The formation of blood serves as a paradigm for understanding the mechanisms of self-renewal and differentiation of tissue-derived adult stem cells. Hematopoietic stem cells (HSC) have been considered as a well-characterized model system to study stem cell biology in aging and oncogenesis. HSCs reside as rare cells in the bone marrow of adult mammals and are the origin of a hierarchical array of multipotential progenitors that become progressively restricted to several or single lineages. Because mature blood cells are predominantly short lived, HSCs are required throughout life to replenish multilineage progenitors and the precursors committed to individual hematopoietic lineages. Therefore, HSC are defined operationally by their capacity to reconstitute the entire hematopoietic system of a recipient as a result of the maintenance of their major properties: self-renewal and multipotential differentiation.
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