Which fixative is isotonic with pH 7.2-7.4 and can be used for electron microscopy?

• A. Modified Millonig's Formalin
• B. 10% Neutralized Formalin
• C. Formalin Ammonium Bromide
• D. Mercuric Chloride

Answer: (A)

Osmolarity and pH stability of a fixative directly affect tissue morphology, especially for electron microscopy where ultrastructural detail must be preserved. A fixative that is near physiologic pH (about 7.2–7.4) and is isotonic with tissue helps prevent osmotic shrinkage or swelling that would blur fine cellular features.

Modified Millonig's Formalin provides that balance. It is formalin buffered with phosphate to a pH around 7.2–7.4 and formulated to be isotonic with tissue, reducing osmotic artifacts and preserving ultrastructure well enough for electron microscopy. The other options either don’t guarantee isotonicity at that pH, or involve fixatives that are not optimized for preserving ultrastructure in EM or come with disadvantages (toxicity, artifacts, or incompatibilities). Thus, this fixative best meets the need for an isotonic, near-neutral fixative suitable for electron microscopy.