Effects of Radiation on The Capacity of The Stem Cell Compartment to Differentiate into Granulocytic and Erythrocytic Progeny

Different methods have been used to measure the survival following radiation of the hematopoietic stem cell pool. Two of these systems measure the stem cell pool by its ability to proliferate and differentiate into mature progeny. In both methods, irradiated recipient mice receive syngeneic bone marrow. A period of time is allowed for the transplanted progenitor cells to divide and differentiate, and then the progeny produced are assayed. Ability to form red blood cells is assessed by the amount of radioactive iron incorporated into newly-formed erythrocytes. Capacity for granulocyte formation is measured by peripheral white blood cell counts following endotoxin stimulation. This latter is a granulocyte response and has been shown to be a measure of the marrow granulocyte reserve. The pool as measured by its ability to produce erythrocytic progeny appears to be more sensitive initially than as measured by its ability to produce granulocytic progeny. Erythropoietic repopulating ability begins recovery more promptly than the granulopoietic. These effects appear to be due to the host milieu rather than any direct effect of radiation on the stem cells, resulting in initial conservation of granulopoiesis relative to erythropoiesis with subsequent compensatory recovery of erythropoiesis. Because of recent evidence suggesting a common stem cell, these results are interpreted as consistent with the notion that radiation affects not only stem cell proliferation, but also the direction and extent of differentiation.