Erythropoiesis

Erythropoiesis (from Greek ἐρυθρός, erythros, meaning red, and ποίησις, poiēsis, meaning creation, production, making) is the process which produces red blood cells (erythrocytes), which is the development from erythropoietic stem cell to mature red blood cell.

It is stimulated by decreased O₂ in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin. This hormone stimulates proliferation and differentiation of red cell precursors, which activates increased erythropoiesis in the hemopoietic tissues, ultimately producing red blood cells (erythrocytes). In postnatal birds and mammals (including humans), this usually occurs within the red bone marrow. In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac. By the third or fourth month, erythropoiesis moves to the liver. After seven months, erythropoiesis occurs in the bone marrow. Increased levels of physical activity can cause an increase in erythropoiesis. However, in humans with certain diseases and in some animals, erythropoiesis also occurs outside the bone marrow, within the spleen or liver. This is termed extramedullary erythropoiesis.

The bone marrow of essentially all the bones produces red blood cells until a person is around five years old. The tibia and femur cease to be important sites of hematopoiesis by about age 25; the vertebrae, sternum, pelvis and ribs, and cranial bones continue to produce red blood cells throughout life. Up to the age of 20 years, RBCs are produced from red bone marrow of all the bones (long bones and all the flat bones). After the age of 20 years, RBCs are produced from membranous bones such as vertebrae, the sternum, ribs, scapulas, and the iliac bones. After 20 years of age, the shaft of the long bones becomes yellow bone marrow because of fat deposition and loses the erythropoietic function.

Comparison of erythrocyte production by marrow stem cell lines from old and young adult donors shows no significant differences. This finding implies that little or none of the proliferative capacity of the erythropoietic stem cells is exhausted by a lifetime of normal functioning.

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:

The cell is released from the bone marrow after Stage 8, and so in newly circulating red blood cells there are about 1% reticulocytes. After one to two days, these ultimately become "erythrocytes" or mature red blood cells.

These stages correspond to specific appearances of the cell when stained with Wright's stain and examined by light microscopy, and 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 fL to an enucleated disc with a volume of 95 fL. By the reticulocyte stage, the cell has extruded its nucleus, but is still capable of producing hemoglobin.