Combined melanocytoma-mastocytoma in a case of nodular mastocytosis.

A patient with long-standing nodular mastocytosis developed a slate-blue lesion on the scalp which produced symptoms of histamine release on mechanical and thermal stimulation. Light and electron microscopic examination of the lesion showed zones of mast cells and zones of melanized cells. A transition zone showing cells with dual (melanosomal and mast) granulation, as well as granules representing intergrades between classical mast granules and melanosomes, was present. Features of this lesion add to the accumulated evidence that there is a histogenetic relationship between mast cells and melanocytes.