Which stain reacts with aldehyde groupings in tissue samples?

• A. PAS
• B. Hematoxylin
• C. Alcian Blue
• D. Congo Red

Answer: (A)

Stains that rely on aldehyde groups come from a chemical reaction where carbohydrates are oxidized to reveal aldehydes, which are then targeted by a color-producing reagent. In this stain, periodic acid oxidizes vicinal diols in carbohydrates (like glycogen, mucins, and basement membranes) to form aldehyde groups. The Schiff reagent then reacts with those aldehydes, producing a magenta (pink) color wherever such carbohydrates are present. This makes the stain particularly good for highlighting glycogen, mucopolysaccharides, and mucosubstances in tissue sections.

Other stains use different chemical interactions: Hematoxylin colors nuclei by binding to nucleic acids; Alcian Blue binds acidic mucopolysaccharides through charge interactions rather than aldehyde chemistry; Congo Red binds amyloid fibers and is evaluated for apple-green birefringence under polarized light. So the stain that specifically reacts with aldehyde groupings is the one that uses the periodic acid–Schiff reaction.