Which substances are reported to show positivity with the PAS reaction?

• A. Glycogen, neutral mucins, certain epithelial sulfomucins and sialomucins, colloid material of thyroid and pars intermedia of the pituitary, basement membranes and fungal walls
• B. Lipids and pigments only
• C. Nucleic acids only
• D. Proteins only

Answer: (A)

Periodic acid–Schiff (PAS) staining highlights substances that contain carbohydrate groups because the oxidation of sugars creates aldehyde groups that react with the Schiff reagent to give a magenta color. This makes PAS positive for glycogen and for mucosubstances that have carbohydrate chains, including neutral mucins and certain sulfomucins and sialomucins, which are glycoproteins with carbohydrate side chains. The colloid material in thyroid and in the pars intermedia of the pituitary is rich in glycoproteins, so it also stains PAS positive. Basement membranes are packed with proteoglycans and glycoproteins that are full of carbohydrate residues, and fungal cell walls are composed largely of polysaccharides such as glucans and chitin, which respond strongly to PAS.

In contrast, lipids and pigments do not rely on carbohydrate oxidation for staining, so they are not PAS-positive. Nucleic acids and proteins alone lack the carbohydrate-rich structures that PAS detects, so they are not the typical targets of this reaction.