Osmium tetroxide

Osmium tetroxide (OsO4) is a heavy tetrahedral, symmetrical molecule composed of osmium, a third-row transition metal. Osmium tetroxide is both a dye, and a non-coagulant fixative agent in electron microscopy primarily used for the examination of biological tissue components in the histopathology laboratory. Specimens are postfixed with osmium testroxide to ensure preservation of the lipids.

Osmium tetroxide has been used as a fixative reagent for Brevibacterium mcbrellneri, and curcumin treated human lung cancer cell lines′ observation with a scanning electron microscope. It is also suitable as a fixative for ultrastructural analysis of Toxoplasma gondii′s skeletal muscle cells.

Osmium tetroxide chemically combines with lipids making them insoluble; only 7% of the lipids can be extracted after the fixation. Therefore, the cell membranes comprising phospholipids become black and electron-dense after fixation in osmium tetroxide. It is also used to fix small amount of fat maintained in sections during paraffin processing.

Due to the hazardous nature of osmium tetroxide, it is recommended to use an ampule opener when using this chemical.