For electron microscopy, what fixative provides superior ultrastructural preservation?

• A. Glutaraldehyde
• B. Formalin
• C. Bouin
• D. Osmium Tetroxide

Answer: (A)

In electron microscopy, maintaining the exact architecture of cellular structures during processing hinges on strong, rapid cross-linking of proteins. Glutaraldehyde achieves this best: its difunctional aldehyde groups form robust cross-links between amino groups on proteins, creating a stable, rigid network that preserves fine ultrastructural details of organelles, cytoskeletal elements, and membranes through dehydration and embedding. Formalin and Bouin fixatives provide slower or less comprehensive cross-linking and can introduce artifacts or shrinkage, making them less ideal for high-resolution ultrastructure. Osmium tetroxide is excellent for fixing and contrasting membranes, but it’s typically used after the initial fixation to enhance lipid preservation and membrane visibility, not as the sole fixative for overall ultrastructural preservation. Therefore, glutaraldehyde offers superior ultrastructural preservation in EM, often followed by osmium tetroxide for membrane contrast.