Mast cells in loose connective tissue are identified by metachromatic granules that stain reddish purple with toluidine blue despite the dye being blue. Which biochemical basis explains this metachromasia, and which preformed mediator in these granules acts on endothelial cells?

• A Acid phosphatase polymers change dye color; platelet-activating factor aggregates platelets
• B Neutral proteases bind dye covalently; bradykinin causes vasodilation
• C Phospholipids shift dye spectra; leukotriene C4 induces chemotaxis
• D Sulfated glycosaminoglycans alter dye absorption peak; histamine increases venular permeability ✓

Explanation

Mast cell granules contain highly sulfated heparin glycosaminoglycan whose closely spaced anionic groups shift the absorption maximum of thiazine dyes such as toluidine blue, producing metachromatic reddish purple staining. Histamine, stored preformed in the same granules, binds H1 receptors on venules to increase permeability in immediate hypersensitivity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.