Which tissue component has granules that stain metachromatically?

• A. Mast cells
• B. Adipose tissue
• C. Collagen
• D. Elastic fibers

Answer: (A)

Mast cell granules stain metachromatically because they are packed with highly sulfated glycosaminoglycans, mainly heparin. When a basic dye like toluidine blue binds to these granules, the dye’s color shifts from blue to a purple/violet hue, a phenomenon called metachromasia. This distinctive staining helps identify mast cells in connective tissue. Other tissues listed don’t contain granules with the same highly charged components, so they don’t show the metachromatic color shift: adipose tissue is lipid-rich and stains differently, collagen fibers stain pink with eosin and do not exhibit metachromasia, and elastic fibers are demonstrated with specific stains that highlight fibers rather than producing metachromatic color changes.