2 models of embryonic and adult blood formation: Figure by Ueno and Weissman

If you have previously thought (in your spare time) that the conventional wisdom concerning blood formation is that the yolk sac’s embryonic blood-forming cells serve only the embryo, while the source of adult blood-forming stem cells is the region called aorta-gonad-mesonephros (AGM), it’s time to think it again due the elegant experiments of Samokhalov et al.: Cell tracing shows the contribution of the yolk sac to adult haematopoiesis Nature 446, 1056-1061 (26 April 2007)

Legend: a, The ‘separate’ model. Embryonic blood-forming cells, generated in a region called the ‘primitive streak’, proliferate in the yolk sac, migrate to the embryo proper, but eventually die out. The adult blood-forming cells, whose origin in the early-stage embryo is unknown, are separately generated in the aorta–gonad–mesonephros (AGM) region and later seed the adult bone marrow. b, The ‘common’ model. Adult blood-forming cells in the AGM regions at least partly originate from Runx1-positive progenitors in the yolk sac. Black and blue arrows show pathways of embryonic and adult blood-forming cells, respectively; bold blue arrow is the pathway described by Samokhvalov and colleagues¹. Regions of cells at the primitive-streak stage: notochord (blue); somites (yellow); heart and cranial mesoderm (green); extraembryonic mesoderm (pink).

Source: Ueno-Weissman: Blood lines from embryo to adult Nature 446, 996-997 (26 April 2007)