Mast cells are found in the connective tissue and their cytoplasm contains granules (metachromatic) composed of heparin and histamine. Toluidine Blue Stain Kit is designed for staining of formalin fixed, paraffin embedded tissue sections. Mast cells will be stained dark blue/red purple (metachromatic staining) and the background will be stained light blue (orthochromatic staining).