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ABSTRACT A comparison of the exocolonizing and autorepopulating tests for haemopoietic stem‐cell assay indicate that the ‘overshoot’in splenic colony formation, observed 12–14 days after 150 rad total‐body radiation (TBR), only occurs with the auto‐repopulation assay. The explanation is that the priming dose of 150 rad increases the absolute seeding rate of stem cells from the marrow. A seeding rate significantly greater than normal can ‘take’only if the spleen is available—it can expand and accommodate stem cells while the bone marrow cannot. If, however, the absolute number of colony‐forming cells are decreased in the femur, a relative increase in seeding rate can take place even in the splenectomized animal. Evidence is presented concerning the different turnover states of exo‐ and autorepopulating stem cells (CFU) and those responsible for erythropoietic response (ERC), and the precursors of agar colony‐formers.
Lajtha et al. (Wed,) studied this question.