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The morphology and prevalence of different forms of cell degeneration in hair follicles in acute alopecia areata were investigated. In addition to apoptosis and necrosis, a third morphological pattern of cell degeneration, dark-cell transformation, was evident. Fifteen patients with untreated acute alopecia areata and three normal adults without hair loss were studied. Electron microscopy revealed that although apoptosis of outer root sheath keratinocytes produces normal hair follicle involution (catagen), increased levels of apoptosis. necrosis, and darkcell formation appear to be related to the pathology of alopecia areata. Although cell degeneration was generally restricted to keratinocytes of the lower follicle, melanocytes, Langerhans' and dermal papilla cells were also affected. Keratinocytic degeneration may affect layers of matrix cells in alopecia areata, unlike the apoptosis of scattered outer root sheath cells in normal catagen. The extent of cell death suggests a pathological rather than a physiological event in alopecia areata.