTRACT FROM AUTHOR]