Kdra and Kdrb cooperate to mediate vasculogenesis during zebrafish embryogenesis. Fli-gfp transgenic embryos were injected with combinations of morpholinos against kdra and kdrb or 5-bp mismatch control morpholinos. (A) Twenty-eight hpf embryos injected with either of 2 5-bp mismatch control morpholinos (4.5 ng each) corresponding to separate 5′UTR kdrb morpholinos (m1-kdrb [n = 32] and m2-kdrb [n = 44]) or the previously published kdra morpholino (m-kdra [n = 38]) had no vascular abnormalities. (B) Injection of 4.5 ng kdra or kdrb morpholino has no demonstrable effect on vasculogenesis at 28 hpf (107 embryos). Vascular defects were seen at higher concentrations of kdrb morpholinos; however, similar defects, but at a reduced penetrance, were also seen with equivalent amounts of the mismatched controls, suggesting these effects were not directly related to Kdrb function. (C) Coinjection of either 4.5 ng m1-kdrb or m2-kdrb with 4.5ng kdra morpholino caused variable loss of intersegmental arteries in 42 (24%) of 178 kdra/m1-kdrb or 53 (38%) of 138 kdra/m2-kdrb morpholino–injected embryos. (D) RBCs are seen in the anterior axial vasculature but cannot circulate. (E,F) Kdra/kdrb double morphant defects are still seen at 2 dpf. (G-I) At 4 dpf, injection of 4.5 ng kdra morpholino led to defects in the angiogenesis (◀, ) in 34 (33%) of 101 injected embryos. Embryos coinjected with morpholinos targeting both kdra/b had severe axial vessel defects, although some subintestinal vasculature is apparent. The results are combined from at least 4 separate experiments and the photomicrographs are representative of the visible defects.