Hemopoiesis (hematoiesis) is the process that produces the formed elements of the blood. Hemopoiesis takes place in the red bone marrow found in the epiphyses of long bones (for example, the humerus and femur), flat bones (ribs and cranial bones), vertebrae, and the pelvis. Within the red bone marrow, hemopoietic stem cells (hemocytoblasts) divide to produce various “blast” cells. Each of these cells mature and becomes a particular formed element.
Erythropoiesis
Erythropoiesis, the process of making erythrocytes, begins with the formation of proerythroblasts from hemopoietic stem cells. Over three to five days, several stages of development follow as ribosomes proliferate and hemoglobin is synthesized. Finally, the nucleus is ejected, producing the depression in the center of the cell. Young erythrocytes, called reticulocytes, still containing some ribosomes and endoplasmic reticulum, pass into the bloodstream and develop into mature erythrocytes after another one or two days.
Erythroprotein
Erythroprotein (EPO), a hormone produced mostly by the kidneys, stimulates bone marrow to produce erythrocytes. When inadequate amounts of oxygen are delivered to body cells, a condition called hypoxia, the kidneys increase EPO secretion, which, in turn, stimulates an increase in erythrocyte production.
The average production rate of erythrocytes in healthy individuals is two million cells per second. Normal production requires adequate amounts of iron and vitamin B12 and folic acid.
Leukopoiesis
Leukopoiesis, the process of making leukocytes, is stimulated by various colony-stimulating factors (CSFs), hormones produced by mature white blood cells. The development of each kind of white blood cell begins with the division of themopoietic stem cells into one of the following “blast” cells.
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Myeoblasts divide to form eosinophilic, neutrophilic, or basophilic myelocytes, which lead to the development of the three kinds of granulocytes.
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Monoblasts lead to the development of monocytes.
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Lymphoblasts lead to the development of lymphocytes.
Thrombopoiesis
Thrombopoiesis, the process of making platelets, begins with the formation of megakaryoblasts from hemopoietic stem cells. The megakaryoblasts divide without cytokinesis to become megakaryocytes, huge cells with a large, multilobed nucleus. The megakaryocytes then fragment into segments as the plasma membrane infolds into the cytoplasm.
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