The development of the vertebrate limb is dependent upon two signaling centers, the apical ectodermal ridge (AER), which provides the underlying mesenchyme with essential growth factors, and the zone of polarizing activity (ZPA), the source of the Sonic hedgehog (SHH) product. Recent work involving gain and loss of function of Hox genes has emphasized their impact both on AER maintenance and Shh transcriptional activation. Here, we describe antagonistic interactions between posterior Hoxd genes and Gli3, suggesting that the latter product protects the AER from the deleterious effect of the formers, and we present evidence that Fgf10 is the mediator of HOX-dependent AER expansion. Furthermore, the striking similarity between some of the hereby observed Hox/Gli3-dependent morphogenetic defects and those displayed by fetuses with severely altered retinoic acid metabolism suggests a tight connection between these various pathways. The nature of these potential interactions is discussed in the context of proximal-distal growth and patterning.
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