Suppression of rap1b expression increases ISV sensitivity to VEGFR2 inhibition. (A) Low- (i-ii,vii-x) and high- (iii-vi) power images of the lateral view of the mid-trunk (i-vi) and anterior (vii-x) region of 28 hpf flk1:EGFP zebrafish embryos injected at 1- to 2-cell stage with 2 ng of MO and treated at 20 hpf with 0.25μM SU5416 or DMSO control for 1 hour, as indicated. Treatment of embryos with a combination of sub-effective doses of SU5416 and rap1b-MO (vi,x) but not control MO (v,ix) led to a complete inhibition of ISV formation in the mid-trunk and decreased ISV length in anterior region of the trunk. Arrows indicate missing ISVs and asterisks indicate truncated ISVs; scale bars are 75 μm. (B) ISV defects in the mid-trunk region of rap1b or control morphants (boxed areas in Ai-ii) were quantified by counting the number of affected embryos (numerator) in each group (denominator). (C) Cartoon represents heterogeneous ISV phenotype of zebrafish embryos treated with a combination of sub-effective doses of SU5416 and rap1b-MO (top, image shown in panel Ax) compared with control embryos treated with SU5416 and control-MO (bottom, image shown in panel Aix). (D) ISV length in the anterior region of the trunk was quantified as diagrammed in panel C. Trajectories of 7 adjacent ISVs from the anterior region (boxed areas in panel Avii-x) were measured in 4-12 embryos per group, as indicated in parentheses above bars.