Fig. 3. T-box gene mutant phenotypes in mice. (A,B) Scanning electron micrographs showing hearts from wild-type (wt; A) and Tbx20^–/– (B) E9.0 embryos (scale bars indicate relative size), showing hypoplasia, lack of looping and abnormal chamber development in the mutant. (C) In situ hybridization showing expression of cardiac marker Nkx2-5 in wild-type and Tbx5^–/– E8.5 embryos, demonstrating a severe reduction of the LV and SA region in the mutant. (D-J) Comparison of Tbx20^–/– embryos with wild-type siblings at E8.5, showing upregulation of Tbx2 (D), reduced proliferation (immunostaining for phosphohistone H3; arrows indicate the heart tube; E-F) and downregulation of cell-cycle gene Nmyc1 (G-J) in mutants. G,H are at the level of the OFT, I,J are at the level of the AVC. Figures reproduced from Bruneau et al., Cai et al. and Stennard et al. (Bruneau et al., 2001; Cai et al., 2005; Stennard et al., 2003), with permission from Elsevier and Company of Biologists Limited. avc, atrioventricular canal; iv, inflow ventricle-like chamber; lv, left ventricle; oft, outflow tract; ov, outflow ventricle-like chamber; rv, right ventricle; sa, sinuatrium.