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The developmental origin of hematopoietic stem cells has been for decades the subject of great interest. Once thought to emerge from the yolk sac, hematopoietic stem cells have now been shown to originate from the embryonic aorta. Increasing evidence suggests that hematopoietic stem cells are produced from an endothelial intermediate designated by the authors as hemangioblast or hemogenic endothelium. Recently, the allantois in the avian embryo and the placenta in the mouse embryo were shown to be a site of hematopoietic cell production/expansion and thus appear to play a critical role in the formation of the hematopoietic system. In this review we shall give an overview of the data obtained from human, mouse and avian models on the cellular origins of the hematopoietic system and discuss some aspects of the molecular mechanisms controlling hematopoietic cell production. [ABSTRACT FROM AUTHOR]

To date three sites of emergence of hemopoietic cells have been identified during early avian development: the yolk sac, the intraaortic clusters and recently the allantois. However, the contributions of the hematopoietic stem cell (HSC) populations generated by these different sites to definitive hematopoiesis and their migration routes are not fully unraveled. Experimental embryology as well as the establishment of the genetic cascades involved in HSC emergence help now to draw a better scheme of these processes. [Copyright &y& Elsevier]