Figure 2.

The hair “cycle.” Schematic diagram of the three main phases of hair cycle: the growth phase (anagen), the dystrophic phase (catagen), an extremely shortened resting phase (telogen), and the ‘shedding’ of the hair (exogen). In anagen, the hair bulb is located deep inside the skin and hair grows towards the skin surface. The dermal papilla survives catagen and moves upward to the lowermost portion of the bulge, which then forms the secondary germ at its base during telogen. In telogen, the hair falls out and the hair bulb relocates down again as the new hair grows. At their cycle end, telogen HFs can be activated through mechanical depilation, pharmacologically, and by specific signaling factors (e.g., Wnt signaling), which stimulates a return to anagen and the generation of the new lower follicle and hair shaft. As the new hair grows in, the old hair is shed during exogen. The duration of each phase depends on the type, site and specific genetic programming of the follicle.