Blood contains large numbers of various cell types. The mature blood cell types exert highly specialized functions such as oxygen and carbon dioxide transport, blood clotting and defense against infections by antibody production, cell mediated immunity and phagocytosis. Most of these mature blood cell types have a limited life span and therefore need to be produced continuously. This process of blood cell formation, termed hemopoiesis, is impressive since daily approximately 1011 new blood cells are generated in man. In steady state situations, the continuous replacement of terminally differentiated cells is tuned with great precision but the hemopoietic system can respond dramatically to environmental stress, such as bleeding or infection. The primary site of hemopoiesis is the bone marrow which permits the formation of all blood cell types i.e., erythrocytes, platelets, monocytes, neutrophils, basophils, eosinophils and lymphocytes. The continuous replenishment of functionally mature hemopoietic cells ill vivo is strictly dependent on the presence of a small but persistent pool of bone marrow plmipotent hemopoietic stem cells. The mechanism(s) controlling hemopoiesis appear to involve regulation mediated by a group of interacting specific glycoproteins designated hemopoietic growth factors. Furthermore, it has been implied that microenvironmental stromal cells support hemopoiesis as well. Several mechanisms through which stromal cells affect hemopoiesis have been postulated, i.e., a direct cell contact regulated mechanism, secretion of CSFs; expression of antagonists of differentiation-inducing factor(s) and/or self-renewal mediators.

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Netherlands Cancer Foundation (KWF), Netherlands Organization for Scientific Research (NWO), EU
D.W. van Bekkum (Dirk)
Erasmus University Rotterdam
Erasmus MC: University Medical Center Rotterdam

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