Fig. 8. Morphogenetic defects in the mid/hindbrain region. (A,B) BrdU incorporation at E17.5 detects proliferating cells in the EGL of wild type (A) and Psen1 mutants (B). In the mutant, proliferation is highest in medial ectopic tissue (white arrow in B) and also in the caudal midbrain (black arrow in B). (C,D) Expression of reelin is detected in the EGL of wild type (C) and mutant (D), and reveals, in addition, a dramatic overgrowth of the caudal midbrain in the mutant (asterisk in D) and a medial mass forming between the developing cerebellum and the caudal midbrain (delineated by the two white lines in D). (E,F) Ephrin A5 expression identifies the inferior colliculus in wild type (E) and its expansion in the mutant (F). Sagittal (A-D) or coronal (E,F) sections through caudal midbrain and cerebellum at E17.5.