The control of hair follicle growth has been an enigma for cell biologists. While growth factors such as EGF are known for their ability to stimulate proliferation in epithelial cells, EGF causes hair loss. In addition, the downstream signals that control hair growth are largely unknown. Now, an Akt relative called Sgk3 is identified as a hair growth kinase.
Similar to EGF treatment, loss of Sgk3 in mice results in short, thin hair coats due to problems in follicle growth. The engine for hair production is the hair bulb, which is composed of epithelial matrix cells at the follicle base. These cells, derived from follicle stem cells, differentiate to produce the hair. Throughout normal adult life, hair follicles undergo cycles of growth and shrinkage. In the sgk3 mutant, the shrinking phase begins prematurely, before the hair has fully developed.
Alonso et al. find that apoptosis, cell cycling, and differentiation timing are all roughly normal in the sgk3 mutant matrix. Recruitment of progenitors into the matrix, however, seems to be impaired, thus accelerating the exhaustion of the matrix cell supply and initiation of shrinkage. The question now is which compartment uses the kinase to control the maintenance of the matrix. It might be the matrix itself, the stem cell niche, or the outer root sheath connecting the two compartments.