HOST NERVE-FIBERS THAT REGENERATE AND RESIDE LONG-TERM IN A REJECTED NERVE ALLOGRAFT ARE NOT PROTECTED BY PERMEABILITY BARRIERS

We investigated whether permeability barriers develop and protect host nerve fibers that regenerate and reside long-term in a rejected nerve allograft. In order for barriers to form, host cells have to enter th e rejected allograft and differentiate into new endothelial and perine urial cells that respectively form the impermeable endoneurial blood-n erve and the perineurium-nerve barriers that are present in normal ner ve. A 2-cm long graft of peroneal nerve was taken from American Cancer Institute (ACI) or Fischer (FR) rats and transplanted to bridge a 2-c m gap between the cut ends of the peroneal nerve of other FR rats. Six months postoperatively, histology revealed that regenerated host nerv e fibers in ACI allografts were compartmentalized into numerous minifa scicles by perineurial cells and that blood vessels were located outsi de rather than inside the perineurial compartments among the nerve fib ers. Administration of the permeability indicator horseradish peroxida se to allograft recipients (intravenously or topically to the graft in situ) revealed that it entered the endoneurium of microcompartments a nd spread around the nerve fibers. In contrast, none of the indicator reached nerve fibers in FR syngrafts or normal ACI or FR nerves which were not microcompartmentalized. We concluded that host nerve fibers t hat regenerate and reside long-term in a rejected nerve allograft are not protected by permeability barriers.