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. Author manuscript; available in PMC: 2008 May 30.
Published in final edited form as: Annu Rev Cell Dev Biol. 2006;22:339–373. doi: 10.1146/annurev.cellbio.22.010305.104357

Figure 5.

Figure 5

The sonic hedgehog (Shh) signaling pathway during hair follicle morphogenesis and adult hair cycle. (a) Schematic of the Shh pathway. In the absence of Shh, its receptor Patched (Ptch) inhibits Smoothened (Smo) activity. Upon Shh binding, Ptch can no longer repress Smo, which activates the translocation of Gli into the nucleus, allowing it to transactivate its target genes. (b) The role of Shh in the hair follicle. Loss-of-function studies in mice have revealed the importance of Shh in sustaining proliferation in the embryonic and adult hair germ. Gain-of-function studies underscore the striking relation between basal cell carcinomas and deregulation of the Shh pathway. (c) Shh is not expressed in the quiescent bulge stem cells. During hair regeneration, there is a lag before Shh is strongly activated in the developing hair germ. Sustained expression of Shh seems to rely on close association with the dermal papilla (DP). Both in embryonic development and the adult, Shh appears to act downstream of the Wnt/β-catenin signaling pathway. Bu, bulge; HG, hair germ.