Nevi, commonly known as moles, are common skin growths that appear as small, pigmented spots. They represent benign collections of melanocytes, the specialized cells responsible for producing the pigment melanin that colors the skin. Nevi are usually harmless and generally develop in childhood and young adulthood. Nearly every adult has at least a few nevi.
The Biology of Nevi
The formation of a nevus begins with a localized proliferation of melanocytes, causing them to cluster rather than remaining evenly distributed across the skin’s surface. These clusters, often referred to as “nevus cells,” are derived from the neural crest during embryonic development and are normally found in the basal layer of the epidermis. Most nevi are acquired, meaning they appear after birth, with a peak in development during adolescence and early adulthood.
Acquired nevi are influenced by genetics and environmental factors, particularly sun exposure. Many acquired nevi harbor specific gene mutations that drive the initial cellular growth. Congenital nevi, which are present at birth or develop shortly thereafter, are less common.
The growth of a benign nevus is halted by a process called oncogene-induced senescence, which is a form of permanent growth arrest. This mechanism ensures the melanocyte cluster remains stable and benign, preventing it from progressing into a malignant tumor. The location of the nevus cells within the skin layers also determines the nevus’s appearance and classification.
Classifying Different Types of Nevi
Nevi are classified based on the architectural location of the melanocyte clusters within the skin layers. A Junctional Nevus is flat and typically dark brown because the nevus cells are confined to the dermal-epidermal junction, the boundary between the epidermis and the dermis. These nevi usually have well-defined borders and a uniform color.
A Compound Nevus develops as nevus cells migrate downwards, existing in both the junction and the dermis. This migration causes the nevus to become slightly raised, often appearing as a dome-shaped papule. An Intradermal Nevus is a common type where the cells have completely migrated into the dermis, resulting in a raised, often flesh-colored or lightly pigmented bump.
A clinically distinct category is the Dysplastic Nevus, also known as an atypical mole. These nevi are benign but display irregular features that make them difficult to distinguish from melanoma. Dysplastic nevi are generally larger than common nevi, often exceeding 5 millimeters, and have uneven color and indistinct borders. Individuals with multiple dysplastic nevi have an increased lifetime risk of developing melanoma and require vigilant monitoring.
Monitoring for Change
Regular self-examination of the skin is important for detecting potential skin cancer early. The ABCDE rule is the standard method for monitoring nevi for signs of transformation, focusing on five observable characteristics used to identify moles that may be melanoma.
- Asymmetry: One half of the nevus does not mirror the other half when an imaginary line is drawn through the center.
- Border: Suspicious lesions may have irregular, notched, or blurred borders, unlike the smooth, well-defined edges of benign moles.
- Color: A normal nevus is typically a single shade of brown, while a concerning lesion often has multiple colors, including shades of black, red, white, or blue.
- Diameter: Any nevus larger than six millimeters (roughly the size of a pencil eraser) should be closely inspected.
- Evolving: Any change in the mole’s size, shape, color, or elevation over time. Changes such as itching, bleeding, or crusting of an existing lesion require immediate professional evaluation.
Professional Evaluation and Removal Options
If a nevus exhibits any ABCDE warning signs, a dermatologist will perform a professional evaluation, often utilizing a dermatoscope. This handheld device provides a magnified, illuminated view of the mole’s subsurface structures, helping the clinician identify specific patterns characteristic of benign or malignant lesions. This aids in the decision of whether a biopsy is required.
If the nevus is deemed suspicious, a biopsy is performed to obtain a tissue sample for microscopic examination by a pathologist. The two primary methods for removing nevi are shave removal and surgical excision. Shave removal uses a blade to shave the lesion off at the skin’s surface, often used for raised, benign nevi for cosmetic purposes and usually results in minimal scarring.
Surgical excision is the preferred method for any nevus suspected of being cancerous or highly atypical. This procedure involves cutting out the entire lesion, along with a small surrounding margin of normal skin, and then closing the wound with stitches. Excising the full depth ensures the pathologist can accurately determine the nevus’s architecture and, if it is melanoma, its depth of invasion, which is a determining factor for subsequent treatment and prognosis.