Melanocytic nevi, or moles, consist of a benign proliferation of melanocytes in the skin. Recall that melanocytes produce the pigment melanin. Melanocytes that are not part of a melanocytic nevus typically reside within the basal layer of the epidermis and are organized in a single layer, whereas melanocytes that are part of a nevus are clustered in nests within the epidermis or dermis. Melanocytes that help form a nevus are also sometimes referred to simply as "nevus cells".
While melanocytes not involved in the formation of nevi are arranged in a single layer along the basal membrane of the epidermis, nevus cells (melanocytes that form part of a nevus) are arranged as clusters or nests along the dermal-epidermal junction and within the dermis. The resulting nevi are referred to as junctional and intradermal nevi respectively.
Junctional nevi, which are composed of nevus cells located at or near the dermal-epidermal junction, are characteristically macular, or flat, and dark in color. The dark pigmentation is conferred by melanin.
Nevi formed by melanocytes clustered together within the dermis, referred to as intradermal nevi, appear grossly as flesh-colored papules. Intradermal nevus cells typically lose their ability to produce melanin, hence therefore they will have a similar pigmentation as the surrounding skin.
Genetics play a large role in the predilection of an individual to develop congenital or acquired melanocytic nevi. Multiple genes have been implicated, such as interferon regulatory factor 4 (IRF4) and telomerase reverse transcriptase (TERT).
Degree of sun exposure during childhood plays a role in the development of melanocytic nevi. Particularly, intense and intermittent exposure is thought to increase incidence.
Although the vast majority of nevi a person has remains benign, each one carries a small potential risk of becoming a melanoma, which is malignant. It is important to remember however that over half of melanomas arise de novo, i.e., not in association with any nevus. Although each nevus carries this small risk, it is not recommended that unremarkable nevi be removed prophylactically. Instead, individuals with many nevi are advised to receive routine full-body skin examinations on a regular basis.
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