Abstract
Most organ or tissue allografts with viable cells are stored in solutions ex vivo for hours to several days. Most allografts then require rapid host revascularization upon transplantation to maintain donor-cell functions (e.g., cardiac muscle contractions, hepatic secretions). In contrast, peripheral nerve allografts stored ex vivo do not require revascularization to act as scaffolds to guide outgrowth by host axons at 1-2 mm/d, likely aided by viable donor Schwann cells. Using current storage solutions and protocols, axons in all these donor organ/tissue/nerve transplants are expected to rapidly become non-viable due to Wallerian degeneration within days. Therefore, ex vivo storage solutions have not been assessed for preserving normal axonal functions, i.e., conducting action potentials or maintaining myelin sheaths. We hypothesized that most or all organ storage solutions would maintain axonal viability. We examined several common organ/tissue storage solutions (University of Wisconsin Cold Storage Solution, Normosol-R, Normal Saline, and Lactated Ringers) for axonal viability in rat sciatic nerves ex vivo as assessed by maintaining: (1) conduction of artificially-induced compound action potentials; and (2) axonal and myelin morphology in a novel assay method. The ten different storage solution conditions for peripheral nerves with viable axons (PNVAs) differed in their solution composition, osmolarity (250-318 mOsm), temperature (4°C vs. 25°C), and presence of calcium. Compound action potentials and axonal morphology in PNVAs were best maintained for up to 9 days ex vivo in calcium-free hypotonic diluted (250 mOsm) Normosol-R (dNR) at 4°C. Surprisingly, compound action potentials were maintained for only 1-2 days in UW and NS at 4°C, a much shorter duration than PNVAs maintained in 4°C dNR (9 days) or even in 25°C dNR (5 days). Viable axons in peripheral nerve allografts are critical for successful polyethylene glycol (PEG)-fusion of viable proximal and distal ends of host axons with viable donor axons to repair segmental-loss peripheral nerve injuries. PEG-fusion repair using PNVAs prevents Wallerian degeneration of many axons within and distal to the graft and results in excellent recovery of sensory/motor functions and voluntary behaviors within weeks. Such PEG-fused PNVAs, unlike all other types of conventional donor transplants, are immune-tolerated without tissue matching or immune suppression. Preserving axonal viability in stored PNVAs would enable the establishment of PNVA tissue banks to address the current shortage of transplantable nerve grafts and the use of stored PEG-fused PNVAs to repair segmental-loss peripheral nerve injuries. Furthermore, PNVA storage solutions may enable the optimization of ex vivo storage solutions to maintain axons in other types of organ/tissue transplants.