Hematopoiesis is the process of blood cell formation in the adult human, the bone marrow (BM) is the primary hematopoietic organ. Each day, the BM produces billions of leukocytes, erythrocytes, and thrombocytes, which enter the circulation. Production of such enormous numbers of mature blood cells results from the exponential expansion and differentiation of pluripotent hematopoietic stem cells through a series of increasingly more differentiated hematopoietic progenitor cells (HPCs) Regulation of hematopoiesis is accomplished by the transduction of signals that follow interactions of the HPCs and developing blood cells with BM stromal cells, cytokines, and the extracellular matrix.