Mucosal melanosis.

The labial melanotic macule has an increased number of dendritic melanocytes along the basal layer. Numerous counterparts of the labial melanotic macule occur on the other mucosal and cutaneous surfaces. Most melanoacanthomas of the lip show a similar number of melanocytes along the junctional zone. Melanoacanthoma of the lip differs histologically from the labial melanotic macule by the presence of intraepithelial dendritic melanocytes. It also has histologic counterparts that occur on other oral mucosal surfaces. Primary acquired melanosis of the conjunctiva is biologically different from the other mucosal melanoses because it frequently progresses to malignant melanoma. All mucosal melanoses must be taken seriously by the physician, because it is usually not possible to exclude malignant melanoma on clinical inspection alone. A biopsy is always indicated. The labial melanotic macule and melanoacanthoma demonstrate histologic features strikingly different from those of melanoma. Although intraepithelial dendritic melanocytes are seen in melanoacanthoma, they are mostly present in the lower portions of the epithelium, and there is no full-thickness haphazard dispersion of atypical single and nested melanocytes. Along the basal layer in melanotic macules and melanoacanthomas of the lip, melanocytes are arranged as solitary units without a tendency to form abnormal nests or become confluent, in contrast to the picture in melanoma. Melanocytes in this zone show no pleomorphism, in contradistinction to the often bizarre, atypical melanocytes seen in melanoma.