The ultrastructure of congenital nevocytic nevi. II. Nevus cell morphology and contacts with nerve fibers.

As only few previous investigations considered size-dependent differences in the histology of congenital nevocytic nevi (CNN) or focused on their ultrastructure, a light- and electron-microscopic study of six giant and six medium-sized CNN was performed. Histologically, giant CNN showed minimal junctional nevus elements and a pandermal, diffuse nevus cell extension including the subcutaneous tissue, while medium-sized CNN exhibited more abundant junctional nests and a rather focal pattern of nevus cell extension into the reticular dermis. Ultrastructurally, giant CNN often showed nevus cells with irregular and indented nuclei and also nevus cells with highly complicated dendrites. Nuclear inclusions, multiple cilia and centrioles, and collagen phagocytosis in nevus cells were found in both groups of CNN. Direct contacts between nevus cells and nerve fibers were observed in medium-sized CNN only. The findings provided no clear morphological indication why giant CNN have a higher risk for malignant degeneration than medium-sized CNN or acquired nevocytic nevi (ANN). On the other hand, the histological differences speak in favor of a different ontogenesis of the various size classes of CNN and ANN.