The hair cycle

The stages of the hair cycle are depicted, starting from the first postnatal anagen, when the hair shaft is growing and protruding through the skin surface. Follicles progress synchronously to the destructive (catagen) phase, during which the lower two-thirds of the follicle undergo apoptosis and regress. The dermal papilla is brought to rest below the bulge-stem-cell compartment, and after the resting (telogen) phase, a critical threshold of activating factors is reached and the stem cells become activated to regrow the hair.