Satisfactory electron microscopy can be obtained on a specimen from the wet tissue file if it has been fixed and stored in which solutions?

• A. Millonig formalin or formaldehyde-glutaraldehyde (4CF-1G).
• B. Formalin only.
• C. Osmium tetroxide only.
• D. Bouin's solution.

Answer: (A)

For electron microscopy, preserving ultrastructure depends on fixation that cross-links proteins and stabilizes membranes, while also maintaining a near-neutral pH to prevent artifacts. Millonig formalin is buffered formalin that stays around neutral pH, which helps keep tissue architecture intact during processing. A mixture of formaldehyde and glutaraldehyde (the 4CF-1G formulation) provides strong protein cross-linking, which better preserves fine cellular details than formalin alone. Together, these fixatives yield tissue that remains suitable for the rigorous demands of EM sample preparation.

Bouin’s solution and formalin alone lack the combination of buffering and robust cross-linking that glutaraldehyde offers, which is why they’re less ideal for EM. Osmium tetroxide is used later as a post-fixative to fix lipids and add membrane contrast, rather than as the primary fixative for long-term storage of tissue intended for EM.