Fig. 1. Axon fate predisposition is present at the bipolar stage. (A) Neocortical slice of an E18 mouse brain that had been transfected with a cytoplasmic, fluorescent protein (Venus-EGFP) at E15 by in utero electroporation, showing migrating cells (lower panel). The upper panel shows the distribution of the axonal marker tau-1. The lower panel shows Venus-EGFP fluorescence merged with the tau-1 image. (B) Maximal intensity projection of a confocal z-stack of a cell in the upper intermediate zone (boxed in A). The GFP-positive cell has a typical bipolar morphology of migrating neurons (arrowheads indicate trailing neurite). The numbers at the left (1-7) mark the places along the neuron that were selected for y-z scans shown in C. (C) Views along the y-z plane. Arrowheads show a colocalization of GFP and tau-1. (D) GFP and tau-1 intensity-profiles from the selected places 1-7 confirm a colocalization of GFP and tau-1 immunoreactivity in the distal portion of the trailing process of the migrating neuron. The profile was obtained by drawing a line that crossed the middle of the GFP signal. Bar, 100 µm (for A) and 10 µm (for B,C).