Fig. 3 Pvr is required for proper CNS morphogenesis. (A,B) Stage 16 (A) wild-type (Canton-S) and (B) Pvr⁵³⁶³ embryos stained with MAb BP102 to visualize CNS axons. ac, anterior commissure. pc, posterior commissure. lo, longitudinal tracts. Red line, CNS midline. In wild type, the commissures and longitudinal tracts form a rectangular axon scaffold in each segment (arrow), whereas in Pvr, the longitudinal tracts are closer together than in wild type, giving the axon scaffold a rounded appearance in each segment (arrow). (C,D) Stage 16 (C) wild type (Canton-S) and (D) Pvr⁵³⁶³ embryos stained with anti-Repo to visualize glia. In wild type, Repo-positive glia are arranged in a patterned array along the two longitudinal tracts, with a single row of Repo-positive glia between the longitudinal tracts in each segment (arrowhead). In Pvr mutants, Repo-positive glia accumulate at the midline (arrowheads), and there are fewer Repo-positive glia in the longitudinal tract region (arrow). The longitudinal tracts appear in outline in the photograph, which was taken using differential interference contrast (DIC) optics. (E,F) Longitudinal axon tracts of stage 17 embryos are stained with the anti-Fasciclin 2 monoclonal antibody 1D4. (E) Wild type. Three tightly bundled axon tracts can be seen on each side of the midline. (F) Pvr⁵³⁶³ mutants also form three axon tracts on either side of the midline. No ectopic axon crossing of the midline can be detected, although minor defasciculation of axon tracts can be seen in some segments (arrowhead).