Defective preplate splitting and subplate development. (a, a') ISH for Tbr1 transcript on E15.5 coronal sections (medial is to the right). (b, b') Immunohistochemical co-detection of Tbr1 and BrdU on E15.5 sagittal sections (rostral is to the left). Mutants lack well separated/positioned Tbr1+, putative subplate cells at E15.5 (arrow and asterisk in (a, a')). At this stage, BrdU+ (injected E11) cells co-labeled with Tbr1 form a superplate-like structure by populating the upper CP/MZ in cKO (arrow in (b)). In controls, those cells settle in the subplate area (arrow in (b)). Arrows indicate the dislocation of BrdU+/Tbr1+ cells in mutants (b'). Co-detection of Reelin- and Tbr1 protein in the MZ on coronal sections at E18.5 (c, c') (medial is to the right) cKO showed more Reelin+/Tbr1+ (putative Cajal-Retzius cells) in the MZ (c'). The PPL does not split properly in cKO; additionally Gap43+ axons (assayed on E18.5 coronal sections) form bundles around the internal capsule (arrowheads in (d, d') and some project ectopically into the MZ (arrow in (d, d')).