Fig. 1. Axons from Wg-expressing neurons form pathways for glial migration to different optic lobe target layers. (A) Schematic diagram of developing optic lobe architecture and glial migration viewed from a lateral perspective in a late third instar larval stage brain. Dorsal is upwards and posterior rightwards. Inner chiasm glia (Xi, red), medulla neuropile glia (MNG, purple), marginal and epithelial lamina glia (Ma/Ep, yellow), and lobula neuropile glia (LoG, light blue) exit the Wg domains (Wg, dark blue) via distinct scaffold axon fascicles. The eye disc (ed), photoreceptor axons (R-cell) and optic stalk (os) are indicated. The medulla (med, dark green), lamina (lam, light green) and lobula (lob, brown) neuropiles are indicated. (A') An enlarged schematic view of the ventral Wg domain, showing the distinct scaffold axon pathways for different glial cell types. (B) Optic lobe architecture and glial migration viewed from the horizontal perspective. Distal is upwards. The scaffold axon fascicles are distributed on the proximal distal axis. The pathway for lamina glia is the most distally positioned, while the tract for lobula neuropile glia is most proximal. Labeling as in A. (B') An enlarged view of the ventral Wg domain in horizontal view, illustrating the lamination of scaffold axon fascicles and glial cell types. (C) Two scaffold axon fascicles (orange arrowheads) are observed extending from each of the dorsal and ventral Wg domains (Wg; dorsoventral boundary indicated by the orange line in C-F), which are visualized by expression of wg-lacZ (anti-ß-gal; blue) in this late third instar stage brain. The two pairs of scaffold axon fascicles visible in this plane extend to the medulla neuropile (med n'pil, tract for MNG glia) and lobula neuropile (lob n'pil, tract for lobula neuropile glia). The neuropiles are visualized by anti-HRP antibody staining (green). The location of medulla cortex (med cortex) is indicated. (C') Image in C showing only wg-lacZ expression, to permit visualization of the scaffold axon fascicle pairs (orange arrows). (D) Glia (anti-Repo staining; red) seen during migration (white arrowheads) from the Wg domains (wg-lacZ visualized by anti-ß-gal staining; blue). The layer of lamina marginal glia (Ma) is visible in this plane, as well as some inner chiasm glia (Xi) headed towards a more proximal destination. (D') Image shown in D with only wg-lacZ expression shown in order to detail the scaffold axon fascicles (orange arrowheads). (E) On their pathway towards the lamina (lam), glial differentiation is marked by the onset of Gcm expression (anti-Gcm, white, cells between arrowheads). The glia emerge from a region in which Optomotor Blind (Omb; anti-Omb, purple) is expressed under Wg control, and continue to express Omb as they migrate toward neuropile destinations. The area demarcated by white arrowheads, in which most cells express Gcm, can be compared with the onset of Repo expression in the same region, indicated by white arrowheads in D. (F) Scaffold axon fascicles (orange arrowheads) originate from a subset of Wg expressing neurons (wg-lacZ; anti-ß-gal staining; white), which co-express the neuronal marker Elav (anti-Elav; red). Neuronal populations and neuropiles are labeled as in C. Scale bar: 20 µm.