Age Related Differences in Erythropoiesis
1. During intrauterine life, in the early embryo, erythropoiesis occurs in the mesoderm (the middle of the three germ layers of an embryo) of yolk sac. In a few weeks, the process shifts to the mesoderm of the body. During these stages, erythropoiesis is intravascular. It takes place by some endothelial cells undergoing transformation into blood cells and detaching themselves from the vessel wall to enter the circulation.
2. After the third intrauterine month, erythropoiesis takes place in the liver and spleen, in the mesenchymal tissue (embryonic connective tissue) between the blood vessels and tissue cells. After the fourth intrauterine month, the fetal bone marrow also starts manufacturing blood cell
3. By the time of birth, bone marrow takes over erythropoiesis completely.
Thus, all extrauterine (occurring outside the uterus) erythropoiesis is medullary, i.e., in the bone marrow. Red cells, white cells and platelets are made in the marrow of Bones.