Regeneration Distances Research Articles

Improving outcomes in traumatic peripheral nerve injuries to the upper extremity.

Peripheral nerve lesions of the upper extremity are common and are associated with devastating limitations for the patient. Rapid and accurate diagnosis of the lesion by electroneurography, neurosonography, or even MR neurography is important for treatment planning. There are different therapeutic approaches, which may show individual differences depending on the injured nerve. If a primary nerve repair is not possible, several strategies exist to bridge the gap. These may include autologous nerve grafts, bioartificial nerve conduits, or acellular nerve allografts. Tendon and nerve transfers are also of major importance in the treatment of nerve lesions in particular with long regeneration distances. As a secondary reconstruction, in addition to tendon transfers, there is also the option for free functional muscle transfer. In amputations, the prevention of neuroma is of great importance, for which different strategies exist, such as target muscle reinnervation, regenerative peripheral nerve interface, or neurotized flaps. In this article, we give an overview of the latest methods for the therapy of peripheral nerve lesions.

A single session of brief electrical stimulation enhances axon regeneration through nerve autografts

Nerve graft reconstruction of gap defects may result in poor clinical outcomes, particularly with long regeneration distances. Electrical stimulation (ES) of nerves may improve outcomes in such patients. A single session of ES at 20 Hz for 1 h significantly enhances axon regeneration in animals and human subjects after nerve crush or nerve transection and repair. The objectives of this study were to evaluate if ES enhances axon regeneration through nerve grafts and if there is added benefit of a second, delayed session of ES (serial ES) on axon regeneration as compared to a single session only of ES. In female rats, a gap defect was created in the hindlimb common peroneal (CP) nerve and immediately reconstructed with a 10 mm nerve autograft (Experiment 1) or a 20 mm nerve autograft (Experiment 2). In Experiment 1, rats were randomized to 1 h of CP nerve ES or sham stimulation. In Experiment 2, rats were randomized to control (sham ES + sham ES), single ES (ES + sham ES), or serial ES (ES + ES), which consisted of an initial 1 h session of either ES or sham stimulation of the CP nerve, followed by a second 1 h session of ES or sham stimulation of the CP nerve 4 weeks later. In both experiments, after a 6 week period of nerve regeneration, CP neurons that had regenerated axons distal to the autograft were retrograde labelled for enumeration, and the CP nerve distal to the autograft was harvested for histomorphometry. In Experiment 1, rats that received CP nerve ES had statistically significantly more motor (p < .05) and sensory (p < .05) neurons that regenerated axons distal to the 10 mm nerve autograft, with more myelinated axons on histomorphometry (p < .001). Similarly, in Experiment 2, significantly more motor (p < .01) and sensory (p < .05) neurons regenerated axons distal to the 20 mm nerve autograft after a single session or two sessions of CP nerve ES. There was no significant difference in the number of regenerated motor or sensory neurons between rats with 20 mm CP nerve autografts receiving either one or two sessions of CP nerve ES (p > .05). In conclusion, a single session of ES enhances axon regeneration following nerve autografting with no added effect of a second, delayed session of ES. These findings support previous studies in animals and humans of the robust effect of a single session of ES in promoting nerve regeneration following injury and repair.

Reconstruction of the spinal accessory nerve with selective fascicular nerve transfer of the upper trunk.

Spinal accessory nerve palsy is frequently caused by iatrogenic damage during neck surgery in the posterior triangle of the neck. Due to late presentation, treatment regularly necessitates nerve grafts, which often results in a poor outcome of trapezius function due to long regeneration distances. Here, the authors report a distal nerve transfer using fascicles of the upper trunk related to axillary nerve function for reinnervation of the trapezius muscle. Five cases are presented in which accessory nerve lesions were reconstructed using selective fascicular nerve transfers from the upper trunk of the brachial plexus. Outcomes were assessed at 20 ± 6 months (mean ± SD) after surgery, and active range of motion and pain levels using the visual analog scale were documented. All 5 patients regained good to excellent trapezius function (3 patients had grade M5, 2 patients had grade M4). The mean active range of motion in shoulder abduction improved from 55° ± 18° before to 151° ± 37° after nerve reconstruction. In all patients, unrestricted shoulder arm movement was restored with loss of scapular winging when abducting the arm. Average pain levels decreased from 6.8 to 0.8 on the visual analog scale and subsided in 4 of 5 patients. Restoration of spinal accessory nerve function with selective fascicle transfers related to axillary nerve function from the upper trunk of the brachial plexus is a good and intuitive option for patients who do not qualify for primary nerve repair or present with a spontaneous idiopathic palsy. This concept circumvents the problem of long regeneration distances with direct nerve repair and has the advantage of cognitive synergy to the target function of shoulder movement.

Brief Electrical Stimulation Promotes Nerve Regeneration Following Experimental In-Continuity Nerve Injury.

Brief electrical stimulation (ES) therapy to the nerve may improve outcome in lacerated, repaired nerves. However, most human nerve injuries leave the nerve in continuity with variable and often poor functional recovery from incomplete axon regeneration and reinnervation. To evaluate the effect of brief ES in an experimental model for neuroma-in-continuity (NIC) injuries in rodents. Lewis rats were randomly assigned to 1 of 4 groups: NIC injury immediately followed by brief (1 h) ES; NIC injury without ES; sham-operated controls; sciatic nerve transection without repair. Outcome measures included serial behavioral evaluation and electrophysiology together with terminal retrograde spinal cord motor neuron labeling and histomorphological analysis for axonal regeneration. Applying brief ES immediately after in-continuity nerve injury resulted in earlier recovery and significantly improved locomotion function at 4 and 6 wk. At 8 wk, brief ES resulted in higher compound action potential amplitude. By 12 wk there was no significant difference between the 2 groups in behavior or electrophysiology. Histomorphological analysis demonstrated a significantly higher percentage of neural tissue in the brief ES group. Spinal cord motor neuron pool cell counts revealed a preference for regeneration into a motor over a sensory nerve, for the group receiving ES. The application of brief ES for in-continuity nerve injury promotes faster recovery, although in a rat model where regeneration distances are short the control group ultimately recovers to a similar degree. Brief EF requires further evaluation as a promising therapy for in-continuity nerve injuries in humans.

Surgery for nerve injury: current and future perspectives.

In this review article, the authors offer their perspective on nerve surgery for nerve injury, with a focus on recent evolution of management and the current surgical management. The authors provide a brief historical perspective to lay the foundations of the modern understanding of clinical nerve injury and its evolving management, especially over the last century. The shift from evaluation of the nerve injury using macroscopic techniques of exploration and external neurolysis to microscopic interrogation, interfascicular dissection, and internal neurolysis along with the use of intraoperative electrophysiology were important advances of the past 50 years. By the late 20th century, the advent and popularization of interfascicular nerve grafting techniques heralded a major advance in nerve reconstruction and allowed good outcomes to be achieved in a large percentage of nerve injury repair cases. In the past 2 decades, there has been a paradigm shift in surgical nerve repair, wherein surgeons are not only directing the repair at the injury zone, but also are deliberately performing distal-targeted nerve transfers as a preferred alternative in an attempt to restore function. The peripheral rewiring approach allows the surgeon to convert a very proximal injury with long regeneration distances and (often) uncertain outcomes to a distal injury and repair with a greater potential of regenerative success and functional recovery. Nerve transfers, originally performed as a salvage procedure for severe brachial plexus avulsion injuries, are now routinely done for various less severe brachial plexus injuries and many other proximal nerve injuries, with reliably good to even excellent results. The outcomes from nerve transfers for select clinical nerve injury are emphasized in this review. Extension of the rewiring paradigm with nerve transfers for CNS lesions such as spinal cord injury and stroke are showing great potential and promise. Cortical reeducation is required for success, and an emerging field of rehabilitation and restorative neurosciences is evident, which couples a nerve transfer procedure to robotically controlled limbs and mind-machine interfacing. The future for peripheral nerve repair has never been more exciting.

"Pathway protection" - enhanced motoneuron regeneration by end-to-side coaptation of sensory axons

Proximal nerve injuries and delayed nerve repair lead to reduced peripheral nerve regeneration. Poor functional results after nerve injury are clinically a very challenging problem. Experimental studies defined "chronic axotomy" of the neuron and the proximal part of a dissected nerve that are separated from their distal target, as well as "chronic denervation" of the distal nerve stump as an independent factors that reduce regenerative capacity of injured nerves. The denervated distal nerve undergoes changes associated with Wallerian degeneration. Denervated Schwann cells change their phenotype to interact with in-growing axons and increase the expression of growth factors. These changes lead to a pro-regenerative environment in the distal nerve stump. This growth-permissive environment deteriorates with time leading to significantly reduced nerve regeneration. Clinically, delayed nerve repair and long regeneration distances often result in inadequate functional outcomes. Different "pathway protection" techniques were developed to improve nerve regeneration and reduce the chronic denervation of the distal nerve. Most of these "babysitter" procedures used a motor donor nerve which was coapted usually end-to-side to the denervated distal nerve stump. Experimental studies showed that in-growing donor axons increase neurotrophic factor levels and improved reinnervation of distal targets. Motor "babysitter" procedures are, however, associated with a motor donor nerve deficit. In recent years, sensory "pathway protection" was investigated to avoid such motor deficit. Motoneuron regeneration of its axons can, in experimental animal models, be improved by end-to-side coaptation of sensory donor axons to either a denervated distal nerve stump or to a long autologous nerve graft, both of which undergo changes associated with chronic denervation. Sensory "pathway protection" has already been successfully clinically applied, however long-term functional analysis awaits.

Collagen (NeuraGen®) nerve conduits and stem cells for peripheral nerve gap repair

Collagen nerve guides are used clinically for peripheral nerve defects, but their use is generally limited to lesions up to 3cm. In this study we combined collagen conduits with cells as an alternative strategy to support nerve regeneration over longer gaps. In vitro cell adherence to collagen conduits (NeuraGen® nerve guides) was assessed by scanning electron microscopy. For in vivo experiments, conduits were seeded with either Schwann cells (SC), SC-like differentiated bone marrow-derived mesenchymal stem cells (dMSC), SC-like differentiated adipose-derived stem cells (dASC) or left empty (control group), conduits were used to bridge a 1cm gap in the rat sciatic nerve and after 2-weeks immunohistochemical analysis was performed to assess axonal regeneration and SC infiltration. The regenerative cells showed good adherence to the collagen walls. Primary SC showed significant improvement in distal stump sprouting. No significant differences in proximal regeneration distances were noticed among experimental groups. dMSC and dASC-loaded conduits showed a diffuse sprouting pattern, while SC-loaded showed an enhanced cone pattern and a typical sprouting along the conduits walls, suggesting an increased affinity for the collagen type I fibrillar structure. NeuraGen® guides showed high affinity of regenerative cells and could be used as efficient vehicle for cell delivery. However, surface modifications (e.g. with extracellular matrix molecule peptides) of NeuraGen® guides could be used in future tissue-engineering applications to better exploit the cell potential.

Vitamin E reduces sensory neuronal loss and improves nerve regeneration after sciatic nerve injury

Background: Nerve injury results in axonal degeneration and loss of sensory neurons in dorsal root ganglia (DRG). Enhanced oxidative stress was also observed following injury, suggesting potential efficacy of antioxidant therapy. Objective: To examine the effects of vitamin E on nerve regeneration after nerve injury in rats. Method: Unilateral sciatic nerve crush was performed on rats which were divided into three groups: control and 2 doses of vitamin E. The control group received only vehicle (corn oil) and vitamin E groups received ?-tocopherol 500 and 1,000 mg/kg/day once daily by gavage. Regeneration was evaluated at 10 days post-injury. In a second experiment, there were two groups, vehicle and vitamin E 500 mg/kg/day, and the regeneration duration was extended to 3 weeks. Result: The first experiment showed that vitamin E 500 and 1,000 mg/kg/day could significantly reduce DRG neuronal loss from 31.5% in the control group to 17.1% and 13.5%. However, the regeneration distances were not significantly different among groups. In the second experiment, although vitamin E 500 mg/kg/day significantly increased the plasma vitamin E level, motor recovery of the hind limb, as assessed by walking track analysis, was not significantly improved. By contrast, nerve morphometry revealed increased density and number of myelinated fiber after vitamin E treatment. Conclusion: Vitamin E may be beneficial to sensory neuronal loss and nerve regeneration after nerve trauma. Keywords: Nerve injury, nerve regeneration, neuronal loss, vitamin E, ?-tocopherol

Nerve Transfers for the Upper Extremity

Nerve transfers are key components of the surgeon's armamentarium in brachial plexus and complex nerve reconstruction. Advantages of nerve transfers are that nerve regeneration distances are shortened, pure motor or sensory nerve fascicles can be selected as donors, and nerve grafts are generally not required. Similar to the principle of tendon transfers, expendable donor nerves are transferred to denervated nerves with the goal of functional recovery. Transfers may be subdivided into intraplexal, extraplexal, and distal types; each has a unique role in the reconstructive process. A thorough diagnostic workup and intraoperative assessment help guide the surgeon in their use. Nerve transfers have made a positive impact on the outcomes of nerve surgery and are essential tools in complex nerve reconstruction.

Expression of ATF3 and axonal outgrowth are impaired after delayed nerve repair.

BackgroundA delay in surgical nerve repair results in impaired nerve function in humans, but mechanisms behind the weakened nerve regeneration are not known. Activating transcription factor 3 (ATF3) increases the intrinsic growth state of injured neurons early after injury, but the role of long-term changes and their relation to axonal outgrowth after a delayed nerve repair are not well understood. ATF3 expression was examined by immunohistochemistry in motor and sensory neurons and in Schwann cells in rat sciatic nerve and related to axonal outgrowth after transection and delayed nerve repair (repair 0, 30, 90 or 180 days post-injury). Expression of the neuronal cell adhesion molecule (NCAM), which is expressed in non-myelinating Schwann cells, was also examined.ResultsThe number of neurons and Schwann cells expressing ATF3 declined and the length of axonal outgrowth was impaired if the repair was delayed. The decline was more rapid in motor neurons than in sensory neurons and Schwann cells. Regeneration distances over time correlated to number of ATF3 stained neurons and Schwann cells. Many neurofilament stained axons grew along ATF3 stained Schwann cells. If nerve repair was delayed the majority of Schwann cells in the distal nerve segment stained for NCAM.ConclusionDelayed nerve repair impairs nerve regeneration and length of axonal outgrowth correlates to ATF3 expression in both neurons and Schwann cells. Mainly non-myelinating Schwann cells (NCAM stained) are present in distal nerve segments after delayed nerve repair. These data provide a neurobiological basis for the poor outcomes associated with delayed nerve repair. Nerve trunks should, if possible, be promptly repaired.

Physiological and clinical advantages of median nerve fascicle transfer to the musculocutaneous nerve following brachial plexus root avulsion injury

Loss of biceps muscle function is a significant disability after brachial plexus root avulsion injuries. Nerve grafting techniques to reestablish anatomical and functional continuity between the spinal cord and the avulsed root have not proven successful. Using nerve transfers for functional restoration of root avulsion injuries appears to be effective and has physiological advantages for reducing regeneration distances. Since the early 1990s, the Oberlin technique of transferring ulnar nerve fascicles to the motor branch of the musculocutaneous nerve has been the preferred operative technique for reinnervation and restoration of biceps muscle function. In the current study the authors examine the efficacy of an alternative technique using median nerve fascicles transferred to the musculocutaneous nerve to reinnervate the biceps muscle. Forty consecutive patients with combined C5-6 brachial plexus root avulsions were evaluated pre- and postoperatively according to the British Medical Research Council Motor Grading Scale. Personal interviews concerning quality of life (QOL) after surgery were conducted and scored based on standards set by the World Health Organization. All patients showed some degree of improvement in biceps muscle function. Thirty-six (90%) of the 40 patients regained movement against gravity. The patients had a 77% improvement in overall QOL after the surgery; most notably, 92% of the patients reported their lack of need for medication and 75% a significant lessening of postoperative pain. Redirection of part of the healthy median nerve resulted in no measurable functional deficits, and only 28 patients reported minor sensory disturbances in the first web space for an average of 3 months after surgery. Median nerve fascicle transfer resulted in a significant improvement in biceps muscle function with an acceptable level of morbidity and should be considered an effective, and in many cases preferable, alternative to ulnar nerve fascicle transfer.

Green Fluorescent Protein Is a Stable Morphological Marker for Schwann Cell Transplants in Bioengineered Nerve Conduits

Bioengineered systems incorporate cultured cells to mimic the substituted tissue. A labeling method is necessary to monitor the survival of transplanted cells within the host. This labeling method must be compatible with the histochemical methods used for morphological analysis. This study assessed (1) The in vitro characteristics of Schwann cells (SCs) labeled with green fluorescent protein (GFP), (2) the in vivo effect of transplanted GFP-SCs in a model of peripheral nerve injury, and (3) the compatibility of GFP-SCs with immunofluorescence histochemical techniques. SCs were retrovirally labeled with GFP and their growth characteristics were compared with those of nontransduced SCs (ntSCs). GFP-SCs were seeded in a resorbable nerve conduit for grafting into a 1-cm gap in rat sciatic nerve. Grafts were harvested after 2 weeks and immunofluorescent staining was performed to measure axonal and SC regeneration distances and to identify GFP-SCs. Results of GFP-SC vitality assays did not vary significantly from those of ntSC assays. GFP-SCs were readily located ex vivo and stimulated significantly better axonal and SC regeneration distances in comparison with empty conduits. These findings show that GFP labeling does not have a deleterious effect on SCs and that it is a useful labeling method for the study of bioengineered systems.

Tourniquet compression: a non-invasive method to enhance nerve regeneration in nerve grafts.

One hindlimb of a rat was subjected to tourniquet compression (150, 200 and 300 mmHg; 2 h). After 6 days a 10 mm sciatic or tibial nerve graft from the compressed limb was sutured to bridge a 3-4 mm gap in the sciatic nerve of the non-compressed limb. The distances of regenerating sensory axons were measured 6 days post surgery (tibial grafts, 8 days). Compression at 200 and 300 mmHg led to significantly longer regeneration distances than those seen in controls. Incorporation of BrdU and expression of p75 receptor by non-neuronal cells (Schwann cells) in sciatic nerves 6 days after compression (150 and 300 mmHg; 2 h) was also increased as a sign of Schwann cell activation. Tourniquet compression may be used as a non-invasive method to enhance nerve regeneration in nerve grafts.

NERVE REGENERATION ENHANCEMENT BY TOURNIQUET

The use of tourniquet compression as a non-invasive method to enhance axonal regeneration was assessed in the rat sciatic nerve. One hind limb of the rat was subjected to compression by a tourniquet set at 300 mmHg for 30 or 120 min followed by bilateral test crush lesions performed either directly or after a conditioning interval of 3 or 6 days, with the non-compressed side serving as a control. Axonal regeneration distances were evaluated after 3 days by the pinch reflex test. We found that compression caused an increased outgrowth length of sensory axons compared to the controls. The effect was most obvious after 120 min of compression with a conditioning interval of 6 days. Tourniquet compression has a conditioning lesion effect on peripheral nerve and may enhance nerve regeneration.

Hyperbaric Oxygen Treatment Enhances Regeneration of the Rat Sciatic Nerve

The effect of hyperbaric oxygen (HBO) treatment on regeneration of the rat sciatic nerve was studied. The sciatic nerve was crushed with a pair of pliers and the animals were either left untreated or subjected to a series of 45-min exposures to 100% O2at 3.3 atm absolute pressure at 0, 4, and 8 h postoperatively and then every 8 h. Regeneration was evaluated using the pinch-reflex test at 3, 4, or 5 days following surgery and with neurofilament staining at 4 days. The regeneration distances at all time points were significantly longer in animals exposed to hyperbaric oxygen treatment independent of the evaluation procedure. A short initial period of the same HBO treatment schedule, with no more treatments after 25 h, appeared as effective as when treatments were maintained being given every 8 h until evaluation. We conclude that HBO treatment stimulates axonal outgrowth following a nerve crush lesion.

Nerve injury induced by vibration: prevention of the effect of a conditioning lesion by D600, a Ca2+ channel blocker.

OBJECTIVES: Exposing a hind leg of a rat to vibration induces an injury to the sciatic nerve--a so called conditioning lesion. After such injury induced by vibration the regenerative capacity...

Role of macrophages in the stimulation and regeneration of sensory nerves by transposed granulation tissue and temporal aspects of the response.

Application of granulation tissue, which is rich in macrophages, to a peripheral nerve induces a conditioning effect, in that it enhances the regeneration capability of peripheral nerves after a test crush lesion. The temporal aspects of this response and the role of macrophages and interleukin-1 beta (IL-1 beta) were studied in the sciatic nerves of 71 rats. Granulation tissue was implanted close to the sciatic nerve and test crush lesions were applied after various periods of time (0-21 days). Regeneration was evaluated after an additional two, three, four, or six days. Regeneration distances were longer in granulation-treated nerves than in nerves treated with subcutaneous tissue. Furthermore, in animals in which the test crush lesion was made at the same time as the granulation tissue was implanted (n = 6), regeneration distances were longer, 8.1 (0.8) mm compared with 7.2 (0.6), than those in which the crush was made after conditioning intervals of 3 (n = 6, 7.6 (0.4) compared with 6.9 (0.4), p = 0.03); 7 (n = 6, 7.4 (0.4) compared with 6.6 (0.1), p = 0.03); and 21 days [(n = 8, 7.2 (0.6) compared with 6.4 (0.5)]. Inactivation of the granulation tissue by freezing suppressed the conditioning effect. There were numerous ED1 and ED2 positive macrophages as well as positive staining for IL-1 beta in the granulation tissue on day 0. Positive staining for IL-1 beta was also seen in nerve fibres as well as in non-neuronal cells after a conditioning interval. The results suggest that regeneration is stimulated by factors released from the cells of the granulation tissue, and that the amount of factors released or the responsiveness of the regenerating nerve change during the conditioning interval.

Axonal regeneration of sensory nerves is delayed by continuous intrathecal infusion of nerve growth factor

While it is well established that nerve growth factor is growth promoting for sensory neurons in culture, it is unclear whether it serves such a function in vivo. In fact, our previous studies led to the hypothesis that nerve growth factor could actually impair axonal regeneration by reducing the neuronal cell body response to injury. In the present study, the consequence of continuous intrathecal infusion of nerve growth factor on regeneration of sensory neurons was examined in rats given a bilateral sciatic nerve crush. Rats received nerve growth factor (125ng/h) as a continuous infusion into the subarachnoid space of the lumbar spinal cord via an osmotic minipump (Alzet); controls received cytochrome C. At seven or 10 days, the pump was removed and L4 or L5 dorsal root ganglion exposed and injected with 50μCi of [3H]leucine. Animals were killed 24h later, the sciatic nerves removed, cut into 3mm segments and the radioactivity in each segment determined by liquid scintillation spectrophotometry. Maximal regeneration distances (determined from the front of the resultant transport curves) were similarly reduced (by approximately 6mm) in nerve growth factor-infused compared to cytochrome C-infused rats. Thus, regeneration rates (determined between eight and 11 days) were unaltered by nerve growth factor infusion; regeneration rates from cytochrome C-infused and nerve growth factor-infused animals were 2.8mm/day and 3.1mm/day, respectively. However, nerve growth factor significantly (P<0.005) increased the delay to onset for regeneration by two days.Taken together, the present study demonstrates that nerve growth factor delays the onset of regeneration without affecting the rate of regeneration. The results implicate the involvement of at least two signals in the regulation of axonal regeneration in dorsal root ganglion neurons. It is suggested that the loss of nerve growth factor serves as an early, induction signal regulating the onset of regeneration and that a second, unidentified signal independently serves to maintain regeneration.

Impaired regeneration in rat sciatic nerves exposed to short-term vibration

We have studied the effects of vibration on the regeneration capacity of the peripheral nerve. A rat model was used where one hind limb was subjected to vibration of defined magnitude and duration while the contralateral hind limb was not exposed to vibration. Seven days later, the sciatic nerves were transected bilaterally and cross-joined giving the following groups: group A, a proximal vibrated nerve end sutured to a non-vibrated distal nerve end; group B, a non-vibrated proximal nerve end sutured to a distal vibrated nerve end, and group C, non-vibrated proximal nerve end sutured to a non-vibrated distal nerve end. The regeneration distances were measured 3, 6 and 8 days after surgery. The control group showed a normal linear outgrowth. The outgrowth in the two experimental groups was initially not different to controls but later became significantly different, indicating a retardation of outgrowth in these groups. It is concluded that short-term exposure to vibration can impair nerve regeneration after transection and nerve repair.

Stimulation of nerve regeneration by macrophages in granulation tissue

The effects of granulation tissue exposure on regeneration of sensory axons after a test crush lesion in rat sciatic nerves were studied. Chromic catgut threads were applied subcutaneously in the back of rats. Three weeks later the surrounding granulation tissue was isolated and transposed to the sciatic or tibial nerve in the same rat. Immunocytochemical, light and electron microscopic evaluation of the isolated granulation tissue revealed numerous EDI and ED2 positive macrophages. After an additional three weeks the sciatic nerves were crushed and the axonal outgrowth lengths were measured by the pinch reflex test and by neurofilament staining 3 or 6 days later. The regeneration distances in the sciatic nerve exposed to granulation tissue were significantly longer compared with that of nerves to which only subcutaneous tissue was transposed. Thus, the nerve had become conditioned by the transposure of macrophage rich tissue. After the transposure macrophages were present close to the nerve at the time point of the test crush lesion, but no signs of degeneration or inflammation in the endoneurial space were observed. It was also found that the outgrowth lengths in the tibial nerve branch were consistently longer than in the peroneal branch of the sciatic nerve. It is concluded that granulation tissue, containing macrophages, can induce a conditioning effect in the sciatic nerve. We suggest that this effect is related to factors emanating from macrophages in the transposed tissue.