EGFR-GFP+ cells migrate preferentially along axonal tracts. A, EGFR-GFP+ cells migrate along axonal tracts in corpus callosum when crossing to the contralateral side (asterisk indicates side of the retroviral injection). When invading the striatum, EGFR-GFP+ cells were following the route of pencil fibers (B) and were observed in the close vicinity of the pencil fibers in the striatum (coronal section) (C). wm, White matter; cc, corpus callosum; str, striatum; pf, pencil fibers.