The renewal and differentiation of hemopoietic stem cells.

Blood-forming tissues are organized in well-defined microenvironments composed of hemopoietic cells and a supportive stroma of connective tissue and endothelium. Hemopoietic cells segregate to various lineages, all derived from a small population of pluripotent stem cells residing in the bone marrow. Regulation of growth and differentiation, particularly under conditions of perturbations, damage, and disease, is mediated by inducer colony-stimulating factors and interleukins counteracted by inhibitory cytokines. Whereas much is known about the mode of induction of differentiation, insufficient information is available to explain the process of stem cell renewal that is crucial for the longevity of the hemopoietic system. It is also only partially known how inhibition of hemopoietic processes occurs, and what molecules in blood-forming tissues signal organization into discrete patterns. This paper reviews recent progress that has opened new avenues to a better understanding of this highly complex issue.