A mole is a blemish on the skin, which may be present at birth or may develop later. Moles are clusters of nevus cells, which are specialized epithelial cells that contain melanin, causing a darker appearance.
Intradermal or “common” moles (in which the melanin-forming cells are in the lower layer of the skin or dermis) are benign. They are elevated and often have hair in them. These intradermal nevi need not be removed except for cosmetic purposes, as they are not precursors of melanomas.
Another type of mole, the junctional nevi, arises from nevus cells at the junction between the outer layer of skin and the dermis. They are flat and deep brown or even black in color. They are more susceptible to activation, but yet only a tiny percentage becomes malignant. A sudden increase in size or color, or bleeding or ulceration, is an indication for surgical removal.
Moles rarely become malignant, or cancerous. In general, malignant melanomas develop more readily from moles on the lower legs and on mucous membranes than from those elsewhere. Pigmented moles subjected to constant irritation or trauma show a relatively high incidence of malignant changes.
During pregnancy, a benign increase in mole size and color is very common. Large speckled, flat, rough lesions on the exposed parts in the elderly are best removed surgically because of their potential for malignant transformation.