In electron microscopy fixation, which postfixation agent is used after glutaraldehyde in the protocol?

• A. Osmium tetroxide
• B. Phosphate buffer
• C. Sucrose solution
• D. Glutaraldehyde

Answer: (A)

In electron microscopy, the tissue is first fixed to stabilize overall structure with a protein cross-linker like glutaraldehyde. After that, a postfixation step with osmium tetroxide is used because OsO4 reacts with the lipids in membranes, stabilizing them and adding heavy metal contrast. This makes membranes—such as the plasma membrane, endoplasmic reticulum, and mitochondrial membranes—more electron-dense and clearly visible in TEM images. Glutaraldehyde alone preserves proteins well but doesn’t lock in lipids as effectively, so postfixation with OsO4 is essential for good membrane preservation and contrast.

The other options aren’t postfixatives: a buffer maintains pH, not fixation; sucrose solution may be used for osmotic balance or cryoprotection in other contexts but isn’t a postfixative; repeating glutaraldehyde would reiterate the primary fixation, not provide the lipid stabilization and contrast that OsO4 adds.