Retinal Ganglion type specific Axon Targeting to the Superior Colliculus in EphrinA2/EphrinA5 Knock-out Mice

Retinal ganglion cells (RGCs) are responsible for sending visual information from the retina to the brain.  In mice, there are about 20 different types of RGCs, each of which transmits a specific aspect of the visual field.  Each type also terminates in specific lamina of the SC where they form synapses with specific sets on neurons. RGCs of each type send axons to the superior colliculus (SC) where they form precise topographic connections, such that neighboring cells in the retina make adjacent connections in the SC.   Ephrins and Ephs exist in concentration gradients in the SC and retina respectively; as a result, direct RGCs to target in the topographically correct location.  In ephrinA2/ephrinA5 double knock-out mice, the topographic maps in the SC are disturbed, although some RGCs still target correctly. We wondered if these RGCs  are composed of one RGC type or whether all types of RGCs have the ability to correctly target in the absence of ephrin-As. To distinguish between these possibilities we examined the topographic maps of specific RGC types by anterograde tracing RGC axons in mice that also express GFP in a specific RGC type.  We find that each RGC type can be targeted correctly in ephrin-A mutant mice.  Thus these RGC subtypes rely on mechanisms other than EphrinA2/EphrinA5 gradients for axon targeting to the SC.