Which fixative is isotonic and suitable for electron microscopy due to its pH of 7.2-7.4?

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

Answer: (A)

Preserving ultrastructure for electron microscopy relies on fixing tissues under near-physiological conditions so structures remain true to life. An isotonic fixative keeps cells from swelling or shrinking during fixation, which is crucial for maintaining membranes, organelles, and fine details that EM reveals. Modified Millonig's Formalin is formalin buffered with phosphate to be isotonic and to maintain a pH around 7.2–7.4. That combination minimizes osmotic and pH-related artifacts, helping to preserve membranes and other fine structures for accurate EM analysis.

The other fixatives either don’t provide consistent isotonic conditions or have properties that can distort ultrastructure for EM: neutralized formalin can alter osmolarity, formalin-alcohol can extract lipids and degrade membranes, and mercuric chloride, while a strong fixative, is not inherently isotonic and carries safety and artifact concerns.