Fig. 6. Tbx5 expression depends on WNT/ß-catenin signaling and is sufficient for limb induction in chick embryos. All panels show anterior to the top. (A,B) Axin- and EGFP-expressing adenoviruses were co-injected into the LPM of stage 8 chick embryos. At stage 15, Tbx5 expression was downregulated in the injected side (A, arrow). EGFP expression marks the spatial distribution of the adenovirus and the integrity of the injected tissue (B, arrow). A and B are images of the same embryo. (C-F) tbx5 regulates Fgf10 expression and mediates limb outgrowth in chick embryos. RCAS expressing an N-terminal truncated mutant of Tbx5 (Tbx5N; C,E,F) or control RCAS (D) was injected into the presumptive wing field of stage 8 embryos. In stage 16 embryos injected with RCAS-Tbx5N, Fgf10 expression is downregulated on the injected side (C, arrow). Cartilage staining of stage 36 injected embryos revealed that truncations occurred in the zeugopodal and autopodal structures of the wing, and consisted of hypoplasia of the radius and ulna as well as the complete absence of anterior digits (E,F, arrows). Control RCAS infection caused no obvious wing phenotypes (D). (G,H) Tbx5 is sufficient for limb induction in chick embryos. RCAS expressing full length Tbx5 was injected into stage 7 chick embryos. Five days after injection, cartilagenous elements of the embryos were visualized by Alcian Blue staining. Ectopic limb-like structures were induced (arrows), and cartilage staining revealed additional autopod- and zeugopod-like elements.

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