Stem cell proliferation: ex vivo and in vivo observations.

The contribution of stem cells to the early phases of hematopoietic engraftment is controversial, as is the issue of the number of stem cells required for successful long-term engraftment in man. An extensive body of data exists in the murine system and the degree to which this can be extrapolated to man is discussed. Other variables involving stem cells include their efficiency of homing to the bone marrow, their previous proliferative history influencing telomere length and their ability to upregulate telomerase. The extent to which stem cell numbers increase in vivo upon transplantation, or ex vivo upon cytokine stimulation, is discussed and the question as to the ability of nonstochastic extrinsic influences to alter stem cell self-renewal probability is raised.