Axonal-ependymal associations during early regeneration of the transected spinal cord in Xenopus laevis tadpoles.

The nature and organization of the cellular substrate supporting axonal outgrowth during early regeneration of the spinal cord following transection and/or segment removal were examined in Xenopus tadpoles. Longitudinal axonal compartments, formed by radial ependymal processes in unoperated spinal cords, were maintained within the rostral and caudal stumps throughout the early post-operative period. The first neuritic sprouts to appear near the cut ends of the cord were frequently associated with these processes. Between 5 and 7 days after transection either single or multiple cellular aggregates, which consisted primarily of ependyma, began occupying the lesion zone and were encompassed by numerous small fibres. Ependymal processes subsequently extended among the neurites within the lesion gap and re-established longitudinal axonal channels and a surrounding glia limitans. A concurrent outgrowth of fibres and ependyma from the rostral and caudal stumps was also initiated by 5-7 days following resection of the cord. Axons were seen further within the ablation gap at 10 and 12 days either coursing along the surfaces of the emerging ependyma or organized into small fascicles by the radial processes of these cells. Intermediate stages of axonal fasciculation by the underlying ependyma were also seen. In contrast with previous reports, axonal elongation beyond the cut ends of the cord did not consistently entail the formation of pre-existing ependymal channels. These observations provide additional evidence emphasizing the importance of the ependymal cell surface during axonal outgrowth in the regenerating spinal cord. Guided neuritic elongation, however, does not appear to be dependent upon the prior establishment of a specific type of cytoarchitecture.