Two hemogenic sites are present in mouse embryos before the onset of fetal liver hematopoiesis. While the yolk sac provides for immediate erythropoiesis, an intraembryonic region encompassing the dorsal aorta produces definitive hematopoietic stem cells. At early developmental stages this region, called the paraaortic splanchnopleura, produces multipotent progenitors. At the time of fetal liver colonisation, the paraaortic splanchnopleura further evolves into aorta, gonads and mesonephros (AGM) and contains progenitors capable of long term multilineage reconstitution (26). The ma—jority of hematopoietic cells generated in the yolk sac are nucleated erythroid cellscalled primitive erythroid cells which express an embryonic type of hemoglobins. After their switch to fetal liver, the majority of hematopoietic cells are erythroid cells, but they are enucleated cells containing adult type hemoglobins (ti-globin) in mice, whereas they contain fetal type (r—globin) hemoglobin in human. In the adult mouse, bone marrow is the major hematopoietic organ where hematopoietic stem cells and their progenitors develop and myelopoiesis seems to dominate; spleen is another hematopoietic organ where the stem cells are predominantly committed to erythroid development (3). Self—renewal and commitment of hematopoietic stem cells are believed to be regulated by the hematopoietic inductive microenvironment (HIM) created in these hematopoietic organs and the stem cells may migrate to and localize in appropriate microenvironments during ontogeny. Dexter (4) developed a culture for bone mar—