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Fig. 10. Schematic of the mapping of nasotemporal retina to rostrocaudal colliculus. (A) Normal case. The bar chart along the horizontal axis shows the EphA receptor densities of cells positioned equidistantly along nasotemporal retina. According to an induction model, a monotonically rising profile of ephrinA is distributed across the rostrocaudal axis of the colliculus (also shown as a bar chart). The pattern of connections can be seen by finding, for each retinal cell, the collicular cells that have an ephrinA density that is the inverse of the retinal EphA density. Thus retinal cells with the largest amount of EphA connect to collicular cells with the smallest amount of ephrinA, and vice versa. (B) EphA3 homozygous knockin. There are two populations of retinal cells, each following a different EphA receptor density profile. Green: EphA3⁺ cells; black: EphA3^- cells. According to an induction model, the distribution of ephrinA across the colliculus will also form a monotonically rising profile, with retinal cells with the largest amount of EphA projecting to collicular cells with the smallest amount of ephrinA, and vice versa. The variation of receptor density over the retina now involves both profiles, stretching from its largest value (in EphA3⁺ cells) to its smallest (in EphA3^- cells). Because of the interleaved nature of the two overall EphA receptor density profiles in the retina, the retina will be projected twice over the colliculus, the colour coding indicating which contacts are made by the EphA3⁺ RGCs and which by the EphA3^- RGCs.

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