Which fixative is specifically associated with electron microscopy fixation?

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

Answer: (A)

Electron microscopy relies on fixatives that not only preserve tissue structure but also provide electron-dense contrast, especially for membranes. Osmium tetroxide fits this role perfectly because it acts as a heavy-metal postfix fixative that binds to lipids in membranes, stabilizing the lipid bilayers and imparting strong electron density. This makes fine ultrastructural details, such as membranes and organelles, clearly visible under the electron microscope. In contrast, fixatives like Bouin solution or formalin are tailored for light microscopy and protein cross-linking, but they don’t preserve or contrast membranes as effectively for electron microscopy. Cryofixation (frozen) is a separate approach that preserves structure by rapid freezing rather than chemical cross-linking, and isn’t the standard chemical fixative used for routine EM preparation.