Regulation of Blood Cell Production

Hemoglobin is the protein inside red blood cells that carries oxygen. Red blood cells also remove carbon dioxide from your body, transporting it to the lungs for you to exhale. Red blood cells are made inside your bones, in the bone marrow. They typically live for about 120 days, and then they die. [1]

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How Regulation of Blood Cell Production

In children the marrow of most bones produces blood cells. By adulthood, however, only the bones of the chest, the base of the skull, and the upper portions of the limbs remain active. The bone marrow in an adult weighs almost as much as the liver, and it produces cells at an enormous rate.

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All blood cells are descended from a single population of bone marrow cells called pluripotent hematopoietic stem cells, which are undifferentiated cells capable of giving rise to precursors (progenitors) of any of the different blood cells. When a pluripotent stem cell divides, its two daughter cells either remain pluripotent stem cells or become committed to a particular developmental pathway; what governs this “decision” is not known.

The first branching yields either lymphoid stem cells, which give rise to the lymphocytes, or so called myeloid stem cells, the progenitors of all the other varieties. At some point, the proliferating offspring of the myeloid stem cells become committed to differentiate along only one path, for example into erythrocytes.

Proliferation and differentiation of the various progenitor cells is stimulated, at multiple points, by a large number of protein hormones and paracrine agents collectively termed hematopoietic growth factors (HGFs). Thus, erythropoietin, the hormone described earlier, is an HGF. (Nomenclature can be confusing in this area since the HGFs belong to a still larger general family of messengers termed “cytokines".

Physiology of hematopoietic growth factors

The physiology of the HGFs is very complex because (1) there are so many of them, (2) any given HGF is often produced by a variety of cell types throughout the body, and (3) HGFs often exert other actions in addition to stimulating blood cell production. There are, moreover, many interactions of the HGFs on particular bone marrow cells and processes. For example, although erythropoietin is the major stimulator of erythropoiesis, at least 10 other HGFs cooperate in theprocess. Finally, in several cases the HGFs not only stimulate differentiation and proliferation of progenitor cells, they inhibit the usual programmed death of these cells.

The administration of specific HGFs is proving tobe of considerable clinical importance. Examples are the use of erythropoietin in persons having a deficiency of this hormone due to kidney disease, and the use of granulocyte colony stimulating factor (G-CSF) to stimulate granulocyte production in individuals whose bone marrow has been damaged by anticancer drugs.