Fig. 2. Mesodermal ablation of Tbx1 causes severe pharyngeal and cardiovascular abnormalities. (A-K) Mesp1^Cre/+;Tbx1^+/flox control embryos compared with (A'-K') Mesp1^Cre/+;Tbx^E5/flox M-ko mutant embryos. Hypoplastic external ears (arrow A,A'), absence of thymus (T and asterisk, B,B'), persistent truncus arteriosus (PTA, C,C') originating from the right ventricle (D,D') and ventricular septal defect (VSD, D,D') in M-ko embryos at E18.5. (E,E') Hypoplasia of the 2nd pharyngeal arch (arrowhead) in an E10.5 M-ko embryo. (F,F') Cardiac ink injection in E10.5 embryos visualized the 3rd, 4th and 6th pharyngeal arch arteries in cotrols (F); M-ko embryos have only one pair of arteries connecting the aortic sac with the dorsal aorta (F'). (G,G') Coronal sections of E10.5 embryos revealed severe hypoplasia of the pharynx (Ph) in M-ko mutants. (H,H') Wholemount RNA in situ hybridization on E10 embryos using a Crabp1 probe as a neural crest marker. Conditional deletion mutants showed abnormal Crabp1 expressing pattern. (I,I') Whole-mount immunohistochemistry on E10 embryos with an anti-neurofilament M antibody revealed abnormalities of cranial nerve pathways in the mutant. (J,J') Whole-mount RNA in situ hybridization on E10 embryos with a Pax1 probe revealed a small 1st pouch and loss of labeling of pouches 2 and 3 in M-ko mutants. (K,K') Whole-mount immunohistochemistry on E10 embryos with an anti-Hoxb1 antibody revealed a normally specified, albeit slightly smaller, 4th pouch in M-ko mutants. The insets show coronal sections of the same embryos; the 4th pouches of M-ko embryos are not fully developed and not as close to the surface ectoderm as those of control embryos. T, thymus; H, heart; Ph, pharynx; Ao, aorta; P, pulmonary trunk; Pa, pulmonary artery; VSD, ventricular septum defect; RV/LV, right/left ventricle; RSA/LSA, right/left subclavian artery; RCA/LCA, right/left carotid artery; I, II and III, 1st, 2nd and 3rd pharyngeal arch; 3, 4 and 6, 3rd, 4th and 6th pharyngeal arch artery; 1p, 2p, 3p and 4p, 1st, 2nd, 3rd and 4th pharyngeal pouches; V, trigeminal nerve; VII/VIII, facial/acoustic nerve; IX, glossopharyngeal nerve; X, vagus nerve; XI, accessory nerve. Scale bars: 2 mm in A'; 1 mm in B',C',E',F',I',K'; 100 µm in D' and G'.