Distinct WNT Pathways Regulating AER Formation and Dorsoventral Polarity in the Chick Limb Bud

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Abstract
The apical ectodermal ridge (AER) is an essential structure for vertebrate limb development. Wnt3a is expressed during the induction of the chick AER, and misexpression of Wnt3ainduces ectopic expression of AER-specific genes in the limb ectoderm. The genes β-catenin and Lef1 can mimic the effect of Wnt3a, and blocking the intrinsic Lef1activity disrupts AER formation. Hence, Wnt3a functions in AER formation through the β-catenin/LEF1 pathway. In contrast, neither β-catenin nor Lef1 affects theWnt7a-regulated dorsoventral polarity of the limb. Thus, two related Wnt genes elicit distinct responses in the same tissues by using different intracellular pathways.