Which fixative contains formaldehyde?

• A. Helly
• B. Zenker
• C. Bouin
• D. Carnoy

Answer: (A)

Formaldehyde fixes tissue by cross-linking proteins, locking structures in place so cells and nuclei retain their appearance under the microscope. Helly’s fixative is a classic example that includes formaldehyde as its fixing agent, so it contains formaldehyde and will preserve morphology through those protein cross-links. This cross-linking helps maintain overall architecture and nuclear details, which is why formaldehyde-based fixatives are often preferred for routine light microscopy.

In contrast, the other fixatives listed achieve preservation with different chemistry. Zenker’s uses mercuric chloride and potassium dichromate with acetic acid, not formaldehyde. Carnoy’s fixative relies on a strong organic solvent system (ethanol, chloroform, acetic acid). Bouin’s fixative is a picric acid–containing solution that also includes formaldehyde as part of its mixture, though it’s not a pure formaldehyde-based fixative.