Which fixation form is most directly associated with crosslinking NH2 groups?

• A. NH2 crosslinking
• B. COOH crosslinking
• C. SH crosslinking
• D. PO4 crosslinking

Answer: (A)

Aldehyde-based tissue fixation works by creating covalent links between amino groups on proteins. Formaldehyde reacts with primary amines (NH2) on proteins to form a hydroxymethyl adduct, and then two such sites can link together to form a methylene bridge, effectively crosslinking neighboring proteins or different parts of the same protein. This direct attack on NH2 groups is why NH2 crosslinking is the correct association for this fixation method—it reflects the primary chemistry that preserves tissue structure.

COOH, SH, and PO4 groups are not the main targets for crosslinking in standard formaldehyde fixation. Carboxyl groups (COOH) and phosphate groups (PO4) are less reactive with aldehydes under routine fixation conditions, and sulfhydryl groups (SH) are not the primary crosslinking partners in this context, so the characteristic crosslinking pattern centers on amino groups.