Fig. 3. The prospective thalamus is replaced by the pretectum in the Six3-null brain. Hematoxylin and Eosin staining of sagittal sections from E14.5 wild-type (A) and Six3-null (A') mouse brains. At E14.5, Tcf4 is expressed in the pretectum and thalamus of wild-type embryos (B) and in the anterior-most region of the Six3-null brain (B'). At E13.5, Lhx2 is normally expressed in the thalamus (C), but is absent from the Six3-null brain (C'). Lhx2 is also expressed between the tegmentum and pretectum of the wild-type brain (arrow, C,C'). At E14.5, Lim1 is normally expressed in the pretectum (D), but is shifted anteriorly in the Six3-null brain (D'). Ebf1 is expressed in the anterior pretectum of wild-type embryos (E) and in the anterior region of the Six3-null brain (E'). As indicated by the absence of Dxl2 (F') and Nkx2.1 (G') expression at E14.5, the prethalamus (F) and the hypothalamus (G) were not present in the Six3-null brain. Instead, as indicated by Nkx6.1 expression at E12.5, the tegmentum is located in the ventral region of the mutant brain (H', arrows point to the limit of Nkx6.1 expression; compare with H). The ZLI, as revealed by Shh expression at E12.5, is not present in the Six3-null brain (I'; compare with I). Therefore, instead of the thalamus (J), the pretectum and the tegmentum are present in the anterior region of the Six3-null brain (J'). Anterior is to the right. GE, ganglionic eminence; HY, hypothalamus; P, pons; PT, prethalamus; PTC, pretectum; T, thalamus; TG, tegmentum.