Fig. 9. Schematic diagram illustrating the relationship between the developing reciprocal TCAs and CFA and ß-galactosidase stained early subdivisions of the embryonic forebrain in Pax6^+/- (upper row) and in Pax6^-/- (lower row) mice at E14.5 and E18.5. Blue represents the ß-galactosidase staining, which reflects the Pax6 promoter activity. Red dots represent cells with dorsal thalamic projections. In wild-type and heterozygote mice at E14.5, early thalamic and corticofugal fibres synchronously approach the internal capsule. The growing CFAs (dark blue) pause at the border of the Pax6-positive compartment before they continue their journey towards the diencephalon and encounter the growing TCAs (green). By E18.5, both sets of axons have almost reached their destination and the Pax6 expression is gradually reduced in the PSPB and it is limited to the cortical intermediate zone, but continues to be present in the ventral pallium. In the Pax6^-/- mice, the cells with dorsal thalamic projection are displaced into the ventral IC and some to hypothalamus. TCAs descend into the hypothalamus and a subset of fibres cross the telencephalic-diencephalic junction at altered ventral or dorsal sites. The majority of the CFAs fail to turn into the internal capsule and continue to descend into the ventral pallium. The two sets of fibres fail to interact at the PSPB.