In vivo morphological changes in migrating neurons in contact with the marginal zone. Time course of cell migration after E16 electroporation. A, At E17, 1 d after electroporation, GFP⁺ cells in sections of the neocortex are found primarily in the ventricular zone (VZ). By E19 (B) and E20 (C), GFP⁺ cells have entered the cortex and are approaching the MZ. D, By P0, typically 5 d after electroporation, GFP⁺ cells are found in a tight layer underneath the MZ. E, F, Examples of the first group of GFP⁺ cells contacting the MZ from an E19 section of E16 pCAG-GFP-electroporated (green) and E16 bromodeoxyuridine (BrdU)-labeled (red) cortex. E, The white box identifies the migrating cells at the leading edge of the GFP⁺ cells that are examined at higher magnification in F. The cell on the left has a clear branch (yellow arrow) approximately two cell diameters above the CP, but the cell on the right is not branched. G, Arrows identify the BrdU⁺ nuclei from the cells shown in F. H, A higher percentage of cells show leading process branching if the process was in the MZ as opposed to the cortex (CP) (***p < 0.001, χ² test; n = 30 leading processes in MZ; n = 25 leading processes in CP). I, Histogram of the location of the principal branch point with respect to the CP–MZ boundary (red line, x = 0). The x-axis is in nuclear diameters (7 μm). A majority of branch points in MZ-contacting cells were found in the first two cell diameters above the CP–MZ boundary. The leading cell population (n = 55 cells) was derived from z-series from four E16 electroporated brains fixed at E19 (n = 1) or E20 (n = 3). Scale bars: D, 500 μm; G, 25 μm.