Fate of Thymocytes: Studies With ¹²⁵i‐iododeoxyuridine and ³h‐thymidine in Mice

Cortical thymocytes of young adult mice were labeled in situ with radioactive DNA precursors. As a result of cell emigration and cell death, total thymic radioactivity decreased within 8 days to 10% or less of that present on day 1. Accumulation of thymic migrants in peripheral lymphoid organs was estimated by computing the net thymus‐derived radioactivity in these tissues. Thymic cell death was assessed by comparing values obtained with 125 I‐UdR to those acquired with 3 H‐TdR. The results indicate that cortical thymocytes migrate to the spleen, mesenteric lymph node, femurs and intestine; nevertheless, only a small fraction of the activity originally present in the thymus was recovered in these organs; the vast majority of newly formed cortical thymocytes apparently die after a relatively short life span. Exclusive of the fraction which dies in situ , evidence for thymocyte death is seen in bone marrow; however, most migrants appear to terminate in the intestine.