Red cell production.

Red blood cells (erythrocytes) are normally produced only in the bone marrow of adults, but during embryonic and fetal life erythropoiesis occurs in the liver, and during the second to the fifth month of fetal life it occurs in the spleen.1 From the fifth month after gestation marrow production increases gradually, having begun to take over from the liver during late fetal and early neonatal life. During the early years of life erythropoiesis occurs in the marrow of most bones but as age progresses active marrow is replaced by fat cells, starting from the distal bones of the limbs and proceeding towards the trunk, so that by adult life activity is confined to the vertebrae, ribs, sternum, skull, and iliac crests.