Which fixative is NOT typically used for electron microscopy?

• A. Bouin Solution
• B. Zambini and osmium tetroxide
• C. Osmium tetroxide
• D. Formalin

Answer: (A)

Electron microscopy relies on fixatives that create strong protein cross-links and preserve membranes, followed by osmium tetroxide to stabilize lipids and provide the contrast needed for ultrastructural detail. Bouin solution is designed for light microscopy and contains picric acid, which can extract lipids and disrupt delicate membranes, making it incompatible with resin embedding and osmium staining. Because of these incompatibilities, Bouin is not typically used for electron microscopy. In contrast, osmium tetroxide is a standard post-fixative in EM to fix lipids and enhance contrast, and fixatives such as Zamboni’s fixative used with osmium tetroxide are compatible with the ultrathin-sectioning process. Formalin is more suited to light microscopy and does not routinely provide the level of ultrastructural preservation required for EM.