James L.Campbell Jr., M.D., M.S.
Adjunct Assistant Professor of Medicine (Dermatology), Dartmouth Medical School
Nevi, or moles, are benign tumors composed of nevus cells that are derived from melanocytes. Nevi exist in a variety of characteristic forms that must be readily recognized to distinguish them from malignant melanoma. Except for certain types, such as large congenital nevi and atypical moles, most nevi have a very low malignant potential. Nevi vary in size, shape, surface characteristics, and color. The important fact to remember is that each individual nevus tends to remain uniform in color and shape. Although various shades of brown and black may be present in a single lesion, the colors are distributed over the surface in a uniform pattern.
Malignant melanoma is a malignancy of melanocytes that occurs in the skin, eyes, ears, gastrointestinal tract, leptomeninges, and oral and genital mucous membranes. Malignant pigment cells grow and extend with little constraint through the epidermis and into the dermis. Such unrestricted growth produces a lesion with a haphazard or disorganized appearance, which varies in shape, color, and surface characteristics. One of the most dangerous tumors, melanoma has the ability to metastasize to any organ, including the brain and heart.