Origin of the radioresistant precursor cells responsible for the initial phase of thymic regeneration after X irradiation

The origin of the precursor cells responsible for the initial phase of thymic regeneration after whole-body X-irradiation was investigated. The extent and timing of this phase of thymic regeneration was unaffected by shielding or transplanting substantial amounts of bone marrow, suggesting that the precursor cells responsible for effecting regeneration were not derived directly from the bone marrow. Female albino mice were first exposed to a dose of whole-body X-irradiation (750 rads) which would permit a substantial degree of thymic regeneration on the tenth day post-irradiation then the thymus was exposed to additional doses of local X-irradiation while the remainder of the mouse was shielded. Additional doses of up to 600 rads to the thymus only did not affect the extent of regeneration on the tenth day, suggesting that, in this mouse strain, regeneration was effected by a cell population derived from an extra-thymic source. This experiment was repeated using CBA mice. In CBA mice increasing doses of thymic irradiation decreased the extent of thymic regeneration observed on the tenth day. This could indicate that in CBA mice some of the precursor cells effecting regeneration were intra-thymic at the time of irradiation or, alternatively, moderately high doses of X-irradiation (1,200 rads plus) impaired the intra-thymic proliferation of the progeny of the thymic precursor cell population. Since the initial phase of thymic regeneration was observed in thymus grafts which are repopulated by host bone marrow cells, this indicated that the precursor cell population was ultimately, if not immediately, derived from the bone marrow. The data would appear to be consistent with the hypothesis that the precursor cell population responsible for the initial phase of thymic regeneration after X-irradiation is ultimately derived from the bone marrow, but is in transit to the thymus at the time of irradiation.