Erythropoiesis

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Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. In human adults, this usually occurs within the bone marrow; in humans with certain diseases, in fetuses and in some animals, erythropoeiesis also occurs outside the bone marrow, within the spleen, liver, or yolk sac. This is termed extramedullary erythropoiesis.

The tibia and femur cease to be important sites of hematopoiesis by about age 25; the vertebrae, sternum, and ribs continue to produce red blood cells throughout life.

[edit] Erythrocyte differentiation

In the process of red blood cell maturation, a cell undergoes a series of differentiations. The following stages of development all occur within the bone marrow:

pluripotent hematopoeiticstem cell
multipotent stem cell
unipotential stem cell
pronormoblast
basophilic normoblast/early normoblast
polychromatophilic normoblast/intermediate normoblast
orthochromatic normoblast/late normoblast
reticulocyte

After these stages, the cell is released from the bone marrow, and ultimately becomes an "erythrocyte" or mature red blood cell circulating in the peripheral blood.

These stages correspond to specific appearances of the cell when stained with Wright's stain and examined by light microscopy, but correspond to other biochemical changes.

In the process of maturation a basophilic pronormoblast is converted from a cell with a large nucleus and a volume of 900 µm³ to an enucleated disc with a volume of 95 µm³.

[edit] Regulation of erythropoesis

A feedback loop involving erythropoietin helps regulate the process of erythropoiesis so that, in non-disease states, the production of red blood cells is equal to the destruction of red blood cells and the red blood cell number is sufficient to sustain adequate tissue oxygen levels but not so high as to cause sludging, thrombosis, or stroke. Erythropoeitin is produced in the kidney and liver in response to low oxygen levels. In addition, erythropoeitin is bound by circulating red blood cells; low circulating numbers lead to a relatively high level of unbound erythropoeitin, which stimulates production in the bone marrow.