DORSAL-VENTRAL SIGNALING IN LIMB DEVELOPMENT

In both Drosophila wings and vertebrate limbs, signaling between dorsa l and ventral cells establishes an organizer that promotes limb format ion. Significant progress has been made recently towards characterizin g the signaling interactions that occur at the dorsal-ventral limb bor der, Studies of chicks have indicated that, as in Drosophila, this sig naling process requires the participation of Fringe. Studies of Drosop hila have indicated that Fringe functions by inhibiting the ability of Notch to be activated by one ligand, Serrate, while potentiating the ability of Notch to be activated by another ligand, Delta. Recent stud ies of both Drosophila and vertebrates have also shed new light on the signaling activity of the dorsal-ventral boundary limb organizer, and have highlighted how this organizer is maintained by feedback mechani sms with neighboring cells.