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Hematopoietic stem cells: concepts, assays, and controversies.

Mature cells of the hematopoietic system, including B and T lymphocytes, have a limited life span and must be continuously replenished by differentiation from immature precursors. The stability of the hematopoietic system depends on the maintenance of a pool of pluripotent hematopoietic stem cells. During the past 30 years, the development of quantitative assays for stem cells and their progeny has led to an explosion of information concerning the stages of differentiation, regulatory factors and receptors, and characterization of surface markers. Nevertheless, there remain many controversies about the properties of cells in the stem cell compartment. The degree of heterogeneity within the stem cell compartment is unknown. The detailed mechanisms for regulation of stem cells are uncertain. Finally, the factors which control the recruitment of quiescent stem cells are unknown and controversial.