FORMATION OF BLOOD CELLS
Because many blood cells die within hours, days, or weeks, the body must constantly replace them. The total number of each cellular element is controlled by negative feedback mechanisms. The process of blood cell formation is called hemopoiesis or hemato-poiesis.
The blood cells are formed from pluripotent stemcells found in the bone marrow. These stem cells arecapable of multiplying. They also differentiate into precursor cells that can form all the formed elements of the blood. The myeloid stem cells give rise to the red blood cells, platelets, and white blood cells (all except lymphocytes). The lymphoid stem cells give rise to the lymphocytes; however, some of the lym-phocytes complete their development in lymphoid tissue located outside the bone marrow.
A number of growth factors regulate the differentia-tion and multiplication of the stem cells. For example, erythropoietin (a hormone produced by the kidney)regulates red blood cell production. Thrombopoietin, a hormone produced by the liver, regulates the produc-tion of platelets. Cytokines,small glycoproteins pro-duced in such different areas as the bone marrow, white blood cells, fibroblasts, and endothelial cells, also help regulate cell function and proliferation. Examples of cytokines include colony-stimulating factors and interleukins.