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Human melanocytic naevi. I. Electron microscopy and 3-dimensional computer reconstruction of naevi and basement membrane zone from ultrathin serial sections.

Ultrastructural studies were carried out to investigate epidermal and dermal components of human naevi. Serial sections from forty-three acquired human melanocytic naevi were examined in the electron microscope. Control naevi were obtained from guinea pigs, and melanoma from human subjects and guinea pigs. It was found that the epidermal melanocytes which form naevus nests, do not "drop down" and penetrate into the dermis, but remain in continuity with the epidermis. The naevus "nests" actually remain as part of the epidermis and are contained by the basement membrane derived from the epidermal-dermal junction. The three-dimensional aspect of naevus protrusion into the dermis was confirmed by computer reconstruction throughout the ribbons of serial sections for all forty-three biopsies examined. The results indicate that the present differentiation of melanocytic naevi into junctional, compound, dermal and blue naevus is not adequate.