Early malignant melanoma manifested as longitudinal melanonychia: subungual melanoma may arise from suprabasal melanocytes.

A 51-year-old woman developed longitudinal melanonychia of 3 months' duration on the right index fingernail. A biopsy specimen revealed that atypical melanocytes were distributed in the lower third of the matrix epithelium but were few in number at the basal layer. The involved nail matrix was resected because of continual growth of the lesion after the biopsy. It has been proven in normal nail matrices that melanocytes are distributed not only in the basal layer but also in the lower half of the epithelium. It is therefore understandable that malignant melanoma of the nail matrix can arise from melanocytes situated in the squamous epithelium above the basal layer. The present case is a good example in which malignant melanoma of the nail matrix may arise from the intraepithelial region where melanocytes normally reside.