Bone marrow failure syndromes and refractory cytopenia of childhood

__Abstract__ Hematopoiesis, or blood cell production, is sustained through hematopoietic stem cells, which are self-renewing cells that reside in the bone marrow, and that are capable of producing daughter cells that proliferate and mature to provide all adult blood effector cells, including erythrocytes or red blood cells, leukocytes or white blood cells, and thrombocytes or platelets. Erythrocytes are the most numerous cell types in the blood, and have as main task to transport oxygen through the lungs to peripheral tissues; leukocytes are responsible for elimination of bacteria and viruses, and thrombocytes for blood clotting. Hematopoiesis can be subdivided into myelopoiesis and lymphopoiesis. Myelopoiesis comprises the generation of granulocytes, monocytes, mast cells, mega- karyocytes (the precursors of thrombocytes), and erythrocytes. Lymphopoiesis encompasses the generation of B cells, T cells, and NK cells; collectively referred to as lymphocytes. Although recent insights suggest that cell fate determination during hematopoiesis is more plastic than previously thought, the classic, hierarchical model of determination of hematopoietic cell lineages is depicted in Figure 1 (adapted from Orkin and Zon