The Morphology and Cell Biology of the Hair Apparatus : Recent Advances

Recent advances in knowledge of the morphology and biology of hair apparatuses are introduced. The hair apparatus morphologically shows a cyclic change "hair cycle" from anagen through catagen to telogen. In anagen, the hair apparatus is composed of eight epithelial cell layers, one of which has been very recently discovered: the innermost cell layer of the outer root sheath. When the ultrastructures of these cell layers are compared with each other, the cells of each layer reveal unique ultrastructural characteristics in differentiation and keratinization. By bromodeoxyuridine stain, the germinative cells in anagen hair and hair follicles can be divided into four groups. The basal lamina in the hair follicle gradually becomes thickened during anagen. In catagen, as the inner hair structures atrophy and retract, the hyaline membrane including the thickened basal lamina becomes corrugated. Immunoblot analyses and immunohistochemical studies using several anti-hair keratin monoclonal antibodies have shown that the cells of the hair shaft and inner root sheath express hard keratin and that the innermost cell layer of the outer root sheath is an independent cell layer. The various anti-keratin monoclonal antibodies that are available are useful for either differentiation of normal skin appendages or diagnosis of skin epithelial tumors. Remaining for further biological and biochemical investigation are the roles of cytoplasmic interfilamentous substances in filament formation in hair cells and the nature of the cell membranes of the hair cells. Proper cell culture methods for the cells deriving from hair apparatuses are required for further advances in studies of hair biology