Which fixative is least satisfactory for PAS staining due to its chemical properties?

• A. Glutaraldehyde
• B. Osmium tetroxide
• C. Formalin
• D. Acetic acid

Answer: (A)

For PAS staining, you want to oxidize carbohydrate residues to form aldehydes that can then react with Schiff reagent to give color. Fixatives that preserve tissue by heavy crosslinking can block these carbohydrate sites and hinder the oxidation step. Glutaraldehyde, being a dialdehyde, creates extensive crosslinks between proteins and other tissue components. This tight crosslinking masks carbohydrate moieties and reduces their accessibility to periodic acid, so fewer aldehyde groups form and the Schiff reaction is weakened. That makes glutaraldehyde-fixed tissue yield weak or inconsistent PAS staining, making it the least satisfactory option among the choices. In contrast, formalin is a milder fixative that preserves carbohydrate groups more accessibly, osmium tetroxide mainly fixes lipids and tends not to interfere with PAS to the same extent, and acetic acid coagulates without the same level of crosslinking, so they’re generally more compatible with PAS staining.