Positive colloidal thorium dioxide as an electron microscopical contrasting agent for glycosaminoglycans, compared with ruthenium red and positive colloidal iron.

In electron microscopy Thorotrast has been used as a specific contrasting agent for acid glycosaminoglycans. Because of its high atomic number, thorium (Z=90) gives good contrast in the electron microscope, but at present it is less frequently used for this purpose. We prepared a positive colloidal solution of ThO2 without stabilizers to compare its properties with those of ruthenium red and positive colloidal iron for contrasting fetal mouse epiphyseal cartilage. The results indicate that colloidal ThO2, which is easy to prepare in any laboratory, gives better results than ruthenium red and colloidal iron do in this kind of cartilage. Furthermore, as judged from data in the literature and obtained in our laboratory, it penetrates this tissue better than Thorotrast does, probably because of the absence of stabilizers.