Graded expression of EphA3 in the retina and ephrin-A2 in the superior colliculus during initial development of coarse topography in the wallaby retinocollicular projection

We describe the expression of EphA3 and EphA7 receptors and ephrin-A2 ligan d in the retina and the superior colliculus during the development of the r etinocollicular projection in the marsupial wallaby (Macropus eugenii), usi ng immunoblotting and immunohistochemistry. EphA3 in the retina was in a st riking, low central to high peripheral gradient, superimposed on which was a high temporal to low nasal level of expression. This distribution was evi dent from postnatal day 30, when axons are growing into the colliculus and forming a coarsely organized topographic projection, to postnatal day 65, w hen axons have arborized in their correct retinotopic positions. EphA7 show ed a shallow centroperipheral gradient with no nasotemporal differences in expression. In the superior colliculus no rostrocaudal differences in distr ibution were detected for either of these receptors. Ephrin-A2 was distribu ted in a gradient increasing from the rostral to the caudal pole in the sup erficial layers of the superior colliculus only up to postnatal day 30. Eph rin-A2 was evenly distributed in the retina throughout development of the p rojection. Expression of EphA3 in the retina increased, while the expressio n of ephrin-A2 in the colliculus was downregulated over time. The graded ex pression of EphA3 and ephrin-A2 early in the development of the projection suggests that they play a role in establishment of coarse topography of ret inal axons along the rostrocaudal axis of the superior colliculus. However, the gradients were not complementary, meaning that EphA3 alone cannot medi ate the repulsive interactions with ephrin-A2 that have been postulated to underlie formation of the topographic map.