The amount of mobilizable stem cells in perturbed hemopoiesis.

The level of mobilizable 9-day colony-forming units (CFU-s), which represents a constant fraction of the normal mouse bone marrow CFU-s pool, was assayed in BDF1 mice with perturbed hemopoiesis (i.e., during increased turnover of CFU-s or increased CFU-s traffic after irradiation). After low-level irradiation, regeneration of the mobilizable CFU-s fraction was significantly slower than that of bone marrow CFU-s. Depletion of the mobilizable CFU-s pool was observed if a permanently increased outflow of CFU-s from the bone marrow was induced by endotoxin injection. After 40% withdrawal of the blood volume, the mobilizable CFU-s pool expanded marginally. Assuming that the level of mobilizable CFU-s is a consequence of production and outflow from the bone marrow compartment, changes in the pool size of mobilizable CFU-s may be a sensitive indicator of balanced or unbalanced hemopoiesis.