Hemopoiesis ( hematopoiesis) 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 matures and becomes a particular formed element.
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.
Erythropoietin (EPO), a hormone produced mostly by the kidneys, stimulates bone marrow to produce erythrocytes (stimulates erythropoiesis). 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, vitamin B 12, and folic acid. Vitamin B 12 and folic acid are necessary for the proper development of DNA in the erythroblasts. This DNA is responsible for the organization of the heme molecule of which iron will become a component. Proper DNA development is also necessary for erythroblast reproduction. A lack of either vitamin B 12 or folic acid can result in pernicious anemia.
Leukopoiesis, the process of making leukocytes, is stimulated by various colony‐stimulating factors (CSFs), which are hormones produced by mature white blood cells. The development of each kind of white blood cell begins with the division of the hemopoietic stem cells into one of the following “blast” cells:
- Myeloblasts divide to form eosinophilic, neutrophilic, or basophilic myelocytes, which lead to the development of the three kinds of granulocytes.
- Monoblasts lead to the development of monocytes.
- Lymphoblasts lead to the development of lymphocytes.
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.