The Fate of ' Thorotrast ' ( Thorium Dioxide ) injected into the Dorsal Lymph

Thorotrast (a colloidal suspension of thorium dioxide) injected into the dorsal lymph sac of the common frog, Rana temporaria, can be shown by radiological methods to pass through the anterior lymph hearts and so into the blood system. From the blood the thorotrast is removed, as in mammals, by cells of the reticulo-endothelial system and by macrophages, the sites of greatest activity being the liver, spleen, and bone-marrow. Whereas in mammals almost all uptake of such colloidal substances takes place in these organs, in the frog there is considerable macrophage activity elsewhere, notably in the submucosa of the alimentary canal. Our results suggest that macrophages which ingest thorium particles in the wall of the alimentary canal migrate through the blood system to the liver where the thorium is ultimately deposited. This may be correlated with the possession by the frog of a macrophage and reticuloendothelial system less highly organized than that possessed by mammals.