Melanocytic nevi

Melanocytic Nevi - Moles, Not Monsters

• Benign melanocyte proliferations; nests of uniform cells with regular nuclei & inconspicuous nucleoli.
• Types by nest location:
  • Junctional: Flat macule; nests at dermo-epidermal junction (DEJ).
  • Compound: Raised papule; nests at DEJ and in dermis.
  • Intradermal: Dome-shaped papule; nests only in dermis.
• Dysplastic (Atypical) Nevus: Potential melanoma precursor. Larger (>5 mm), irregular pigment/borders.

⭐ Activating mutations in BRAF or NRAS are common drivers.

Common Acquired Nevi - The Usual Suspects

Benign proliferations of melanocytes (nevus cells) grouped in nests. Evolve through a predictable maturation sequence with age.

• Junctional Nevus

  • Flat (macular), small, brown-to-black.
  • Nevus cell nests confined to the dermo-epidermal junction (DEJ).
  • Typically appear in childhood.

• Compound Nevus

  • Raised (papular), may be pigmented or dome-shaped.
  • Nests at both the DEJ and within the dermis.
  • Common in adolescence and young adults.

• Intradermal Nevus

  • Dome-shaped or pedunculated, often skin-colored or light brown.
  • Nests located exclusively in the dermis.
  • Most common type in adults.

⭐ Nevus Maturation: Over time, nevi transition from junctional → compound → intradermal. This involves nests migrating downwards into the dermis, often leading to a less pigmented and more raised appearance as the nevus ages and undergoes fibrosis.

Atypical Moles - Watchlist Wonders

Clinically and histologically intermediate between a benign nevus and melanoma. While most are stable, they are markers for increased melanoma risk.

• Clinical Features: Often >5 mm, with a flat (macular) and raised (papular) component. Exhibit the 📌 ABCDEs of melanoma concern:
  • Asymmetry
  • Border irregularity
  • Color variegation (e.g., tan, brown, black, pink)
  • Diameter >6 mm
  • Evolving or changing over time

• Histopathology:
  • Architectural Disorder: Asymmetric, poorly circumscribed, nests may bridge between adjacent rete ridges.
  • Cytologic Atypia: Nuclear enlargement, irregular nuclear contours, hyperchromasia.

⭐ Individuals with Familial Atypical Mole and Melanoma (FAMM) syndrome often have mutations in the CDKN2A gene on chromosome 9p21, significantly ↑ melanoma risk.

Nevi vs. Melanoma - Spot The Impostor

📌 ABCDE Rule for differentiating benign nevi from malignant melanoma.

⭐ The most common driver mutation in melanoma is BRAF V600E. This is a key target for therapies like vemurafenib.

High‑Yield Points - ⚡ Biggest Takeaways

• Melanocytic nevi are benign melanocyte proliferations, categorized by location: junctional, compound, or intradermal.
• Junctional nevi are flat macules; compound and intradermal nevi are raised papules.
• Key benign features include symmetry, uniform color, and sharp borders.
• Large congenital nevi carry an increased risk of melanoma.
• Dysplastic nevi are atypical, larger, irregular moles considered melanoma precursors.
• Nevus cell maturation (decreasing size with dermal depth) is a crucial sign of a benign lesion.