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Hemogenic endothelium--ontogenesis and role in blood production.

Endothelial and hematopoietic lineages have long been thought to develop from a common ancestor, the hemangioblast. Alternatively, clusters of hematopoietic cells in the dorsal aorta were observed to form in a close association with endothelial wall of the aorta, leading to the hypothesis that a special subset of endothelial cells, called the hemogenic endothelium, generates hematopoietic cells. Recent advances in time-lapse imaging, conditional labeling of cells in vivo and embryonic stem cell differentiation provided new evidence for the existence of both, the hemangioblast and hemogenic endothelium. Importantly, these seemingly contradictory theories can be merged into one model of hematopoietic differentiation from mesoderm.