(A,B) Macroscopic view of Mef2c-AHF-Cre (Mef2c-Cre, control) and Hoxb1GoF;Mef2c-Cre embryos at E9.5. (A’,B’) High magnification of embryo in A and B showing GFP activity in the Cre-recombinase driven cells. (C,D) Immunofluorescence with MF20 (red) in control (C) and Hoxb1GoF;Mef2c-Cre (D) hearts at E10.5. (E–J) Hematoxylin and Eosin (H and E) staining on transversal section of control (E,I) and Hoxb1GoF;Mef2c-Cre (F,J) hearts at E12.5 and E15.5. The right ventricle (asterisk) in Hoxb1GoF;Mef2c-Cre hearts is hypoplastic compared to control hearts (n = 8). (G,H) Whole-mount views of control (G) and Hoxb1GoF;Mef2c-Cre (H) hearts at E15.5. (K,L) Immunofluorescence with Isl1 (red) and Phospho-histone H3 (pHH3, green) on Mef2c-Cre (K) and Hoxb1GoF;Mef2c-Cre (L) embryos at E9.5. (M) Quantification of pHH3-positive cells in the aSHF Isl1+, showed a reduced of the mitotic index in Hoxb1GoF;Mef2c-Cre (n = 3; GoF) compared to Mef2c-Cre (Ctr) (n = 7) embryos. (N,O) Immunofluorescence with Isl1 (red) and Caspase 3 (Cas3, green) on Mef2c-Cre (N) and Hoxb1GoF;Mef2c-Cre (O) embryos at E9.5. Arrowheads indicate Cas3-positive cells. (P) Quantification of Cas3-positive cells revealed increased cell death in the aSHF of Hoxb1GoF;Mef2c-Cre embryos. (Q) Quantification of TUNEL staining performed on Mef2c-Cre (Ctr) and Hoxb1GoF;Mef2c-Cre embryos (GoF). ao, aorta; avc, atrioventricular canal; ba, branchial arch; dpw, dorsal pericardial wall; f, foregut pocket; la, left atrium; lv, left ventricle; nt, neural tube; oft, outflow tract; PM, pharyngeal mesoderm; pt, pulmonary trunk; ra, right atrium; rv, right ventricle. Scale bars: 100 μm (A’,B’,D,E,F); 200 μm (A’,B’,E,F); 500 μm (A,B,G-J).