Fig. 4. Cardiomyocyte differentiation defects in had mutants. (A,B) Transverse sections of two-day-old wild-type (A) and had mutant (B) heart. Arrow indicates myocardium; arrowhead, endocardium. (C-H) Ventral views of 50 hpf embryos, head to the top, solid line marks the site of the constriction between the ventricle (v) and the atrium (a). Cardiac expression of vmhc (C,D), irx1 (E,F) and versican (G,H) was detected by in situ hybridization. The hearts of had mutants are smaller and dysmorphic (B,D,F,H) compared with those in wild types (A,C,E,G). By 50 hpf, expression of vmhc is restricted to ventricles in wild types (C), but expression of this gene extends to the atrium in had mutants (D). Cardiac expression of irx1 is restricted to ventricles in wild-type embryos (E), but is severely reduced in the heart of had mutants (F, arrow). versican is expressed at the boundary of the atrium and the ventricle in wild-type embryos (G), but is expressed throughout had mutant hearts (H).