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To restore function after nerve injury, axons must regenerate from the injury site to the periphery, then reinnervate appropriate end organs when they arrive. Only 10 % of adults who suffer nerve injury will regain normal function, often because axons regenerate to functionally inappropriate targets (Brushart, 2011). The peripheral destination of these axons is largely determined by the pathways they enter at the site of nerve repair. To improve clinical outcomes, it is thus critical to improve the accuracy of axon pathfinding. In rodents, motor axons regenerating in mixed nerve preferentially reinnervate pathways leading to muscle, a process termed preferential motor reinnervation (PMR). Previous experiments have shown that PMR can be enhanced by predegenerating nerve grafts to enhance growth factor production and remove inhibitory factors (Abdullah et al., 2013). The current experiments explore the relative contributions of motor pathways, sensory pathways, and the repair environment to this enhancement. Sensory and/or motor pathways within rat femoral nerve grafts were predegenerated for 3 weeks to optimize growth factor production (Brushart et al., 2013) or for 12 weeks to deplete it. Optimizing the environment within previously motor Schwann cell tubes promoted PMR, regardless of whether adjacent sensory pathways were optimized or chronically denervated. However, this positive effect was abolished when sensory pathways were undergoing acute Wallerian degeneration immediately after nerve repair. The repair environment thus precluded motor axon pathfinding in spite of an optimized distal motor pathway. When sensory pathways were optimized and motor pathways were chronically denervated, not only was PMR abolished, but motoneurons failed to respond to the greater volume of growth factors in the sensory nerve. Small sensory neurons, however, selectively reinnervated cutaneous nerve under these conditions. These experiments thus strengthen the concept that, in adult rats, sensory and motor pathways have unique identities capable of influencing both sensory and motor axon regeneration. Furthermore, they demonstrate that, in the rat, delaying nerve repair for 3 weeks to enhance growth factor production and clear the products of acute Wallerian degeneration can enhance regeneration specificity without the need for exogenous treatments., Competing Interests: Declaration of competing interest I certify that none of the authors of this paper have any conflict of interest of an academic or financial nature., (Copyright © 2024 Elsevier Inc. All rights reserved.)

During mammalian embryonic development sensory and motor axons interact as an integral part of the pathfinding process. During regeneration, however, little is known of their interactions with one another. It is thus possible that sensory axons might influence motor axon regeneration in ways not currently appreciated. To explore this possibility we have developed an organotypic model of post-natal nerve regeneration in which sensory and motor axons are color-coded by modality. Motor axons that express yellow fluorescent protein (YFP) and sensory axons that express red fluorescent protein (RFP) are blended within a three-dimensional segment of peripheral nerve. This nerve is then transected, allowing axons to interact with one another as they grow out on a collagen/laminin gel that is initially devoid of directional cues. Within hours it is apparent that sensory axons extend more rapidly than motor axons and precede them during the early stages of regeneration, the opposite of their developmental order. Motor axons thus enter an environment already populated with sensory axons, and they adhere to these axons throughout most of their course. As a result, motor axon growth is reduced dramatically. Physical delay of sensory regeneration, allowing motor axons to grow ahead, restores normal motor growth; direct axonal interactions on the gel, rather than some other aspect of the model, are thus responsible for motor inhibition. Potential mechanisms for this inhibition are explored by electroporating siRNA to the neural cell adhesion molecule (NCAM) and the L1 adhesion molecule (L1CAM) into dorsal root ganglia (DRGs) to block expression of these molecules by regenerating sensory axons. Although neither maneuver improved motor regeneration, the results were consistent with early receptor-mediated signaling among axons rather than physical adhesion as the mechanism of motor inhibition in this model., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Functional testing has assumed a progressively dominant role in validating the success of experimental nerve repair. Results obtained in one model, however, cannot predict the results in others because they reflect the coordinated interaction of several muscles across multiple joints. As a result, many combinations of topographically correct and incorrect muscle reinnervation could produce the same result. We have developed a binary model in which elbow flexors and extensors are reinnervated, and elbow flexion and extension are the functions tested. The musculocutaneous and radial nerves of Lister-Hooded rats were subjected to axonotmetic injuries that produced increasing degrees of axonal misdirection at the site of injury ranging from simple crush to transection and rotational offset of proximal and distal stumps. Elbow function was tested with a device that requires coordinated elbow extension to reach sugar pellets and flexion to return them to the mouth. After 12 weeks of regeneration, motoneurons projecting to the distal musculocutaneous nerve were retrogradely labelled with WGA-Ruby and scored regarding their location within musculocutaneous or radial motoneuron pools. The severity of axonal misdirection resulting from the initial surgery was mirrored by progressive degrees of inappropriate reinnervation of the musculocutaneous nerve by radial nerve axons. The specificity of reinnervation predicted elbow function (r = 0.72), whereas the number of motoneurons regenerating did not. This model is thus well suited to study the interaction of regeneration specificity and function across a single joint, and to produce data that can be generalized more broadly than those obtained from more complex models., (© 2015 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Regeneration in the peripheral nervous system offers unique opportunities and challenges to medicine. Compared to the central nervous system, peripheral axons can and do regenerate resulting in functional recovery, especially if the distance to target is short as in distal limb injuries. However, this regenerative capacity is often incomplete and functional recovery with proximal lesions is limited. Furthermore, regeneration of axons to the appropriate targets remains a challenge with inappropriate reinnervation being an impediment to full recovery. The reviews and selected original research papers in this Special Issue will address some of these challenges and highlight new opportunities for development of effective therapies for nerve regeneration., (Copyright 2009 Elsevier Inc. All rights reserved.)

Clinical outcomes of nerve grafting are often inferior to those of end-to-end nerve repair. This may be due, in part, to the routine use of cutaneous nerve to support motor axon regeneration. In previous work, we have demonstrated that Schwann cells express distinct sensory and motor phenotypes, and that these promote regeneration in a modality-specific fashion. Intra-operative modification of graft Schwann cell phenotype might therefore improve clinical outcomes. This paper demonstrates the feasibility of electroporating genes into intact nerve to modify Schwann cell gene expression. Initial trials established 70 V, 5 ms as optimum electroporation parameters. Intact, denervated, and reinnervated rat tibial nerves were electroporated with the YFP gene and evaluated serially by counting S-100 positive cells that expressed YFP. In intact nerve, a mean of 28% of Schwann cells expressed the gene at 3 days, falling to 20% at 7 days with little expression at later times. There were no significant differences among the three groups at each time period. Electronmicroscopic evaluation of treated, intact nerve revealed only occasional demyelination and axon degeneration. Intra-operative electroporation of nerve graft is thus a practical means of altering Schwann cell gene expression without the risks inherent in viral transfection.

Youth is a strong predictor of functional recovery after peripheral nerve repair, while adulthood is commonly associated with poor outcome. Identification of the factors responsible for this difference could form the basis for strategies to improve regeneration in adults. Preferential reinnervation of motor pathways by motor axons (PMR) occurs strongly in young rats, but is often absent in older animals, and thus parallels the overall trend for superior results in young individuals. These experiments evaluate the individual contributions of peripheral nerve age and motoneuron age to the decline in regeneration specificity (PMR) which accompanies the aging process. The femoral nerves of young and old Lewis rats were removed as inverted "Y" grafts from the femoral trunk proximally to the terminal muscle and cutaneous branches distally. These grafts were transferred from (1) old to young, (2) young to old, (3) old to old, and (4) young to young bilaterally in 10 individuals per group. After 8 weeks of regeneration, reinnervation of cutaneous and muscle branches was assessed by dual labeling with HRP and Fluoro-Gold. Motor neuron regeneration was random in old to old (mean muscle branch (M) = 159, mean cutaneous branch (C) = 168), but PMR was seen when young pathways were used in old animals (M = 163, C = 116). PMR was vigorous when either type of graft was used in young animals (young graft, M = 218, C = 134; old graft, M = 204, C = 127). In this model, motoneuron age appears to be the primary determinant of specificity. However, the pathway also makes significant contributions, as shown by the ability of young pathways to generate specificity in old animals. Manipulation of graft Schwann cell behavior might therefore be an appropriate strategy to improve outcome in older individuals.

Electrical stimulation promotes the speed and accuracy of motor axonal regeneration. The positive effects of stimulation are mediated at the cell body. Here we characterize the effect of electrical stimulation on motoneuronal expression of BDNF and its receptor, trkB, two genes whose expression levels in motoneurons correlate with regeneration and are regulated by electrical activity in a variety of neurons. We used semiquantitative in situ hybridization to measure expression of mRNA encoding BDNF and the full-length trkB receptor at intervals of 8 h, 2 days and 7 days after unilateral femoral nerve cut, suture, and stimulation. Expression in regenerating motoneurons was compared to that of contralateral intact motoneurons. BDNF and trkB signals were not significantly upregulated 8 h and 2 days after femoral nerve suture and sham stimulation. By 7 days, there was a 2-fold increase in both BDNF and trkB mRNA expression. In contrast, stimulation of cut and repaired nerves for only 1 h led to rapid upregulation of BDNF and trkB mRNA by 3-fold and 2-fold, respectively, within the first 8 h. The stimulation effect peaked at 2 days with 6-fold and 4-fold increases in the signals, respectively. Thereafter, the levels of BDNF and trkB mRNA expression declined to equal the 2-fold increase seen at 7 days after nerve repair and sham-stimulation. We conclude that brief electrical stimulation stimulates BDNF and trkB expression in regenerating motoneurons. Because electrical stimulation is known to accelerate axonal regeneration, we suggest that changes in the expression of BDNF and trkB correlate with acceleration of axonal regeneration.

Functional recovery is often poor despite the capacity for axonal regeneration in the peripheral nervous system and advances in microsurgical technique. Regeneration of axons in mixed nerve into inappropriate pathways is a major contributing factor to this failure. In this study, we use the rat femoral nerve model of transection and surgical repair to evaluate (1) the effect of nerve transection on the speed of regeneration and the generation of motor-sensory specificity, (2) the efficacy of electrical stimulation in accelerating axonal regeneration and promoting the reinnervation of appropriate muscle pathways by femoral motor nerves, and (3) the mechanism of action of electrical stimulation. Using the retrograde neurotracers fluorogold and fluororuby to backlabel motoneurons that regenerate axons into muscle and cutaneous pathways, we found the following. (1) There is a very protracted period (10 weeks) of axonal outgrowth that adds substantially to the delay in axonal regeneration (staggered regeneration). This process of staggered regeneration is associated with preferential motor reinnervation (PMR). (2) One hour to 2 weeks of 20 Hz continuous electrical stimulation of the parent axons proximal to the repair site dramatically reduces this period (to 3 weeks) and accelerates PMR. (3) The positive effect of short-term electrical stimulation is mediated via the cell body, implicating an enhanced growth program. The effectiveness of such a short-period low-frequency electrical stimulation suggests a new therapeutic approach to accelerate nerve regeneration after injury and, in turn, improve functional recovery.

We developed a new quantitative rat sciatic nerve model to test whether free fat grafts can reduce postoperative perineural scar formation. Epineurectomies of sciatic nerves were performed to create scar. The force required to distract the nerve a unit distance was measured after surgery to determine the time of maximal scar formation. Nerve stiffness normalized for rat weight was statistically greater at 2 months after the initial dissection (0.097+/-0.009 g/mm/g rat weight; n = 10 limbs) than rat limbs that had not undergone a previous dissection (0.075+/-0.012 g/mm/g rat weight). Perineural scar thickness was thicker at 2 months than the perineural tissue in preoperative controls. Free fat grafts decreased nerve stiffness at 2 months (0.078+/-0.012 g/mm/g rat weight) in comparison to the contralateral surgical control limb without a fat graft (0.094+/-0.014 g/mm/g rat weight). Free fat grafts reduced the strength of postoperative perineural scar in this surgical model; however, they were associated with an unexpected finding of substantial postoperative neuropathy.

Motor axons regenerating after transection of mixed nerve preferentially reinnervate distal muscle branches, a process termed preferential motor reinnervation (PMR). Motor axon collaterals appear to enter both cutaneous and muscle Schwann cell tubes on a random basis. Double-labeling studies suggest that PMR is generated by pruning collaterals from cutaneous pathways while maintaining those in motor pathways (the "pruning hypothesis"). If all collaterals projecting to muscle are saved, then stimulation of regenerative sprouting should increase specificity by increasing the number of motoneurons with at least one collateral in a muscle pathway. In the current experiments, collateral sprouting is stimulated by crushing the nerve proximal to the repair site before suture, a maneuver that also conditions the neuron and predegenerates the distal pathway. Control experiments are performed to separate these effects from those of collateral generation. Experiments were performed on the rat femoral nerve and evaluated by exposing its terminal cutaneous and muscle branches to HRP or Fluoro-Gold. Crush proximal to the repair site increased motor axon collaterals at least fivefold and significantly increased the percentage of correctly projecting motoneurons, consistent with the pruning hypothesis. Conditioning the nerve with distal crushes before repair had no effect on specificity. A graft model was used to separate the effects of collateral generation and distal stump predegeneration. Previous crush of the proximal femoral nerve significantly increased the specificity of fresh graft reinnervation. Stimulation of regenerative collateral sprouting thus increased PMR, confirming the pruning hypothesis. However, this effect was overshadowed by the dramatic specificity with which predegenerated grafts were reinnervated by fresh uncrushed proximal axons. These unexpected effects of predegeneration on specificity could involve a variety of possible mechanisms and warrant further study because of their mechanistic and clinical implications.

The effects of denervated muscle and Schwann cells on collateral sprouting from peripheral nerve were studied in the peroneal and tibial nerves of 48 Sprague-Dawley rats. Three groups were prepared. In group MSW (muscle-Schwann cell-window), the peroneal nerves were transected 3 mm below the sciatic bifurcation. The proximal stumps were sealed in a blocked tube to prevent regeneration and the distal stumps were implanted into denervated muscle cells that were wrapped around the ipsilateral tibial nerve, which had a window of perineurium resected. Schwann cells from the ipsilateral sural nerve were implanted into the muscle. Group MS (muscle-Schwann cell) was similar to group MSW, except that the tibial nerve perineurium was kept intact. In group MW (muscle-window), the muscle was prepared without Schwann cells and the tibial nerve perineurium was windowed. S-100 immunostain was used to identify the Schwann cells surviving 1 week after transplantation. After 16 weeks of regeneration, horseradish peroxidase tracer was used to label motor neurons and sensory neurons reinnervating the peroneal nerve. Myelinated axons of the reinnervated peroneal nerves were quantified with the Bioquant OS/2 computer system (R&M Biometrics, Nashville, TN). A mean of 169 motor neurons in group MSW, 64 in group MW, and 26 in group MS reinnervated the peroneal nerve. In the dorsal root ganglion, the mean number of labeled sensory neurons was 1,283 in group MSW, 947 in group MS, and 615 in group MW. The mean number of myelinated axons in the reinnervated peroneal nerve was 1,659 in group MSW, 359 in group MS, and 348 in group MW. Reinnervated anterolateral compartment muscles in group MSW were significantly heavier than those in group MS or MW. This study demonstrates that the transplantation of denervated muscle and Schwann cells promotes motor and sensory nerve collateral sprouting through a perineurial window.

Neurotization--reinnervation of muscle by direct nerve implantation--has been the subject of several reports. The underlying neurobiology, however, has not been adequately studied. The use of a combined silver-acetylcholinesterase stain was used in this study to identify reinnervated motor endplates and to quantify motor endplates reinnervated by the neurotization process. This study examined the effect of distance between nerve implantation and native motor endplate zone on the formation of ectopic motor endplates and on the total number of motor endplates reinnervated. Experiments were performed on the rat soleus muscle. The transected tibial nerve was implanted directly into the motor endplate zone (near, n = 10) or distally, far from the motor endplate zone (far, n = 10). After a reinnervation interval, frozen sections were processed to demonstrate both axons and motor endplates. In the near group, a mean of 566 motor endplates were reinnervated in the native motor endplate zone and a mean of only 13 in distant locations. In the far group, a mean of 362 motor endplates were reinnervated in the native zone, while a mean of 477 were reinnervated in distant locations. Significantly more ectopic motor endplates were generated by far implantation, and native motor endplates were increased by near implantation. The total number of motor endplates was independent of implant location. These experiments demonstrate that the distance between implanted nerve and the native motor endplate zone influences the morphology of reinnervation.

Previous studies in the rat femoral nerve have shown that regenerating motor neurons preferentially reinnervate a terminal nerve branch to muscle as opposed to skin, a process that has been called preferential motor reinnervation. However, the ability of sensory afferent neurons to accurately reinnervate terminal nerve pathways has been controversial. Within the dorsal root ganglia, sensory neurons projecting to muscle are interspersed with sensory neurons projecting to skin. Thus, anatomical studies assessing the accuracy of sensory neuron regeneration have been hampered by the inability to reliably determine their original innervation status. A sensory neuron that regenerated an axon into a terminal nerve branch to muscle might represent either an appropriate return of an original sensory afferent to muscle stretch receptors or the inappropriate recruitment of a cutaneous sensory afferent that originally innervated skin. The current experiments used a labeling strategy that effectively labels motor and sensory neurons projecting to a terminal nerve branch before experimental manipulation of the parent mixed nerve. Our results confirm previous observations concerning preferential motor reinnervation for motor neurons, and show for the first time anatomical evidence of specificity during regeneration of sensory afferent projections to muscle. In addition, the accuracy of sensory afferent regeneration was highly correlated with the accuracy of motor regeneration. This suggests that these two distinct neuronal populations that project to muscle respond in parallel to specific guidance factors during the regeneration process.

Tubular prostheses support peripheral axon regeneration across gaps of up to 3 cm in the primate. However, the precision with which axons cross a gap and reinnervate the periphery remains controversial. These experiments use continuous tracing of regenerated rat sciatic nerve axons with HRP-WGA to examine the dispersion of axons as they cross a gap, and the effects on this dispersion of gap distance and fascicular orientation. Proximal and distal tibial and peroneal fascicles were precisely oriented about the longitudinal midplane of a silicon tube, with correct or reversed fascicular alignment and gaps of 2 mm and 5 mm. After 6 weeks of regeneration, HRP-WGA was applied to the distal peroneal fascicle to continuously label its reinnervating axons. These axons tended to grow straight across the tube, with dispersion increasing as a factor of distance when correct fascicular alignment was maintained. However, when fascicular alignment was reversed, axonal dispersion was determined by fascicular size rather than fascicular identity. These experiments provide no evidence for neurotropic interactions promoting "correct" fascicular reinnervation. Progressive axonal dispersion and the absence of factors to promote fascicular specificity should result in an increase of random reinnervation and functional disruption with larger gaps. An enclosed gap is not an acceptable substitute for nerve graft when reconstructing a nerve that serves multiple functions.

The carbohydrate epitope L2/HNK-1 (hereafter designated L2) is expressed in the adult mouse by myelinating Schwann cells of ventral roots and muscle nerves, but rarely by those of dorsal roots or cutaneous nerves. Since substrate-coated L2 glycolipids promote outgrowth of cultured motor but not sensory neurons, L2 may thus influence the preferential reinnervation of muscle nerves by regenerating motor axons in vivo. In the present study, we have analyzed the influence of regenerating axons on L2 expression by reinnervated Schwann cells by directing motor or sensory axons into the muscle and cutaneous branches of femoral nerves of 8-week-old mice. We observed that regenerating axons from cutaneous branches did not lead to immunocytochemically detectable L2 expression in muscle or cutaneous nerve branches. Axons regenerating from muscle branches led to a weak L2 expression by few Schwann cells of the cutaneous branch, but provoked a strong L2 expression by many Schwann cells of the muscle branch. Myelinating Schwann cells previously associated with motor axons thus differed from previously sensory axon-associated myelinating Schwann cells in their ability to express L2 when contacted by motor axons. This upregulation of L2 expression during critical stages of reinnervation may provide motor axons regenerating into the appropriate, muscle pathways with an advantage over those regenerating into the inappropriate, sensory pathways.

Nephropathic cystinosis is a lysosomal storage disorder leading to renal failure by age 10 years. Prolonged patient survival following renal transplantation has allowed the development of previously unknown long-term complications. Muscle involvement has been reported in a single posttransplant cystinosis patient, but the range of clinical, electrophysiologic, and histologic features has not been fully described. Thirteen of 54 post-renal-transplant patients that we examined developed weakness and wasting in the small hand muscles, with or without facial weakness and dysphagia. Tendon reflexes were preserved and sensory examinations were normal. Electrophysiologic studies in 11 affected patients showed normal nerve conduction velocities and preserved sensory action potentials. The voluntary motor units in the affected distal muscles had reduced amplitude and brief duration, confirmed with quantitative electromyography in 4 patients. Biopsy of the severely affected abductor digiti minimi or extensor carpi radialis brevis muscles in 2 patients revealed marked fiber size variability, prominent acid phosphatase-positive vacuoles, and absence of fiber type grouping or inflammatory cells. Crystals of cystine were detected in perimysial cells but not within the muscle cell vacuoles. The muscle cystine content of clinically affected muscles was markedly elevated. We conclude that a distal vacuolar myopathy is a common late complication of untreated nephropathic cystinosis. Although the cause is unclear, the general lysosomal defect in this disease may also affect the lysosomes within muscle fibers.

Motor axons regenerating after transection of mixed nerve preferentially reinnervate distal motor branches and/or muscle, a process termed "preferential motor reinnervation." Collaterals of a single motor axon often enter both sensory and motor Schwann cell tubes of the distal stump; specificity is generated by pruning collaterals from sensory pathways while maintaining those in motor pathways. Previous experiments in the rat femoral nerve model evaluated reinnervation of the femoral motor branch and quadriceps muscle as a unit. In this study, pathway contributions are analyzed separately by denying muscle contact, or by reinnervating muscle through inappropriate, formerly sensory pathways. Motor axons preferentially reinnervate motor pathways, even when these pathways end blindly in a silicon tube. If the femoral nerve is removed as a graft and reinserted with correct or reversed alignment of the sensory and motor branches, more motoneurons reinnervate muscle through correct motor than through incorrect sensory pathways. Motor pathways thus differ from sensory pathways in ways that survive Wallerian degeneration and transplantation as a graft, and that can be used by regenerating motor axons as a basis for collateral pruning and specificity generation.

The traditional view that axons are not functionally grouped within proximal human nerve is based on the interfascicular dissections of Sunderland ('45). However, microstimulation and microneurography (Schady et al., '83a; Hallin, '90) reveal proximal grouping of cutaneous sensory axons from small areas of skin. In the present studies, conjugates of horseradish peroxidase with wheat germ agglutinin (HRP-WGA) were used to trace the course of digital nerve axons within the median nerve of Macaca mulatta. The electrophysiologic findings were confirmed, suggesting the potential for precise surgical realignment of functionally related axons even after proximal nerve transection. Radial digital nerves were labeled in the thumb (bilateral, 1 animal), the index finger (unilateral, 2 animals), and the middle finger (bilateral, 1 animal). Median nerve cross sections were cut at 1-cm intervals, treated with tetramethyl benzidine to demonstrate HRP-WGA within axons, and compiled to form maps of each digital nerve "territory" within the median nerve. These territories were limited to a single, densely labeled fascicle at the wrist level. They expanded somewhat in the forearm to encompass clusters of labeled axons within a matrix of unlabeled axon profiles. The clusters were more loosely packed in the arm, occupying 1/3 to 1/6 of the nerve cross section at the entrance to the brachial plexus. The three digital nerve territories studied were widely separated at the wrist level. In the proximal arm, there was moderate intermingling of axons from adjacent digits, but those to the middle finger and thumb remained segregated. Territory configuration differed widely overall, but was moderately constant for each digit. The location of territories within the nerve was often strikingly similar from right to left and from animal to animal, with occasional prominent variations reflecting isolated rotation of one nerve.

Previous experiments have shown that motor axons regenerating in mixed nerve will preferentially reinnervate a distal motor branch. The present experiments examine the mechanism through which this sensory-motor specificity is generated. An enclosed 0.5 mm gap was created in the proximal femoral nerves of juvenile rats. Two, three or eight weeks later the specificity of motor axon regeneration was evaluated by simultaneous application of horseradish peroxidase (HRP) to one distal femoral branch (sensory or motor) and Fluoro-Gold to the other. Motoneurons were then counted as projecting (i) correctly to the motor branch, (ii) incorrectly to the sensory branch, and (iii) simultaneously to both branches (double-labeled). Motor axon regeneration was random at 2 weeks, with equal numbers of motoneurons projecting to sensory and motor branches. However, the number of correct projections increased dramatically between 2 and 3 weeks. Twenty-six percent of neurons labeled at 2 weeks contained both tracers, indicating axon collateral projections to both sensory and motor branches. This number decreased significantly at each time period. Axon collaterals were thus 'pruned' from the sensory branch, increasing the number of correct projections at the expense of double-labeled neurons. These findings suggest random reinnervation of the distal stump, with specificity generated through trophic interaction between axons and the pathway and/or end organ.

Transganglionic transport of horseradish peroxidase (HRP) was greatly improved by conjugating the enzyme with wheat germ agglutinin (WGA) as described by Gonatas et al. [7]. Intense dorsal horn projections from muscle groups and even discrete skin patches were labeled after peripheral injection of the HRP-WGA conjugate. These projections could not be consistently demonstrated when HRP was injected either alone or in combination with DMSO. Conjugation with WGA thus enhances the uptake and/or transport of HRP and constitutes a valuable advance in HRP technique.

We experimentally analyzed the specificity of muscle reinnervation after suture and regeneration of rat sciatic nerve. We used a horseradish peroxidase (HRP) technique of axon tracing to compare the number and location of motoneurons that innervate muscle via the peroneal nerve after epineurial and individual fascicular suture of the parent sciatic nerve. These motoneurons are significantly reduced in number from control levels and are often in spinal cord locations that indicate previous innervation of antagonistic muscle via the tibial nerve. This inappropriate reinnervation of peroneal muscle by tibial motoneurons is minimized by individual fascicular suture without compromise of overall reinnervation. Our findings thus support the hypothesis that individual fascicular suture may avoid distortion of the central connections of peripheral units.

Banked allograft bone has been used for metacarpal reconstruction in 10 patients. In one patient, allograft replaced the shafts of the fourth and fifth metacarpals that were lost as the result of an injury from an explosion. In the remaining nine, allograft was used as an intercalary bone after distraction lengthening of the metacarpal. The first metacarpal was lengthened in seven patients; four had hypoplasia of the thumb and three had partial thumb amputation. One patient with hypoplasia of the ulnar side of the hand had the fourth and fifth metacarpals lengthened, and the fifth metacarpal was lengthened in another. Of the 12 allografts used, radiographic and clinical evidence of bone union occurred at 23 of the 24 allograft/recipient bone interface sites. Most patients had no callus formation. This suggested primary bone healing. Recipient new bone appeared to cross the recipient/donor interface and replace the allograft by "creeping substitution" within 6 months to 1 year. The density and trabecular pattern of the allograft bone gradually assumed that of the recipient's bone. There was no clinical or radiographic evidence of rejection and no evidence of infection, abnormal swelling, or bone resorption. Nine of the 10 patients had improved function as determined by objective and subjective evaluation. Banked allograft bone offers two advantages over autogenous bone for reconstructive hand surgery: There is no donor site morbidity, and large defects can be filled with cylindric cortical bone of appropriate size and shape, which offers excellent stability as an intercalary graft.

Regeneration of axons into inappropriate distal nerve branches may adversely affect functional recovery after peripheral nerve suture. The degree to which motor axons reinnervate sensory nerves, and vice versa, has not been determined. In these experiments, HRP is used to quantify the sensory and motor neurons that reinnervate sensory and motor branches of the rat femoral nerve after proximal severance and repair. Motoneurons preferentially reinnervate the motor branch in juveniles and adults, even if the repair is intentionally misaligned or a gap is imposed between proximal and distal stumps. A specific interaction thus occurs between regenerating motor axons and the Schwann cell tubes that lead to the motor branch. This interaction is independent of mechanical axon alignment.

One approach to repair of transected nerves is to attempt extrinsic guidance of axons across the gaps. We inserted the proximal and distal stumps of severed mouse sciatic nerves into opposite ends of biodegradable polyester tubes. The nerves and ensheathing tubes were examined after postoperative survival times of as long as 2 years. Myelinated fiber number in each successfully regenerated nerve was measured and correlated with the tube's residual lumen size. In selected regenerated nerves axonal sizes and myelin sheath widths were sampled and compared with control values. Swelling and deformation of tube walls occurred in nearly all tubes. Successful regeneration was obtained through more than half of the implants, and was more probable in tubes with larger initial lumens. Myelinated fiber number in regenerated nerves ranged from 231 to 3561 (normally 3900 to 4200); larger values again were found in tubes with larger initial lumens. Mean axonal areas in regenerated nerves were roughly half of normal, though myelin sheaths became appropriately thick. We concluded that the more biodegradable a tube, the more likely it was to incur distortion and luminal narrowing. Tube composition per se seemed of importance mainly as it related to maintenance of adequate luminal size over the length of the degrading tubes; luminal adequacy, not tube composition, seemed paramount in determining the extent of nerve regeneration.

Random matching of regenerating axons with Schwann tubes in the distal nerve stump is thought to contribute to the often poor results of peripheral nerve repair. Motor axons would be led to sensory end organs and sensory axons to motor end plates; both would remain functionless. However, the ability of regenerating axons to differentiate between sensory and motor environments has not been adequately examined. The experiments reported here evaluated the behavior of regenerating motor axons when given equal access to distal sensory and motor nerve stumps across an unstructured gap. "Y"-shape silicon chambers were implanted within the rat femoral nerve with the proximal motor branch as axon source in the base of the Y. The distal sensory and motor branches served as targets in the branches of the Y, and were placed 2 or 5 mm from the axon source. After 2 months for axon regeneration, horseradish peroxidase was used to label the motoneurons projecting axons into either the motor or the sensory stump. Equal numbers of motoneurons were labeled from the sensory and motor stumps at 2 mm, but significantly more motoneurons were labeled from the motor stump at 5 mm. (P = 0.016). This finding is consistent with selective reinnervation of the motor stump. Augmentation of this phenomenon to produce specific reunion of individual motor axons could dramatically improve the results of nerve suture.

Ninety-one thumbs with symptomatic trapeziometacarpal arthritis were studied and classified by the severity of trapeziometacarpal arthritis, as seen on the roentgenogram. Thirty-five patients with mild changes had satisfactory relief of pain after treatment with anti-inflammatory drugs and a C-splint. Operation, consisting of resection of the trapezium and shortening by 1.5 cm of the abductor pollicis longus tendon was performed on 16 thumbs. In six patients a rolled palmaris longus tendon was inserted between the metacarpal base and the scaphoid. All patients had relief of pain, improvement of pinch and grip strength, and an increased range of thumb pronation after operation. There was no difference in the results of those patients treated with or without tendon interposition. Trapezial resection was found to be an effective means of treating patients with symptomatic trapeziometacarpal arthritis who failed to improve with conservative care.

Somatotopic arrangements of cells and fibers within the dorsal columns and the dorsal column nuclei have been mapped most precisely by electrophysiological recording methods. This study uses an anatomical approach to evaluate the precision of individual digital nerve projections to the cuneate nucleus (CN) in young macaque monkeys. Digital nerves supplying about one-half the palmar skin of a digit were surgically exposed, cut, and treated with wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) on 3 successive days. After 2 additional days, animals were killed and medullas were recovered for study of serial sections reacted to display axons labeled by transganglionic transport of label. Labeled afferent fibers from each digit were found within a circumscribed columnar zone extending through the caudal CN and rostrally throughout the pars rotunda of CN. At caudal levels, diffuse projections reach the dorsal edge of the CN; more rostrally, they shift into deeper parts of the nucleus and are heaviest along its ventral and medial edges at levels near the obex. Fibers from the thumb (digit 1) project lateral (and ventral) to those from digit 2, and projections from digit 3 are medial to those from 2. Each digital projection field is closely adjacent to that from the adjacent digit. Few fibers extend to the rostral CN. Projection fields of homologous digits are quite symmetrical on the two sides. Although there do seem to be some differences in the somatotopic arrangement of digital input in macaques compared to other nonprimate mammals studied previously, these observations (precisely organized, circumscribed fields for separate digits) define a system well designed for transmission of data encoding spatial relationships.

The connections between the spinal cord and lower leg muscles of the rat are significantly altered by repair of the intervening sciatric nerve. Muscles supplied by the peroneal branch of the sciatic are innervated by fewer motoneurons after sciatic repair. Many of these neurons originally innervated the peroneal muscles, and others formerly served the antagonistic tibial muscles. Perikarya in the size range of alpha motoneurons regained peripheral connections with greater frequency than those in the gamma range. There are thus postoperative defects in the extent and specificity of alpha reinnervation as well as in the degree of gamma control.