Fig. 3. In the absence of Gliolectin, commissural axons cross the midline at the dorsal boundary of the nerve cord rather than interdigitating between the midline glial cells. All panels are transverse sections through the ventral nerve cord (20 µm, ventral towards the bottom) of stage 12/1 embryos (equivalent to Fig. 2A,F) at the position of maximal commissural axon density. Axons are labeled with mAb BP102 (brown) and the entire width of the nerve cord is presented. (A) Wild-type embryo double stained with mAb 1B7 (black) to visualize the midline glial cells (MG) demonstrates the intimate association between diffuse commissural axons and Gliolectin-expressing glial cells. The forming longitudinal comprises a distinct lateral mass of BP102 staining axons (arrows). (B) Without 1B7 staining, the fine distribution of commissural axons between the midline glia is apparent in a wild-type embryo as is the density of fibers in the forming longitudinal (arrows). (C) However, in 3013/3013 embryos, commissural axons fasciculate into a dense bundle that crosses the midline at the dorsal boundary of the nerve cord (arrowhead) and longitudinal mass is greatly reduced (arrows). Scale bar: 5 µm.