The PAS reaction demonstrates which tissue components?

• A. Polysaccharides and basement membranes
• B. Proteins
• C. Nucleic acids
• D. Lipids

Answer: (A)

The PAS reaction highlights carbohydrate-rich structures in tissue. Periodic acid oxidizes vicinal diols in sugars to aldehydes, and the Schiff reagent then binds these aldehydes to produce a magenta color. In practice, this makes polysaccharides and mucosubstances visible, especially basement membranes that are rich in glycoproteins and proteoglycans. That’s why the PAS reaction demonstrates polysaccharides and basement membranes—the primary carbohydrate-containing components in tissue. Proteins, nucleic acids, and lipids don’t produce the same strong PAS staining unless they’re heavily glycosylated, so they’re not the main targets of this stain. (Glycogen, a carbohydrate, can be distinguished by diastase digestion if needed, since it’s removed by the enzyme.)