Fig. 3. Delineation of outflow tract proliferation, apoptosis and myocardialization in E13.5 wild-type and Rxra^–/– embryos. (A,B) Transverse sections of the outflow tracts of E13.5 wild-type and Rxra^–/– embryonic hearts, respectively. Broken line depicts the boundary between endocardial cushion tissue (ec) and myocardializing myocytes. ao, aortic outlet. pa, pulmonary outlet. (C) Adjacent section to A immunostained for MLC2v (red) and BrdU (green). Note less cells are BrdU-positive in the myocardial cuffs (asterisks) and endocardial cushion tissue compared with the ventricular myocardium. Myocardializing myocytes are readily identified (arrowheads) and were more easily visualized at higher magnification (G, arrows). (D) Adjacent section to B immunostained for MLC2a (red) and BrdU (green). Similar to wild-type hearts, less cells are BrdU-positive in the myocardial cuffs (asterisks) and endocardial cushion tissue compared with the ventricular myocardium in the Rxra^–/–. (E) Adjacent section to C immunostained for MLC2a and apoptotic cells using the TUNEL assay. Apoptosis was largely confined to the endocardial cushion tissue between the myocardial cuffs (asterisks) in the outflow tract. (F) Adjacent section to D immunostained for MLC2v and apoptotic cells using the TUNEL assay. Apoptosis was again largely confined to the endocardial cushion cells between the myocardial cuffs (asterisks). Myocardializing myocytes were also identifiable in the Rxra^–/– (arrowheads) and were more easily visualized at higher magnification (H, arrows).