Osmium dependent argentaffin staining of lysosomes

Lysosomes stain with the argentaffin reaction after fixation with glutaraldehyde followed by osmium tetroxide. The reaction works well both at the level of the light and electron microscope. Control experiments show that this argentaffinity is caused by reduced osmium tetroxide. No staining could be observed in freeze-dried material, in tissues fixed only with glutaraldehyde, or after bleaching of the sections with hydrogen peroxide solutions. In the electron microscopy, the population of lysosomes appears heterogeneous as related to the density of silver deposits over the organelles. No correlation is found between size and argentaffinity of lysosomes. X-ray microanalysis of sections from glutaraldehyde/osmium tetroxide fixed material reveals significantly higher amounts of osmium in lysosomes, as compared to other cell organelles (e.g. peroxisomes or mitochondria). A significant peak for silver is observed in lysosomes after treatment of the sections with ammoniacal silver solution, whereas the signal for osmium is reduced. Amounts of sulphur are too low to be detected in lysosomes. It is concluded that argentaffin staining of lysosomes is an osmium dependent reaction.