Fig. 6. Analysis of primary myocardium in Tbx20^–/– hearts. Whole-mount in situ hybridization analysis in E7.5 to E9.5 embryos shows the presence of markers for primary myocardium (Tbx2, Bmp2) and endocardium (vinexin ) in the heart tube of Tbx20 mutant embryos. (G-H') Anti-PECAM immunohistochemistry for endocardial endothelium. Images are of whole embryos (A,A') and magnifications of anterior body regions, including the heart (B-N'), in views from the right lateral side (A-B',D,D',F-I',K-L',N,N'), or from the ventral side (C,C',E,E',J,J',M,M'). Anterior is up. Embryos are E7.5 (A,A'), E8.25 (B-C'), E8.5 (D-E',G,G',I-J',L-M') and E9.5 (F,F',H,H',K,K',N,N'). Arrow in A' marks Tbx2 expression in the cardiac crescent in Tbx20 mutant embryos. Arrows in all other figures point to the left ventricle that is free of hybridization signals for markers of primary cardiac phenotype. By contrast, corresponding mutant hearts show expression of these genes in the primitive ventricle (arrowheads in lateral views of E8.5 and 9.5 mutant hearts). Note the increase of Tbx2 expression (C') but decrease of Bmp2 expression (M') in the primitive inflow tract of the E8.5 mutant heart. In wild-type embryos, Tbx2 expression is restricted to the atrioventricular canal (avc) and the septum transversum (st). al, allantois. Markers and genotypes are indicated in the figure.