Fig. 5. Retinal innervation is locally required for scaffold axon outgrowth. (A-A'') A wild-type late third instar stage optic lobe (lateral view) in which the Wg domains (indicated by a pair of arrows) are marked by wg-lacZ (anti-ß-gal staining; blue). Glia are marked by anti-Repo staining (red). MNG and Xi glia are visible in this focal plane. These two glial types are more easily discerned when anti-HRP staining (green, A) is omitted in A' and A''. (A'') An enlarged view of the area boxed in A', where the alignment of MNG glia (yellow arrowheads) with a scaffold axon fascicle is evident. (A'') wg-lacZ staining is shown in grayscale. (B-B'') An so¹, wg-lacZ late third instar stage optic lobe in which all photoreceptor axon innervation is absent. The medulla neuropile (arrow in B) is somewhat disorganized (med n'pil; anti-HRP, green). Glia (red) are mostly absent from the neuropile region, and appear accumulated at the edge of the Wg domains (yellow arrowheads in B',B''). Scaffold axons are completely absent, as is clear in the higher magnification view (B'') of the boxed area in B'. (C-C'') An so¹, wg-lacZ late third instar specimen in which photoreceptor axons have innervated only the dorsal half of the optic lobe. The dorsal scaffold axon fascicles (anti-ß-gal staining, blue) have extended (yellow arrow in C', C'') and glia (red) migrate into the neuropile. By contrast, ventral scaffold axons are missing and the glia stall (yellow arrowhead) as in B. Medulla neuropile organization is disrupted in the ventral half of the optic lobe. White or yellow lines indicate the boundary between dorsal and ventral Wg domains. Scale bars: 20 µm in all panels.