Toluidine Blue staining for mast cells is metachromatic; what color do mast cells appear when stained?

• A. Deep violet
• B. Blue
• C. Red
• D. Green

Answer: (A)

Metachromasia is the key idea here. Toluidine blue is a basic dye that normally stains structures blue, but when it binds to the highly sulfated glycosaminoglycans in mast cell granules, the dye molecules interact and shift color to a purple–violet hue. That is why mast cells stained with toluidine blue appear deep violet, a hallmark of metachromatic staining. Other colors like blue, red, or green don’t reflect this specific dye-substrate interaction in mast cells.