Which fixative cannot be used with Schiff's reagent and is frequently used for fixation of specimens for electron microscopy, preserving ultrastructure?

• A. Glutaraldehyde
• B. Mercuric Chloride
• C. Formalin Ammonium Bromide
• D. 10% Formalin Saline

Answer: (A)

Schiff's reagent detects aldehyde groups created when tissue carbohydrates are oxidized by periodic acid. If the tissue is fixed with a fixative that uses up or masks those aldehyde groups, Schiff's reaction won’t develop the magenta color properly.

Glutaraldehyde is a dialdehyde fixative that cross-links proteins very rapidly, preserving ultrastructure well for electron microscopy. During fixation, its aldehyde groups participate in crosslinking and become tied up, leaving few free aldehyde groups available for Schiff’s reagent to react with. That’s why glutaraldehyde-fixed tissue is not compatible with Schiff’s reagent.

Other fixatives listed are not as disruptive to the Schiff reaction in typical protocols, or are used for different purposes in EM, so they don’t carry the same incompatibility.