Fig. 4. Defects in CFAs in the Pax6^-/- brains revealed by tracing with carbocyanine dyes. (A-D) Early CFA were labelled from single DiI crystal placement in the dorsal cortex (ctx) (asterisks in A and C) of an E15.5 wild-type (A,B) and Pax6^-/- brain (C,D). CFA in both phenotypes descend through the intermediate zone and turn towards the primitive internal capsule (arrows in B,D). (E) ß-galactosidase staining in a coronal section of an E18.5 Pax6^+/- brain. (F) Trajectories of fibres labelled from cortical DiI placement (asterisk) in a similar section to that shown in E, of an E18.5 Pax6^+/- brain. (G) ß-galactosidase staining is displaced ventrolaterally in the PSPB (arrows) in the Pax6^-/- brain. Note the presence of an abnormal cellular mass (asterisk) protruding into the lateral ventricle (LV). (H) Same section as G showing the abnormal pathway of CFA in Pax6^-/- brains. (J-M) Coronal sections from more rostral (J) to more caudal (L) levels, showing abnormal CFA in an E18.5 Pax6^-/- brain. (M) High-power image from the region indicated by the box in K. Some fibres descend towards the ventral pallium and cross the PSPB at a more ventral position in large fascicles (arrows in J-L). (N) Numerous backlabelled cells are seen in the marginal zone (mz) of the paleocortex (plx) (arrowheads), suggesting that they develop abnormal projections to the tracing site. (O) Fibres of the corpus callosum (cc) labelled by cortical DiI crystal placements. (P-U) Adjacent serial sections from an E18.5 Pax6^-/- brain in which double carbocyanine dyes were placed, DiA in the cortex and DiI in the dorsal thalamus. Note that the two sets of fibres failed to encounter each other at the PSPB (arrowheads indicate DiI-labelled TCAs, and arrows indicate DiA-labelled CFA in Q,S,U). (P) ß-Galactosidase staining of the section shown in Q; (R,T) The bizbenzimide staining of sections in S,U, respectively. GE, ganglionic eminence; cc, corpus callosum; se, septal eminence; hp, hippocampus; dt, dorsal thalamus. Scale bars: 200 µm in A,C,I; 100 µm in B,D,N,M,O; 400 µm in E-H,J-L,P,Q.

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