The fate of thymic radioactivity after local labeling with 125iododeoxyuridine

The thymic cortex was locally labeled with 125Iododeoxyuridine (125IUdR) in young adult mice in an attempt to provide a simple quantitative assessment of the fate of cortical thymocytes. Similarly operated and nonoperated mice given 125IUdR intravenously were used for comparison. Analogous experiments were performed in adrenalectomized animals. More than 90% of thymic activity present 1 day after labeling had been lost by day 8. That proportion of radioactivity contributed to a given organ by accumulation of labeled thymic migrants was estimated by comparison of values obtained after local labeling with those acquired after systemic labeling. Thymic cell accumulation was apparent in the intestine, spleen, mesenteric lymph node, and femurs of locally labeled mice; however, only a few per cent of the total activity lost from the thymus was accounted for in these lymphoid organs. The pattern of fecal and urinary elimination of 125I did not markedly differ in the various experimental groups, the bulk of the activity being recovered in the urine. The intestine could not be ruled out as a major site of thymocyte loss. Since significant radiation or pharmacologic toxicity was unlikely with the doses of 125IUdR used, the data indicated that the vast majority of newly formed thymocytes dies after a short life and only a small fraction of thymic migrants is longer lived.