TAILIEUCHUNG - Chapter 058. Anemia and Polycythemia

Hematopoiesis is the process by which the formed elements of the blood are produced. The process is regulated through a series of steps beginning with the pluripotent hematopoietic stem cell. Stem cells are capable of producing red cells, all classes of granulocytes, monocytes, platelets, and the cells of the immune system. The precise molecular mechanism—either intrinsic to the stem cell itself, or through the action of extrinsic factors—by which the stem cell becomes committed to a given lineage is not fully defined. . | Chapter 058. Anemia and Polycythemia Hematopoiesis is the process by which the formed elements of the blood are produced. The process is regulated through a series of steps beginning with the pluripotent hematopoietic stem cell. Stem cells are capable of producing red cells all classes of granulocytes monocytes platelets and the cells of the immune system. The precise molecular mechanism either intrinsic to the stem cell itself or through the action of extrinsic factors by which the stem cell becomes committed to a given lineage is not fully defined. However experiments in mice suggest that erythroid cells come from a common erythroid megakaryocyte progenitor that does not develop in the absence of expression of the GATA-1 and FOG-1 friend of GATA-1 transcription factors Chap. 68 . Following lineage commitment hematopoietic progenitor and precursor cells come increasingly under the regulatory influence of growth factors and hormones. For red cell production erythropoietin EPO is the regulatory hormone. EPO is required for the maintenance of committed erythroid progenitor cells that in the absence of the hormone undergo programmed cell death apoptosis . The regulated process of red cell production is erythropoiesis and its key elements are illustrated in Fig. 58-1. Figure 58-1 The physiologic regulation of red cell production by tissue oxygen tension. Hb hemoglobin. In the bone marrow the first morphologically recognizable erythroid precursor is the pronormoblast. This cell can undergo 4-5 cell divisions that result in the production of 16-32 mature red cells. With increased EPO production or the administration of EPO as a drug early progenitor cell numbers are amplified and in turn give rise to increased numbers of erythrocytes. The regulation of EPO production itself is linked to O2 availability. In mammals O2 is transported to tissues bound to the hemoglobin contained within circulating red cells. The mature red cell is 8 pm in diameter anucleate discoid in shape

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