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Vertebrate blood development occurs in two phases. A transient embryonic (“primitive”) phase of hematopoiesis, beginning on E7.5 in the mouse embryo, is followed by the definitive (“adult”) phase, which begins on E9.5 in the liver of the developing mouse embryo. The embryonic phase of hematopoiesis is thought to initiate the circulation that provides the embryo with its initial blood cells and with its capillary network connecting it to the yolk. The definitive phase of hematopoiesis generates more cell types and provides the stem cells that will last throughout the vertebrate's lifetime.
In the mouse embryo, red blood cell formation, or erythropoiesis, is seen in the blood islands in the mesoderm surrounding the yolk sac. However, the embryonic hematopoietic cell population is transitory. The hematopoietic stem cells that last the lifetime of the organism form within nodes of the mesoderm that line the mesentery and the major blood vessels. The aorta-gonad-mesonephros (AGM) region in the aortic wall appears to be the most important source of new blood cells, and it has been found to contain numerous hematopoietic stem cells by day 11 of mouse embryonic development. These hematopoietic stem cells later colonize the liver and constitute the fetal and adult circulatory system. Near the time of birth, stem cells from the liver populate the bone marrow, which then becomes the major site of blood formation throughout adult life.
Blood formation in the human embryo begins in the liver at about week 5 and at week 12 in the spleen, red bone marrow, and thymus.