Fig. 8. Chrd regulates Tbx1 and Fgf8 expression in mutant mice and Xenopus ventral marginal zones. (A-E) Wild-type embryos. (A'-E') Mutant littermates. In mutants, the levels of Tbx1 expression are diminished at E7.5 (A,A'), E8.0 (B,B'), E8.5 (C,C') and E9 (D,D'). Arrowheads indicate the level of sections depicted in F-H'. At E9 Tbx1 transcripts are absent of otic vesicle (arrowhead in D'). At E9, Fgf8 expression in the mutant endoderm has disappeared and mesodermal expression is restricted to a small area in the most posterior part of the neck (arrowhead in E'). (F-H') Nomarski optic photographs of section through wild-type (F-H) and Chrd mutant littermates (F'-H'). Note the reduction of Tbx1 expression in the splanchnic (arrowheads) and somatic mesoderm (arrows), as well as in the mesenchyme of the head and peripharyngeal region. I, first pharyngeal arch; II, second pharyngeal arch; fp, frontonasal prominence; is, isthmus; pe, pharyngeal endoderm; sv, sinus venosus; bc, bulbus cordis; v, ventricular chamber; a, atrial chamber; fe, foregut endoderm; hd, hepatic diverticulum. (I) Xenopus VMZ assays. RT-PCR analyses of RNAs isolated from whole embryos (WE), dorsal marginal zones (DMZ), uninjected ventral marginal zones (VMZ co) and Chrd injected ventral marginal zones (VMZ chrd). Tbx1 and Fgf8 transcripts are high in the whole embryo and DMZ, but not in uninjected VMZ (lanes 1, 2 and 3, respectively). Lane 4 shows that Tbx1 and Fgf8 are induced in VMZs by Chrd injection. Note that the levels of endodermin (Edd), a pan-endodermal marker, are not affected. Ornithine decarboxylase (ODC) is expressed uniformly during embryonic development and serves as a loading control.