The formation of methylene bridges can be reversed using the tap water. Which statement best reflects this?

• A. Tap water can reverse the bridges
• B. Acetic acid reverses the bridges
• C. Ethanol reverses the bridges
• D. Formalin reverses the bridges

Answer: (A)

Formalin fixation creates methylene cross-links between proteins, which stabilizes tissue structure but masks antigen sites. Reversing these cross-links relies on hydrolysis, and water provides the hydrolytic environment needed to break the bonds. When tissue is exposed to water, especially with heat (as in antigen retrieval steps), the methylene bridges can be broken, restoring accessibility to staining targets. Tap water embodies this mild aqueous environment that promotes de-crosslinking without introducing additives that would promote further fixation. In contrast, acetic acid, ethanol, or formalin do not promote reversal of these cross-links in the same way: acetic acid can alter tissues and may not effectively hydrolyze the cross-links; ethanol dehydrates and preserves the cross-links; formalin would continue to fix and form more cross-links.