Effects of hemopoietic growth factors on stem cells in vitro.

The central feature of hemopoiesis is the lifelong, stable cell renewal. This process is supported by hemopoietic stem cells, which in the steady-state appear to be dormant in cell cycling. The entry into cell cycle of the dormant stem cells may be promoted by such factors as IL-1, IL-6, and G-CSF. Available evidence indicates that the effects of IL-1 on stem cells are indirectly mediated in part by IL-6 and G-CSF. Once the stem cells leave G0 and begin proliferation, the subsequent process is characterized by continued proliferation and differentiation. While several models of stem cell differentiation have been proposed, micromanipulation studies of individual progenitors suggest that the commitment of multipotential progenitors to single lineages is a stochastic (random) process. The proliferation of early hemopoietic progenitors appears to be supported by IL-3, IL-4, and/or GM-CSF. Once the progenitors are committed to individual lineages, the subsequent maturation process appears to be supported by late-acting, lineage-specific factors such as Ep (for erythropoiesis), G-CSF (for neutrophil production), and IL-5 (for eosinophilopoiesis). Thus, hemopoietic proliferation appears to be regulated by a cascade of factors directed at different developmental stages.