Under healthy physiological conditions, the production of new blood cells is tightly controlled. Hematopoietic stem cells in adult bone marrow self proliferate and differentiate into erythroid, lymphoid (B cells and T cells) and myeloid lineages (granulocytes, megakaryocytes, and macrophages). Commitment and maturation of hematopoietic stem cells toward a specific lineage cell type is governed by a wide range of growth factors. Cytokines that influence hematopoiesis include those that can be classified into the Common beta chain, Common gamma chain, and IL-6 cytokine families. Growth factors such as EGF, FGF, GDF, IGF, PDGF, and VEGF also affect hematopoietic stem cell differentiation. In addition, the fate of hematopoietic stem cells is modified by Activins, Bone Morphogenetic Proteins (BMPs), Hedgehog molecules, and Wnt-related molecules.