Why are aldehyde fixatives used in electron microscopy?

• A. To preserve cell ultrastructure; they must be followed by osmium tetroxide fixation to preserve lipids.
• B. They rapidly fix tissues without preparing for EM.
• C. They dissolve lipids to enhance contrast.
• D. They have no effect on ultrastructure.

Answer: (A)

Aldehyde fixatives are used in electron microscopy because they stabilize the protein framework by cross-linking amino groups, which preserves fine cellular ultrastructure and details of organelles and membranes. However, lipids are not well preserved by aldehydes alone, so a subsequent post-fixation with osmium tetroxide is applied to fix and contrast membranes by reacting with lipids. This combination yields well-defined ultrastructure and clear membrane visualization essential for EM images. The other statements don’t fit: aldehydes are not chosen for rapid fixation without EM preparation, they don’t dissolve lipids to enhance contrast, and they do affect ultrastructure, especially when followed by an osmium postfixation.