Common Acquired Melanocytic Nevi

Common Acquired Melanocytic Nevi
Christine J. Ko, MD
Key Facts
Clinical Issues
  • Majority develop in adolescence
Microscopic Pathology
  • Flat surface to slightly raised to polypoid
  • Epidermis varies (e.g., thin or seborrheic keratosis-like)
  • Well-circumscribed
  • Melanocytes arranged in regular clusters/nests, particularly at junction of epidermis and dermis and superficial dermis
  • Nest defined as 3-5 clustered melanocytes
  • Generally symmetrical from side to side
  • Orderly arrangement of nests at junction and in superficial dermis
  • Mitoses generally absent
  • Melanin pigment often limited to junctional or superficial dermal nests
  • Dermal maturation: Type A nevus cells superficially, type B and C nevus cells with descent into dermis
    • Type A nevus cells: Epithelioid
    • Type B nevus cells: Lymphocytoid
    • Type C nevus cells: Spindled, neuroid
    • May see pseudonuclear inclusion: Lighter staining round area within nucleus
Top Differential Diagnoses
  • Atypical/”dysplastic”/Clark nevus
  • Congenital melanocytic nevus
  • DDx of junctional lentiginous melanocytic nevus
    • Lentigo (simple)
  • DDx of intradermal nevus (especially neurotized)
    • Neurofibroma
This junctional melanocytic nevus is an oval macule of even, light brown pigment. There is overall symmetry with a smooth border; the size was measured to be 2 × 1 mm.
This junctional melanocytic nevus shows nests at the tips and sides of rete ridges. The presence of pigmented parakeratosis and pigment incontinence image likely corresponds to a dark color clinically.
TERMINOLOGY
Synonyms
  • Benign melanocytic nevus, junctional melanocytic nevus, compound melanocytic nevus, intradermal melanocytic nevus, common mole, common melanocytic nevus, nevocellular nevus
ETIOLOGY/PATHOGENESIS
Exact Etiology Unknown
  • Believed by some to arise from intraepidermal melanocytes
  • Others suggest that melanocytic nevi arise from nerves or pluripotential cells
Tumor vs. Hamartoma?
  • Still debated
  • Evidence for tumor
    • Studies showing that some nevi are clonal
    • Growth advantage of nevus cells over epidermal, dendritic melanocytes in cell culture
    • Similar expression of different markers by nevi and malignant melanoma
    • Presence of mutations in BRAF oncogene in majority of nevi
  • Evidence for hamartoma
    • Other proliferative elements: Epidermal, follicular, connective tissue
    • Studies showing that some nevi are polyclonal
CLINICAL ISSUES
Epidemiology
  • Incidence
    • Number of acquired nevi
      • Increases with age up to 3rd decade
      • May be related to familial predisposition
      • Sun exposure in childhood also linked to development of nevi
      • More common in lighter skinned individuals
  • Age
    • Not present at birth
    • Majority develop in adolescence
    • Density of nevi generally decreases after 4th decade
Presentation
  • Junctional lesions
    • Flat to minimally raised macule/very thin papule
    • Generally < 6 mm, but may be larger
    • Color often a variation of brown
  • Compound lesions
    • Slightly raised papule
    • Variable color (brown, flesh-colored, pink)
    • May contain hair
  • Intradermal lesions
    • Papule, may be pedunculated
    • Variable color (brown, flesh-colored, pink)
    • May contain hair
Natural History
  • Abtropfung hypothesis of Unna
    • Melanocytes are initially junctional
    • With chronologic time (aging), melanocytes “drop off” into dermis, creating compound melanocytic lesions
    • With more time, the junctional melanocytes have all “dropped off,” creating intradermal melanocytic lesions
    • Intradermal melanocytic lesions may eventually “shed” or become acrochordons
      • Degenerative changes include balloon cell change and fatty change
  • Alternative hypothesis
    • Nevi begin in the dermis
  • Not all melanocytic nevi display progressive changes; some arrest at a given stage indefinitely
Treatment
  • Not necessary
  • Conservative removal (e.g., shave removal) generally sufficient
    • Residual dark pigment may remain
    • Pigment may recur irregularly within scar (recurrent nevus)
Prognosis
  • Benign
MICROSCOPIC PATHOLOGY
Histologic Features
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Jul 8, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Common Acquired Melanocytic Nevi

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