Which stain demonstrates mast cells metachromatically?

• A. Toluidine blue
• B. Hematoxylin
• C. Eosin
• D. Methylene blue

Answer: (A)

Metachromasia occurs when a dye binds to the highly sulfated proteoglycans in mast cell granules, causing a color shift that is different from the dye’s usual hue. Toluidine blue is the classic basic dye that exhibits this metachromatic reaction. When tissue sections are stained with toluidine blue, mast cell granules turn a reddish-purple color while the surrounding tissue remains blue, making mast cells easy to identify. Hematoxylin, eosin, and methylene blue do not produce this distinctive metachromatic change in mast cell granules, so they won’t highlight mast cells in the same way.