ABSTRACT
H ematopoiesis is a complex process in which hematopoietic stem cells self-replicate and differentiate to generate the different mature cell types in the blood and lymphoid organs. Under steady-state conditions, stem cells and hematopoietic progenitor cells reside in the bone marrow (BM) medullary cavity in contact with the bone marrow microenvi ronment, which includes a diversity of mesenchymal cells that secrete hematopoietic cytokines and extracellular matrix components. Most of the factors required for the orderly development of stem cells are present in the BM microenvironment. More than 40 different growth factors, cytokines, and chemokines interact with stem and progenitor cells through specific receptors and regulate proliferation, differentiation, and cell fate. Hematopoietic growth factors are pro duced by both mesenchymal and hematopoietic cells and can be present in soluble, cell-bound forms, or bound to the extracellular matrix (ECM ).1’2 Stem and progenitor cells express adhe sion molecules that provide specific cell-cell and cell-ECM interactions. In addition, cytokines and growth factors affect adhesive interactions between stem and progenitor cells and their adhesive ligands in the BM providing a further level of regulation.
