Fig. 2. Effects on branchial arch development of the RARß agonist BMS453. External (A-D) and histological (E,F) aspects of the pharyngeal region of wild-type embryos after 48 hours in culture without retinoid (vehicle; A,C,E) or in the presence of the RARß-selective agonist (B,D,F). Embryos in C and D were injected with ink. Frontal histological sections (E,F) were obtained from comparable levels of the embryos. A1-A3, aortic arches 1 to 3, respectively; A1-2 fused first and second aortic arches; B1-B3, branchial arches 1 to 3, respectively; B1-2, fused first and second branchial arches; D, dorsal aorta; H, heart; P1, first pharyngeal pouch. White arrowhead (A) indicates the first branchial cleft. Scale bar in F: 200 µm in E,F.