DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0022] A method of acellularization and the acellularized peripheral nerve construct formed thereby are described herein. The acellularization method of the present invention is a chemical, rather than a mechanical, process that generally involves submersion of peripheral nerve tissue in a glycerol solution followed by a series of detergents and other reagents to disrupt and digest the cellular elements of the nerve. As shown and described herein, the removal of cellular elements by this method does not disrupt the endoneurial matrix which is necessary to support and guide axonal regeneration. Therefore, the immunogenicity of the peripheral nerve construct of the present invention is eliminated by the removal of cellular elements, but the ability to support nerve regeneration is maintained.

[0023] The following description and experimental results illustrate the efficacy of the chemical acellularization method of the present invention and the resulting peripheral nerve construct using nerve tissue harvested from and subsequently implanted in rats. Of course, it is fully contemplated that peripheral nerve tissue from any mammal, including human beings, could be similarly acellularized and grafted using the method described herein.

[0024] To produce the acellular nerve constructs according to the present invention, 100 mm culture dishes are first prepared by mixing and pouring approximately 20 ml of SYLGARD® (Dow Coming, Midland, Mich.) into the dishes, wherein the SYLGARD® is allowed to air dry and harden for at least one week prior to use of the dishes. Under general anesthesia and aseptic conditions, rat peripheral nerve segments are then surgically removed, pinned at slack length (straight, but not taut) within the culture dishes using minutien pins, and immediately submersed in Dulbecco's Phosphate Buffered Saline (PBS) or another suitable balanced salt solution.

[0025] The following acellularization method is carried out at room temperature (˜21° C.) within covered culture dishes. Advantageously, the acellularization method described herein is simple, inexpensive, uses commonly available chemicals of low toxicity, and does not require mechanical agents of any kind. Accordingly, in order to remove the cellular material, the following solutions are prepared and stored at room temperature, wherein NaN ₃ is used in each solution as a preservative: 1

[0026] First, the peripheral nerve segments are submersed with Solution 1 for approximately 72 hours in order to disrupt the cell membrane. Second, the nerve segments are submersed with Solution 2 for approximately 72 hours to begin intracellular protein dissociation. Between each solution change, the nerve segments were rinsed at least once with distilled water. Next, the nerve segments are again submersed with Solution 1 , this time for approximately 48 hours to complete the removal of lipid-soluble cell structures. The nerve segments are then submersed with Solution 3 for approximately 48 hours for additional protein denaturing. Subsequently, the nerve segments are submersed with Solution 4 for approximately 48 hours in order to remove denatured proteins from the extracellular matrix, leaving the extracellular matrix intact. The nerve segments are next submersed with Solution 3 for approximately 48 hours to accomplish final protein denaturing and removal. Lastly, the nerve segments are submersed with Solution 5 where they can be stored for at least 4 weeks until use, wherein Solution 5 may be added as necessary to prevent evaporation.

[0027] Due to the technical design of the acellularization method according to the present invention, there should be no limitations on the length or diameter of nerve grafts that can be created. Larger diameter nerve grafts can be acellularized by simply employing correspondingly longer immersion times for each solution.

[0028] Of course, it is understood that all reagent measurements and submersion times described above are approximate, and can be varied slightly without affecting the resulting acellularization. The chemical acellularization method according to the present invention was utilized in the context of acellularizing muscle tissue in commonly assigned U.S. Pat. No. 6,207,451 which is incorporated by reference herein.

[0029] To functionally and histologically evaluate the acellularized nerve construct produced by the chemical acellularization method of the present invention, several experiments were performed. By examining the corresponding muscle force generation following nerve graft repair as well as muscle and nerve histology, microscopic structure can be correlated with functional results.

[0030] Experiments were performed using adult male, specific-pathogen-free Fischer-344 rats (Charles River Laboratory, Kingston, N.Y.). For each animal, the left peroneal nerve was exposed and a 2 cm segment was excised to create a nerve gap and serve as a model for traumatic nerve injury. An identical length of acellularized nerve was used to repair the nerve gap. The proximal and distal ends of the peroneal nerve stumps were coapted to the proximal and distal ends of the graft in the standard end-to-end fashion using 10-0 nylon epineurial sutures. A 15 week recovery period was allowed following the initial surgery prior to measuring muscle contractile properties and analyzing muscle and nerve histology.

[0031] Walking tracks were used to evaluate integrated motor function preoperatively and at 15 weeks postoperatively using standard protocols. Individual walking track records were digitally scanned into high-resolution computer graphic files, and measurements were performed at 2X resolution using SigmaScan Pro (Version 4.10.003, 1997, SPSS Inc.) image analysis software. For each record, 3 or 4 footprints during a period of brisk walking were used for analysis. For each footprint, the intermediate toe spread (ITS) was measured from the tip of the second toe to the tip of the fourth toe bilaterally on both the nerve grafted leg (left) and unoperated leg (right). The functional ITS index was then calculated for each walking track record using the formula ITS index=(Left ITS−Right ITS)/Right ITS. The results showed that the average ITS index value at 15 weeks following acellular nerve grafting was decreased only approximately 20% compared with the preoperative baseline value, thereby confirming at least partial recovery of integrated function using the acellular nerve grafts of the present invention.

[0032] Extensor digitorum longus (EDL) muscle contractile function was also analyzed in situ 15 weeks following the initial nerve graft surgery to determine the extent of muscle reinnervation following peripheral nerve injury and repair. Each rat was anesthetized, the left EDL was isolated, and the distal tendons of the EDL were divided and folded to create a tendon loop which was secured at the musculotendinous junction with 3-0 silk suture. The tibial and sural nerves, as well as the distal tendon of the tibialis anterior muscle were then divided to avoid motion artifact. The rat was placed on a platform maintained at ˜37° C. by a temperature-controlled water circulator, and the EDL tendon loop was secured to the force transducer. Throughout the evaluation, muscle temperature was monitored and maintained between approximately 35° and 37° C.

[0033] The EDL muscles were activated indirectly by delivering supramaximal electrical stimuli (square pulses, 0.2 msec pulse duration, 6-10 V) to the peroneal nerve proximal to the graft site. Stimuli were generated by a Grass S88 Stimulator (Grass Instrument Co., Quincy, Mass.) and delivered with a shielded bipolar silver wire electrode (Harvard Apparatus, South Natick, Mass.). Output from the force transducer was sampled by means of an analog-to-digital converter (Data Translation, Marlboro, Mass.) interfaced with a microcomputer. Custom software (Asyst Software Technologies, Inc., Rochester, N.Y.) was used to control data collection and to perform signal analysis. At optimal muscle length (L ₀ ), maximum tetanic isometric force (F ₀ ) was evaluated by stimulating the EDL muscle for 250 msec at increasing frequencies (from 30 to 350 Hz) and determining the highest force generated. Following the force measurements, the muscles were excised, the tendons trimmed, and the muscles were weighed. The results showed that at 15 weeks postoperatively the EDL muscle mass was 72.8±22.6 mg and F ₀ was 726.3±608. 1 mN. Although decreased compared with values for sham-operated control animals, the recovery of force generating capabilities of EDL muscles reinnervated using acellularized nerve grafts according to the present invention assures that the regenerating axons have made functional connections with the target muscle.

[0034] Subsequent to the above procedures, the nerve grafts were fixed in a formaldehyde/glutaraldehyde solution, then rinsed within 24 hours and transferred to storage buffer. The fixed nerves were then hydrated using graded ethanol baths prior to embedding in epoxy. Thin (10 μm) sections taken from the distal end of the nerve grafts were then mounted and stained with toluidine blue for quantitative analysis. Nerve sections also underwent electron microscopic analysis to assess nerve graft axon population as well as degree of myelination.

[0035] Referring now to FIGS. 1 a and 1 b , electron micrographs of representative peripheral nerves prior to and following, respectively, the chemical acellularization method of the present invention are shown. As one skilled in the art will readily observe, FIG. 1 a depicts myelinated axons traversing the length of the nerve, while FIG. 1 b shows a lack of cellular elements and preserved endoneurial sheaths and basal lamina within the nerve. This analysis indicates that the majority of the intracellular and cell membrane components are removed by the chemical acellularization method utilized herein. The remaining material is principally extracellular matrix (ECM) which provides the structural support for nerve regeneration. Therefore, the acellularization method of the present invention appears to remove the Schwann cells in order to reduce the immunogenicity of the construct, but preserves the basal lamina in order to maintain the appropriate molecular signals and adhesion molecules to enhance axonal regeneration. Presumably, the preserved basal lamina provides the appropriate adhesion molecules and molecular signals to promote Schwann cell migration, differentiation, and ultimately axonal elongation.

[0036] With reference to FIG. 2, a toluidine blue stained section of the distal end of a representative acellular nerve graft is shown 15 weeks after the construct was used to reconstruct a gap in a rat peroneal nerve, wherein the section was taken approximately 2 cm distal to the nerve coaptation site. Multiple large and small myelinated axons can be readily observed by one skilled in the art, thereby demonstrating the ability of the acellularized nerve grafts to support axonal regeneration and to allow end-organ reinnervation.

[0037] Following the force measurements, the EDL muscles were covered with cryopreservative, frozen with isopentane cooled by liquid nitrogen (−160° C.), and stored at −60° C. for subsequent processing. Whole EDL muscle cross-sections (12 μm thick) were cut using a cryotome (−20° C.), then stained with hematoxylin & eosin and myosin ATPase using standard techniques for subsequent light microscopic fiber type analysis. With reference to FIG. 3, a myosin ATPase stained section (pH=4.3) of a representative EDL muscle is shown, wherein areas of large polygonal muscle fibers loosely grouped according to muscle fiber type are readily evident to those skilled in the art. Such a spatial distribution of fiber types is characteristic of reinnervated muscle.

[0038] Therefore, the functional and histological data described above clearly demonstrate that the chemically acellularized nerve grafts of the present invention support axonal regeneration across at least a 2 cm nerve gap and that functional muscle reinnervation can be achieved.

[0039] To evaluate immunogenicity, acellularized nerve constructs according to the present invention were transplanted into subcutaneous pockets across a major histocompatibility barrier, from ACI rat donors to Lewis rat recipients. Referring now to FIG. 4, a light micrograph of a section of a representative acellular nerve construct displays no evidence of an inflammatory response or any other acute or chronic rejection response. This preliminary experiment demonstrates that the acellularization method of the present invention produces a nerve construct that is nonimmunogenic when transplanted across major histocompatibility antigen barriers, thus immunosuppressive drugs should not be required. Such grafts also demonstrate axonal regeneration and successful reinnervation of the motor end organ via the nerve graft as described for the previous experiments.

[0040] Therefore, the chemically acellularized nerve constructs described herein support axonal regeneration and functional reinnervation for the reconstruction of peripheral nerve gaps. The chemical acellularization method according to the present invention provides a scaffold for axonal regeneration that is more suitable than that left by mechanical acellularization processes while removing all of the immunologically reactive cellular components. More particularly, the chemical acellularization method appears to preserve the delicate ultrastructure of peripheral nerves while removing the most antigenic components, namely the Schwann cells and myelin. Furthermore, the acellularization method of the present invention produces a nerve construct that induces no histologically evident acute or chronic rejection response when transplanted across major histocompatibility antigen barriers.

[0041] Schwann cells produce many neurotrophic factors that aid in axonal regeneration, including NGF, IGF, and CNTF. By reintroducing Schwann cells into the acellularized peripheral nerve grafts of the present invention, axonal regeneration and muscle reinnervation may be further enhanced. Additional strategies to enhance the efficacy of the acellular grafts of the present invention might include reintroducing critical growth factors into the construct through tissue culture techniques or gene derived matrices.

[0042] The chemical acellularization method discussed herein could easily be adapted for use in clinical situations where nerve gaps are identified. Applications for the nerve construct and acellularization method of the present invention include peripheral nerve reconstruction after traumatic injury, reconstruction of nerve defects resulting from the resection of malignant tumors, functional reconstruction for patients with congenital syndromes where peripheral nerves are absent (e.g., Mobius Syndrome), and for the amelioration of facial paralysis and other movement disorders in some circumstances of CNS injury including strokes and spinal cord injuries. Under these circumstances, sufficient amounts of autologous nerve graft may not be present, and a nonimmunogenic, acellularized human, cadaveric allograft could be utilized for this peripheral nerve reconstruction. In addition, the acellularized nerve grafts could be created from donor nerves that match the recipient nerve to be reconstructed, thus the basic architecture of the nerve graft would be specific to the site of transplantation.

[0043] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.