Methylene bridges are formed during the reaction of certain tissue groups and Formaldehyde. Which group is primarily involved?

• A. -OH
• B. -NH2
• C. -COOH
• D. -SH

Answer: (B)

Methylene bridges form when formaldehyde reacts with amino groups on proteins, creating covalent cross-links that stabilize tissue structure. The primary amino groups (-NH2), such as those on lysine residues and at N-termini, are highly reactive with formaldehyde. The reaction begins with the formation of a hydroxymethyl adduct on the amino group, which can then condense with another amino group to produce a -CH2- linkage between two sites. This cross-linking between protein molecules (or within a molecule) is what creates the methylene bridges seen during fixation. Other groups like -OH, -COOH, or -SH are not as capable of forming these inter-molecular methylene cross-links under standard fixation conditions, so the amino group is the key player.