Rejection and regeneration through peripheral nerve allografts: immunoperoxidase studies with laminin, S100 protein...

The pattern and temporal sequence of histopathological events in a rat nerve allograft model were evaluated. Following grafting and varying survival periods (from 1 to 30 weeks), the host and donor nerve were removed and assessed by light and electron microscopy. Nerve allografts underwent Wallerian degeneration and rejection. Wallerian degeneration was the dominant pathologic process at weeks 1 and 2 after engraftment. Histologic rejection started as an epineurial process at weeks I and 2, became progressively endoneurial and was most prominent at 4 and 6 weeks after engraftment. Rejection was accompanied by evidence of graft Schwann cell and endoneurial tube loss. The rejection process delayed, but did not prevent, nerve regeneration by the host. Regeneration of fine neurofilament-positive axonal sprouts into the proximal portions of the graft was observed as early as week 2. Subsequently, regeneration occurred through the periphery and around the exterior of the rejected nerve allograft fascicle. Regenerating axons were accompanied by S100 protein reactive Schwann cells and newly synthesized laminin-positive endoneurial tubes. Regenerating axons reinnervated the distal host segment at week 8 and increased in number and myelination thereafter. The observations of rejection and regeneration through nerve allograft segments are discussed in reference to previous studies.