Summary. The mouse yolk sac has been shown to contain in-vivo colony forming cells capable of producing granulocytic, megakaryocytic and erythroid spleen colonies; in-vitro colony forming cells producing granulocytic and mononuclear-macrophage colonies in agar; and cells capable of repopuiating the lymphoid and myeloid tissue of lethally irradiated hosts. Similar haemopoietic precursor cells were also demonstrated in the blood at the time of initiation of the circulation and in the early embryonic liver.
Organ cultures of 7 day embryos with intact yolk sacs, and embryos or yolk sacs after separation have shown the autonomous nature of the development of haemopoiesis in the yolk sac and the dependence of intra-embryonic haemopoiesis, particularly in embryonic liver, on colonization by yolk sac haemopoietic cells.
Both in-vivo and in-vitro colony forming cells have been involved in the first migration stream, between yolk sac and embryonic liver, and evidence has been presented for the role of local environmental factors in controlling the differentiation of these cell types.
These results support the view that development of haemopoietic organs in both embryo and adult is dependent on colonization by circulating cells and that these circulating stem cells originate initially in the yolk sac. This indicates that the yolk sac is the only site of genuine de novo formation of haemopoietic stem cells.