Radiation sensitivity and cycling status of mouse bone marrow prothymocytes and day 8 colony forming units spleen (CFUs).

Mouse bone marrow prothymocytes as determined in an in vivo thymus regeneration assay have an in vitro gamma radiation sensitivity which is different from that of spleen colony forming cells (CFUs). Determination of Do according to in vivo irradiation revealed similar but insignificant differences. Prothymocytes in normal bone marrow maintain a low but slightly different proliferative state as compared to CFUs, according to determinations using the 3H-TdR suicide technique. In regenerating bone marrow prothymocytes were found to be sensitive to an inhibitory effect of in vitro incubation with cold thymidine. CFUs and normal bone marrow prothymocytes were not affected by cold thymidine. Taking into account the cold thymidine effect it can be concluded that prothymocytes and CFUs in regenerating bone marrow are fully in cycle. These results are best explained when prothymocytes and CFUs are considered to be different cells.