The role of bone marrow in different phases of the cellular repopulation of irradiated mouse thymus.

In one series of experiments, the cellular repopulation of the thymus was investigated in mice exposed first to 200 R on the whole body and, after various intervals, to 700 R with one leg protected or unprotected during the exposure. When no protection was made, the mice were transplanted with syngeneic bone marrow cells in a defined number immediately after irradiation. Repopulation was fastest when the interval between exposures was 5 days, and most delayed when it was 14 days; with a 30 day interval the speed of repopulation was intermediate, and resembled that of a control group exposed to only the second dose. In another experimental series, thymus repopulation was studied after exposure of mice first to 200 R with one leg protected or unprotected and, after an interval of 5, 14 or 30 days, to a second dose of 700 R on the whole body. In all cases, syngeneic bone marrow cells were grafted intravenously after the second irradiation. The thymus repopulation was enhanced by protection of the leg when 14 day interval separated the exposures. In the other cases, no enhancement was noted. The findings were interpreted to indicate that the cellular composition of the thymus and, in particular, the frequency of the proliferating stem cells at the time of the exposure determines thymic repopulation for about two weeks after irradiation. After this period, repopulation is due to new precursors from the bone marrow which seeded the thymus.