Immunology | Immune system| Immunity

        All blood cells arise from a type of cell called the hematopoietic stem cell(HSC).Stem cells are cells that can differentiate into other cell type;they are self renewing-thy maintain their population level by cell division.In humans,hematopoeisis,the formation and development of red and white blood cells,begins in the embryonic yolk sac during the first weeks of development.Here,yolk sac stem cells differentiate into primitive erythroid cells that contain embryonic hemoglobin.In the third month of gestation, hematopoietic stem cells migrate from the yolk sac to the fetal liver and then to the spleen;these two organs have major roles in hematopoiesis from third to the seventh months of gestation.After that,the differentiation of HSCs in the bone marrow becomes the major factor in hematopoiesis,and by birth there is little or no hematopoiesis in the liver and spleen.
       It is remarkable that every functionally specialized,mature blood cell is derived from the same type of stem cell.In contrast to a uni-potent cell,which differentiates into a single cell type,a hematopoeitic stem cell is multipotent,or pluripotent,able to differentiate in various ways and thereby generate erythrocytes,granulocytes,monocytes,mast cells,lymphocytes,and megakaryocytes.These stem cells are few,normally fewer than one HSC per 5 X 100000 cells in the bone marrow.
      The study of hematopoietic stem cells is difficult bot because of their scarcity and because they are hard to grow in vitro.As a result,little is known about how their proliferation for self-renewal,hematopoietic stem cells are maintained at stable levels throughout adult life;however,when there is an increased demand for hematopoiesis,HSCs display an enormous proliferative capacity.This can be demonstrated in mice whose hematopoietic systems have ben completely destroyed by a lethal dose of x-rays.Such irradiated mice will die within 10 days unless they are infused with normal bone-marrow cells from a syngeneic(genetically identical) mouse.
        Early in hematopoiesis, a multipotent stem cell differentiates along one of two pathways,giving rise to either a common lymphoid progenitor cell or common myeloid progenitor cell.The types and amounts growth factors in the microenvironment of a particular stem cell or progenitor cell control its differentiation.During the development of the lymphoid and myeloid lineages,stem cells differentiate into progenitors cells,which have lost the capacity for self-renewal and are committed to a particular cell lineage.Common lymphoid progenitor cells give rise to B,T,and NK(natural killer)cells and some dendritic cells.Myeloid stem cells generate progenitors of red blood cells(erythrocytes),many of the various white blood cells(neutophils,eosinophils,basophils,monocytes,mast cells,dendritic cells.),platelets.Progenitors commitment depends on the acquisition of responsiveness to particular growth factors and cytokines.when the appropriate factors and cytokines are present,progenitor cells proliferate and differentiate into corresponding cell type,either a mature erythrocyte,a particular type of leukocyte,or a platelet-generating cell(the megakaryocyte).Red and white blood cells pass into bone-marrow channels,from which they enter the circulation.
        In bone marrow,hematopoietic cells grow and mature on a meshwork of stromal cells,endothelial cells,fibroblasts,and macrophages.Stromal cells influence the differentiation of hematopoietic stem cells by providing a hematopoietic-including micro-environment (HIM) consisting of a cellular matrix and factors are soluble agents that arrive at their target cells by diffusion,others are membrane-bound molecules on the surface of stromal cells that require cell-to-cell contact between the responding cells and the stromal cells.During infection,hematopoiesis is stimulated by the production of hematopoietic growth factors by activated macrophages and T cells.