OBSERVATIONS ON THE DEPOSITION OF THOROTRAST IN RAT TISSUES

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Abstract
Young growing rats were injected intravenously (tail vein) with colloidal thorium dioxide (thorotrast, 0.4-0.75 ml.) and the disposition of the material in the body was traced at various times after injection by autoradiographic techniques. In bone, the injected material was filtered from the blood and sequestered by macrophages in the marrow and intertrabecular regions. The distribution of these cells was nonuniform, and the autoradiographic pattern seemed to reflect growth processes. In the metaphysis of the long bone, the labeled cells (in fixed macrophages?) formed a band of high activity which became further removed from the epiphyseal cartilages with time, due to continued endochondral ossification. The rate at which the bones of the thorotrast-treated rats grew in length was normal. Nevertheless, irregular foci of endochondral bone formation were observed due to the formation of plugs of degenerate cartilage in the growth cartilages during the first 6 weeks, and focal hypertrophy of endosteal bone occurred by 12 months. The etiology of the histologic change is obscure.