Mast cells in the skin differentiate from perivascular fibroblasts. The cells nearest the walls of the blood-vessels contain mostly sparse and small mast granules; in those farther removed from the blood-vessels the granules are more numerous and coarse. With weak solutions of toluidine blue, mast granules reveal maximal chromotropy at pH 5-0. At lower pH values not all of the granules stain; at higher ones the granules and the intergranular cytoplasm stain progressively more orthochromatically. After digestion with ribonuclease and staining with toluidine blue buffered to pH 4.0 or 5.0 the mast granules are cherry red and all traces of orthochromatic staining are abolished; when stained at pH 60 or above, however, the cytoplasm and the granules attain a strong blue stain as if they had not been digested in the enzyme. Preparations fixed in Helly's fluid may be washed in running water overnight and the mast granules show no diminution in chromotropy. The same sections may be stained, destained, and stained again at any desired pH with excellent results. Both the cytoplasm and the granules are Schiff-reactive, but the granules stain more intensely than the background. Sections stained with the periodic acid/Schiff technique and subsequently stained with toluidine blue reveal the mast granules brilliantly metachromatic, suggesting that the metachromatic and the Schiff-reactive substances, although coexistent, may be in fact separate elements. Mast granules, according to these tinctorial reactions, then, may contain 4 substances: (a) a protein cytoskeleton stainable with toluidine blue buffered to pH 6.0 or above; (b) some ribonucleic acid removable with ribonuclease and stainable with toluidine blue buffered to pH 5.0 or below; (c) an acid mucopolysaccharide which stains metachromatically; and (d) a Schiff-reactive substance.

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