Melanocytic naevus of the nail organ

Naevi may be located at any site in or around the nail organ, and may pose considerable differential diagnostic problems when they cause longitudinal melanonychia. This sign may be due to a focus of functionally active melanocytes (as in ethnic pigmentation), to an accumulation of active melanocytes (as in the Laugier-Hunziker-Baran syndrome), or to a common lentigo, junctional melanocytic naevus, compound naevus or malignant melanoma.

Most naevi are of the junctional type. A few barely visible cells or large numbers of distinctly pigmented melanocytes may be seen singly or in clusters within the basal and suprabasal matrix epithelium. Mitoses are absent. A few melanophages may occur in the upper papillary. The nail plate contains intracellular fine melanin granules. Despite clinically obvious pigmentation, pigment visualization under the microscope often requires staining with the Fontana-Masson argentaffin reaction. Suprabasal location of melanocytes is common in the matrix and nail bed and must not be confused with melanoma.

Congenital and acquired compound naevi of the nail organ are similar to those of the skin.

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