Fig. 4. Interstitial branches extended within ectopic domains of ephrin-B1 are directed away from their correct TZ. (A) Medial half of an E13 tectum transfected with RCAS-ephrin-B1-IRES-eGFP. A focal injection of DiI was made in NV (nasal-ventral) retina (arrowhead, inset). RGC axons form a TZ in MP (medial-posterior) tectum. Ectopic domains of ephrin-B1 are marked by the green eGFP reporter and RGC axons, interstitial branches and arborizations are labeled red by anterogradely transported DiI. The boxed area lateral to the TZ is enlarged in B-D. (B-D) Most branches that extend within an ectopic domain of ephrin-B1 are directed laterally, away from their TZ (arrows). Branches that extend outside ectopic domains of ephrin-B1 are appropriately directed toward their TZ, as most branches are in control cases. The disruption in the guidance of interstitial branches is confined to ectopic domains of ephrin-B1. Along a single axon, a branch outside an ectopic domain of ephrin-B1 extends toward the TZ (lower arrowhead) whereas a branch inside a domain of ectopic ephrin-B1 is directed aberrantly away from the TZ (upper arrowhead). Scale bar in D: 800 µm (A) and 250 µm (B-D).