Mobilizable stem cells: characteristics and replacement of the pool after exhaustion.

Upon i.v. injection of 1.5 mg trypsin per mouse, a constant amount of pluripotent stem cells (CFUs) enters the circulation. These "mobilized" CFUs have higher self-renewal capacity than blood stem cells but lower than bone marrow CFUs. Consequently, they may be regarded as a transit subpopulation between sessile bone marrow and mobile blood CFUs. The mobilizable CFUs pool can be exhausted by rapid mobilization: within 48 h a second challenge can not induce CFUs mobilization. Replacement of mobilizable pool is preceded by an increased proliferation rate of bone marrow CFUs. The findings suggest the existence of a preformed, distinct, easily mobilizable CFUs pool that seems to be more mature than sessile bone marrow CFUs but younger than blood stem cells.