Retinal ganglion cell and nonneuronal cell responses to a microcrush lesion of adult rat optic nerve.

Injury of the optic nerve has served as an important model for the study of cell death and axon regeneration in the CNS. Analysis of axon sprouting and regeneration after injury by anatomical tracing are aided by lesion models that produce a well-defined injury site. We report here the characterization of a microcrush lesion of the optic nerve made with 10-0 sutures to completely transect RGC axons. Following microcrush lesion, 62% of RGCs remained alive 1 week later, and 28% of RGCs, at 2 weeks. Optic nerve sections stained by hematoxylin-based methods showed a thin line of intensely stained cells that invaded the lesion site at 24 h after microcrush lesion. The lesion site became increasingly disorganized by 2 weeks after injury, and both macrophages and blood vessels invaded the lesion site. The microcrush lesion was immunoreactive for chondroitin sulfate proteoglycans (CSPG), and an adjacent GFAP-negative zone developed early after the lesion, disappearing by 1 week. Luxol fast blue staining showed a myelin-free zone at the lesion site, and myelin remained distal to the lesion at 8 weeks. To study the axonal response to microcrush lesion, anterograde tracing was used. Within 6 h after injury all RGC axons retracted back from the site of lesion. By 1 week after injury, axons regrew toward the lesion, but most stopped abruptly at the injury scar. The few axons that were able to cross the injury site did not extend further in the optic nerve white matter by 8 weeks postlesion. Our observations suggest that both the CSPG-positive scar and the myelin-derived growth inhibitory proteins contribute to the failure of RGC regeneration after injury.