Acquired melanocytic nevi will be discussed here. Congenital melanocytic nevi and other benign pigmented lesions are discussed separately. (See "Congenital melanocytic nevi and speckled lentiginous nevi" and "Benign pigmented skin lesions other than melanocytic nevi (moles)".)
Melanocytic nevi are benign proliferations of a type of melanocyte known as a "nevus cell." The two major differences between ordinary melanocytes that reside in the basal layer of the epidermis and nevus cells are:
- Nevus cells cluster as nests within the lower epidermis and/or dermis, whereas epidermal melanocytes are evenly dispersed as single units.
- Nevus cells do not have dendritic processes (with the exception of those within blue nevi).
Both melanocytes and nevus cells are capable of producing the pigment melanin. Melanocytic nevi may be congenital or acquired. Acquired nevi can be classified as common (banal) or atypical, and there are several additional variants, including halo nevi, blue nevi, and Spitz nevi.
Terminology — The names applied to acquired nevi reflect the location of the nests of melanocytes (figure 1). In junctional nevi, the nests of melanocytes are at the dermal-epidermal junction. In compound nevi, the nests of melanocytes are at the dermal-epidermal junction and in the dermis. In intradermal nevi, the nests of melanocytes are in the dermis. With progressive migration of melanocytes from the dermal-epidermal junction into the dermis, nevi become more elevated and less pigmented.