||Electron microscopy. Mast cells (mastocytes) are frequently found perivascularly or perineurally. A part of this mast cell shows thin microvilli at the surface and the cytoplasm is provided with a moderate amount of organelles. At (*) the Golgi area (1) is seen in close association with granules that vary in shape and size. These membrane-bound vesicles (so-called compound granules) (2) 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. These structures contain among others heparin, histamine, enzymes such as serine proteases, superoxide dismutase, beta-hexosaminidase, tryptase, factors such as neutrophil- and eosinophil-chemotactic factors, vasoactive mediators, prostaglandin, leukotriene C4 and lymphokines.