(A–E) Lateral views of flk1:GFP transgenic zebrafish embryos at different developmental stages. Anterior to the left. (ye) yolk sac extension. (A) Tail of a control zebrafish embryo at 48 hpf. Note the regular, organized patterning of intersegmental blood vessels and the size of the caudal vein plexus. (B–E) zbmper morpholino injected embryos. (B) Tail of zbmper morphant at 48 hpf. Note the dysmorphic caudal vein with a localized reduction of vessel size (arrow) and aberrant ISV formation (arrowhead). (C) Tail of a zbmper morphant at 72 hpf displaying a severe tail phenotype. Intersegmental vessels cross segmental boundaries (arrowheads). The caudal vein is absent. At the tip of the tail the normal vascular organization is completely lost (*). In this area endothelial cells are detectable but do not form patent blood vessels. (D) Tail of a zbmper morphant at 72 hpf displaying a weak tail phenotype. The indicated intersegmental vessel crosses the respective vessel of the adjacent somite before it connects dorsally. The caudal vein is severely dysmorphic (arrow). (E) Tail of zbmper morphant at 7 dpf. Note the absence of the dorsal ends of ISVs and aberrant sprouting. ye = yolk extension, CA = caudal artery, CV = caudal vein, DLAV = dorsal longitudinal anastomotic vessel, ISV = intersegmental vessel.