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Recently, there has been much interest in the embryonic origins of the adult hematopoietic system in mammals (1). The controversy surrounding the potency and function of hematopoietic cells produced by the yolk sac compared to those produced by the intrabody portion of the mouse embryo has prompted much new research in the field of developmental hematopoiesis (2-8). While the yolk sac is the first tissue in the mammalian conceptus to visibly exhibit hematopoietic cells, the intrabody region-which at different stages of development includes the splanchnopleural mesoderm, para-aortic splanchnopleura (PAS) and the aorta-gonad-mesonephros (AGM) region-clearly contains more potent undifferentiated hematopoietic progenitors and stem cells before the yolk sac. Furthermore, the most interesting dichotomy revealed by these studies is that terminally differentiated hematopoietic cells can be produced in the mouse embryo before the appearance of cells with adult repopulating capacity. Thus, the accepted view of the adult hematopoietic hierarchy with the hematopoietic stem cell (HSC) at its foundation does not reflect the hematopoietic hierarchy in the developing mouse embryo (9). Because this field offers many questions concerning the types of hematopoietic cells present in the embryo, the lineage relationships between these cells, and the molecular programs necessary for the development of the embryonic and adult hematopoietic systems, this section presents the approaches taken and the materials and methods necessary to explore the mouse embryo for the presence of the first adult repopulating HSCs.

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