Vascularization of the mouse embryo is accomplished via the collaboration of two major cellular processes. Differentiation of vascular endothelial cells de novo from their precursors, called the angioblasts, has been termed vasculogenesis (Risau 1997). Somewhat confusingly, the term vasculogenesis has also been used in a broader sense, covering all aspects of vascular development (Dumont et al. 1994; Noden 1991; Sherer 1991; Wilting and Christ 1996). Subsequent expansion of the endothelium by remodelling, migration and proliferation is called angiogenesis. Vasculogenesis is known to occur intraembryonically within the splanchnopleural and paraxial mesoderm, as well as extraembryonically in the yolk sac mesoderm, and is responsible for laying down the primitive vascular network (Pardanaud et al. 1996; Wilting and Christ 1996). Vasculogenesis is characterized by angioblast differentiation and by either the immediate aggregation of angioblast cells to give rise to endothelium, or the directed migration of these cells through the embryo to segregate eventually into vascular cords. Interestingly, angioblasts appear to have different characteristics depending on their site of origin (Pardanaud et al. 1996). Vasculogenesis both in the yolk sac and in the splanchnic mesoderm appears closely linked to haemangioblast (Pardanaud et al. 1989; Shalaby et al. 1995, 1997).