The relationship between melanocytes and peripheral nerve sheath cells (Part II): blue nevus with peripheral nerve sheath differentiation.

Peripheral nerve sheath differentiation was studied in 120 specimens of blue nevi (112 specimens of "common" blue nevi and 8 specimens of "cellular" blue nevi). In 10 of 112 common blue nevi, fascicles of pigmented dendritic melanocytes or less pigmented spindle-shaped melanocytes with S-shaped nuclei were associated with wavy delicate collagen bundles. In 4 of these 10 specimens, the melanocytes showed a perifollicular arrangement. These nerve fascicle-like structures were seen in some cellular blue nevi (5 of 8 specimens). Structures closely resembling authentic nerve fascicles were not observed in common or cellular blue nevi. The fascicles with S-shaped nuclei and fibrillary collagenous tissue observed in blue nevi (which were well detected in the cellular type but rarely found in the common type) may be peripheral nerve sheath features and perhaps evidence that dermal dendritic melanocytes in the reticular dermis may have arisen within a peripheral nerve sheath milieu from primitive fibroblast-like precursors. Some of the examples presented here may be identical to those reported for "pilar neurocristic hamartoma" or "neurocristic hamartoma." I also speculate on the pathogenesis of blue nevi based on the observations made in this study.