Osmium tetroxide is commonly used in electron microscopy but has limited tissue penetration. Which statement is true?

• A. It penetrates tissues rapidly
• B. It penetrates poorly
• C. It preserves lipids poorly
• D. It fixes proteins rapidly

Answer: (B)

Osmium tetroxide is a postfix fixative and stain that binds strongly to unsaturated lipids in membranes, giving membranes strong electron density for electron microscopy. Its diffusion into tissue is slow because the molecule is relatively large and hydrophobic, so penetration into thicker specimens is limited. In practice, this means OsO4 works best on small blocks or thin sections, after initial fixation, rather than rapidly penetrating whole, thick tissues. It primarily fixes lipids and preserves membrane structure rather than fixing proteins quickly, so statements about rapid protein fixation or poor lipid preservation aren’t accurate. Therefore, the true statement is that it penetrates poorly.