Fig. 4. Tcf3^-/- embryos display defective patterning of anterior neural regions. In situ hybridization of E8.5 wild-type (WT; A,A',C,C',E,E') and Tcf3^-/- (KO; B,B',D,D',F,F') embryos using cRNA probes as indicated. Wild-type embryos display prominent staining for Hesx1 (A,A') in the forebrain whereas mutant embryos (B,B') display very weak Hesx1 staining. (C,C') En1 is expressed in the midbrain-hindbrain boundary caudal to the midbrain. The distance of expression to the rostral end of the embryo is marked by the curved bracket (C). (D,D') En1 expression is observed in the mutant embryos, but the distance from the rostral end is diminished (bracket). Note also the deformed neural groove in mutant (D'). (E,E') Robust Krox20 is present in rhombomeres 3 and 5 of the wild-type embryo. (F,F') Weak Krox20 expression is observed in only one stripe of neurectoderm in the mutant embryos. In addition, the expression of Krox20 is expanded laterally compared with the pattern seen in the wild type.