The transition from telogen to anagen occurs when one or twoquiescent stem cells at the base of the telogen follicle, nearthe DP, are activated to produce a new hair shaft (Blanpainet al., 2004
; Tumbar et al., 2004). These cells now begin toproliferate rapidly, and become the transit-amplifying daughtercells that are fated to form the new hair follicle. The newfollicle forms adjacent to the old pocket that harbors the clubhair, which will eventually be shed (exogen). This creates the`bulge' and adds a layer to the stem cell reservoir. The newhair emerges from the same upper orifice as the old hair. Inmany ways, the telogen-to-anagen transition resembles the activationof embryonic skin stem cells that are stimulated to make thefollicle de novo. Signaling by Wnts (Gat et al., 1998; Huelskenet al., 2001; Lo Celso et al., 2004; Lowry et al., 2005; VanMater et al., 2003) and Shh (Callahan et al., 2004; Mill etal., 2003; St-Jacques et al., 1998) is indispensable for newanagen, whereas Bmps (Botchkarev et al., 1999; Kulessa et al.,2000) have been implicated in follicle differentiation. Themolecular steps involved are likely to hold clues to understandingthe activation and specification of stem cells.
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