Fig. 2. Developmental profile of scaffold axon outgrowth, glial migration and retinal innervation. Wg expression commences in the mid-1st instar larval stage in a pair of domains in the outer anlagen, preceding retinal innervation by several days. Scaffold axon fascicle elaboration was visualized by wg-lacZ expression (blue in A-E or white in A'-E'). Glia were visualized by anti-Repo staining (red in all panels). Neuronal cell bodies and neuropiles were visualized by anti-HRP staining (green in A-E). Above each optic lobe image, which shows a different time point (youngest at left in A), is a schematic diagram indicating the state of ommatidial development in terms of the number of ommatidial columns. The developing optic lobe is outlined in white in A-E and yellow in A'-E'. (A,A') An early 2nd instar stage specimen, lacking any retinal innervation (0 columns), in which the two Wg domains contain a small number of cells. A small island of Repo-positive glia are observed in the prospective photoreceptor axon target region (arrowhead). Scaffold axon fascicles are not detected. (B,B') A mid-2nd instar specimen, a time point just prior to ommatidial development. The anlage have grown in size, but scaffold axon tracts are not yet detectable. (C,C') A late 2nd instar specimen in which one ommatidial column is detected in the retina, and a small amount of retinal innervation has occurred. The medulla neuropile has just begun formation. Scaffold axon fascicles are clearly visible emanating from the Wg domains and extending towards the neuropile (green arrowheads in C'). There is an increase in glia number within the neuropile region at this time point (yellow arrowhead). (D,D') An early 3rd instar stage animal in which three columns of developing ommatidia were present in the retina. Scaffold axon fascicles begin to elaborate an architecture reminiscent of the late third instar (compare with Fig. 1C), in which tracts for medulla neuropile glia and lobula neuropile glia are distinguishable (green arrowheads in D'). Considerably more glia are visible in the neuropile region (yellow arrowhead in D') in the focal plane shown. (E,E') A mid-3rd instar stage specimen in which seven columns of ommatidia were detected in the retina. Scaffold axon fascicles for MNG glial migration are indicated (green arrowheads in E').