Which fixative is compatible with osmium for secondary fixation in electron microscopy?

• A. Zamboni's solution
• B. Bouin's solution
• C. Zenker's solution
• D. Orth's solution

Answer: (A)

In electron microscopy, the post-fixative step with osmium tetroxide is used to stabilize and stain lipids in membranes, so the preceding fixative should crosslink proteins well without removing or interfering with lipids or reacting unfavorably with osmium. Zamboni's fixative fits this role well because it is an aldehyde-based mixture that provides strong protein crosslinking while compatible with subsequent osmium postfixation, preserving membrane structure and ultrastructure cleanly for good osmium staining and visualization.

Bouin's solution contains picric acid, which can cause lipid extraction and unpredictable interactions with osmium, leading to artifacts. Zenker's solution includes heavy metals and chromates that can interfere with osmium staining and introduce artifacts. Orth's solution, while used in some contexts, is not as consistently compatible with osmium postfixation for preserving membrane detail as Zamboni's fixative.