||Electron microscopy. Mast cells (mastocytes) are oval (12 m) or spindle-shaped and show few thin microvilli at the surface. They are frequently found perivascularly or perineurally within the connective tissue (1). The cytoplasm is provided with a moderate amount of organelles. Most obvious is the presence of granules varying in shape and size. These membrane-bound vesicles (so-called compound granules) show a metachromatic reaction in light microscopy and ultra structurally a granule exhibits a heterogeneous content (different with species) e.g. osmiophilic granular, filamentous, whorl-like substances. The granules contain among others heparin, histamine, enzymes such as superoxide dismutase, beta-hexosaminidase, tryptase, factors such as neutrophil- and eosinophil-chemotactic factors, vasoactive mediators.