Erythropoiesis

Key points

• • The site moves through life: yolk sac and then liver and spleen in the fetus, all marrow in infancy, and mainly the axial skeleton and proximal long bones in adults.
• • The line runs stem cell, proerythroblast, normoblast stages, reticulocyte after the nucleus is extruded, and finally the mature biconcave erythrocyte.
• • Erythropoietin, made by interstitial cells of the kidney in response to tissue hypoxia, is the master regulator of the rate of production.
• • The essential raw materials are iron, vitamin B12 and folate; a shortage of B12 or folate blocks DNA synthesis and gives large megaloblastic cells, while a shortage of iron blocks haemoglobin and gives small pale cells.
• • The mature red cell has no nucleus and no mitochondria, so it depends on anaerobic glycolysis, survives about 120 days, and is finally removed by splenic macrophages.
• • The reticulocyte count reads the marrow's reply: high means the marrow is responding, low in the face of anaemia points to a marrow or raw-material problem.