Degeneration and sprouting of identified descending supraspinal axons after contusive spinal cord injury in the rat

Contusive spinal cord injury (SCI) results in the formation of a chronic le sion cavity surrounded by a rim of spared fibers. Tissue bridges containing axons extend from the spared rim into the cavity dividing it into chambers . Whether descending axons can grow into these trabeculae or whether fibers within the trabeculae are spared fibers remains unclear. The purposes of t he present study were (1) to describe the initial axonal response to contus ion injury in an identified axonal population, (2) to determine whether and when sprouts grow in the face of the expanding contusion cavity, and (3) i n the long term, to see whether any of these sprouts might contribute to th e axonal bundles that have been seen within the chronic contusion lesion ca vity. The design of the experiment also allowed us to further characterize the development of the lesion cavity after injury. The corticospinal tract (CST) underwent extensive dieback after contusive SCI, with retraction bulb s present from 1 day to 8 months postinjury. CST sprouting occurred between 3 weeks and 3 months, with penetration of CST axons into the lesion matrix occurring over an even longer time course. Collateralization and penetrati on of reticulospinal fibers were observed at 3 months and were more extensi ve at later time points. This suggests that these two descending systems sh ow a delayed regenerative response and do extend axons into the lesion cavi ty and that the endogenous repair can continue for a very long time after S CI (C) 2001 Academic Press.