Differential expression of Eph receptors and ephrins correlates with the formation of topographic projections in primary and secondary visual circuits of the embryonic chick forebrain

Repulsion plays a fundamental role in the establishment of a topographic ma p of the chick retinotectal projections. This has been highlighted by studi es demonstrating the role of opposing gradients of the EphA3 receptor tyros ine kinase on retinal axons and two of its ligands, ephrin-A2 and ephrin-A5 , in the tectum. We have analyzed the distribution of these two ephrins in other retinorecipient structures in the chick diencephalon and mesencephalo n during the period when visual connections are being established. We have found that both ephrin-A2 and ephrin-A5 and their receptors EphA4 and EphA7 are expressed in gradients whose orientation is consistent with the topogr aphy of the nasotemporal axis of the respective retinofugal projections. In addition, their distribution suggests that receptor-ligand interactions ma y be involved in the organization of connections between the different prim ary visual centers and, thus, in the topographic organization of secondary visual projections. Interestingly, where projections lack a clear topograph ic representation, a uniform expression of the Eph-ephrin molecules was obs erved. Finally, we also show that a similar patterning mechanism may be imp licated in the transfer of visual information to the telencephalon. These r esults suggest a conserved function for EphA receptors and their ligands in the elaboration of topographic maps at multiple levels of the visual pathw ay. (C) 2001 Academic Press.