The relationship between melanocytes and peripheral nerve sheath cells (Part I): melanocytic nevus (excluding...

Among thousands of specimens of melanocytic nevi, not including giant congenital melanocytic nevus or blue nevus, 42 melanocytic nevi that showed peripheral nerve sheath differentiation were collected. The patterns of melanocytic nevi with peripheral nerve sheath differentiation may be classified into three groups: 1) "neurotized and neural nevi" with nests of "neuroid cords" and "nevic corpuscles" (the most common pattern); 2) nerve fascicle-like structures with no relation to neurotized and neural nevi; and 3) palisading melanocytes of a nevus in nests of conventional melanocytic nevi (a rare pattern). Each pattern may represent a different expression of nerve sheath differentiation in melanocytic nevi. Some melanocytic nevi with nerve fascicle-like structures show discrete structures closely resembling authentic nerve fascicles, confirming a close relationship between melanocytes and peripheral nerve sheath cells (Schwann cells and probably perineurial cells in part) and suggesting derivation of the two types of cells from common precursor cells of the neural crest and their de novo development in the dermis rather than by Abtropfung of melanocytes from the epidermis. In addition, the high prevalence of Unna, Miescher, and superficial congenital nevi in melanocytic nevi with peripheral nerve sheath differentiation suggests a different character or process for these congenital melanocytic nevi than for Clark and Spitz nevi (junctional and compound types).