Ultrastructural features of mast cells in systemic mastocytosis.

A 3-year-old boy with systemic mastocytosis has been observed since the age of 4 months when he was first diagnosed as suffering from urticaria pigmentosa. Involvement of skin, liver, spleen and bones was observed. The electron microscopy of skin and liver revealed varied alterations in the morphology of mast cells. The most important findings were irregularly-shaped cells and unusual long and interdigitated cytoplasmic villi, with consequent aggregation of mast cells which was more prominent in the dermis. Proliferation and accumulation of mitochondria in one part of the cell and deeply indented nuclei were frequent. The problem, whether the morphological changes encountered--especially the complex interdigitation of villi--should be interpreted as a sign of expected neoplastic development, is discussed.