Mast cells (Mast cell) is a common cell in loose connective tissue, often in groups along the distribution of small blood vessels and lymphatic vessels, but also commonly found in the bronchial tubes and pancreas around the interlobular ducts, the cells are generally larger, diameter of about 20-30 μm, round or oval, the nucleus is small, the cytoplasm is filled with coarse and heterochromatic eosinophilic particles.

Toluidine blue (Toluidine Blue O) in the cation has a staining effect, the tissue cells of the acidic substances and the cation combination and be stained, toluidine blue also contains two chromophores, can promote the dye ionization into salts, to help the chromophore on the tissue to produce staining power, so that the tissue cells on the section coloring, can be stained nuclei so that it is blue; mast cell particles are purple-red, the others are Blue color.