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Manipulation of outer root sheath cell survival perturbs the hair‐growth cycle

Transgenic mice that overexpress the anti‐apoptotic gene bcl‐xL under the control of the keratin 14 promoter have significantly shorter hair than non‐transgenic littermates. The deficit in hair length correlated with a decrease in the duration of anagen, the growth phase of the hair cycle. A prolongation in telogen, the resting phase of the hair cycle, was also observed in adult animals. In the developing hair bulb, bcl‐xL transgene expression was observed exclusively in the outer root sheath (ORS) cells. Bcl‐xL expression enhanced the survival of ORS cells treated with apoptotic stimuli. The results suggest that preventing the apoptotic death of ORS cells during anagen leads to a more rapid termination of progenitor cell commitment/proliferation, while the increased survival of ORS cells during telogen delays the initiation of a new hair cycle. ORS cells produce fibroblast growth factor‐5 (FGF‐5), which acts in a paracrine fashion to terminate precursor cell division during anagen. The short hair phenotype of bcl‐xL transgenic mice was substantially reversed in FGF‐5‐deficient mice. Thus, the production of growth inhibitory factors by ORS cells may provide a mechanism through which the hair‐growth cycle is regulated by cell survival.