Hair Growth

Hair growth is not continuous but cycles through repeated growing and resting phases. In the growing phase, the proliferation and differentiation of cells in the germinal matrix cause the hair to elongate. In the resting phase, the germinal matrix becomes inactive and may atrophy. The hair detaches from the bulb, moving upward as the external root sheath retracts toward the surface. Eventually, the hair is shed. During the next growing phase, the lower part of the external root sheath grows downward again, either forming a new germinal matrix over the old papilla or stimulating formation of a new papilla. The bulb re-forms, and the next phase of the cycle--proliferation in the matrix and renewed hair growth--begins. Hair growth cycles do not occur synchronously over the entire body surface. Rather, they occur in patches, a pattern called growth in mosaic. Several hormones, especially androgens, influence the pattern of terminal hair distribution and growth rate.