Tbx-2, Tbx-3, Tbx-4 and Tbx-5 chick genes have been isolated and, like the mouse homologues, are expressed in the limb regions. Tbx-2 and Tbx-3 are expressed in anterior and posterior domains in wings and legs, as well as throughout the flank. Of particular interest, however, are Tbx-5, which is expressed in wing and flank but not leg, and Tbx-4, which is expressed very strongly in leg but not wing. Grafts of leg tissue to wing and wing tissue to leg give rise to toe-like or wing-like digits in wing and leg respectively. Expression of Tbx-4 is stable when leg tissue is grafted to wing, and Tbx-5 expression is stable when wing tissue is grafted to leg. Induction of either extra wings or legs from the flank by applying FGF-2 in different positions alters the expression of Tbx-4 and Tbx-5 in such a way that suggests that the amount of Tbx-4 that is expressed in the limb determines the type that will form. The ectopic limb always displays a limb-like Tbx-3 expression. Thus Tbx-4 and Tbx-5 are strong candidates for encoding ‘wingness’ and ‘legness’.
Tbx genes and limb identity in chick embryo development
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A. Isaac, C. Rodriguez-Esteban, A. Ryan, M. Altabef, T. Tsukui, K. Patel, C. Tickle, J.C. Izpisua-Belmonte; Tbx genes and limb identity in chick embryo development. Development 15 May 1998; 125 (10): 1867–1875. doi: https://doi.org/10.1242/dev.125.10.1867
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