DORSOVENTRAL LIMB POLARITY AND ORIGIN OF THE RIDGE - ON THE FRINGE OFINDEPENDENCE

Molecular and developmental studies of limb pattern formation have rec ently gained widespread attention. The fact that vertebrate limbs are amenable to both genetic and embryological manipulations has establish ed this model system as a valuable paradigm for studying vertebrate de velopment. Limb buds are polarised along all three major axes and the establishment of the dorso-ventral (DV) polarity is dependent upon cue s localised in the trunk, where a DV ectodermal interface is produced by confrontation of dorsal and ventral identities. By analogy to Droso phila imaginal disc development, this interface has been proposed to d etermine and position an ectodermal organising centre, the Apical Ecto dermal Ridge (AER), controlling limb bud outgrowth. Recent fate mappin g studies((1)) and studies of genes regulating DV limb polarity((2-6)) , AER formation((7,8)) and differentiation((9)) suggest, however, that DV patterning and AER induction, though coordinately regulated during limb bud outgrowth, may early on be more dissociated than expected.