Figure 1.

Schematic representation of hair follicle morphogenesis and cycling. For follicle induction (1) the important early molecular signals involve the Edar and Wnt-Catenin pathways. The first morphological change is placode formation (2) controlled by a balance between inhibitor and activator molecules. Details of molecular events related to early morphogenesis and differentiation (5) are reviewed in Millar (11) and Schmidt-Ullrich and Paus (105). With time a mature growing hair follicle forms. After that, the hair follicle initiates its cyclic growth, experiencing periods of growth and shaft formation, anagen, of regression, the catagen phase mediated by apoptosis, and then a quiescent or resting phase, telogen, before the start of the next anagen. Many of the molecules involved in morphogenesis are also expressed during the growth cycle of the mature hair follicle.