(A–A') Representative E19 coronal sections after in utero electroporation at E14.5 with Neurod1-Cre and Z/EG plasmids. Electroporated cells were visualized with anti-EGFP (green), and nuclei were stained with DAPI (blue). Gsk3-deleted neurons remain in the deeper layers of the cortex but elaborate a long pial-directed process (yellow arrowheads). Gsk3-deleted neurons elaborate axons projecting towards the corpus callosum (orange arrows). Scale bar = 200 μm (n = 5, two independent litters). (B–B') Coronal sections at P10 after E14.5 electroporation, as in A. Gsk3-deleted neurons remain in the deeper layers of the cortical plate and fail to reach layer 2/3 (denoted with yellow bars). Scale bar = 200 μm (n = 3, 2 independent litters). (C) Higher magnification of Gsk3-deleted neurons in B' (box). Gsk3-deleted neurons (green) in deeper layers co-label with Cux (red) (orange arrows). Nuclei were stained with Dapi. (D) Quantification of control and Gsk3-deleted neurons in upper (layer 2–3) vs deeper layers of the cortex at P10. (n = 3, 4209 total neurons counted, 2234 control vs 1975 Gsk3 deleted) **p=0.003, unpaired t-test. (E) Gsk3 deletion delays the multipolar to bipolar transition. Still images from time-lapse imaging of slice cultures at 3DIV. pCAG-dsRED or Neurod1-Cre;Z/EG was injected into the ventricles of Gsk3a−/−Gsk3bloxp/loxp embryos and electroporated at E15. Representative images were taken at time 0, 6, and 12 hr. Control dsRed neurons migrate through the cortical plate (yellow, red, and blue arrows show individual neurons at the different time points). (n = 2 controls). Gsk3-deleted neurons fail to migrate through the cortical plate and exhibit persistent multi-polar morphology (yellow arrowheads). (n = 4 mutants).