Fig. 1. Tbx1 expression in the pharyngeal region. (A,A') X-gal staining pattern generated by a Tbx1-lacZ knock-in allele (Tbx1^+/–) at E8 (A), and in a transverse section through the pharynx of the same embryo (A'). D, dorsal; V, ventral; da, dorsal aorta; se, surface ectoderm; pe, pharyngeal endoderm; pm, pharyngeal mesoderm; p, pharynx; h, heart. (B,B') Tbx1 expression in surface ectoderm overlying the presumptive caudal arches (arrowheads) at E9. A, anterior; P, posterior; II, III, second and third pharyngeal arch. (C,C') Radioactive in situ hybridization with an Fgf8 antisense probe on coronal sections of wild-type (C) and Tbx1^–/– (C') embryos at E9. There is a loss of Tbx1 expression in the pharyngeal endoderm (arrowhead) but not in the surface ectoderm (arrow) of Tbx1^–/– mutants. (D,D') Fate map of Tbx1-expressing cells in the fourth pharyngeal arch: X-gal staining pattern in coronal sections of a Tbx1^mcm/+;R26R embryo at E10.5. Red arrowhead in D' indicates X-gal staining in endothelial cells lining the fourth pharyngeal arch artery (PAA); no staining is observed in the surrounding vascular smooth muscle (red arrow). A, anterior; P, posterior; III, IV, third and fourth pharyngeal arch. Scale bars: 100 µm in A,A',B',C-D'; 10 µm in B.