Fig. 2. Aberrant projections of retinal ganglion cell (RGC) axons from the retina to the superior colliculus (SC) in Vax2 mutants, as assessed by anterograde axonal tracing with DiI. (A) A focal DiI injection into the ventral temporal (VT) quadrant of the retina (indicated by the circle on the upper panel retinal flatmount schematic) labels RGC axons that terminate at the medial-rostral (MR) border of the SC in wild-type and heterozygous mice [+/–, middle panel, arrow indicates termination zone (TZ)], but at the rostral-lateral border of the SC in the mutant (–/–, lower panel, arrow). Note that for all anterograde labeling we analyzed, the projections of heterozygous Vax2^+/– axons were indistinguishable from the previously described projections of wild-type RGCs. (B) A mid-ventral injection (split circle on schematic) labels RGC axons that terminate near the medial border of the SC at the midpoint of the collicular RC axis in both heterozygous (+/–, middle panel, arrow) and mutant mice (–/–, lower panel, arrow). Arrowheads indicate additional multiple ectopic TZs in the mutant, some of which are lateralized. (C) A focal injection into the ventral nasal quadrant of the retina (circle, upper panel) labels RGC axons that terminate in the medial-caudal SC in both heterozygous (+/–, middle panel) and Vax2 mutant mice (–/–, lower panel). (D-W) SC diagrams depicting expected (squares) and observed collicular TZs (gray) from focal DiI injections into the ventral retinae of Vax2 mutants, illustrating the progressive change in misprojection phenotype from extremely strong for ventral temporal injections (D-F) to extremely weak for ventral nasal injections (U-W). The position of the expected TZ in the SC is plotted from the observed position of the retinal injection site, assuming a linear map from extreme ventral retina=extreme medial SC to extreme dorsal retina=extreme lateral SC. Squares and gray shapes are scaled to indicate relative sizes of retinal injection sites and collicular TZs, respectively. With the exception of S, all panels illustrate results from ventral retinal injection between 60-85% of the DV axis, with extreme dorsal defined as 0%. c, caudal; l, lateral; m, medial; r, rostral.