Fig. 6. Chimera analysis and Fgf10 regulation by Tbx1. (A-B) Contribution of Tbx1^–/– cells to OFT (arrow) and SHF (square box magnified in A') in a chimeric E10.5 embryo, compared with the same region of a Tbx1^+/– chimera. Note that Tbx1^–/– cells appear more intensely stained because they carry two lacZ knock-in alleles. (C) Tbx1 and Tbx5 activate the Fgf10 promoter carrying a wild-type (Fgf10Luc) but not mutant (Fgf10MutAluc) T-box binding element. Scale bars: 100 µm in A; 50 µm in A' and B. (D-F) RNA in situ hybridization on sagittal sections (E10.25) of control embryos (Tbx1^flox/+) showing overlap of Tbx1 expression (D) and Fgf10 expression (E) in the SHF region (arrowheads). Fgf10 expression is reduced or absent in a conditional mutant (F). (G) Sagittal section (at the same stage) of an Nkx2.5^Cre/+;R26R embryo showing extensive recombination that includes the region in which Tbx1 is expressed. (H) Schematic drawing of the proposed model for Tbx1 function in the SHF. Scale bars: 100 µm in D-G.