Mast cells are bone marrow-derived cells that are widely distributed in the tissue. They are found predominantly in the subepithelial tissue near blood vessels and nerves and usually are sprinkled diffusely without forming clusters. In tissue sections stained with hematoxylin and eosin, normal mast cells usually display a round-to-oval nucleus with clumped chromatin and indistinct or no nucleoli. They have moderately abundant cytoplasm and are oval, spindle, or polygonal in shape. The cytoplasm is amphophilic, and sometimes small slightly eosinophilic granules may be visible. Hematoxylin and eosin staining is not a specific or reliable method for detecting mast cells in tissue sections because of variable cellular morphology. For confirmation of mast cells, special stains, such as mast cell tryptase or CD117, are required.