The hair cycle.

The hair coat, which keeps most mammals warm, dry and protected from harmful elements, requires a constant supply of new hairs throughout the lifetime of the animal. To produce new hairs, existing follicles undergo cycles of growth (anagen), regression (catagen) and rest (telogen). During each anagen phase, follicles produce an entire hair shaft from tip to root; during catagen and telogen, follicles reset and prepare their stem cells so that they can receive the signal to start the next growth phase and make the new hair shaft. The hair cycle represents a remarkable model for studies of the regulation of stem cell quiescence and activation, as well as transit-amplifying cell proliferation, cell-fate choice, differentiation and apoptosis in a regenerative adult epithelial tissue. Here we summarize the major events of the hair cycle, and touch on known regulators of the transitions. Detailed reviews of the hair cycle and its regulation can be found elsewhere (Lavker et al., 2003; Millar, 2002; Muller-Rover et al., 2001).