Stem cells give rise to all the different types of blood cell. These pluripotent cells undergo self-renewal and differentiation to produce more developmentally restricted cells, which proliferate and develop to form mature cells. Hematopoietic stem cells are defined by their ability to establish long-term reconstitution of hematopoiesis when transplanted to irradiated recipient animals (Till and McCulloch, 1961). Committed progenitor cells have a restricted developmental capacity; e.g., granulocyte---macrophage colony-forming cells (GM-CFC) or erythroid burst-forming units (BFU-e) can form only neutrophil, macrophage, or erythroid cells, respectively. Hematopoiesis occurs in the microenvironment of adult bone marrow where proliferation and differentiation of hematopoietic stem and progenitor cells occurs in close contact with the bone marrow stromal cells and the associated extracellular matrix (ECM) (Adams and Watt, 1993; Yoder and Williams, 1995). These interactions have been termed the local area network (LAN).