Fig. 5. Chamber formation in Tbx20 mutant hearts. Whole-mount in situ hybridization analysis of expression of cardiac chamber markers in wild-type and Tbx20^–/– hearts at E9.5. All views are from the left lateral side, anterior up. Expression of markers for ventricular (arrows in wild-type embryos in A-F) and for atrial chamber myocardium (arrowheads in A-C) are missing in the corresponding stage-matched Tbx20 mutant hearts. Chisel is expressed very weakly in the primitive ventricle of the Tbx20^–/– heart (arrow in B'). Vessel formation is affected in the mutant, as shown by Cx40 expression, which marks branchial arch vessels and dorsal aorta in the wild type (white arrows in C), but reveals only scattered cells in the branchial arch region in the mutant (white arrow in C'). Tbx3 expression is only found in the atrioventricular canal of the wild-type heart (arrow in G). (H,H') Tbx18 expression in the proepicardial organ (arrow) and the septum transversum (arrowhead) is unchanged in Tbx20^–/– embryos. Note that expression of markers for pharyngeal mesoderm, Hey2 and Tbx3 (arrowheads in D and G) is preserved in Tbx20 mutants (arrowheads in D' and G'). Markers and genotypes are indicated in the figure.