Fig. 1. A failure in growth of diencephalic and midbrain primordia in Shh mutants. Morphology of wild-type (A,C) and Shh mutant (B,D) brains at 8.5 dpc (13 somite; A,B) and 9.5 dpc (26 somite; C,D). At 8.5 dpc, the presumptive diencephalon is recognized as a narrow region between two constrictions (Region I). Region II corresponds to the midbrain and Region III to the anterior hindbrain between the isthmus and otic vesicle (ot). A sharp ventral constriction was observed in Shh mutants (arrows in B,D). At 9.5 dpc, regions I and II of Shh mutant were smaller than those of wild type, but Region III of the mutants was still comparable with that of wild type (see text, Table1). (E-J) Expression patterns of Emx2, En1 and Pax6. A gap between the Emx2 and En1 expression domains represents the anterior midbrain and P1/2 (brackets in E-H). No difference was observed in this region at the 14-somite stage (E,F) but by the 21-somite stage, the gap was greatly reduced in Shh mutants (G H). The anterior midbrain, a gap between the Pax6 and En1 expression domains, was also reduced in Shh mutants (I,J). Arrowheads in I,J indicate the forebrain-midbrain boundary. Scale bars: 200 µm.

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