The Notch pathway in the developing hematopoietic system.

The main function of the Notch signaling pathway is to generate cell diversity during both embryonic development and adult tissue homeostasis. The extended use of this pathway, together with its conservation during evolution, is indicative of its importance. During embryonic development, the vascular and hematopoietic systems are intimately associated and Notch signals are responsible for the correct specification of both systems. More explicitly, Notch is required for the induction of the arterial program; however, it is simultaneously or consecutively also involved in the generation of hematopoietic stem cells. Although both genetic programs are different, they are both implemented in endothelial cells of the dorsal aorta in the midgestation embryo. This close association during the development of arteries and blood has hindered our understanding of Notch function in the generation of hematopoietic stem cells. Here, we will review the work from recent years showing how Notch participates in the embryonic development of hematopoiesis in the mouse, but also in other organisms such as chick, zebrafish and flies.