Fig. 4. The relationship between the position of GFP⁺ neurons in the cerebral cortex and their rate or orientation. (A,B) The average speed of migration for individual cells plotted against the distance from the ventricular surface. Each point represents a migrating neuron at (A) E13.5 and (B) E15.5 mouse. (C,D) The orientation of migration as measured from deflection from a line drawn vertical to the ventricular surface was plotted against the distance from the ventricular surface at (C) E13.5 and (D) E15.5. Inset illustrates the method of angle measurement. The green line represents 90°. Note that a substantial proportion of cells in the IZ, SP and CP migrate with angles of more than 90°, indicating that they deflect towards the pial surface. Negative angles indicate the occurrence of medial-to-lateral migration. All cells that could be traced were plotted. The high density of GFP⁺ neurons, particularly in the SVZ and the MZ, allowed measurement of only a fraction of the cells. The speed of migration is underestimated, because rapidly migrating cells in cell-dense regions could not be followed and curved trajectories of migrating cells were approximated by straight lines. Although most MZ neurons were stationary, some slight dislocations can be observed. This is largely due to a drift of the slice during observation. Thus, in C and D, cells that moved less than 10 µm per hour were excluded from the analysis.

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