Transplanted GDAs promote regeneration of rubrospinal axons. (a) Confocal montage scanned through a depth of 60 μm, showing a small population of BDA+ rubrospinal tract (RST) axons (green) that have traversed a GDA-bridged (red) lesion of the dorsolateral funiculus and entered caudal white matter at 8 days after injury. The majority of RST axons, however, have sprouted to within 300 μm of the lesion center (LC) but failed to extend beyond the site of injury. Note the absence of BDA-labeled axons within the dorsal-most regions of the injury site. (b) Confocal montage showing the complete failure of axotomized BDA+ RST axons to cross control lesions at 8 days after injury and that the majority of axons have remained within rostral lesion margins at a distance of 500–800 μm from the lesion center (LC). (c) At 5 weeks after injury and transplantation, a small population of BDA+ RST axons have traversed GDA-bridged injury sites and extended within caudal white matter. Note that BDA+ axons have also sprouted into the dorsal regions of the lesion center and even extended beyond the pial surface (arrowhead; see also the high-power image in (d)). Note the lower levels of GFAP immunoreactivity (red) in more ventral regions of the injury margins and center, coincident with the presence of BDA+ axons. (e) Two examples of RST axons displaying growth cones within white matter 2 mm caudal to a GDA-treated lesion, at 5 weeks after transplantation. Note the collateral branch (asterisk). (f) Confocal image of a BDA+ terminal field-like axonal plexus within layer 5 spinal cord gray matter, immediately adjacent to the dorsolateral funiculus white matter at 5 weeks after injury and transplantation. In contrast, in all GDA-transplanted rats and controls injected with medium alone at 8 days after injury, no BDA labeling was observed within gray matter beyond the injury site. Scale bars: (a-c) 200 μm; (d) 100 μm; (e) 5 μm; (f) 10 μm.