Ultrastructural Relationship of Melanocytes to Mast Cells and "Melanophages" in a Lesion of Alopecia Mucinosa

Mast cells were observed in the basal layer of epidermis and external hair sheaths in a case of alopecia mucinosa with ultrastructural examination, paralleling the light microscopic observation of cells containing metachromatic granules in these areas. Intermediate cells which combined features of mast cells and melanocytes were also present in focal areas of the basal layer. These were characterized ultrastructurally by the presence of both mast cell granules and melanosomes in their cytoplasm and by the presence of atypical granules combining ultrastructural features of both granule types. An electron density consistent with melanization was noted in some mast cell granules. Cells were present in the dermis containing individual melanosomes in various stages of melanization melanosome complexes, or both individual melanosomes and melanosome complexes. True vacuoles (phagosomes) containing melanin or endocytotic vesicles containing melanin were not observed. Melanosomes in the intercellular spaces were extremely rare. These observations support the hypotheses a) that mast cells and melanocytes may be histogenetically related, and b) that at least some “melanophages” are melanocytes in which melanosome complexes form by intracellular reorganization