A comparative study of acellular nerve xenografts and allografts in repairing rat facial nerve defects

The effectiveness of acellular nerve allografts compared to autografts in animal models: A systematic review and meta-analysis

BACKGROUND

Treatment of nerve injuries proves to be a worldwide clinical challenge. Acellular nerve allografts are suggested to be a promising alternative for bridging a nerve gap to the current gold standard, an autologous nerve graft.

OBJECTIVE

To systematically review the efficacy of the acellular nerve allograft, its difference from the gold standard (the nerve autograft) and to discuss its possible indications.

MATERIAL AND METHODS

PubMed, Embase and Web of Science were systematically searched until the 4th of January 2022. Original peer reviewed paper that presented 1) distinctive data; 2) a clear comparison between not immunologically processed acellular allografts and autologous nerve transfers; 3) was performed in laboratory animals of all species and sex. Meta analyses and subgroup analyses (for graft length and species) were conducted for muscle weight, sciatic function index, ankle angle, nerve conduction velocity, axon count diameter, tetanic contraction and amplitude using a Random effects model. Subgroup analyses were conducted on graft length and species.

RESULTS

Fifty articles were included in this review and all were included in the meta-analyses. An acellular allograft resulted in a significantly lower muscle weight, sciatic function index, ankle angle, nerve conduction velocity, axon count and smaller diameter, tetanic contraction compared to an autologous nerve graft. No difference was found in amplitude between acellular allografts and autologous nerve transfers. Post hoc subgroup analyses of graft length showed a significant reduced muscle weight in long grafts versus small and medium length grafts. All included studies showed a large variance in methodological design.

CONCLUSION

Our review shows that the included studies, investigating the use of acellular allografts, showed a large variance in methodological design and are as a consequence difficult to compare. Nevertheless, our results indicate that treating a nerve gap with an allograft results in an inferior nerve recovery compared to an autograft in seven out of eight outcomes assessed in experimental animals. In addition, based on our preliminary post hoc subgroup analyses we suggest that when an allograft is being used an allograft in short and medium (0-1cm, > 1-2cm) nerve gaps is preferred over an allograft in long (> 2cm) nerve gaps.

Physical processing for decellularized nerve xenograft in peripheral nerve regeneration

In severe or complex cases of peripheral nerve injuries, autologous nerve grafts are the gold standard yielding promising results, but limited availability and donor site morbidity are some of its disadvantages. Although biological or synthetic substitutes are commonly used, clinical outcomes are inconsistent. Biomimetic alternatives derived from allogenic or xenogenic sources offer an attractive off-the-shelf supply, and the key to successful peripheral nerve regeneration focuses on an effective decellularization process. In addition to chemical and enzymatic decellularization protocols, physical processes might offer identical efficiency. In this comprehensive minireview, we summarize recent advances in the physical methods for decellularized nerve xenograft, focusing on the effects of cellular debris clearance and stability of the native architecture of a xenograft. Furthermore, we compare and summarize the advantages and disadvantages, indicating the future challenges and opportunities in developing multidisciplinary processes for decellularized nerve xenograft.

A novel decellularized nerve graft for repairing peripheral nerve long gap injury in the rat

Decellularized nerve allografts are an alternative to autograft for repairing severe nerve injuries, since they have higher availability and do not induce rejection. In this study, we have assessed the regenerative potential of a novel decellularization protocol for human and rat nerves for repairing nerve resections, compared to the gold standard autograft. A 15-mm gap in the sciatic nerve was repaired with decellularized rat allograft (DC-RA), decellularized human xenograft (DC-HX), or fresh autograft (AG). Electrophysiology tests were performed monthly to evaluate muscle reinnervation, whereas histological and immunohistochemical analyses of the grafts were evaluated at 4 months. A short-term study was also performed to compare the differences between the two decellularized grafts (DC-RA and DC-HX) in early phases of regeneration. The decellularization process eliminated cellularity while preserving the ECM and endoneurial tubules of both rat and human nerves. Higher amount of reinnervation was observed in the AG group compared to the DC-RA group, while only half of the animals of the DC-HX showed distal muscle reinnervation. The number of regenerating myelinated axons in the mid-graft was similar between AG and DC-RA and lower in DC-HX graft, but significantly lower in both DC grafts distally. At short term, fibroblasts repopulated the DC-RA graft, supporting regenerated axons, whereas an important fibrotic reaction was observed around DC-HX grafts. In conclusion, the decellularized allograft sustained regeneration through a long gap in the rat although at a slower rate compared to the ideal autograft, whereas regeneration was limited or even failed when using a decellularized xenograft.

Biomimicry in 3D printing design: implications for peripheral nerve regeneration

Nerve guide conduits (NGCs) connect dissected nerve stumps and effectively repair short-range peripheral nerve defects. However, for long-range defects, autografts show better therapeutic effects, despite intrinsic limitations. Recent evidence shows that biomimetic design is essential for high-performance NGCs, and 3D printing is a promising fabricating technique. The current work includes a brief review of the challenges for peripheral nerve regeneration. The authors propose a potential solution using biomimetic 3D-printed NGCs as alternative therapies. The assessment of biomimetic designs includes microarchitecture, mechanical property, electrical conductivity and biologics inclusion. The applications of 3D printing in preparing NGCs and present strategies to improve therapeutic effects are also discussed.

Facial Nerve Repair: Bioengineering Approaches in Preclinical Models

Injury to the facial nerve can occur after different etiologies and range from simple transection of the branches to varying degrees of segmental loss. Management depends on the extent of injury and options include primary repair for simple transections and using autografts, allografts, or conduits for larger gaps. Tissue engineering plays an important role to create artificial materials that are able to mimic the nerve itself without extra morbidity in the patients. The use of neurotrophic factors or stem cells inside the conduits or around the repair site is being increasingly studied to enhance neural recovery to a greater extent. Preclinical studies remain the hallmark for development of these novel approaches and translation into clinical practice. This review will focus on preclinical models of repair after facial nerve injury to help researchers establish an appropriate model to quantify recovery and analyze functional outcomes. Different bioengineered materials, including conduits and nerve grafts, will be discussed based on the experimental animals that were used and the defects introduced. Future directions to extend the applications of processed nerve allografts, bioengineered conduits, and cues inside the conduits to induce neural recovery after facial nerve injury will be highlighted.

Decellularization of Nervous Tissues and Clinical Application

The nervous system is an ensemble of organs that transmit and process external information and are responsible for the adaption to the external environment and homeostasis control of the internal environment. The nervous system of vertebrates is divided into the central nervous system (CNS) and peripheral nervous system (PNS) due to its structural features. The CNS, which includes the brain and the spinal cord, processes information from external stimuli and assembles orders suitable for these stimuli. The CNS then sends signals to control other organs/tissues. On the other hand, the PNS connects the CNS to other organs/tissues and functions as a signal pathway. Therefore, the decline and loss of various functions due to injuries of the nervous system cause an impaired quality of life (QOL) and eventually the termination of life activities. Here, we report mainly on decellularized neural tissue and its application as a substrate for the regeneration of the nervous system.

Nerve Repair Using Decellularized Nerve Grafts in Rat Models. A Review of the Literature

Peripheral nerve regeneration after severe traumatic nerve injury is a relevant clinical problem. Several different strategies have been investigated to solve the problem of bridging the nerve gap. Among these, the use of decellularized nerve grafts has been proposed as an alternative to auto/isografts, which represent the current gold standard in the treatment of severe nerve injury. This study reports the results of a systematic review of the literature published between January 2007 and October 2017. The aim was to quantitatively analyze the effectiveness of decellularized nerve grafts in rat experimental models. The review included 33 studies in which eight different decellularization protocols were described. The decellularized nerve grafts were reported to be immunologically safe and able to support both functional and morphological regeneration after nerve injury. Chemical protocols were found to be superior to physical protocols. However, further research is needed to optimize preparation protocols, including recellularization, improve their effectiveness, and substitute the current gold standard, especially in the repair of long nerve defects.

Tissue-engineered nerve graft with tetramethylpyrazine for repair of sciatic nerve defects in rats

A tissue-engineered nerve with tetramethylpyrazine (TMP) was repaired for sciatic nerve defects in rats. A total of 55 adult Sprague Dawley (SD) rats were classified into 4 groups, with 15 rats in each of groups A, B, and C as well as 10 rats in group D. About 1.5cm of a sciatic nerve of the right hind limb located 0.5cm below the inferior margin of the piriformis was resected to form the defects. Four types of nerve grafts used for bridging nerve defects in the SD rats corresponded to the 4 groups: tissue-engineered nerves with TMP in group A, tissue-engineered nerves without TMP in group B, acellular nerve grafts (ANGs) in group C, and autologous nerves in group D. Twelve weeks post-surgery, the sciatic functional index, nerve conduction velocity, and gastrocnemius wet weight of groups A and D were higher than those of groups B and C (P<0.05). Results of fluorescence microscopy and histological staining indicated that group A performed better than groups B and C (P<0.05). Similarly, the number of horseradish peroxidase-labeled positive cells was significantly larger in group A than in groups B and C. Regenerative nerve fibers were abundant in group A and consisted mainly of myelinated nerve fibers, which were better than those in groups B and C (P<0.05). The study demonstrated that tissue-engineered nerves constructed by ANGs seeded with neural stem cells and combined with TMP can effectively repair sciatic nerve defects in rats.