Haematopoietic stem cells (HSCs), responsible for blood production in the adult mouse, are first detected in the dorsal aorta starting at embryonic day 10.5 (E10.5)(1-3). Immunohistological analysis of fixed embryo sections has revealed the presence of haematopoietic cell clusters attached to the aortic endothelium where HSCs might localize(4-6). The origin of HSCs has long been controversial and several candidates of the direct HSC precursors have been proposed (for review see ref. 7), including a specialized endothelial cell population with a haemogenic potential. Such cells have been described both in vitro in the embryonic stem cell (ESC) culture system(8,9) and retrospectively in vivo by endothelial lineage tracing(5,10) and conditional deletion experiments(11). Whether the transition from haemogenic endothelium to HSC actually occurs in the mouse embryonic aorta is still unclear and requires direct and real-time in vivo observation. To address this issue we used time-lapse confocal imaging and a new dissection procedure to visualize the deeply located aorta. Here we show the dynamic de novo emergence of phenotypically defined HSCs (Sca1(+), c-kit(+), CD41(+)) directly from ventral aortic haemogenic endothelial cells.