Histologic sections showed a normal-appearing nail plate and matrix, with rare normal-appearing basal layer melanocytes. The deep papillary and superficial reticular dermis contained a well-circumscribed collection of spindled cells containing copious amounts of coarse melanin granules. Intracytoplasmic melanin obscured the nuclei of the spindled cells. Melanin was also noted outside the cells, lying freely in the dermis and within the cytoplasm of epithelioid cells. Pigment incontinence was present in the dermis in close apposition to the epithelium; however, junctional melanocytic foci or increased basal layer melanocytes were not observed. The sections were subjected to a bleaching reagent, revealing vesicular nuclei with prominent nucleoli within the spindled cells and vesicular nuclei with inconspicuous nucleoli in the epithelioid cells. Special stains were negative for iron (Prussian blue) and positive for melanin (Fontana-Masson). Immunostaining performed with adequate controls revealed a population of spindled cells that were weakly positive for S100 protein and strongly positive for HMB-45, findings that are consistent with dendritic melanocytes, as well as epithelioid histiocytes that were positive for KP-1. The absence of atypical features, such as lesional asymmetry, a proliferation of junctional melanocytes, pagetoid extension, cytologic atypia, or mitoses, made the diagnosis of melanoma unlikely. The microscopic findings were consistent with the diagnosis of common blue nevus of the nail.
Longitudinal Melanonychia—Diagnosis. Arch Dermatol. 2003;139(9):1209-1214. doi:10.1001/archderm.139.9.1209-a