Fig. 2. The prospective thalamus is the most anterior structure of the Six3-null brain at the 22- to 24-somite stage. (A) Wtn3a is normally detected in the roof and alar plate (arrow) of the thalamus at the 22- to 24-somite stage; Fgf15 (B) and Otx1 (C) are localized in the alar plate of the thalamus (arrows) and prethalamus. In Six3-null mouse embryos, the prospective thalamus is located in the anterior region of the brain, as indicated by the expression of Wnt3a (arrow, A'), Fgf15 (arrow, B') and Otx1 (arrow, C'). The prospective prethalamus, as shown by Fezf1 (D) and Dlx2 (E) (arrows), is reduced in the Six3-null brain (arrows, D',E'). The alar plate-basal plate border, as indicated by Nkx2.2 expression (F), is also reduced (arrow, F'; the arrow in F shows the equivalent region to F'). Bmp4 is normally expressed in the basal plate of the posterior hypothalamus (arrow, G) and is absent in the Six3-null brain (G'). In the wild-type basal plate, Wnt1 is expressed in the ventral midline (H), and Ngn2 (I) and Foxa2 (J) are expressed more laterally in the tegmentum. The expression of these genes is expanded anteriorly in the Six3-null brain (H',I',J'). Arrows in H-J' indicate the most-anterior limit of the expression domain. The size of the mesencephalon, as indicated by the Fgf15 expression domain (B), and the region between the Pax6⁺ and En1⁺ territories (K, bracket), are normal in the Six3-null brain (B',K'). Schemes of the 22- to 24-somite stage wild-type (L) and Six3-null (L') brain. The pretectum, thalamus and prethalamus are located dorsally to the hypothalamus in the wild-type brain. In the Six3-null brain, the thalamus is the most anterior structure, and only a small portion of the prethalamus remains at this stage. The basal plate is shown in blue. Lines delimit the prospective regions. Anterior is to the right. HY, hypothalamus; PT, prethalamus; PTC, pretectum; T, thalamus.