Abstract
Introduction: A number of clinical and experimental studies suggest that an intact nervous system is essential for normal fracture healing. In the present study, we analysed the occurrence of regenerating and mature nerve fibres over time in fracture callus. Using antibodies against neuronal proteins specific for nerve regeneration (growth associated protein – GAP-43) and nerve maturity (protein gene product – PGP 9.5) it is possible to demonstrate regeneration and end differentiation of nerves by immunohistochemistry.
Methods: Twelve male Sprague Dawley rats, weighing 230–290 g were used. The right tibias were fractured under HypnormÒ anaesthesia and fixed with a 17-G cannula needle in the medullary canal. The left un-fractured tibia served as an internal control. X-rays was used to monitor progress of fracture healing. Three rats were killed at 3 days, 1, 2 and 3 weeks post-fracture and right and left tibia were prepared for immunohistochemistry. The tissue sections (15 mm thick) were incubated with antiserum to GAP-43 and then with biotinylated antibodies. Cy2-conjugated avidin was used for the fluorescent staining. For double staining, after the staining with first antibody, the sections were incubated with avidin blocking solution followed by biotin blocking solution. Incubation with the second antiserum to PGP 9.5 was performed in the same manner as for the first peptide. For fluorescent staining of PGP 9.5, the sections were incubated with Cy3-conjugated avidin. A Nikon epifluorescence microscope was used for photog
Results: In the un-fractured tibia. PGP 9.5-positive nerve fibres were consistently identified in periosteum, muscles and connective tissues. A number of nerve fibres also expressed GAP-43, although there were no signs of nerve sprouting, i.e. regeneration. In the fractured tibia, many GAP-43-positive nerves were identified already at 3 days post-fracture in the hematoma and periosteum. At 1 week, abundant sprouting of these nerves was seen in cartilaginous callus and hyperplastic periosteum. A number of nerve terminals were observed very close to the chondroid cells in the fibrocartilage of the fracture gap. At 2 and 3 weeks, GAP 43-positive fibres gradually shifted from the fibrocartilage area towards the outlying hyperplastic periosteum. Double staining studies showed that an increased expression of GAP-43 as compared to PGP 9.5 occurred in the early period of fracture healing. This relationship changed at 3 weeks when enhanced PGP 9.5 and less GAP 43 expression was found.
Discussion: Our study suggests that there was an intense nerve regeneration in the early phase of fracture healing. Thus, a prominent expression of GAP-43 was seen in sprouting nerves in the hyperplastic periosteum and the callus fibrocartilage as early as 1 week post-fracture. This expression remained high in the fractures up to 3 weeks, when healing was essentially completed. Possibly, this persistent occurrence of GAP-43 is necessary for the ensuing ossification and bone remodeling. PGP 9.5 expression was markedly low at one week, but became pronounced at 3 weeks, probably reflecting functional maturation of the regenerated nerve in the healing fracture. It may prove that strong regenerative capability of nerves seen in the fractures is a prerequisite for normal fracture healing. Our results point to the possibility that regenerating nerves provide the delivery system for GAP-43 and other neuronal mediators required for normal callus formation and/or neovascularization.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
