Fig. 1. Expression of Pitx2c in the branchial arch mesoderm and aortic sac. (A,B) Whole-mount images of 9.5 dpc wild-type embryos hybridized to a Pitx2c-specific probe showing Pitx2c in left branchial arch mesoderm (arrow in B). (C-E) Transverse serial sections through a 9.5 dpc wild-type embryo hybridized to Pitx2c probe. Pitx2c expression is denoted by arrows. (F,G) Coronal sections through a 9.5 dpc embryo hybridized to a Pitx2c probe showing Pitx2c expression in left branchial arch mesoderm near junction of the aortic sac with the branchial arch artery (arrow in F). Pitx2c expression within left aortic sac myocardium is denoted by arrowhead. In G, arrow designates Pitx2c expression in dorsal branchial arch mesoderm in proximity to forming branchial arch arteries. (H,I) Parasagittal section through a 10.5 dpc embryo hybridized to Pitx2c probe. Arrow denotes ventral Pitx2c expression while arrowhead in I indicates less abundant dorsal Pitx2c expression. (J-M) Transverse sections through hearts of 10.5 dpc embryos hybridized to Pitx2c probe. Arrows in J,K,L indicate Pitx2c expression in dorsal mesocardium, while arrowhead in L indicates Pitx2c expression in left atrium. Arrows in M indicate Pitx2c expression in right ventricular and interventricular myocardium. as, aortic sac; ba, branchial arch; baa, branchial arch artery; da, dorsal aorta; oft, outflow tract.