MOST THYMOCYTES DIE IN THE ABSENCE OF DNA FRAGMENTATION

Most thymocytes are known to be depleted from the thymus during T cell development, with the process of thymocyte death considered to be apo ptosis. In this study we examined the mechanism of thymocyte death in the thymus of 6-week-old mice by using terminal deoxynucleotidyl trans ferase to detect DNA fragmentation or double strand breaks (TUNEL meth od). The TUNEL positive thymocytes were scattered throughout the corte x. Double staining of the section with the TUNEL method and acid phosp hatase (ACP) activity showed that all the TUNEL positive cells were ph agocytosed by ACP positive macrophages. An ultrastructural study revea led the presence of a substantial number of extremely small, unphagocy tosed thymocytes throughout the cortex. These small unphagocytosed thy mocytes were apparently dead cells, as based on several morphological features: 1) The majority were much smaller than red blood cells; 2) t he nuclei were also considerably small; and 3) the extent of chromatin condensation was enormous. Importantly, these unphagocytosed dead thy mocytes were TUNEL negative. These results indicate that: 1) DNA fragm entation, which is detected by the TUNEL method, is not involved in th e cell death process of small unphagocytosed dead thymocytes shown in the present study; and that 2) typical apoptosis, which is characteriz ed by DNA fragmentation, is not the dominant type of cell death in the normal murine thymus. Processes of cell death other than typical apop tosis taking place in most thymocytes require further investigation.