Ultrastructural study of macromelanosomes in a unique case of spindle and epithelioid cell nevus.

Macromelanosomes in a spindle and epithelioid cell nevus, which occurred in the electron beam-damaged skin of a woman with mycosis fungoides, were ultrastructurally investigated. The melanosomes in the spindle and epithelioid cells were varied in size but uniformly spherical in shape and composed of a core and cortex structure. Electron-lucent bodies were observed in the small melanosomes as well as in the macromelanosomes. Positive dopa-oxidase reaction was revealed on the cortex of these melanosomes and partly on their electron-lucent bodies. The electron-lucent bodies were not vesicular structures but were aggregates of filamentous elements, which had a continuity with fibrous structures in the melanosome cortex. It is concluded that the electron-lucent bodies may be one of the matrices of melanization. Anomalously large accumulations of these bodies and concentrical melanization seem to lead to the formation of macromelanosomes.