Erytropoëse

Erythropoiesis, or erytropoëse, is the biological process by which new red blood cells (erythrocytes) are produced. In adult humans it occurs mainly in the bone marrow and begins with hematopoietic stem cells differentiating along the erythroid lineage. The erythroid maturation sequence includes proerythroblasts, basophilic erythroblasts, polychromatic erythroblasts, and orthochromatic erythroblasts, followed by enucleation to form reticulocytes, which mature into erythrocytes in the circulation. The process is regulated by erythropoietin (EPO), a hormone produced primarily by the kidneys in response to low oxygen tension. Adequate iron, folate, and vitamin B12 are essential for heme and DNA synthesis during erythropoiesis.

Developing erythroblasts mature within erythroblastic islands in the bone marrow, where a resident macrophage supports maturation, recycles nuclei expelled during enucleation, and helps deliver iron to developing cells. Iron is transported to erythroid precursors bound to transferrin, and systemic iron availability is controlled in part by hepcidin.

Normal erythropoiesis maintains a high production rate, with the human body producing roughly 2 million red blood cells per second to replace senescent cells and sustain oxygen delivery. The average erythrocyte lifespan in circulation is about 120 days. During fetal development, erythropoiesis occurs primarily in the liver and spleen; in certain conditions, extramedullary hematopoiesis can occur outside the bone marrow.

Clinical aspects include anemia, polycythemia, and disorders of iron metabolism. Therapeutic use of erythropoiesis-stimulating agents, particularly EPO, is common in chronic kidney disease and other contexts of impaired red cell production.