Summary of motor projections to bidorsal hindlimbs. (a) Observed trajectories of motor axons. In Brn4-creTg/-, BmprIaflox/-mutant embryos with bidorsal hindlimbs, medial LMC axons do not invade the limb mesenchyme but are deflected to the ventral flank. Lateral LMC axons innervate both the dorsal and the ventral half of the bidorsal limb. (b) Trajectory preferences. LMC axons normally choose among three mesenchymal trajectories at the base of the limb, where they have different relative trajectory preferences (I > II > III for each branch; as it is unclear if lateral LMC axons ever enter the ventral flank, this branch is represented as a broken line). Medial LMC: ventral limb > ventral flank > dorsal limb. Lateral LMC: dorsal limb > ventral limb > ventral flank. (c) Model of guidance interactions. All medial LMC axons express receptor for a repellant guidance cue. A rostral subset of axons expresses high levels of receptor. In normal embryos, target mesenchyme expresses a step gradient of repellant cues: high levels in the dorsal limb, intermediate levels in the ventral limb, low levels in the ventral flank. Medial LMC axons expressing receptor at low levels are directed to the ventral limb by the high level of dorsal limb repellant, while the medial LMC axons expressing the receptor at high levels are directed to the flank by the intermediate levels of ventral limb repellant. All medial LMC axons also express receptor for an attractant cue expressed in limb mesenchyme. In bidorsal embryos, both limb halves express the repellant cue at high levels. Consequently, both subpopulations of medial LMC axons are directed to the flank. Blue (Y): receptor for repellant cue. (-): repellant guidance cue. (+): attractant guidance cue. Attractant receptor expressed on all medial LMC axons is not shown. Red axons: medial LMC. Green axons: lateral LMC.