Fig. 6. Bifunctional action of ephrin-B1 as a repellent and attractant to direct interstitial axon branches and limit arborization to develop the DV retinotectal map. A and B summarize our findings and C and D summarize a model of the bifunctional action of ephrin-B1 during normal map development in chick and mouse, and the graded expression of EphB and EphA receptors by RGCs and ephrin-B1 and ephrin-As in the tectum (or SC). (A) Interstitial branches of primary RGC axons are normally directed towards their future termination zone (TZ; open circle) either medially or laterally, dependent upon primary axon location along the LM tectal axis. Within ectopic domains of ephrin-B1 (green ovals, and indicated as peaks on the endogenous LM gradient of ephrin-B1), normal bidirectional branch extension is disrupted and branches preferentially extend laterally, regardless of axon position. (B) Ectopic domains of ephrin-B1 shape and inhibit the arborization of interstitial branches at the TZ (red shape). These data suggest that a high level of ephrin-B1 is a repellent for interstitial branches and their arbors. (C) Proposed bifunctional action of ephrin-B1 during normal development of the DV retinotopic map. RGC axons initially extend posteriorly past their future TZ and preferentially form branches along their shafts at the level of the AP location of their TZ (Yates et al., 2001). Both the initial axon overshoot and the formation of interstitial branches are controlled, in part, by a repellent action of ephrin-As on EphA-expressing RGC axons (Yates et al., 2001). Branches extended by RGC axons positioned lateral to their TZ are attracted medially by ephrin-B1 up its gradient toward the TZ (Hindges et al., 2002). Branches extended by RGC axons positioned medial to their TZ are repelled laterally by ephrin-B1 down its gradient toward the TZ. Together these findings suggest that the response to ephrin-B1 of interstitial branches extended by the same DV population of RGC axons, and therefore expressing the same subtypes and levels of EphB receptors, is context-dependent. If an interstitial branch forms along an axon positioned lateral to its TZ, the branch initially extends in a domain of lower ephrin-B1 than found at its TZ. At this level of ephrin-B1, for a given axon, it acts as an attractant and guides branches medially up the gradient of ephrin-B1. Conversely, an interstitial branch that forms along an axon positioned medial to its TZ, encounters a level of ephrin-B1 higher than that at its TZ. At this level, ephrin-B1 acts as a repellent and directs branches laterally down the gradient of ephrin-B1. Therefore, ephrin-B1 may act as a bifunctional guidance molecule to control the position-dependent bidirectional extension of interstitial branches of RGC axons originating from the same DV retinal site. Alternatively, EphB receptor signaling may act as a `ligand-density sensor' and titrate signaling pathways that promote branch extension toward the optimal ephrin-B1 concentration found at the TZ; branches located either medial or lateral to the TZ would encounter a gradient of increasingly favored attachment in the direction of the TZ. (D) Arbors are formed at the TZ exclusively by interstitial branches (Yates et al., 2001). Overshooting segments of the primary RGC axons are eliminated during this process. Based on our findings, ephrin-B1 may also function to help restrict the extent of an arbor along the LM tectal axis. Ephrin-As may help restrict the posterior extent of the arbor (Nakamoto et al., 1996; Yates et al., 2001). Retinal axes: D, dorsal; V, ventral; T, temporal; N, nasal. Tectal axes: L, lateral; M, medial; A, anterior; P, posterior.