Building limb morphology through integration of signalling modules

Growth and patterning of the vertebrate limb relies on signals produced by three discrete signalling centres: the Apical Ectodermal Ridge (AER), the Zone of Polarising Activity (ZPA) and the dorsal ectoderm. The molecular identities of these signals and their associated downstream pathways have begun to be uncovered. In this review, we focus on recent work that has highlighted the importance of cross-talk between these signalling centres and how mesenchymal progenitors integrate multiple signalling inputs. We also discuss recent evidence suggesting how modulations of key signalling pathways have been used to generate the morphological diversity seen between different vertebrate limb appendages.