Motor axon projections are topographically ordered. Medial motor column axons project to axial muscles, whereas lateral motor column axons project to limb muscles and, along the rostrocaudal axis of the animal, the more rostral motor neuron pools project to more rostral muscle targets. We have shown that EphA3 is specifically expressed in the developing medial motor column and have postulated that EphA3 might be responsible for directing their axons to axial muscle targets. This hypothesis was supported by our demonstration that EphA3 can direct retinal ganglion cell axon targeting and by studies of ephrin-A5−/− mutants that show that EphA receptor signaling controls the topographic innervation of the acromiotrapezius. To test the role of EphA3 in motor axon guidance, we generated an EphA3 null mutant. Retrograde labeling studies in EphA3−/− embryos and adults indicate that, contrary to our predictions, EphA3 is not necessary to direct motor axons to axial muscle targets. Our results also demonstrate that ephrin A5's ability to direct topographic innervation of the acromiotrapezius must be mediated through EphA receptors other than, or in addition to, EphA3.
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