Erythropoiesis

Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. It is stimulated by decreased O₂ delivery to the kidneys, which then secrete the hormone erythropoietin ^[1]. This activates increased erythropoiesis in the hemopoietic tissues^[1]. In postnatal birds and mammals (including humans), this usually occurs within the red bone marrow^[1]. 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 spleen and liver.^[2] After seven months, erythropoiesis occurs in the bone marrow. ^[3] 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.