Transit amplifying cells (TACs) are highly proliferative in nature and tend to be sensitive to ionizing radiation. Due to the abundance of TACs that support the elongation of hair shafts, growing hair follicles are highly sensitive to radiation injury. How hair follicles repair themselves after radiation injury is unclear. In this study, we observed that in 4 Gy irradiated mice, hair follicle dystrophy was induced with apoptosis-driven loss of hair matrix cells, which are the TACs that fuel hair growth. The dystrophy was repaired within 96 h without significant hair loss, indicating that a regenerative attempt successfully restored the TAC population to resume anagen growth. Soon after irradiation, mTORC1 signaling was activated in the TAC compartment and its activation was maintained until the regeneration process was completed. Inhibition of mTORC1 by rapamycin treatment increased radiation-induced cell apoptosis, reduced cell proliferation and delayed restoration of Wnt signaling in the hair matrix after radiation injury, leading to prolonged dystrophy and hair loss. These results demonstrate that mTORC1 signaling is activated after irradiation and is required for timely regeneration of the TAC pool of hair follicles, so that hair growth can resume after radiation injury.

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