Loss of alk1 dysregulates expression of flow-responsive genes but does not alter shear stress. (A) Whole-mount in situ hybridization using klf2a, edn1 and cxcr4a riboprobes. Vascular klf2a expression is strongly downregulated in the absence of blood flow (tnnt2a-/- or tricaine treatment, 32-40 hpf) but is unaltered in alk1-/- embryos. By contrast, vascular edn1 expression is downregulated and cxcr4a expression is upregulated both in the absence of flow and in alk1 mutants. Changes in expression are primarily in alk1-positive arteries: ICA (blue arrow) and CaDI (white arrow), BCA (white arrowhead) and LDA (blue arrowhead), as well as AA1 (not shown). Lateral expression of both edn1 and cxcr4a is in the pharyngeal arches and is unchanged in all conditions. All embryos are 36 hpf except tricaine treated, which are 40 hpf. klf2a and edn1, lateral views, anterior leftwards. cxcr4a, dorsal view, anterior leftwards. Scale bar: 100 μm. (B) Blood flow in the distal region of AA1 was imaged in chrna1-/- (paralyzed) Tg(kdrl:GFP)la116;Tg(gata1:dsRed)sd2 alk1 mutant embryos and wild-type/heterozygous siblings at 32-36 hpf by high-speed confocal microscopy, and particle image velocimetry used to calculate wall shear stress. Results represent mean±s.d., n=8 wild type and n=5 alk1 mutants. Differences were not significant according to Student′s t-test.