A connective tissue smear stained with toluidine blue shows scattered cells whose cytoplasmic granules stain purple-red against a blue background. These granules contain heparin and histamine. What property of the granules explains this colour change?

• A Eosinophilia due to basic protein content
• B Argyrophilia due to silver reduction
• C Metachromasia due to highly sulphated glycosaminoglycans ✓
• D PAS positivity due to neutral mucopolysaccharides

Explanation

Mast cell granules are packed with heparin, a highly sulphated glycosaminoglycan. Such polyanionic molecules shift the absorption maximum of basic dyes like toluidine blue, so the granules appear purple-red instead of blue. This property is called metachromasia. Eosinophilia applies to eosinophil granules, argyrophilia describes silver-staining of reticular fibres and neuroendocrine granules, and PAS stains neutral carbohydrates rather than acidic sulphated ones.

Reference: Junqueira's Basic Histology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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