[Humoral regulation of stem cell proliferation]

The central feature of hematopoiesis is life-long, stable cell renewal. This process is supported by hemopoietic stem cells which, in the steady state, appear to be dormant in cell cycling. The recruitment of the dormant stem cells into cell cycle may be promoted by such factors as interleukin (IL)-1, IL-6, granulocyte-colony stimulating factor (G-CSF), and newly discovered IL-11. The effects of IL-1 on stem cells may be indirect. Once the stem cells leave Go and begin proliferation, the subsequent process is characterized by continued proliferation and differentiation. Though 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 process. The proliferation of early hemopoietic progenitors requires the presence of IL-3 and/or IL-4, and the intermediate process appears to be supported by granulocyte/macrophage-CSF (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 erythropoietin (erythropoiesis), G-CSF (neutrophil production), and IL-5 (eosinophilopoiesis). Thus, hemopoietic proliferation appears to be regulated by a cascade of factors directed at different developmental stages.