Effects of delayed transplantation of cultured schwann cells on axonal regeneration from central nervous system cholinergic neurons
Authors: Neuberger Timothy J. , Cornbrooks Carson J. , Kromer Lawrence F. 
Published: January 2005

The introduction of transplants consisting of cultured Schwann cells and their associated extracellular matrix (Sc/ECM) into a central nervous system (CNS) lesion cavity facilitates axonal regeneration from injured, adult mammalian neurons with subsequent reinnervation of their appropriate target (Kromer and Cornbrooks: Proceedings of the National Academy of Sciences of the United States of America 82:6330–6334, 1985). In the present study, the effects of a delayed transplantation procedure on the time course of this regenerative response were evaluated. For these experiments, bilateral CNS lesions were created between the septum and hippocampus by removing the fimbria‐fornix pathway. Lesion cavities received either no transplants, transplants of collagen, or Sc/ECM transplants at the time the lesion was created or 6 days later. When no transplants or transplants of collagen were used, axonal sprouts extended for very short distances into the lesion cavity. These axons were not preferentially associated with the collagen transplants nor maintained at long post‐lesion survival times. In animals that received Sc/ECM transplants, the number of sprouting axons and the progression of axonal growth along the transplants was much more extensive than for the collagen transplants. Although more axons were detected in cavities that received transplants immediately after the fimbria‐fornix lesion, axonal regeneration along the transplants was similar regardless of whether there was a delay in transplanting the Schwann cells. By using histochemical techniques to identify acetylcholinesterase (AChE), regenerating AChE‐positive axons were first detected in the cavity at 3 days post‐transplantation, were associated with the Sc/ECM transplants by 5 days, and crossed the cavity within 8 days post‐transplantation. Regenerating, neurofilament‐positive axons crossed the CNS‐Sc/ECM transplant interfaces in association with laminin‐positive, glial fibrillary acidic protein‐positive cellular pathways. Upon reaching the caudal end of the Sc/ECM transplant, the cholinergic axons abandoned the transplant and oriented directly toward the adjacent hippocampus. Both the simultaneous and delayed transplantation paradigms demonstrated a similar reinnervation pattern of AChE‐positive fibers in the hippocampus, but there was a more rapid penetration and more extensive arborization of fibers in animals receiving the delayed transplants. Cholinergic fibers initially invaded the dentate gyrus molecular layer and hilus between 8 and 14 days post‐transplantation. By 45 days post‐transplantation, AChE‐positive axons were detected throughout the dentate gyrus and regio inferior, but few fibers were present in regio superior of the hippocampus.

The results of this study demonstrate that the addition of Sc/ECM transplants, either immediately after a CNS lesion or 6 days later, provides an environment that supports axonal regeneration from injured CNS cholinergic neurons. The cultured peripheral nervous system cells encourage axonal sprouting, attract regenerating axons to the transplant, and facilitate their directed elongation across the lesion cavity into close proximity with the appropriate target. Upon reaching the distal end of the transplant, the AChE‐positive fibers are attracted toward the hippocampus, where they enter the parenchyma and arborize. This reoccupation of terminal domain within the hippocampus is accelerated in specimens with delayed transplants. Thus, it is postulated that a primary effect of the delayed transplantation procedure is to produce a “conditioning lesion‐like” effect that alters the postsynaptic environment of the hippocampus such that it provides a stronger trophic stimulus for reinnervation by the regenerating cholinergic axons. This could possibly be mediated through an elevation in a trophic factor, such as nerve growth factor.