Ephrin-As are required for the topographic mapping but not laminar choice of physiologically distinct RGC types.

The vast number of neurons in the brain connect precisely to one another, with the axons of one neuron contacting the dendrites or other parts of many postsynaptic neurons. One of the puzzles that this complex "connectome" poses is how each neuron is correctly wired to its postsynaptic targets. There are many molecular cues that neurons use to connect to each other; in the visual system one family of cues are the Ephs and ephrins (receptors and ligands respectively) whose graded expression in the retina's output neurons and their target brain regions regions allow for the formation of topographic maps. We wondered whether each of the different subtypes of retinal output neurons (called retinal ganglion cells; RGCs) rely equally on this Eph/ephrin system to target their axons correctly. Furthermore, since each RGC subtypes send their axons to different layers of an early target center, we wondered whether they rely on each other to form their topographic maps in different layers or whether each RGC type forms a map independently of other types. We used a genetic mouse model in which several members of the ephrin fmaily were deleted to ask these questions. All RGC types that we looked at showed mistargeting when Eph/ephrin signaling was disturbed in this way, and misalignment between maps of different RGC types implies that each type forms a map somewhat independently of other types.