Which fixative would make the Feulgen reaction unsuitable?

• A. Bouin solution
• B. Formalin
• C. Alcohol
• D. Acetone

Answer: (A)

Feulgen reaction detects DNA by converting deoxyribose to aldehyde groups via acid hydrolysis, which then reacts with Schiff’s reagent to give a magenta color. The tissue fixative must preserve DNA without masking or removing it so that those aldehyde groups can form and be detected.

Bouin solution, which contains picric acid and acetic acid (and formaldehyde), is very harsh on nucleic acids. The picric acid component causes strong dehydration and can extract or damage DNA, leading to loss or alteration of the deoxyribose structure. As a result, the necessary aldehyde groups aren’t formed reliably, and Schiff’s reagent won’t produce the magenta stain. This makes the Feulgen reaction unsuitable with Bouin-fixed tissues.

Other fixatives listed are more compatible with preserving DNA for Feulgen staining: alcohol and acetone preserve nucleic acids without heavy cross-linking, and formalin-fixed tissue can still undergo the required hydrolysis and staining with proper protocols.