Morphogen gradients in vertebrate limb development

The developing limb is an excellent model for pattern formation in vertebrate embryos. Signalling by the polarizing region controls limb pattern across the antero-posterior axis of the chick limb. It was suggested first on theoretical grounds that signalling by the polarizing region could involve a morphogen gradient. Embryological manipulations provided evidence consistent with this model and, more recently, signalling molecules associated with the polarizing region have been identified and tested for their role as morphogens. It is still not clear whether any of the known molecules act directly as a morphogen. The extension of the morphogen model to patterning along the other axes of the limb has been proposed but this may not be applicable.