Complex hair cycle domain patterns and regenerative hair waves in living rodents.

Single hair follicles go through regeneration and involution cycles. In a population, hair follicles may affect each other during anagen re-entry, thus forming propagating regenerative hair waves. We review these regenerative hair waves and complex hair cycle domains, which were recently reported in transgenic mice. Two non-invasive methods to track the propagating hair wave in large populations of hair follicles in vivo are described. We also reviewed early accounts of "hair growth patterns" from classical literature. We decipher the "behavior rules" that dictate how dynamic hair waves lead to complex hair cycle domains. In general, a single domain expands when a regenerative hair wave reaches a responsive region and boundaries form when the wave reaches a non-responsive region. As mice age, multiple hair cycle domains form, each with its own regeneration rhythm. Domain patterns can be reset by physiological events such as pregnancy and lactation. Longitudinal sections across domains show arrays of follicles in a continuum of hair cycle stages. Hair cycle domains are different from regional specificity domains. Regenerative hair waves are different from the developmental wave of newly forming hair follicles. The study provides insights into the dynamic states of adult skin and physiological regulation of organ regeneration.