Which statement about aldehyde fixatives is true?

• A. They preserve cell ultrastructure and require post-fixation with osmium tetroxide to preserve lipids.
• B. They fix tissues without preserving structure.
• C. They are only used for light microscopy.
• D. They evaporate quickly.

Answer: (A)

Aldehyde fixatives preserve fine cellular structure by forming cross-links between proteins, effectively locking the architecture in place. In electron microscopy, lipids are easily lost during processing, so a post-fixation step with osmium tetroxide is used to stabilize lipids and provide membrane contrast. This combination—aldehyde fixation to preserve ultrastructure followed by osmium tetroxide post-fixation to preserve lipids—is why the statement is true. Aldehydes are suitable for both light and electron microscopy, they do preserve structure, and they do not evaporate quickly under typical lab conditions.