Which staining reagent is commonly used to demonstrate mast cells via metachromasia?

• A. Toluidine Blue
• B. Giemsa
• C. Gomori's Aldehyde Fuchsin
• D. All techniques demonstrates

Answer: (A)

Metachromasia is when a dye changes color after binding to certain tissue components, such as the sulfated glycosaminoglycans in mast cell granules. Mast cells contain granules rich in substances like heparin, which cause basic dyes to shift color. Toluidine blue is the classic metachromatic stain for mast cells: the granules appear reddish-purple while the surrounding tissue stays blue, producing a strong, easy-to-see contrast that makes mast cells stand out under the microscope. It’s widely used because the reaction is reliable, straightforward, and well established in histology teaching. Other stains can highlight mast cells in various ways—Giemsa can stain granules but not via metachromasia, and Gomori’s aldehyde fuchsin can show metachromatic staining as well but is less commonly used by routine labs.