Which property of Osmium Tetroxide makes it valuable for electron microscopy?

• A. Makes lipids insoluble
• B. Stains DNA strongly
• C. Delays dehydration
• D. Dehydrates proteins

Answer: (A)

In electron microscopy, contrast comes from heavy metal binding to cellular components, with membranes being particularly important due to their lipid content. Osmium tetroxide acts as a fixative and stain that specifically reacts with the unsaturated bonds in lipids. This forms electron-dense osmium-containing compounds, making lipid-rich membranes highly visible under the electron beam. Importantly, this lipid reaction also renders those lipids insoluble in the dehydration solvents used during processing, helping preserve membrane structure and maintain contrast through embedding and sectioning. That lipid-targeted property—making lipids insoluble and electron-dense—is what makes osmium tetroxide especially valuable for TEM. It does not primarily stain DNA, nor does it delay dehydration or dehydrate proteins.