[Amyloid K].

Electron-microscopic, histochemical and immunological studies have shown that the deposits of primary and secondary localized cutaneous amyloid (amyloid K) consist mainly of keratin filament material. The amyloid P component is the second most significant constituent. Pathogenetically, defective keratinocyte apoptosis is the most likely mechanism of generation for amyloid K. Apoptosis is a distinct mode of cell death according to which individual basal keratinocytes disintegrate in physiological and pathological conditions to form membrane-bound "apoptotic" bodies consisting mainly of keratin filament aggregates. These lose their protective membrane and drop into the upper dermis, where they present as keratin (Civatte, cytoid) bodies, which are regularly coated with immunoglobulins. Overproduction of keratin bodies and/or a defect in their degradation by enzymatic digestion or phagocytosis by macrophages and fibroblasts, followed by conversion into amyloid fibrils, may be responsible for the generation of deposits of amyloid K.