Which polymerized form of formaldehyde is used as a fixative?

• A. Paraformaldehyde
• B. Formalin
• C. Formaldehyde gas
• D. Glutaraldehyde

Answer: (A)

Paraformaldehyde is the polymerized form of formaldehyde used as a fixative. It can be depolymerized to formaldehyde in solution, delivering a steady release of formaldehyde that crosslinks tissue proteins to preserve structure. This makes paraformaldehyde (often used as 4% paraformaldehyde in buffer, i.e., PFA) a common choice when gentle, well-preserved morphology is needed, and especially when antigen preservation is important for immunohistochemistry. In contrast, formalin is simply an aqueous stabilizer of formaldehyde, formaldehyde gas is the vapour form, and glutaraldehyde is a different aldehyde with distinct fixation properties (and is not the polymerized form of formaldehyde).