A nevus, commonly known as a mole, is a frequent type of skin growth. These growths are collections of melanocytes, the cells responsible for producing the skin pigment melanin. Nevi are usually benign, meaning they are non-cancerous, but understanding their structure and recognizing changes is important for overall skin health. This exploration will detail the biology of these common skin features, how they are categorized, and the signs that warrant a professional medical check.
What Exactly Is a Nevus?
A nevus is a benign proliferation of melanocytes, the pigment-producing cells derived from the neural crest. Instead of being spread uniformly throughout the skin, these cells grow in clusters or clumps, resulting in a visible spot or small growth on the skin surface. The color of a nevus typically ranges from tan or light brown to dark brown or black, determined by the amount of melanin pigment contained within the clustered cells.
The clustering of cells is the fundamental difference that distinguishes a nevus from a freckle. Freckles are flat spots caused by the increased production of melanin pigment in normal skin cells, often due to sun exposure, rather than an abnormal grouping of the pigment cells themselves. Most nevi are acquired, appearing after birth, generally throughout childhood and adolescence. While new moles can occasionally emerge into adulthood, the total number of nevi typically reaches a peak in young adulthood and may decrease later in life as some moles naturally fade.
How Are Nevi Classified?
Nevi are primarily classified based on when they appear in life and the location of the melanocyte clusters within the layers of the skin. The first distinction separates nevi into two groups: acquired (appearing after birth) and congenital (present at birth or developing within the first year of life).
The more detailed classification is based on the microscopic location of the nevus cells relative to the epidermis and the dermis. A junctional nevus is flat because the melanocyte clusters are located exclusively at the dermo-epidermal junction. As the nevus cells migrate deeper, the lesion transitions into a compound nevus, which is slightly raised and contains cells in both the junction and the dermis.
The final stage in this sequence is the dermal nevus, where all the nevus cells have moved entirely into the dermis. These growths are often dome-shaped, can be flesh-colored or lightly pigmented, and may sometimes have hair growing from them.
Recognizing Concerning Skin Changes
While the vast majority of nevi are harmless, monitoring them for changes is important because melanoma, the most serious form of skin cancer, can develop within an existing mole or appear as a new growth. The established method for recognizing potentially malignant changes in a nevus is the “ABCDE” rule. Understanding these five signs helps distinguish a typical, benign mole from one that requires immediate medical evaluation.
The ABCDE rule identifies concerning features:
- Asymmetry: The two halves of the mole do not match each other in shape or appearance.
- Border: The edges are irregular, ragged, notched, or blurred, rather than smooth and well-defined.
- Color: The lesion contains multiple shades, such as different browns, black, red, white, or blue.
- Diameter: The nevus is larger than 6 millimeters (about the size of a pencil eraser), although melanomas can be smaller.
- Evolving: Any change in the mole over time, including changes in size, shape, color, or height.
Evolving is the most critical sign and also refers to new symptoms like itching, tenderness, or bleeding. Any mole that is new or changing, especially in adulthood, should be checked by a healthcare provider.
Medical Evaluation and Treatment Options
A professional evaluation for a concerning nevus typically begins with a thorough skin examination by a dermatologist. This examination often utilizes a specialized handheld microscope called a dermatoscope, which allows the provider to non-invasively examine the structure and pigmentation patterns beneath the skin’s surface. Dermoscopy helps distinguish benign nevi from early-stage melanoma, often preventing unnecessary removal of harmless moles.
If a nevus exhibits concerning features, the next step is usually a biopsy, where a sample of the lesion is taken for pathological testing. This procedure is the only way to definitively determine if the growth is benign or malignant. If the nevus is confirmed to be melanoma, the recommended procedure is a complete surgical excision, which removes the entire lesion along with a margin of surrounding healthy skin. Benign nevi generally do not require treatment unless removed for cosmetic reasons or if they are in an area prone to irritation.