Rapidly mobilizable stem cells. Do they belong to a special subpopulation?

Some characteristics of rapidly mobilized stem cells as a possible distinct subset of the murine bone marrow stem cell population are overviewed. Some of the agents that rapidly mobilize stem cells are toxic and possibly act through disrupting anchorage to the microenvironment. The mobilization occurring days after cytostatics and/or colony-stimulating factors (CSF), however, is a consequence of increased production or differentiation. While stem cells (colony-forming units-spleen; CFU-S) circulating normally in blood have low self-renewal capacity (SRC), SRC of rapidly mobilized CFU-S is closer to that of bone marrow stem cells and is similar to that of the late mobilized stem cells. The survival rate of mice after transplantation of rapidly mobilized stem cells did not differ from that of bone marrow stem cells. One year after transplantation of rapidly mobilized stem cells, the SRC value of bone marrow did not differ from those transplanted with bone marrow cells. Replacement of a rapidly mobilizable stem cell pool requires 48 h under physiological conditions and a longer time after damage to hemopoiesis (irradiation, hydroxyurea injection). Possible physiological mechanisms in the anchorage of stem cells are discussed.