Fig. 1. Ectopic expression of Wnt1 in the rostral hindbrain leads to induction of Otx2 and concomitant generation of ectopic mDA neurons only within the hindbrain FP. (A) Midsagittal sections at the level of the FP of wild-type and En1^+/Wnt1 (n=6) embryos at E10.5 hybridized with probes for Otx2, Aldh1a1, Fgf8 and Wnt1. Top row shows bright-field images; red rectangles depict the region of the dark-field images from consecutive sections shown below, except for those hybridized with Wnt1, which correspond to embryos of a distinct litter. Black arrowheads indicate the normal (wild-type) position of the MHB. White arrows indicate the ectopic expression domains. (B) Midsagittal sections of wild-type and En1^+/Wnt1 (n=4) embryos at E12.5 hybridized with probes for Wnt1, Nr4a2, Th and Pitx3 (white arrows indicate ectopic expression). (C) Pseudocolored overlays of the corresponding dark-field images from coronal sections of E10.5 wild-type and En1^+/Wnt1 (n=4) embryos hybridized with probes for Gbx2 (green), Aldh1a1 (red) and Wnt5a (green). Overlapping domains appear yellow. (D) Immunodetection of BrdU in sagittal sections of E11.5 wild-type and En1^+/Wnt1 (n=2) embryos after 6 hours of cumulative labeling did not reveal in mutant embryos obvious abnormalities in proliferating activity along the FP region of the mid- and hindbrain. (E) Schematic drawing of a cross-section through the rostral hindbrain floor plate and basal plate of En1^+/Wnt1 mutants at two different time points of development, summarizing the events occurring in this region. The ectopic expression of Wnt1 throughout the entire rostral hindbrain (rhombomere 1) of the En1^+/Wnt1 mutants has been omitted for clarity.