Fig. 8. A cellular model for the control of glial migration by retinal innervation. A small number of glia (red) migrate into the target region for photoreceptor axons prior to ommatidial development, and are ready to provide initial guidance cues to the first photoreceptor axons (green; top two panels). These early photoreceptor axons trigger the elaboration of the axonal scaffold for glial migration (blue; third panel from top), which extends in stereotypical fashion to establish the multiple pathways (not depicted). Glia generated in the Wg subdomain 1 (yellow) can then migrate to target destinations, such as the lamina (shown, bottom panel). Subsequent migration may be independent of continued retinal axon ingrowth (see the text). Through this mechanism, the distribution of glia throughout the optic lobe is coordinated with innervation by the photoreceptor axons.