Melanoacanthoma (melanoacanthosis) is a rare, probably reactive, proliferation of both keratinocytes and melanocytes [61, 64]. It is seen most commonly in adult black females, typically involving the labial or buccal mucosa. Lesions may be single or multiple [49] and are usually macular; less frequently, they are slightly raised or papilliferous. Trauma is thought to be the most likely cause and lesions can regress spontaneously or following incisional biopsy.

Microscopy shows acanthosis and frequently spongi-osis that can be florid. There are strongly Fontana silver stain- and HMB45-positive, dendritic melanocytes extending throughout the full thickness of the epithelium (Fig. 3.13). As a consequence of a partial or complete block in pigment transfer the keratinocytes in melano-acanthoma contain little or no melanin in spite of the abundance of melanocytes.

Fig. 3.13. Melanoacanthoma showing dendritic melanocytes extending throughout the epithelial thickness. Inset shows melanocytes stained with Masson-Fontana

form brown, bluish or black macules or sessile papules. Most are less than 6 mm in diameter [24]. Intramuco-sal (intradermal) naevi account for over half of the cases and junctional or compound naevi are uncommon. Blue naevi form 25-35% of cases and usually present in the palate. The large majority are of the common rather than the cellular type.

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