Which statement about Osmium Tetroxide is true?

• A. It is used to preserve lipids
• B. It stains nuclei blue
• C. It fixes by cross-linking proteins
• D. It bleaches pigments during processing

Answer: (A)

Osmium tetroxide’s main action is to preserve and contrast lipids. It reacts with the double bonds in fatty acids, oxidizing and binding to them, which stabilizes lipid-containing membranes and makes them electron-dense for better visualization. This lipid-focused activity is why this chemical is used as a post-fixative in tissue processing, especially when lipid preservation is important.

The idea that it fixes by cross-linking proteins isn’t accurate, because that kind of cross-linking is the hallmark of aldehyde fixatives like formaldehyde or glutaraldehyde. Staining nuclei blue is a job for hematoxylin, not osmium tetroxide. While osmium can oxidize certain pigments, its primary, defining role in histology is lipid preservation and membrane contrast, not pigment bleaching.