Histogenesis of primary localized cutaneous amyloidosis: sequential change of epidermal keratinocytes to amyloid via filamentous degeneration.

Two cases of lichen amyloidosus and 8 cases of macular amyloidosis were examined by electron microscopy. Epidermal keratinocytes showed variable degrees of focal degeneration in the basal or lower Malpighian layer. The primary change was seen in cells which contain fibrillar (30 nm in thickness) cytoplasmic inclusion. The following developments seemed to lead to filamentous degeneration (colloid bodies): (1) aggregation of tonofilaments within the granular or fibrillar cytoplasm, (2) filamentous cells, which are composed of bundles of 7-nm thick filaments surrounded by cell membrane and desmosomes, and (3) filamentous masses composed of bundles or whorls of tightly packed 7-nm thick filaments in the intercellular spaces. At the dermo-epidermal junction, some of the filamentous masses were surrounded by the basal lamina of the epidermis and others were dropping into the dermis. Occasionally, loosened filaments (similar to amyloid filaments) were about to drop into the dermis. Early formation of amyloid islands consisted of electron-dense and electron-light parts. They were located directly beneath the epidermis. In the deeper postion of the papillary dermis and in the upper reticular dermis, the majority were electron-light masses. Electron dense parts were the densely packed 7-nm thick filaments, whereas electron-light parts were the typical straight amyloid filaments. Small tubular filaments were seen in common in the filamentous cells, filamentous masses, and amyloid islands. It is concluded that some of the amyloid substance in primary localized forms of cutaneous amyloidoses derive from the epidermal cells through filamentous degeneration.