Dorsal-ventral signaling in limb development.

In both Drosophila wings and vertebrate limbs, signaling between dorsal and ventral cells establishes an organizer that promotes limb formation. Significant progress has been made recently towards characterizing the signaling interactions that occur at the dorsal-ventral limb border. Studies of chicks have indicated that, as in Drosophila, this signaling process requires the participation of Fringe. Studies of Drosophila 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 studies 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 mechanisms with neighboring cells.