What is a nevus?

What is a Nevus?

A nevus (plural: nevi) is a benign collection of melanocytes—pigment-producing cells—that cluster together in the skin, appearing as a mole or birthmark that can range from flesh-colored to dark brown or blue. 1, 2

Basic Definition and Cell Biology

• A nevus represents a cutaneous lesion where melanocytes group into organized nests, creating visible pigmented spots on the skin or mucosa 1
• These melanocytes are neuroectodermally derived cells that produce melanin pigment and package it into organelles called melanosomes, which are then dispersed to surrounding skin cells 3
• The proposed unifying concept defines nevi as "visible, circumscribed, long-lasting lesions reflecting genetic mosaicism" 2

Types of Nevi

By Timing of Appearance

• Congenital melanocytic nevi (CMN) are present at birth or appear within the first few months of life, occurring in approximately 1-3.6% of newborns 4, 5
• 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 3

By Histologic Location

• Junctional nevus: melanocyte nests located in the epidermis (top layer of skin) 1, 3
• Dermal nevus: melanocyte nests located in the dermis (deeper layer) 1, 3
• Compound nevus: melanocyte nests in both epidermis and dermis 1, 3

By Clinical Subtype

• "Classic" nevi: brown to black macules, papules, patches, or plaques, sometimes with nodules 4, 5
• Blue nevi: gray-blue patches or plaques caused by dermal melanocytes 4, 5
• Nevus spilus: brown patch with overlying darker brown or black spots 4, 5
• Atypical (dysplastic) nevi: nevi with irregular features that can simulate melanoma but are benign 4, 6
• Spitz nevi: benign nevi occurring predominantly in children, with distinct histologic features 4

Clinical Appearance and Natural History

• Nevi vary in color from flesh-colored to light brown, dark brown, reddish-brown, or blue 7
• They range in shape from oval to round and can be flat (macular) or raised (papular/nodular) 7
• Oral melanocytic nevi are uncommon, found in only 0.1% of the population, typically on the palate but occasionally on buccal mucosa, gingiva, or lips 7
• Benign nevi commonly undergo predictable changes over time, including becoming more mottled in pigmentation, more raised or papillated in surface texture, and developing hair growth (hypertrichosis) 4

Relationship to Melanoma

• Nevi are the most important clinical and histologic simulants of melanoma, but can usually be reliably distinguished using established criteria 6
• The lifetime melanoma risk for congenital melanocytic nevi is 0.7-1.7% overall, but increases to 3-8% for giant CMN (>40 cm projected adult size) 4, 5
• Nevi serve as important risk markers—individuals with multiple nevi, especially dysplastic nevi, have greater risk of developing melanoma elsewhere on the body 6
• Despite being vastly more common than melanomas, the rate of progression of individual nevi to malignancy is very low, so prophylactic excision of all nevi is not recommended 6
• Melanoma arising in a preexisting nevus can occur, and when it does, the melanoma should be excised with appropriate margins while also removing the entirety of the residual nevus 4

Common Pitfalls

• Avoid confusing benign changes with malignancy: Normal evolution of nevi includes darkening, lightening, becoming more raised, or developing hair—these do not automatically indicate malignant transformation 4
• Do not perform shave biopsies on suspected melanocytic lesions: This leads to incorrect diagnosis due to sampling error and makes accurate pathological staging impossible 8
• Recognize "nevi of special sites": Nevi on genital skin, acral sites (palms/soles), and flexural areas may show atypical features that mimic dysplastic nevi or melanoma, but are benign and should not be overcalled as malignant 6