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Erythropoiesis
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<p>Erythropoiesis (from Greek <i>erythro</i>, meaning <i>red</i> and <i>poiesis</i>, meaning <i>to make</i>) is the process which produces <a href="/facts/Red_blood_cell/Kggpq8Jh">red blood cells</a> (erythrocytes), which is the development from erythropoietic stem cell to mature red blood cell. </p><p>It is stimulated by decreased O2 in circulation, which is detected by the <a href="/facts/Kidney/HBspPrv9">kidneys</a>, which then secrete the hormone <a href="/facts/Erythropoietin/jgYFo1R6">erythropoietin</a>. This hormone stimulates proliferation and differentiation of red cell precursors, which activates increased erythropoiesis in the <a href="/facts/Hemopoietic/VRDwMymd">hemopoietic</a> tissues, ultimately producing <a href="/facts/Red_blood_cells/Kggpq8Jh">red blood cells</a> (erythrocytes). In <a href="/facts/Postnatal/abLj2JtY">postnatal</a> <a href="/facts/Bird/RJF6zwLP">birds</a> and <a href="/facts/Mammal/phSwTzBo">mammals</a> (including <a href="/facts/Humans/rlcTENsr">humans</a>), this usually occurs within the <a href="/facts/Red_bone_marrow/4dfudk0H">red bone marrow</a>. In the early <a href="/facts/Fetus/evocHd0Y">fetus</a>, erythropoiesis takes place in the mesodermal cells of the <a href="/facts/Yolk_sac/cRSx7H17">yolk sac</a>. By the third or fourth month, erythropoiesis moves to the liver. After seven months, erythropoiesis occurs in the bone marrow. Increased levels of physical activity can cause an increase in erythropoiesis. However, in <a href="/facts/Humans/rlcTENsr">humans</a> with certain <a href="/facts/Diseases/55eeQlGL">diseases</a> and in some <a href="/facts/Animals/4b8J33nd">animals</a>, erythropoiesis also occurs outside the <a href="/facts/Bone_marrow/4dfudk0H">bone marrow</a>, within the <a href="/facts/Spleen/6NKn9d8s">spleen</a> or <a href="/facts/Liver/Rtp4Fk2b">liver</a>. This is termed <i><a href="/facts/Extramedullary_hematopoiesis/fTbVWHSD">extramedullary erythropoiesis</a></i>. </p><p>The <a href="/facts/Bone_marrow/4dfudk0H">bone marrow</a> of essentially all the <a href="/facts/Bone/pCU37jBe">bones</a> produces red blood cells until a person is around five <a href="/facts/Years/51V242hh">years</a> old. The <a href="/facts/Tibia/YXQ0F48L">tibia</a> and <a href="/facts/Femur/uVyTwj4l">femur</a> cease to be important sites of <a href="/facts/Hematopoiesis/VRDwMymd">hematopoiesis</a> by about age 25; the <a href="/facts/Vertebra/qAdLhD5H">vertebrae</a>, <a href="/facts/Human_sternum/qIPMpyIz">sternum</a>, <a href="/facts/Pelvis/8Fq9z3TY">pelvis</a> and <a href="/facts/Rib/cECgXRpF">ribs</a>, and <a href="/facts/Cranial_bones/dzqkVdLQ">cranial bones</a> continue to produce red blood cells throughout life. Up to the age of 20 years, RBCs are produced from red bone marrow of all the bones (long bones and all the flat bones). After the age of 20 years, RBCs are produced from membranous bones such as vertebrae, the sternum, ribs, scapulas, and the iliac bones. After 20 years of age, the shaft of the long bones becomes yellow bone marrow because of fat deposition and loses the erythropoietic function. </p><p>Comparison of erythrocyte production by marrow stem cell lines from old and young adult donors shows no significant differences. This finding implies that little or none of the proliferative capacity of the erythropoietic stem cells is exhausted by a lifetime of normal functioning. </p>