ABSTRACT
Hematopoietic stem cells (HSCs) have the capacity to maintain the entire blood system, while maintaining their own population. Balance between self-renewal and differentiation has been described by the hierarchical model of hematopoiesis in which HSCs are extremely rare, quiescent, pluripotent cells that give rise to increasingly lineage-restricted progeny. However, new evidence, challenging this model, indicates that there are populations of HSC that are less rare, cycling, and exist in a continuum, with the stem cell phenotype influenced by cell cycle state and microenvironmental stimuli. In this chapter, we discuss both this continuum model and the classic hierarchy model of hematopoiesis.
