Physiology of blood
Blood cells originate from a common pluripotential haemopoietic stem cell (PHSC). In the rst 3-4 weeks of embryonic development, primitive erythroblasts are the rst cells that develop from mesenchymal stem cells in the yolk sac. From 6 weeks to 7 months, the liver – and the spleen to a lesser extent – becomes haemopoietically active and reaches its peak activity at 5 months and then declines to the time of birth. Between 6 and 7 months of foetal life, the bone marrow becomes the main source of new blood cells.