Mast cells stained with Geimsa or Toluidine Blue appear as which color due to metachromasia?

• A. Violet - both metachromatic dyes
• B. Pink
• C. Blue
• D. Yellow

Answer: (A)

Metachromasia is the phenomenon at work here: certain dyes, like toluidine blue and Geimsa, change color when they bind to the highly sulfated granule components in mast cells (such as heparin). Although toluidine blue or Geimsa may appear blue in general staining, the mast cell granules cause the dye to form aggregates that shift the color to violet/purple. So these metachromatic dyes stain mast cell granules a violet hue, distinguishing them from the surrounding blue-stained tissue. The other color options don’t reflect this dye-induced color change seen with metachromasia.