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As the early embryos used in most experiments are about 1–2 mm in size, a number of embryos are usually put together in wells and the compound of interest, if small and lipophilic, added to the water. However, test peptides/proteins have to be injected into the yolk sacs of embryos at 20 h post fertilization. Moreover, it is debatable as to whether vasculogenesis or angiogenesis is being measured during such peptide screens. Firstly, because vasculogenesis and angiogenesis are not separated definitively, temporally or spatially in the developing embryo, and secondly, in the trunk of the developing zebrafish embryo, the dorsal aorta and posterior cardinal vein have been clearly shown to be formed by vasculogenesisThe intersegmental vessels that sprout from the dorsal aorta are believed to do so by angiogenesis. However, these vessels are derived from angioblasts that migrate along the dorsal aorta from the lateral posterior mesoderm and sprout at the intersomitic boundaries. It is unknown whether these angioblasts differentiate into endothelial cells before initiating sprouting (angiogenesis), or remain as angioblasts during sprouting in which case vasculogenesis type II is the mechanism by which intersegmental vessels are formed (angioblastic vessel sprouting). Because markers used to observe the formation of intersegmental vessels (Tie-2, Flk-1) are expressed by both angioblasts and endothelial cells, further studies including the monitoring of migratory angioblasts and identification of endothelial cell-specific expression markers may clarify this ambiguity. Alternatively, human endothelial cell cultures may also be used to determine if effects on intersegmental vessel formation in vivo are due to anti-angiogenesis, because such human in vitro cultures contain only differentiated endothelial cells and not angioblasts. The ultimate advantage of such an in vivo screening system is that for each test peptide, only a single assay need be performed. If reliance on human in vitro cultures continues, three individual assays on each peptide are required to monitor the ability of test peptides to inhibit endothelial cell migration, proliferation and differentiation. Using an in vivo system such as the zebrafish that indicates disruption to angiogenesis whether it will be effecting migration, proliferation or differentiation seem less labour intensive, time saving and suitable as an initial screening system following which positive effects can be investigated more thoroughly using human in vitro cultures.