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Erythropoiesis (from Greek 'erythro' meaning "red" and 'poiesis' meaning "to make") is the process which produces red blood cells (erythrocytes). It is stimulated by decreased O2 in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin.〔Sherwood, L, Klansman, H, Yancey, P: ''Animal Physiology'', Brooks/Cole, Cengage Learning, 2005.〕 This hormone stimulates proliferation and differentiation of red cell precursors, which activates increased erythropoiesis in the hemopoietic tissues, ultimately producing red blood cells.〔 In postnatal birds and mammals (including humans), this usually occurs within the red bone marrow.〔 In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac. By the third or fourth month, erythropoiesis moves to the liver. After seven months, erythropoiesis occurs in the bone marrow. Increased level of physical activity can cause an increase in erythropoiesis.〔 〕 However, in humans with certain diseases and in some animals, erythropoiesis also occurs outside the bone marrow, within the spleen or liver. This is termed ''extramedullary erythropoiesis''. The bone marrow of essentially all the bones produces red blood cells until a person is around five years old. The tibia and femur cease to be important sites of hematopoiesis by about age 25; the vertebrae, sternum, pelvis and ribs, and cranial bones continue to produce red blood cells throughout life. == Erythrocyte differentiation == In the process of red blood carpasel maturation, a cell undergoes a series of ''differentiations''. The following stages of development all occur within the bone marrow: # A Hemocytoblast, a multipotent hematopoietic stem cell, becomes # a common myeloid progenitor or a multipotent stem cell, and then # a unipotent stem cell, then # a pronormoblast, also commonly called an proerythroblast or a rubriblast. # This becomes a basophilic or early normoblast, also commonly called an erythroblast, then # a polychromatophilic or intermediate normoblast, then # an orthochromatic or late normoblast. At this stage the nucleus is expelled before the cell becomes # a reticulocyte. The cell is released from the bone marrow after Stage 7, and so in newly circulating red blood cells there are about 1% reticulocytes. After one to two days, these ultimately become "erythrocytes" or mature red blood cells. These stages correspond to specific appearances of the cell when stained with Wright's stain and examined by light microscopy, and correspond to other biochemical changes. In the process of maturation, a basophilic pronormoblast is converted from a cell with a large nucleus and a volume of 900 fL to an enucleated disc with a volume of 95 fL. By the reticulocyte stage, the cell has extruded its nucleus, but is still capable of producing hemoglobin. Essential for the maturation of red blood cells are Vitamin B12 (cobalamin) and Vitamin B9 (Folic acid). Lack of either causes maturation failure in the process of erythropoiesis, which manifests clinically as reticulocytopenia, an abnormally low amount of reticulocytes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Erythropoiesis」の詳細全文を読む スポンサード リンク
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