Thymic Regeneration in Lethally X-Irradiated Mice

The effects of a lethal dose of 750 rad of whole-body X-irradiation on the murine lymphomyeloid complex have been investigated. During the first 4 days after exposure there was a marked decrease in the weight of the thymus, spleen, and lymph nodes and in the cellularity of the femoral bone marrow. Subsequently, a phase of thymic regeneration, maximal on the tenth day, was observed in the absence of detectable regeneration elsewhere in the lymphomyeloid complex. The thymus, from the third to the fifth days postirradiation, although very hypocellular, was a site of extensive tritiated thymidine incorporation indicating the presence of a precursor cell population undertaking DNA synthesis. No comparable changes in the tritiated thymidine incorporation of the spleen, lymph nodes or bone marrow were observed. Thymic regeneration in the absence of detectable regeneration elsewhere in the lymphomyeloid complex occurred over a range of doses within the lethal range in several strains of mice.