Hair growth is unique with a cyclical pattern. The follicle is a highly specialized skin appendage with differentiation-restricted expression of intermediate filaments. The repertoire of stem cells has been localized to the Wulst region. The expression of keratin 19 by these cells argues for a flexible state of differention. There is evidence for the downward transition of daughter cells to the bulb where they provide a pool of amplifying cells with a limited number of cell divisions within the matrix. Regulation of hair follicle growth therefore includes (i) the more general aspect of stem cell control (Wulst or bulge region) and (ii) the fine control of matrix growth and hair formation. Yet not all aspects are clear but regulation is offended on different levels: (a) neurohumoral factors (e.g. hormones, neuropeptides, and neuroendocrine cells), (b) immunological factors and vascular factors. The more complete understanding of these mechanisms may offer new therapeutic modalities in the future.
