Indium and iron as tracers for erythroid precursors.

External visualization and delineation of functional bone marrow is important for diagnostic, prognostic and therapeutic purposes. Because of difficulties in using the isotopes of iron for bone-marrow imaging, indium-111 chloride has been used extensively for this purpose. In this investigation we attempted to solve the problem of cellular localization of indium chloride by employing a rat model with erythropoietic precursors selectively damaged by lethal intracellular radiation from the Auger electrons of Fe-55. In the rat, we have shown that the absolute marrow uptakes of indium and iron are different, whereas the absolute uptakes of indium and sulfur colloid are the same. However, in animals whose erythroid activity was partially destroyed with Fe-55, the fractional depressions of iron and indium uptakes were the same and corresponded to the extent of the remaining erythroid activity. In addition, following an in vitro separation of the cellular elements of marrow with iron carbonyl, both iron and indium were found in the erythroid-rich supernatant, whereas sulfur colloid was in the precipitate. These results indicate that, in the rat, In-111 chloride is an effective in vivo marker for the early phases of iron uptake by the bone marrow.