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Fig. 5. Foxd1 (BF-2 in figure) is expressed in VT retina and its absence leads to expansion of the Foxg1 (BF-1) and ephrin-A domains. (A) X-gal staining (blue) of retinal wholemounts from Foxd1^+/lacZ embryos at E14.5 (a) and at E16.5 (e) indicates that Foxd1 is highly expressed in VT retina at these ages. Flattened retinal wholemounts at E14.5 (b) and E16.5 (f) summarize the relationship between Foxd1 expression domain and Zic2-positive cells over the entire retina. Blue indicates Xgal-Foxd1 staining and dark brown speckles represent Zic2-positive cells. The cartoon shows that the borders of Zic2 and Foxd1 expression overlap, although the Foxd1 expression domain extends more centrally than that of Zic2. Moreover, some Zic2-positive cells are found outside the most ventral lateral border of the Foxd1 expression domain. Higher magnification views of the two boxed regions in b and f are shown in c and d, and g and h, respectively. These pictures show that most Zic2-positive cells (dark brown speckles) are located in the Foxd1-positive domain (blue area) in the VT retina (d,h). c and g show that the peripheral temporal borders of the Zic2 and Foxd1 domains overlap. Scale bars: 100 µm. (B) (a-f) In situ hybridization for Foxg1 in serial horizontal sections from the middle of the retina (a,d) and through the most ventral retina (c,f) in E15.5 Foxd1^+/+ and Foxd1^lacZ/lacZ embryos, indicating that Foxg1 expands into VT retina in the absence of Foxd1. Arrows in b-f indicate that Foxg1 is expressed in the ventrotemporal quandrant of Foxd1 deficient retina but not in normal retina. (g,h) Antibody fusion protein localization in horizontal sections of E12.5 embryos shows that in the absence of Foxd1, ephrin-A is also expressed in the temporal retina (h, arrow), which is in contrast to the normal high-nasal-low-temporal expression in wild-type embryos (g, arrow). Scale bars: 100 µm. (C) Schematic showing that Zic2 and EphB1, thought to regulate the uncrossed projection, are missing in the retina of Foxd1 deficient embryos, and that Foxg1 and ephrin-A proteins expand their territory into VT retina. Top, wild type; bottom, mutant.

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