Mast cells are common cells in loose connective tissues, often distributed in groups along small blood vessels and lymphatic vessels, and are also commonly found in bronchial tubes and around the interlobular ducts of the pancreas. Generally, the cells are large, with diameters of about 20-30 μm, round or oval, with small nuclei, and cytoplasmic cytoplasm filled with coarse and heterochromatic eosinophilic granules.