[Ultrastructural findings in tissue mast cells in urticaria pigmentosa]

In a study on 10 patients suffering from urticaria pigmentosa, biopsies of skin lesions were electron microscopically investigated. In contrast to normal skin, the mast cells showed irregular, partly bilobed nuclei, prominent nucleoli, hypogranulation, and rather small granules; giant granules were rarely observed. In addition, we frequently found degranulation, and the development of microvilli was much more prominent than with normal mast cells. These deviations from normal skin represent at least in part functional alterations. However, some of the findings suggest that urticaria pigmentosa may be considered a neoplastic disease of the tissue mast cells, although clear evidence is still lacking. In any case, our findings show that mast cells in mastocytosis are not identical with those in normal skin.