Fig. 5. Prethalamus but not telencephalon is rescued in Six3^-/-;Wnt1^-/- embryos. Foxg1 is expressed in the telencephalon of wild-type (A) and Wnt1^-/- (A') mouse brains at E10.5. No telencephalic expression of Foxg1 was observed in Six3^-/- (A'') or Six3^-/-;Wnt1^-/- brains (A'''). Emx1 is expressed in the dorsal telencephalon of wild-type (arrow, B) and Wnt1^-/- (arrow, B') brains. Tbr1 is expressed in the telencephalon (arrow) and eminentia thalami (arrowhead) of wild-type (C) and Wnt1^-/- (C') brains. No telencephalic expression of Emx1 and Tbr1 was observed in the Six3^-/- (B'',C'') and Six3^-/-;Wnt1^-/- (B''',C''') brains. Irx1 is expressed in the alar plate of the thalamus (arrow) of wild-type (D), Wnt1^-/- (D') and Six3-null (D'') brains at E10.5. In the Six3-null brain (D''), Irx1 expression expands anteriorly; this ectopic expansion was rescued in the Six3^-/-;Wnt1^-/- brain (D'''). Similar results were obtained with Tcf4, a gene that is expressed in the alar plate of the pretectum and in the thalamus of wild-type (E), Wnt1^-/- (E'), Six3^-/- (E'') and Six3^-/-;Wnt1^-/- embryos (E'''). The prethalamus (arrow), as indicated by Fezf1 expression in the wild-type (F) and Wnt1^-/- (F') brain, is extremely reduced in the Six3-null brain (F''); however, it appears normal in the Six3^-/-;Wnt1^-/- brain (F'''). A model for the Six3^-/-;Wnt1^-/- brain at this stage is shown (G'''). In the Wnt1^-/- brain (G'), the mesencephalon is smaller than in the wild-type brain (G). The prospective thalamus is the most anterior structure in the Six3-null brain (G''), and the prethalamus remains in the Six3^-/-;Wnt1^-/- brain (G''') at this stage. The basal plate is indicated in blue. Lines delimit the prospective regions. Anterior is to the right. H, hindbrain; HY, hypothalamus; M, mesencephalon; PT, prethalamus; PTC, pretectum; T, thalamus; TEL, telencephalon.