Fig. 2. Primitive heart tube extension is perturbed in had mutant embryos. (A-F) All panels show dorsal views, anterior to the bottom. Cardiac cells (arrows) are visualized by in situ hybridization analysis using a vmhc probe. (A) Bilateral cardiac primordia have fused at the midline in wild type zebrafish embryos at 22 hpf. (B) By 24 hpf, the fused heart has grown into a tubular structure, known as the primitive heart tube. (C) By 28 hpf, a long tubular heart is clearly visible in the wild types. In had mutant embryos, the fusion of bilateral cardiac primordia appears normal (D), but primitive heart tube extension is severely defective (E,F). (E) The primitive heart of the had mutant is a cone-shaped structure at the midline at 24 hpf, and the primitive heart tube is significantly shorter at 28 hpf (F), compared with that observed in wild-type siblings (C).