What is a Nevus?
A nevus is a benign collection of melanocytes (pigment-producing cells) that appears as a visible, circumscribed skin lesion, which can be either congenital (present at birth) or acquired (developing later in life). 1
Basic Definition and Cellular Composition
A nevus represents clustering and disorganization of melanocytes in the skin, appearing as pigmented lesions that can be located in the epidermis, dermis, or both layers 2
More comprehensively, nevi are defined as visible, circumscribed, long-lasting lesions reflecting genetic mosaicism, with melanocytic nevi being the most common type 3
Melanocytes are specialized neural crest-derived cells that produce melanin pigment, which is packaged into melanosomes and distributed to surrounding keratinocytes 4
Classification by Location
Junctional nevus: Melanocyte nests located at the junction between epidermis and dermis 4
Dermal nevus: Melanocyte nests located entirely within the dermis 4
Compound nevus: Melanocyte nests present in both epidermal and dermal locations 5, 4
Congenital vs. Acquired Nevi
Congenital melanocytic nevi (CMN) are collections of melanocytes present at birth or appearing within the first few months of life, with variable extension into adipose tissue, muscles, and around skin appendages 1
CMN occur in <1% to 3.6% of newborns and are categorized by projected adult size (small, medium, large, or giant) 1
Acquired melanocytic nevi (AMN) first appear around 1 year of age, peak in number during the second or third decades, and gradually disappear by the seventh to ninth decades 4
Natural Evolution and Expected Changes
Benign nevi undergo predictable changes over time, including pigmentation becoming more mottled or speckled, surface becoming more raised, verrucous, or papillated, and development of hypertrichosis (increased hair growth) 1, 6
CMN typically grow proportionally with the child's somatic growth, which is a normal finding and not necessarily concerning 1, 6
AMN may appear suddenly or become more prominent in response to sun exposure, hormonal changes during puberty or pregnancy, or immunosuppression 4
Clinical Significance and Melanoma Risk
The vast majority of nevi are benign and stable, with very low rates of malignant transformation 7
The lifetime melanoma risk for all CMN is 0.7% to 2.2%, but increases to 3% to 8% for giant CMN (>40 cm projected adult size) 1, 6
Nevi serve as important melanoma simulants (mimicking melanoma clinically and histologically) and as risk markers identifying individuals at higher risk for developing melanoma elsewhere 7
Dysplastic (atypical) nevi are among the most important melanoma risk markers and simulants, requiring careful differentiation from true melanoma 7
Special Variants
Spitz nevi are benign lesions occurring predominantly in children, though atypical Spitz tumors have uncertain malignant potential and can be difficult to distinguish from melanoma even for experienced pathologists 1
Nevus spilus (speckled lentiginous nevus) consists of multiple pigmented macules or papules within a larger pigmented patch, with slight potential for melanoma development 8
Nevi of special sites (genital, acral, flexural skin) may show atypical features mimicking dysplastic nevi or melanoma but are not risk markers and should not be overcalled as malignant 7
Key Clinical Pitfalls
Not all pigmented lesions are nevi—melanoma can present without a visible epidermal component as a dermal nodule, potentially mistaken for a benign nevus 1
Melanoma arising in CMN can present as deep dermal or subcutaneous nodules without overlying color change, making palpation critical during examination 6
Ablative procedures (lasers, curettage, dermabrasion) should be avoided as they obscure future melanoma evaluation and cause frequent pigment recurrence 6, 5