A transmission electron microscopic study of macromelanosomes in sporadic dysplastic naevi

Melanosomes in sporadic dysplastic nevi were examined by transmission electron microscope. In particular the structure of macromelansomes was observed. Macromelanosomes were present in both nevus cells and keratinocytes. They were round and consisted of a core structure and a cortex structure. The core was composed of electron-dense amorphous material and electron-lucent microvesicles. The cortex contained grains of different sizes and electron densities. Occasionally small melanosomes were observed in the cortex. Electron-lucent microvesicles were also present in small spherical and ellipsoid melanosomes. Melanosome complexes observed in nevus cells contained degrading melanosomes with electronlucent microvesicles, dispersed microvesicles and grains. Melanosome complexes with structures resembling those found in macromelanosomes were also present. The result of the investigation suggests that melanosome complexes found in nevus cells might become macromelanosomes through autophagy.