A Measure of Nonreparable Injury to Hematopoietic Stem Cells in Rats Exposed Repeatedly to X-Rays

Erythropoiesis essentially ceases in the polycythemic rat. It has been established that stimulation by exogenous erythropoietin is a measure of the capacity of the stem cell pool in this animal to release cells for erythrocyte proliferation. Polycythemic rats that had been previously exposed to 300 R of X-rays, and for which the rate of erythrocyte stem cell recovery had been established, were given a 3-month rest period and then exposed for a second time in one experiment and for a second and third time in another. The results clearly indicated a significant decrease in the rate of erythropoietic recovery after each repeated exposure during the rapid repair phase from day 1 to day 6 postirradiation. Since this is the time when, owing to the sustained injury, the red cell renewal system is stimulated to its fullest, a reduction in the existing cell population capable of responding to erythropoietin would be noticed and would be an indicator of residual injury. It appears, therefore, that the present experiment further substantiates the hypothesis established in an earlier report that residual injury induced in the erythropoietic system by sublethal ionizing radiation was caused by a reduction in the total stem cell space.