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The first erythrocytes contain nuclei and derive from stem cells (BFU/CFU-E) that arise in the blood islands of the umbilical vesicle. Centrally lying cells of these blood islands join up thereby into nucleus-containing, large erythroblasts, whereas those peripherally located become endothelial cells. One also calls this extraembryonic phase of the blood formation megaloblastic eryhropoiesis. This extraembryonal erythropoiesis is supplanted by thee embryonic erythropoiesis, which arises in the liver. These stem cells have their origin in the aorto-gonado-mesonephros region. The erythrocytes that arise in the liver are nucleus-free - in contrast to those formed outside the embryo - and are produced there up to the 28th week of pregnancy.
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