Erythropoiesis 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.[3] 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.
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.
n the process of red blood corpuscle maturation, a cell undergoes a series of differentiations. The following stages of development all occur within the bone marrow:

A hemocytoblast, a multipotent hematopoietic stem cell, becomes
a common myeloid progenitor or a multipotent stem cell, then
a unipotent stem cell, then
a pronormoblast (also commonly called an proerythroblast or a rubriblast), then
a basophilic or early normoblast (also commonly called an erythroblast), then
a polychromatophilic or intermediate normoblast, then
an orthochromatic or late normoblast. At this stage the nucleus is expelled before the cell becomes
a reticulocyte. (These cells still contain RNA and are also called "immature red blood cells")

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