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Developing tissues dictate the amount and type of innervation they require by secreting neurotrophins, which promote neuronal survival by activating distinct tyrosine kinase receptors. Here, we show that nerve growth factor (NGF) signaling through neurotrophic tyrosine kinase receptor type 1 (TrkA) directs innervation of the developing mouse femur to promote vascularization and osteoprogenitor lineage progression. At the start of primary ossification, TrkA-positive axons were observed at perichondrial bone surfaces, coincident with NGF expression in cells adjacent to centers of incipient ossification. Inactivation of TrkA signaling during embryogenesis in TrkA(F592A) mice impaired innervation, delayed vascular invasion of the primary and secondary ossification centers, decreased numbers of Osx-expressing osteoprogenitors, and decreased femoral length and volume. These same phenotypic abnormalities were observed in mice following tamoxifen-induced disruption of NGF in Col2-expressing perichondrial osteochondral progenitors. We conclude that NGF serves as a skeletal neurotrophin to promote sensory innervation of developing long bones, a process critical for normal primary and secondary ossification., (Published by Elsevier Inc.)

Background Electrical stimulation immediately following nerve lesion helps regenerating axons cross the subsequently grafted nerve repair site. However, the results and the mechanisms remain open to debate. Some findings show that stimulation after crush injury increases axonal crossing of the repair site without affecting regeneration speed. Others show that stimulation after transection and fibrin glue repair doubles regeneration distance. Methods Using a sciatic-nerve-transection-graft in vivo model, we investigated the morphological behavior of regenerating axons around the repair site after unilateral nerve stimulation (20 Hz, 1 hour). With mice expressing axonal fluorescent proteins (thy1-YFP), we were able to calculate the following at 5 and 7 days: percentage of regenerating axons and arborizing axons, branches per axon, and regeneration distance and speed. Results Brief stimulation significantly increases the percentage of regenerating axons (5 days: 35.5 vs. 27.3% nonstimulated, p < 0.05; 7 days: 43.3 vs. 33.9% nonstimulated, p < 0.05), mainly by increasing arborizing axons (5 days: 49.3 [4.4] vs. 33.9 [4.1]% [p < 0.001]; 7 days: 42.2 [5.6] vs. 33.2 [3.1]% [p < 0.001]). Neither branches per arborizing axon nor regeneration speed were affected. Conclusion Our morphological data analysis revealed that electrical stimulation in this model increases axonal crossing of the repair site and promotes homogeneous perilesional branching, but does not affect regeneration speed., (Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.)

Objective: Sports-related peripheral nerve injuries are common among athletes and are often underrecognized because of symptom overlap with more usual sports-related bone, soft-tissue, and joint injuries., Conclusion: MRI plays an increasingly important role in the workup of peripheral nerve injuries and may reveal severe nerve abnormalities before they are diagnosed by electrodiagnostic testing or a clinical examination. Sport-specific peripheral nerve injuries and their MRI appearance will be discussed in this article.

Background: Schwann cells in the distal stump of transected nerve upregulate growth factors that support regeneration on a modality-specific basis. It is unclear, however, which of these preferentially support motor axon regeneration. Identification of these factors will require a model that can isolate growth factor effects to growing axons while reproducing the complex three-dimensional structure of peripheral nerve., New Method: A two-compartment PDMS base is topped by a collagen-coated membrane that supports a spinal cord cross-section above one compartment. Fluorescent motoneurons in this section reinnervate a segment of peripheral nerve that directs axons through a water-tight barrier to the second compartment, where nerve repair is performed., Results: Motoneurons remain healthy for several weeks. The axons they project through the water-tight barrier survive transection and cross a nerve repair in substantial numbers to reinnervate an additional nerve segment. Fluidic isolation of the two compartments was confirmed with a dye leakage test, and the physiologic integrity of the system was tested by retrograde labeling of only those motor neurons to which tracer was exposed and by limitation of toxin effects to a single compartment., Comparison With Existing Methods: Nerve repair cannot be modeled in monolayer cell culture. Our previous organotypic model accurately modeled nerve repair, but did not allow individual control of motoneuron and growth cone environments., Conclusions: This model isolates treatment effects to growing axons while reproducing the complex three-dimensional structure of peripheral nerve. Additionally, it facilitates surgical manipulation of tissues and high-resolution imaging., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Previous studies demonstrated that Schwann cells (SCs) express distinct motor and sensory phenotypes, which impact the ability of these pathways to selectively support regenerating neurons. In the present study, unbiased microarray analysis was used to examine differential gene expression in denervated motor and sensory pathways in rats. Several genes that were significantly upregulated in either denervated sensory or motor pathways were identified and two secreted factors were selected for further analysis: osteopontin (OPN) and clusterin (CLU) which were upregulated in denervated motor and sensory pathways, respectively. Sciatic nerve transection induced upregulation of OPN and CLU and expression of both returned to baseline levels with ensuing regeneration. In vitro analysis using exogenously applied OPN induced outgrowth of motor but not sensory neurons. CLU, however, induced outgrowth of sensory neurons, but not motor neurons. To assess the functional importance of OPN and CLU, peripheral nerve regeneration was examined in OPN and CLU(-/-) mice. When compared with OPN(+/+) mice, motor neuron regeneration was reduced in OPN(-/-) mice. Impaired regeneration through OPN(-/-) peripheral nerves grafted into OPN(+/+) mice indicated that loss of OPN in SCs was responsible for reduced motor regeneration. Sensory neuron regeneration was impaired in CLU(-/-) mice following sciatic nerve crush and impaired regeneration nerve fibers through CLU(-/-) nerve grafts transplanted into CLU(+/+) mice indicated that reduced sensory regeneration is likely due to SC-derived CLU. Together, these studies suggest unique roles for SC-derived OPN and CLU in regeneration of peripheral motor and sensory axons.

Preferential motor reinnervation (PMR) is the tendency for motor axons regenerating after repair of mixed nerve to reinnervate muscle nerve and/or muscle rather than cutaneous nerve or skin. PMR may occur in response to the peripheral nerve pathway alone in juvenile rats (Brushart, 1993; Redett et al., 2005), yet the ability to identify and respond to specific pathway markers is reportedly lost in adults (Uschold et al., 2007). The experiments reported here evaluate the relative roles of pathway and end organ in the genesis of PMR in adult rats. Fresh and 2-week predegenerated femoral nerve grafts were transferred in correct or reversed alignment to replace the femoral nerves of previously unoperated Lewis rats. After 8 weeks of regeneration the motoneurons projecting through the grafts to recipient femoral cutaneous and muscle branches and their adjacent end organs were identified by retrograde labeling. Motoneuron counts were subjected to Poisson regression analysis to determine the relative roles of pathway and end organ identity in generating PMR. Transfer of fresh grafts did not result in PMR, whereas substantial PMR was observed when predegenerated grafts were used. Similarly, the pathway through which motoneurons reached the muscle had a significant impact on PMR when grafts were predegenerated, but not when they were fresh. Comparison of the relative roles of pathway and end organ in generating PMR revealed that neither could be shown to be more important than the other. These experiments demonstrate unequivocally that adult muscle nerve and cutaneous nerve differ in qualities that can be detected by regenerating adult motoneurons and that can modify their subsequent behavior. They also reveal that two weeks of Wallerian degeneration modify the environment in the graft from one that provides no modality-specific cues for motor neurons to one that actively promotes PMR., (© 2013.)

Myelinating Schwann cells express distinct sensory and motor phenotypes as defined by their differing patterns of growth factor production (Hoke et al., 2006). The heterogeneous growth factor requirements of sensory and motor neurons, however, suggest that Schwann cell phenotype might vary across a broad spectrum. To explore this possibility, we selectively denervated six discrete Schwann cell populations: dorsal root, cutaneous nerve, cutaneous unmyelinated axons, muscle nerve afferents, muscle nerve efferents, and ventral root. Real-time RT-PCR for 11 growth factors was performed on the 6 target Schwann cell populations 5, 15, and 30 days after their denervation, and on normal cutaneous nerve, muscle nerve, ventral root, and dorsal root to establish baseline expression levels. Within the denervated axon populations, IGF-1 and VEGF were expressed most prominently in cutaneous nerve, HGF, NGF, and BDNF in cutaneous nerve and dorsal root, GDNF in dorsal root and ventral root, PTN in the ventral root and muscle nerve efferents, and IGF-2 in both afferents and efferents within muscle nerve; expression of CNTF, FGF-2 and NT-3 was not modality or location specific. ELISA for NGF, BDNF, and GDNF confirmed that gene expression correlated with protein concentration. These findings demonstrate that growth factor expression by denervated Schwann cells is not only subject to further regulation within the previously-defined sensory and motor groups, but also varies along a central-peripheral axis. The traditional view of myelinating Schwann cells as a homogenous population is modified by the realization that complex regulation produces a wide variety of Schwann cell phenotypes. Additionally, we found that Schwann cell phenotype is maintained for 2 weeks in vitro, demonstrating that it may survive several cell divisions without instructive cues from either axons or basal lamina., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The role of pathway-derived growth factors in the support of peripheral axon regeneration remains elusive. Few appropriate knock-out mice are available, and gene silencing techniques are rarely 100% effective. To overcome these difficulties, we have developed an in vitro organotypic co-culture system that accurately models peripheral nerve repair in the adult mammal. Spinal cord sections from P4 mice that express YFP in their neurons are used to innervate segments of P4 peripheral nerve. This reconstructed ventral root is then transected and joined to a nerve graft. Growth of axons across the nerve repair and into the graft can be imaged repeatedly with fluorescence microscopy to define regeneration speed, and parent neurons can be labeled in retrograde fashion to identify contributing neurons. Nerve graft harvested from adult mice remains viable in culture by both morphologic and functional criteria. Motoneurons are supported with GDNF for the first week in culture, after which they survive axotomy, and are thus functionally adult. This platform can be modified by using motoneurons from any genetically modified mouse that can be bred to express XFP, by harvesting nerve graft from any source, or by treating the culture systemically with antibodies, growth factors, or pathway inhibitors. The regeneration environment is controlled to a degree not possible in vivo, and the use of experimental animals is reduced substantially. The flexibility and control offered by this technique should thus make it a useful tool for the study of regeneration biology., (Copyright 2009 Elsevier Inc. All rights reserved.)

Objective: Injured peripheral nerves regenerate at very slow rates. Therefore, proximal injury sites such as the brachial plexus still present major challenges, and the outcomes of conventional treatments remain poor. This is in part attributable to a progressive decline in the Schwann cells' ability to provide a supportive milieu for the growth cone to extend and to find the appropriate target. These challenges are compounded by the often considerable delay of regeneration across the site of nerve laceration. Recently, low-frequency electrical stimulation (as brief as an hour) has shown promise, as it significantly accelerated regeneration in animal models through speeding of axon growth across the injury site., Methods: To test whether this might be a useful clinical tool, we carried out a randomized controlled trial in patients who had experienced substantial axonal loss in the median nerve owing to severe compression in the carpal tunnel. To further elucidate the potential mechanisms, we applied rolipram, a cyclic adenosine monophosphate agonist, to rats after axotomy of the femoral nerve., Results: We demonstrated that effects similar to those observed in animal studies could also be attained in humans. The mechanisms of action of electrical stimulation likely operate through up-regulation of neurotrophic factors and cyclic adenosine monophosphate. Indeed, the application of rolipram significantly accelerated nerve regeneration., Conclusion: With new mechanistic insights into the influencing factors of peripheral nerve regeneration, the novel treatments described above could form part of an armament of synergistic therapies that could make a meaningful difference to patients with peripheral nerve injuries.

Objective: Poor functional recovery after peripheral nerve injury is generally attributed to irreversible target atrophy. In rats, we addressed the functional outcomes of prolonged neuronal separation from targets (chronic axotomy for up to 1 year) and atrophy of Schwann cells (SCs) in distal nerve stumps, and whether electrical stimulation (ES) accelerates axon regeneration. In carpal tunnel syndrome (CTS) patients with severe axon degeneration and release surgery, we asked whether ES accelerates muscle reinnervation., Methods: Reinnervated motor unit (MUs) and regenerating neuron numbers were counted electrophysiologically and with dye-labeling after chronic axotomy, chronic SC denervation and after immediate nerve repair with and without trains of 20 Hz ES for 1 hour to 2 weeks in rats and in CTS patients., Results: Chronic axotomy reduced regenerative capacity to 67% and was alleviated by exogenous growth factors. Reduced regeneration to approximately 10% by SC denervation atrophy was ameliorated by forskolin and transforming growth factor-beta SC reactivation. ES (1 h) accelerated axon outgrowth across the suture site in association with elevated neuronal neurotrophic factor and receptors and in patients, promoted the full reinnervation of thenar muscles in contrast to a non-significant increase in MU numbers in the control group., Discussion: The rate limiting process of axon outgrowth, progressive deterioration of both neuronal growth capacity and SC support, but not irreversible target atrophy, account for observed poor functional recovery after nerve injury. Brief ES accelerates axon outgrowth and target muscle reinnervation in animals and humans, opening the way to future clinical application to promote functional recovery.

Brief electrical stimulation enhances the regenerative ability of axotomized motor [Nix, W.A., Hopf, H.C., 1983. Electrical stimulation of regenerating nerve and its effect on motor recovery. Brain Res. 272, 21-25; Al-Majed, A.A., Neumann, C.M., Brushart, T.M., Gordon, T., 2000. Brief electrical stimulation promotes the speed and accuracy of motor axonal regeneration. J. Neurosci. 20, 2602-2608] and sensory [Brushart, T.M., Jari, R., Verge, V., Rohde, C., Gordon, T., 2005. Electrical stimulation restores the specificity of sensory axon regeneration. Exp. Neurol. 194, 221-229] neurons. Here we examined the parameter of duration of stimulation on regenerative capacity, including the intrinsic growth programs, of sensory neurons. The effect of 20 Hz continuous electrical stimulation on the number of DRG sensory neurons that regenerate their axons was evaluated following transection and surgical repair of the femoral nerve trunk. Stimulation was applied proximal to the repair site for 1 h, 3 h, 1 day, 7 days or 14 days at the time of nerve repair. Following a 21-day regeneration period, DRG neurons that regenerated axons into the muscle and cutaneous sensory nerve branches were retrogradely identified. Stimulation of 1 h led to a significant increase in DRG neurons regenerating into cutaneous and muscle branches when compared to 0 h (sham) stimulation or longer periods of stimulation. Stimulation for 1 h also significantly increased the numbers of neurons that regenerated axons beyond the repair site 4 days after lesion and was correlated with a significant increase in expression of growth-associated protein 43 (GAP-43) mRNA in the regenerating neurons at 2 days post-repair. An additional indicator of heightened plasticity following 1 h stimulation was elevated expression of brain-derived neurotrophic factor (BDNF). The effect of brief stimulation on enhancing sensory and motoneuron regeneration holds promise for inducing improved peripheral nerve repair in the clinical setting.

The declining capacity for injured peripheral nerves to regenerate their axons with time and distance is accounted for, at least in part, by the chronic axotomy of the neurons and Schwann cell denervation prior to target reinnervation. A largely unrecognized site of delay is the surgical suture site where, in rats, 4 weeks is required for all neurons to regenerate their axons across the site. Low frequency stimulation for just 1 h after surgery accelerates this axon crossing in association with upregulation of neurotrophic factors in the neurons. We translated these findings to human patients by examining the number of reinnervated motor units in the median nerve-innervated thenar muscles before and after carpel tunnel release surgery in a randomized controlled trial. Motor unit number estimates (MUNE) in patients with moderate and severe carpal tunnel syndrome were significantly lower than normal. This number increased significantly by 6-8 months after surgery and reached normal values by 12 months in contrast to a non-significant increase in the control unstimulated group. Tests including the Purdue Pegboard Test verified the more rapid functional recovery after stimulation. The data indicate a feasible strategy to promote axonal regeneration in humans that has the potential to improve functional outcomes, especially in combination with strategies to sustain the regenerative capacity of neurons and the support of Schwann cells over distance and time.

Schwann cell phenotype is classified as either myelinating or nonmyelinating. Additional phenotypic specialization is suggested, however, by the preferential reinnervation of muscle pathways by motoneurons. To explore potential differences in growth factor expression between sensory and motor nerve, grafts of cutaneous nerve or ventral root were denervated, reinnervated with cutaneous axons, or reinnervated with motor axons. Competitive reverse transcription-PCR was performed on normal cutaneous nerve and ventral root and on graft preparations 5, 15, and 30 d after surgery. mRNA for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor, hepatocyte growth factor, and insulin-like growth factor-1 was expressed vigorously by denervated and reinnervated cutaneous nerve but minimally by ventral root. In contrast, mRNA for pleiotrophin (PTN) and glial cell line-derived neurotrophic factor was upregulated to a greater degree in ventral root. ELISA confirmed that NGF and BDNF protein were significantly more abundant in denervated cutaneous nerve than in denervated ventral root, but that PTN protein was more abundant in denervated ventral root. The motor phenotype was not immutable and could be modified toward the sensory phenotype by prolonged reinnervation of ventral root by cutaneous axons. Retrograde labeling to quantify regenerating neurons demonstrated that cutaneous nerve preferentially supported cutaneous axon regeneration, whereas ventral root preferentially supported motor axon regeneration. Schwann cells thus express distinct sensory and motor phenotypes that are associated with the support of regeneration in a phenotype-specific manner. These findings suggest that current techniques of bridging gaps in motor and mixed nerve with cutaneous graft could be improved by matching axon and Schwann cell properties.

Functional recovery after peripheral nerve injury is often poor despite high regenerative capacity of peripheral neurons. In search for novel treatments, brief electrical stimulation of the acutely lesioned nerve has recently been identified as a clinically feasible approach increasing precision of axonal regrowth. The effects of this stimulation appear to be mediated by BDNF and its receptor, TrkB, but the down-stream effectors are unknown. A potential candidate is the HNK-1 carbohydrate known to be selectively reexpressed in motor but not sensory nerve branches of the mouse femoral nerve and to enhance growth of motor but not sensory axons in vitro. Here, we show that short-term low-frequency electrical stimulation (1 h, 20 Hz) of the lesioned and surgically repaired femoral nerve in wild-type mice causes a motor nerve-specific enhancement of HNK-1 expression correlating with previously reported acceleration of muscle reinnervation. Such enhanced HNK-1 expression was not observed after electrical stimulation in heterozygous BDNF or TrkB-deficient mice. Accordingly, the degree of proper reinnervation of the quadriceps muscle, as indicated by retrograde labeling of motoneurons, was reduced in TrkB+/- mice compared to wild-type littermates. Also, recovery of quadriceps muscle function, evaluated by a novel single-frame motion analysis approach, and axonal regrowth into the distal nerve stump, assessed morphologically, were considerably delayed in TrkB+/- mice. These findings indicate that BDNF/TrkB signaling is important for functional recovery after nerve repair and suggest that up-regulation of the HNK-1 glycan is linked to this phenomenon.

Motor axons regenerating after repair of mixed nerve reinnervate pathways leading to muscle more often than those leading to skin [preferential motor reinnervation (PMR)]. Motoneurons that initially project collaterals to both muscle and skin prune incorrect projections to generate specificity. The number of motor axon collaterals maintained entirely within cutaneous or muscle pathways, however, is unknown. To overcome this shortcoming, dorsal root ganglion excision has been used to allow only motor axons to regenerate after a peripheral lesion. Motor axon number in reinnervated cutaneous and muscle pathways can then be correlated with the number of parent motoneurons determined by retrograde labeling. The number of collaterals per neuron can be calculated for each environment and the relative roles of pathway and end organ assessed by blocking the distal pathways to prevent target reinnervation. Without sensory competition, PMR develops in two stages: a limited response to muscle nerve and then a robust response to muscle that may involve retrograde signaling to the proximal pathway. Motoneurons maintain more collaterals in cutaneous nerve than in muscle nerve, even without muscle contact. This difference could result either from increased collateral formation in cutaneous nerve or from increased collateral pruning in muscle nerve. In either instance, these findings confirm that muscle and cutaneous pathways have functionally significant identities that can be recognized by motor axons and can regulate their arborization. Decreased arborization in muscle pathways could promote regeneration by focusing neuronal resources on high-yield projections; increased arborization in cutaneous pathways, conversely, would enhance pathfinding abilities.

Electrical stimulation at the time of nerve repair promotes motoneurons to reinnervate appropriate pathways leading to muscle and stimulates sensory neurons to regenerate. The present experiments examine the effects of electrical stimulation on the specificity of sensory axon regeneration. The unoperated rat femoral cutaneous branch is served by 2-3 times more DRG neurons than is the muscle branch. After transection and repair of the femoral trunk, equal numbers of DRG neurons project to both branches. However, 1 h of electrical stimulation restores the normal proportion of DRG neurons reinnervating skin and muscle. To ask if the redistribution of stimulated neurons results from enhanced specificity of target reinnervation, we developed a new technique of sequential double labeling. DRG neurons projecting to the femoral muscle branch were prelabeled with Fluoro Gold 2 weeks before the nerve was transected proximally and repaired with or without 1 h of 20-Hz electrical stimulation. Three weeks after repair, the muscle nerve was labeled a second time with Fluororuby. The percentage of regenerating neurons that both originally served muscle and returned to muscle after nerve repair increased from 40% without stimulation to 75% with stimulation. Electrical stimulation thus dramatically alters the distribution of regenerating sensory axons, replacing normally random behavior with selective reinnervation of tissue-specific targets. If the enhanced regeneration specificity resulting from electrical stimulation is found to improve function in a large animal model, this convenient and safe technique may be a useful adjunct to clinical nerve repair.

A century ago, Ramon y Cajal described the generalized response of regenerating peripheral axons to their environment. By using mice that express fluorescent proteins in their axons, we are now able to quantify the response of individual axons to nerve transection and repair. Sciatic nerves from nonexpressing mice were grafted into those expressing a yellow variant of green fluorescent protein, then examined at 5, 7, or 10 days after repair. Regeneration was found to be a staggered process, with only 25% of axons crossing the repair in the first week. In the setting of Wallerian degeneration, this stagger will expose growth cones to an evolving menu of molecular cues upon which to base pathway decisions. Many axons arborize, allowing them to interact simultaneously with several pathways. Arborization could serve as the anatomical substrate for specificity generation through collateral pruning. Axons often travel laterally across the face of the distal stump before choosing a pathway. As a result, the average unbranched axon has access to over 100 distal Schwann cell tubes. This extensive access, however, does not ensure correct matching of axon and end organ, suggesting that pathway choice is made on the basis of factors other than end organ identity. These observations explain the failure of refined surgical techniques to restore normal function after nerve injury. The apparent wandering of axons across the repair defies surgical control and mandates a biological approach to reuniting severed axons with appropriate distal pathways., (Copyright 2005 Wiley-Liss, Inc.)

Predegeneration of nerve enhances its ability to support axon regeneration. Trophic factors are upregulated by reactive Schwann cells while potentially inhibitory molecules are removed. These experiments isolate the effects of one such inhibitory molecule, the myelin-associated glycoprotein (MAG), to determine its role in modifying regeneration after nerve repair. Suture of the mouse femoral nerve was followed by daily intraperitoneal injection of antibodies to MAG, antibodies to HNK-1, a specific muscle pathway marker, or no further treatment. Regeneration was assayed by double-labeling the femoral cutaneous and muscle branches with horseradish peroxidase and fluoro-gold after 4 weeks or 6 weeks of regeneration. Four weeks after nerve repair, selective reinnervation of the muscle branch by motoneurons, or preferential motor reinnervation (PMR), was not seen in either controls or L2-antibody-treated animals. In contrast, treatment with MAG antibodies resulted in dramatic PMR. By 6 weeks, the controls had achieved borderline specificity, substantial PMR developed in the L2 antibody group and the MAG group changed little. Blocking access to MAG in the distal nerve stump thus accelerated and enhanced PMR. Sensory regeneration was depressed by both antibody treatments at 4 weeks but recovered by 6 weeks. Antibody administration has a generalized effect on sensory regeneration that is unrelated to the behavior of motoneurons in the same nerve.

Motoneurons reinnervate the distal stump at variable rates after peripheral nerve transection and suture. In the rat femoral nerve model, reinnervation is already substantial 3 weeks after repair, but is not completed for an additional 7 weeks. However, this "staggered regeneration" can be temporally compressed by application of 20 Hz electrical stimulation to the nerve for 1 hr. The present experiments explore two possible mechanisms for this stimulation effect: (1) synchronization of distal stump reinnervation and (2) enhancement of regeneration speed. The first possibility was investigated by labeling all motoneurons that have crossed the repair at intervals from 4 d to 4 weeks after rat femoral nerve transection and suture. Although many axons did not cross until 3-4 weeks after routine repair, stimulation significantly increased the number crossing at 4 and 7 d, with only a few crossing after 2 weeks. Regeneration speed was studied by radioisotope labeling of transported proteins and by anterograde labeling of regenerating axons, and was not altered by stimulation. Attempts to condition the neuron by stimulating the femoral nerve 1 week before injury were also without effect. Electrical stimulation thus promotes the onset of motor axon regeneration without increasing its speed. This finding suggests a combined approach to improving the outcome of nerve repair, beginning with stimulation to recruit all motoneurons across the repair, followed by other treatments to speed and prolong axonal elongation.

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.

Transected axons in C57BL/Ola mice survive for extraordinary lengths of time as compared to those of normal rodents. The biological difference in the substrain that confers the phenotype of prolonged axonal survival is unknown. Previous studies suggest that 'defect' to be a property of the nervous system itself, rather than one of haematogenous cells. Neuronal or non-neuronal elements could be responsible for this phenotype. This study was undertaken to determine whether Schwann cells, the most numerous of the non-neuronal cells intrinsic to the peripheral nerve, are responsible for delayed degeneration of transected axons. We created sciatic nerve chimeras by transplanting nerve segments between standard C57BL/6 and C57BL/Ola mice, allowing regeneration of host axons through the grafts containing donor Schwann cells. These nerves were then transected and the time course of axonal degeneration was observed. The results show that fast or slow degeneration is a property conferred by the host, and therefore cannot be ascribed to the Schwann cells. Similarly, transected C57BL/Ola axons in explanted dorsal root ganglia cultures survived longer than transected axons from standard mice. Taken together these results indicate that the responsible abnormality is intrinsic to the C57BL/Ola axon.

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.

Using the transected sciatic nerve model in adult mice, regeneration of a large bundle of axons organized into the form of a nerve with myelinated and unmyelinated axons, Schwann cells, fibroblasts, collagen, blood vessels, and connective tissue sheaths has been achieved with bioresorbable microtubular guidance channels over gaps of 5 mm in nonimmobilized animals. After 4-6 wks postoperatively, the regenerated nerve cable contains on the order of 40% as many myelinated axons as were measured in the proximal nerve stumps. With the channels used so far in this model, regenerating axons pass into the distal stump in about 3-6 wks postoperatively. The guidance channels used consist of synthetic polyesters and/or polyester composites including glycolic and lactic acid polymers, and polyesters derived from Krebs Cycle dicarboxylic acids. Inflammatory response to these materials has been minimal. Biodegradation/resorption rates can be controlled so as to be compatible with axon growth rates.

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.

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.

The dorsal horn projection patterns of finger nerves were investigated in four Macaca mulatta monkeys. Proper digital branches of the median nerves, serving the radial aspect of a digit on each hand, were loaded with wheatgerm agglutinin-horseradish peroxidase complex (WGA:HRP). The distribution of the lectin-enzyme complex was mapped in the right and left dorsal horns. The dorsal horn projections of the digital nerves were localized in segments C6-C8 in laminae I-VI, primarily in laminae I-IV. The wedge-shaped termination zones were somatotopically organized, in agreement with the projections of the digits in cats. The fingers are represented medially, as they are in the cat. This similarity suggests that there is a mediolateral gradient of dorsal horn organization similar to that of the cat, with distal skin represented medially and proximal skin represented laterally. The rostrocaudal trajectory of finger representation, with digit 1 most rostral and digit 5 most caudal, is also in agreement with the organization of hindlimb toe projections in the cat. There was a high degree of bilateral symmetry for homologous nerves, and little overlap of projections from nerves innervating adjacent fingers. The sample size was too small to permit us to assess interanimal variation. These results suggest a similar somatotopy of projections, and presumably of dorsal horn cell somatotopy, in monkey and cat.