We show here that bone morphogenetic protein 2 (BMP-2) is involved in patterning the developing chick limb. During early stages of limb development, mesenchymal expression of the Bmp-2 gene is restricted to the posterior part of the bud, in a domain that colocalizes with the polarizing region. The polarizing region is a group of cells at the posterior margin of the limb bud that can respecify the anteroposterior axis of the limb when grafted anteriorly and can activate expression of genes of the HoxD complex. We dissect possible roles of BMP-2 in the polarizing region signalling pathway by manipulating the developing wing bud. Retinoic acid application, which mimics the effects of polarizing region grafts, activates Bmp-2 gene expression in anterior cells. This shows that changes in anteroposterior pattern are correlated with changes in Bmp-2 expression. When polarizing region grafts are placed at the anterior margin of the wing bud, the grafts continue to express the Bmp-2 gene and also activate Bmp-2 expression in the adjacent anterior host mesenchyme. These data suggest that BMP-2 is part of the response pathway to the polarizing signal, rather than being the signal itself. In support of this, BMP-2 protein does not appear to have any detectable polarizing activity when applied to the wing bud. The pattern of Bmp-4 gene expression in the developing wing bud raises the possibility that BMP-2 and BMP-4 could act in concert. There is a close relationship, both temporal and spatial, between the activation of the Bmp-2 and Hoxd-13 genes in response to retinoic acid and polarizing region grafts, suggesting that expression of the two genes might be linked.

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