Congenital melanoma: a case suggesting rhabdomyogenic differentiation.

A newborn girl had some subcutaneous masses within a giant congenital melanocytic nevus in the left parietal area. Microscopic examination revealed melanoma. The tumor consisted of two different patterns, being composed partly of epithelioid cells and partly of spindle-shaped cells. Immunohistochemically the epithelioid cells showed a positive reaction for HMB-45 and NKI/C3, and a negative reaction for desmin and alpha-sarcomeric actin. The spindle-shaped cells showed a negative reaction for HMB-45 and NKI/C3, and a positive reaction for desmin and alpha-sarcomeric actin. Ultrastructurally the spindle-shaped cells had some schwannian features, namely, a basal lamina, interdigitating cell processes, and junctional complexes. The pathologic findings of our case suggest possible direct rhabdomyogenic differentiation of melanoma cells.