Hair growth occurs periodically in a cycle that consists of three different phases: growth, regression, and resting. The length of each phase is regulated by both intrinsic and extrinsic factors throughout life, and influenced by physiological and pathological conditions. Elongation of the resting phase and shortening of the growth phase occur during physiological ageing and in baldness, respectively. In vivo discrimination of each phase of the hair cycle can be used to research for regeneration of hair follicles as well as to evaluate the efficacy of hair regrowth treatments in the same individual. Here we show that NG2+ epithelial cells in the hair follicles encompass bulge stem cells, and that the number of hair follicle NG2 cells underwent dramatic changes during the hair cycle. Transgenic rats with expression of firefly luciferase gene in NG2 cells were generated to monitor the hair cycle in vivo. Hair follicle NG2 cells were clearly visualized via bioluminescence imaging to study each phase of the hair cycle in the rats, from infancy to old age.