A disadvantage of osmium tetroxide fixation is that osmium

• A. penetrates poorly
• B. is volatile
• C. is expensive
• D. fixes rapidly

Answer: (A)

Osmium tetroxide is excellent at preserving membranes and providing heavy electron density, but its main drawback is diffusion into tissue. Its large size and chemical properties slow its penetration, so thick specimens become unevenly fixed or require long processing times. This limited penetration can leave deeper regions under-fixed, compromising structural preservation and image quality in electron microscopy. Handling and safety concerns—its volatility and toxicity—exist, and it is costly, but these relate to practical use rather than the fixation performance itself. It also isn’t a rapidly acting fixative, which reinforces why penetration depth is the standout disadvantage.