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The process of hair formation involves keratinization. Hair and its sheaths undergo keratinization at different speeds and with different modes. In the hair bulb, the matrix of hair and its follicle are composed of similar-looking immature cells, eg, they all contain a small number of wispy tonofilaments and weakly developed desmosomes. Except for their topographic location, there is no clue as to which cell layer they will eventually form. The cells that are located most centrally, just above the dermal papilla, most likely produce the medulla (Fig. 2-1). A large mass of immature cells located peripheral to the medullary matrix and above the shoulders of the dome-shaped dermal papilla are most likely the matrix cells of the future cortex. Further toward the periphery and down the slope of the dermal papilla are the matrix cells of the cuticle of the cortex, cuticle of the inner root sheath, and inner root sheath (Huxley and Henle). Outermost and close to the bottom of the dermal papilla are the matrix cells of the outer root sheath. As the matrix cells move toward the surface of the skin, more definitive alignment of each layer becomes apparent, the timetable of keratinization of each layer is set, and the matrix cells of each layer follow a distinct mode of keratinization.1–3
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© 1988 Published by Elsevier Inc.