A mole, medically termed a melanocytic nevus, is a common, non-cancerous growth on the skin. These lesions form when melanocytes, the pigment-producing cells responsible for skin color, grow in clusters instead of spreading evenly throughout the skin. Most individuals have between 10 and 40 moles that develop primarily during childhood and young adulthood. Moles are classified based on where these cell clusters form within the skin layers, which determines their appearance.
The Three Common Types of Benign Moles
Dermatologists classify common acquired moles based on the depth of the melanocyte nests within the skin’s structure. The three primary types illustrate a natural progression as moles age and migrate deeper into the skin layers. This location determines the mole’s clinical appearance, including its color and elevation.
The first type is the junctional nevus, where melanocyte cells cluster at the junction between the epidermis (outer layer) and the dermis (inner layer). These moles are typically flat (macular) and are often dark brown or black because the pigment is close to the surface. Junctional nevi maintain a regular, round shape and uniform color, usually measuring less than 7 millimeters.
As a mole matures, the melanocyte nests may migrate downward into the dermis, creating a compound nevus. These moles are slightly raised because the cell clusters occupy both the epidermal-dermal junction and the upper dermis. Compound nevi are often lighter brown or tan than junctional moles and may have a smoother, dome-shaped profile.
The final stage is the dermal or intradermal nevus, where the melanocytes reside entirely within the dermis. Because the pigment cells are deep within the skin, these moles are frequently lighter in color, sometimes appearing flesh-toned or pink. Dermal nevi are noticeably raised, often dome-shaped or nodular, and represent the most common type of mole found in adults.
Specific and Atypical Nevi
Beyond the common acquired types, other moles are classified by their origin, unique cell structure, or irregular appearance. Congenital nevi are distinct because they are present at birth or appear within the first few months of life. These birthmarks vary widely in size, and larger congenital nevi carry a slightly elevated lifetime risk of developing into melanoma.
Atypical nevi, also called dysplastic nevi, are important because they can closely resemble melanoma and may indicate an increased melanoma risk, especially when numerous. These moles often display irregular, ill-defined borders and a mix of colors. While most atypical moles remain benign, they are considered precursor lesions that require careful monitoring by a dermatologist.
Less common types include the blue nevus, which gets its characteristic color from deep, densely packed melanocytes that reflect light, appearing blue-gray or blue-black. Another distinct lesion is the Spitz nevus, a benign, dome-shaped growth seen most frequently in children, often appearing pink, red, or brown. Spitz nevi can grow rapidly and sometimes display features that make them difficult for pathologists to distinguish from melanoma.
Distinguishing Moles from Melanoma
The distinction between a benign mole and melanoma, a malignant form of skin cancer, is the most important health consideration. Melanoma is an uncontrolled growth that requires prompt identification and treatment. While a small percentage of melanomas develop from existing moles, most appear as new lesions on previously clear skin.
The most actionable tool for self-examination and monitoring is the ABCDE rule, which highlights five features that differentiate a common mole from a suspicious lesion.
- Asymmetry: If a line were drawn through the mole, the two halves would not match. Benign moles are typically symmetrical and round.
- Border irregularity: The edges of the lesion are ragged, notched, blurred, or poorly defined, unlike the smooth, crisp borders of a common mole.
- Color variation: The mole contains several shades of brown, tan, or black, or includes dashes of red, white, or blue. Common moles usually have a single, uniform color.
- Diameter: Melanomas are typically larger than 6 millimeters (about the size of a pencil eraser) when diagnosed, though they can be smaller in early stages.
- Evolving: Any change in size, shape, color, elevation, or the development of new symptoms like itching or bleeding.
Any mole exhibiting one or more of these ABCDE criteria, or any new spot that looks different from a person’s other moles (the “ugly duckling” sign), warrants immediate evaluation by a healthcare provider.