Decrease in hematopoietic stem cell domains as a delayed effect of x-irradiation

Although the hematopoietic integrity of locally X-irradiated sites can be restored for a time even after fairly large doses, a secondary aplasia often occurs some months later. To gain further insight into this delayed effect within the framework of the stem cell regulatory domain hypothesis, we characterized the growth kinetics of spleen colony forming units (CFU-S) in WBB6FI-+/+ bone marrow transplanted into WBB6FI-W/WV mice in which one leg had been exposed to 10-30 Gy of X rays 4-5 months previously. Compared to unirradiated contralateral marrow, fewer CFU-S either reached the previously irradiated marrow or were seeded into sites that could support growth. The initial exponential growth of effectively seeded CFU-S was unchanged, but growth deceleration (inflection point) occurred at a lower level of CFU-S in marrow previously irradiated with 20-30 Gy. This change in the inflection point indicates a radiation dose-dependent decrease consistent with the decrease in bone marrow cellularity. The decrease in effective stem cell domains after 20 Gy was calculated to be about 35%. We interpret these results to reflect the highly localized nature of delayed radiation damage to the marrow microenvironment.