Active Src expression is induced after rat peripheral nerve injury

The non-receptor-type Src tyrosine kinases are key components of intracellular signal transduction that are expressed at high levels in the nervous system. To improve understanding of the cascades of molecular events underlying peripheral nerve regeneration, we analyzed active Src expression in the crushed or cut rat sciatic nerves using a monoclonal antibody (clone 28) that recognizes the active form of Src tyrosine kinases, including c-Src and c-Fyn. Western blots showed that active Src expressed in the normal sciatic nerve transiently increased up to threefolds after both types of injury. Immunohistochemistry using clone 28 showed that axonal components are the primary sites of active Src expression in the normal sciatic nerve. Soon after both types of injury, active Src was abundantly expressed in Schwann cells of the segments distal to the injury site. The expression of active Src in the cells decreased with restoration of the axon-Schwann cell relationship and eventually became depleted to very low levels after crushing, but was sustained at high levels in the cut model until the end of the experiment. Regenerated axons consistently expressed active Src throughout nerve regeneration and these eventually became the major sites of active Src expression in the crushed nerve. Among the Src tyrosine kinases, active c-Src selectively increased after crushing according to immunoprecipitation and immunoblotting analyses. Due to its potent biological activity, the increased amounts of the active form of Src probably enhance axonal regrowth, the Schwann cell response, and axon-Schwann cell contact for peripheral nerve regeneration.

Poly(L-Lactide) microfilaments enhance peripheral nerve regeneration across extended nerve lesions

After injury, axonal regeneration occurs across short gaps in the peripheral nervous system, but regeneration across larger gaps remains a challenge. To improve regeneration across extended nerve defects, we have fabricated novel microfilaments with the capability for drug release to support cellular migration and guide axonal growth across a lesion. In this study, we examine the nerve repair parameters of non-loaded filaments. To examine the influence of packing density on nerve repair, wet-spun poly(L-Lactide) (PLLA) microfilaments were bundled at densities of 3.75, 7.5, 15, and 30% to bridge a 1.0-cm gap lesion in the rat sciatic nerve. After 10 weeks, nerve cable formation increased significantly in the filament bundled groups when compared to empty-tube controls. At lower packing densities, the number of myelinated axons was more than twice that of controls or the highest packing density. In a consecutive experiment, PLLA bundles with lower filament-packing density were examined for nerve repair across 1.4- and 1.8-cm gaps. After 10 weeks, the number of successful regenerated nerves receiving filaments was more than twice that of controls. In addition, nerve cable areas for control groups were significantly less than those observed for filament groups. Axonal growth across 1.4- and 1.8-cm gaps was more consistent for the filament groups than for controls. These initial results demonstrate that PLLA microfilaments enhance nerve repair and regeneration across large nerve defects, even in the absence of drug release. Ongoing studies are examining nerve regeneration using microfilaments designed to release neurotrophins or cyclic AMP.

Inactivation of myelin-associated glycoprotein enhances optic nerve regeneration

CNS regeneration in higher vertebrates is a long sought after goal in neuroscience. The lack of regeneration is attributable in part to inhibitory factors found in myelin (Caroni and Schwab, 1988a). Myelin-associated glycoprotein (MAG) is an abundant myelin protein that inhibits neurite outgrowth in vitro (McKerracher et al., 1994; Mukhopadhyay et al., 1994), but its role in regeneration remains controversial. To address this role, we performed nerve crush on embryonic day 15 chick retina-optic nerve explants and then acutely eliminated MAG function along the nerve using chromophore-assisted laser inactivation (CALI). CALI of MAG permitted significant regrowth of retinal axons past the site of lesion containing CNS myelin in contrast to various control treatments. Electron microscopy of the site of nerve crush shows abundant regenerating axons crossing the gap. When crushed optic nerve was retrogradely labeled at the nerve stump, no labeling of retinal neurons was observed. In contrast, labeling of CALI of MAG-treated crushed optic nerve showed significant retinal labeling (89 +/- 16 cells per square millimeter), a value indistinguishable from that seen with non-crushed nerve (98 +/- 13 cells per square millimeter). These findings implicate MAG as an important component of the myelin-derived inhibition of nerve regeneration. The acute loss of MAG function can promote significant axon growth across a site of CNS nerve damage.

[A degradable artificial nerve guide to bridge peripheral nerve defects]

The standard technique for bridging a peripheral nerve defect is an autologous nerve graft if the nerve ends cannot be sutured. Recent evidence indicates that an alternative procedure-application of a degradable nerve guide-may be feasible. Currently the use of such a degradable nerve guide for the recovery of peripheral nerve defects in the hand is being tested in a multicenter trial. Conventional suturing as well as autologous nerve grafting are accepted methods in the control group within the protocol of the multicenter study. In the first two patients to receive the implant, a 28-year-old man and a 50-year-old women with tendon and nerve injury due to glass cuts, the operation was technically successful. Functional nerve recovery will be assessed in due course.

Neuropilin and class 3 semaphorins in nervous system regeneration

Injury to the mature mammalian central nervous system (CNS) is often accompanied by permanent loss of function of the damaged neural circuits. The failure of injured CNS axons to regenerate is thought to be caused, in part, by neurite outgrowth inhibitory factors expressed in and around the lesion. These include several myelin associated inhibitors, proteoglycans, and tenascin-R. Recent studies have documented the presence of class 3 semaphorins in fibroblast-like meningeal cells present in the core of the neural scar formed following CNS injury. Class 3 semaphorins display neurite growth-inhibitory effects on growing axons during embryonic development. The induction of the expression of class 3 semaphorins in the neural scar and the persistent expression of their receptors, the neuropilins and plexins, by injured CNS neurons suggest that they contribute to the regenerative failure of CNS neurons. Neuropilins are also expressed in the neural scar in a subpopulation of meningeal fibroblast and in neurons in the vicinity of the scar. Semaphorin/neuropilin signaling might therefore also be important for cell migration, angiogenis and neuronal cell death in or around neural scars. In contrast to neurons in the CNS, neuropilin/plexin positive neurons in the PNS do display long distance regeneration following injury. Injured PNS neurons do not encounter a semaphorin positive neural scar. Furthermore, Semaphorin 3A is downregulated in the regenerating spinal motor neurons themselves. This was accompanied by a transient upregulation of Semaphorin 3A in the target muscle. These observations suggest that the injury induced regulation of Semaphorin 3A in the PNS contributes to successful regeneration and target reinnervation. Future studies in genetically modified mice should provide more insight into the mechanisms by which neuropilins and semaphorins influence nervous system regeneration and degeneration.

Rapid neurorrhaphy with titanium clips

The purpose of this study was to evaluate the usefulness of titanium clips in the repair of transected peripheral nerves. The results of neurorrhaphy using titanium clips (VCS) were compared with the conventional technique of neurorrhaphy using nylon sutures. Transected sciatic nerves in 15 New Zealand white rabbits were repaired with VCS clips on one side and interrupted 9-0 nylon sutures on the other. The average time required for neurorrhaphy using the clip closure was 8.7 +/- 2.6 min, and this was shorter than the suture closure, which took 14.9 +/- 3.6 min (P < 0.01). Electromyographic studies revealed no significant differences in amplitudes of compound motor action potentials in both groups. Also, the number and extent of myelinizations of regenerated axons were not significantly different in both methods of nerorrhaphies. These results demonstrate that microscopic neurorrhaphy using titanium clips is a potential alternative to conventional neurorrhaphy using sutures.

Reaction of spinal cord central canal cells to cord transection and their contribution to cord regeneration

After transection, the spinal cord of the eel Anguilla quickly regrows and reconnects, and function recovers. We describe here the changes in the central canal region that accompany this regeneration by using serial semithin plastic sections and immunohistochemistry. The progress of axonal regrowth was followed in material labeled with DiI. The canal of the uninjured cord is surrounded by four cell types: S-100-immunopositive ependymocytes, S-100- and glial fibrillary acidic protein (GFAP)-immunopositive tanycytes, vimentin-immunopositive dorsally located cells, and lateral and ventral liquor-contacting neurons, which label for either gamma-aminobutyric acid (GABA) or tyrosine hydroxylase (TH). After cord transection, a new central canal forms rapidly as small groups of cells at the leading edges of the transection create flat "plates" that serve as templates for subsequent formation of the lateral and dorsal walls. Profile counts and 5-bromo-2'-deoxyuridine immunohistochemistry indicate that these cells are dividing rapidly during the first 20 days of the repair process. The newly formed canal, which bridges the transection by day 10 but is not complete until about day 20, is greatly enlarged (</=100 times) and is dominated by ependymocytes that are vimentin immunopositive, but cells expressing GABA, TH, and GFAP do not appear until days 11, 13, and 16, respectively. The proliferating ependyma do not provide a supportive scaffold for the regrowing axons, inasmuch as some have crossed the bridge before the canal has formed. However, their modified phenotype suggests a role, possibly trophic, for the central canal region following injury.

Regeneration of primary sensory neurons

Primary sensory neurons have an inherent capacity for regeneration of their cut, crushed, or chemically lesioned axons. This capacity is displayed to a much greater extent after lesions of the peripheral axons than after lesions of their centrally directed axons. Additionally, the surrounding tissue determines to a significant extent the degree of recovery: whereas the peripheral nerve tissue provides neurotrophic support and a favorable environment for axonal growth, the central terminals of primary sensory neurons face a non-permissive and inhibitory glial tissue. Mechanical lesions of the peripheral axons of dorsal root ganglion (DRG) sensory neurons can be repaired by the intrinsic regenerative capacity of the neuron itself, when outgrowing axons from the proximal stump are able to transverse the tissue scar and reach the distal stump of the nerve. Bridging the gap with an autologous nerve graft or a short artificial graft filled with nerve growth factor (NGF) can improve recovery. Neurotoxic lesions of the axon terminals are effectively recovered by intermittent local or systemic NGF injections. A recovery from a diabetic sensory neuropathy probably requires the continuous delivery of NGF or additional neurotrophic factors. A recovery from a dorsal rhizotomy or from a dorsal column lesion can possibly be achieved by the concomitant transgene-mediated overexpression of neurotrophins, the transformation of the DRG neuron cells to a competence for regrowth, and the counteraction of the growth-inhibitory nature of the central nervous system tissue.

Perisynaptic Schwann cells at the neuromuscular junction: nerve- and activity-dependent contributions to synaptic efficacy, plasticity, and reinnervation

Glial cells are increasingly recognized for their important contributions to CNS and PNS synaptic function. Perisynaptic Schwann cells, which are glial cells at the neuromuscular junction, have proven to be an exceptionally useful model for studying these roles. Recent studies have shown that they detect and reciprocally modulate synaptic efficacy in an activity-dependent manner in the short term. In addition, perisynaptic Schwann cells guide reinnervating nerve sprouts after deinnervation, and many important parameters of this are dependent on synapse activity. Thus, it is hypothesized that perisynaptic Schwann cells are key integrators in a continuum of synaptic efficacy, stability, and plasticity at the neuromuscular junction, which is important for maintaining and restoring synaptic efficacy.

Identification of teleost Thy-1 and association with the microdomain/lipid raft reggie proteins in regenerating CNS axons

During regeneration, retinal ganglion cell axons in fish upregulate a cell surface protein that is recognized by the monoclonal antibody (mAB) M802. M802 antigen appeared to be linked to the intracellular, membrane-associated lipid raft/microdomain proteins reggie-1 and reggie-2 that were previously shown to be reexpressed in axon-regenerating neurons [Development 124 (1997), 577]. Here, we report the isolation of the M802 antigen and its identification as the teleost homolog of mammalian Thy-1. Fish Thy-1 is detected in the same detergent-insoluble lipid raft fractions from a fibroblast cell line and from axon regenerating retinae as reggie-1 and 2. Importantly, mAB M802 coimmunoprecipitates reggie-1 and 2 from this lipid raft fraction, implying that fish Thy-1 and reggies interact. This correlates with their colocalization in growing cell processes after M802 antigen/Thy-1 activation with mAB M802. These findings suggest a role of clustered M802 antigen/Thy-1 in reggie raft microdomains for cell growth and axon regeneration.

Loss and recovery of the blood-nerve barrier in the rat sciatic nerve after crush injury are associated with expression of intercellular junctional proteins

The blood-nerve barrier in peripheral nerves is important for maintaining the environment for axons. Breakdown of the barrier by nerve injury causes various pathologies. We hypothesized that the breakdown and recovery of the blood-nerve barrier after injury are associated with the changes in the expression of intercellular junctional proteins. To test this hypothesis, we induced crush injuries in the rat sciatic nerve by ligation and analyzed spatiotemporal changes of claudin-1, claudin-5, occludin, VE-cadherin, and connexin43 by immunoconfocal microscopy and morphometry and compared them with changes in the permeability of the blood-nerve barrier by intravenous and local administration of Evans blue-albumin (EBA). On day 1 after removal of the ligature EBA leaked into the connective tissue in the endoneurium and then the leakage gradually decreased and disappeared on day 7. On day 1 claudin-1, claudin-5, occludin, VE-cadherin, and connexin43 had totally disappeared from the perineurium and endoneurium. Thereafter, claudin-1, claudin-5, occludin, and VE-cadherin recovered from day 2, whereas connexin43 was redetected on day 5. These results indicate that the breakdown and following recovery of the blood-nerve barrier are closely associated with changes in the expression of claudins, occludin, VE-cadherin, and connexin43 and that the recovery time course is similar but nonidentical.

Prolonged low-dose caffeine exposure protects against hippocampal damage but not against the occurrence of epilepsy in the lithium-pilocarpine model in the rat

PURPOSE

Acute caffeine exposure has proconvulsant effects and worsens epileptic and ischemic neuronal damage. Surprisingly, prolonged caffeine exposure decreases the susceptibility to seizures and the extent of ischemic damage. We explored whether the exposure to a low long-term dose of caffeine could protect the brain from neuronal damage and epileptogenesis in the lithium-pilocarpine model of temporal lobe epilepsy.

METHODS

Rats received either plain tap water or water containing caffeine (0.3 g/L) for 15 days before the induction of status epilepticus (SE) by lithium-pilocarpine and for 7 days after SE. The extent of neuronal damage was assessed in the hippocampus and piriform and entorhinal cortices in brain sections stained with thionine and obtained from animals killed 7 days after SE. The latency to spontaneous recurrent seizures was controlled by video monitoring.

RESULTS

Caffeine treatment induced a marked, almost total neuroprotection in CA1 and a very limited protection in the hilus of the dentate gyrus, whereas damage in layers III-IV of the piriform cortex was slightly worsened by the treatment. All rats, whether they received caffeine or plain tap water, became epileptic after the same latency (17-19 days).

CONCLUSIONS

Thus these data extend the neuroprotective effects of low long-term caffeine exposure to epileptic damage and confirm that the sole protection of the Ammon's horn has no influence on the genesis of spontaneous recurrent seizures in this model.

Nerve regeneration inside fresh skeletal muscle-enriched synthetic tubes: a laser confocal microscope study in the rat sciatic nerve model

The search for good conduits for bridging nerve defects is a major challenge of today's tissue engineering research. In this paper we report on a laser confocal microscope study on early nerve regeneration inside a tissue engineered graft made by a poly(DLLA-epsilon-CL) conduit enriched with fresh skeletal muscle. The same biodegradable tubes filled with PBS solution were used as controls. The conduits were placed to bridge unilateral 1-cm-long rat sciatic nerve defects and analysed 10 days after surgery. Results showed that inside the muscle-enriched tubes axon regeneration, labelled by means of anti-neurofilament antibody, was already begun, whilst no axon regeneration was detectable along control tubes. In addition, a-GFAP (glial fibrillar acid protein) immuno-labelling of Schwann cells showed that progression inside muscle-enriched tubes, especially from the distal nerve stump, was much more evident than in control conduits. These results suggest that enrichment of synthetic tubes with fresh skeletal muscle promotes axon regeneration and Schwann cell migration in early nerve repair stages.

Stem therapy for ALS: hope and reality

All are agreed that there is pressing need for an effective treatment for Amyotrophic Lateral Sclerosis (ALS; MND). Such treatment may derive from a combination of therapeutic strategies aimed at different aspects of the disorder, and might include drugs directed at the initial, intermediate or terminal cascade of events leading to cell death, as well as the use of stem cells to replace dead motor neurons, or to protect those that remain. The attraction of cell implantation or transplantation is that it might help to overcome the inability of the CNS to replace lost neurons. It is also clear that neural implantation will yield little benefit if the donor cells fail to integrate functionally into the recipient CNS circuitry. In this respect, ALS poses an especially difficult problem. The recent breakthroughs in stem cell research might nevertheless provide possibilities for neural implantation and cell replacement therapy for patients with ALS. The potential impact of these new approaches to neurodegenerative diseases has been emphasised by the many experiments using human foetal cell grafts in patients affected by Parkinson's and Huntington's disease. Clinical benefits in Parkinson's disease seem to be associated with integration of the donor cells into the recipient brain. Despite promising results, however, significant constraints have hampered the use of foetal cells for neural implantation and transplantation. Besides ethical concerns, the viability, purity, and final destiny of the foetal tissue have not been completely defined. Foetal cells are, in addition, post-mitotic and cannot be expanded or stored for long periods, necessitating close synchronisation of tissue donation and neurosurgery.

The Nogo-66 receptor: focusing myelin inhibition of axon regeneration

CNS myelin inhibits axonal outgrowth in vitro and is one of several obstacles to functional recovery following spinal cord injury. Central to our current understanding of myelin-mediated inhibition are the membrane protein Nogo and the Nogo-66 receptor (NgR). New findings implicate NgR as a point of convergence in signal transduction for several myelin-associated inhibitors. Additional studies have identified a potential coreceptor for NgR as p75(NTR), and a second-messenger pathway involving RhoA that inhibits neurite elongation. Although these findings expand our understanding of the molecular determinants of adult CNS axonal regrowth, the physiological roles of myelin-associated inhibitors in the intact adult CNS remain ill-defined.

Nerve regeneration in a collagen-chitosan tissue-engineered skin transplanted on nude mice

A reconstructed skin made of a collagen-chitosan sponge seeded with human fibroblasts and keratinocytes and grown in vitro for 31 days was developed for the treatment of deep and extensive burns. The aim of this study was to assess whether this tissue-engineered skin could promote nerve regeneration in vivo, since recovery of sensation is a major concern for burnt patients. The human reconstructed skin was transplanted on the back of nude mice and the growth of nerve fibres within it was assessed 40, 60, 90 and 120 days after graft. Nerve growth was monitored by confocal microscopy using immunohistochemical staining of PGP 9.5 and 150 kD neurofilament, while Schwann cell migration was observed using protein S100 expression and laminin deposition. Nerve growth was first detected 60 days after transplantation and was more abundant 90 and 120 days after graft. Linear arrangements of Schwann cells were observed in the graft as early as 40 days after graft. Nerve growth was observed along these Schwann cell extensions 60 days after transplantation. We conclude that the three-dimensional architecture of the collagen-chitosan tissue-engineered skin sponge encourages nerve growth. This result provides new perspectives to increase nerve regeneration within the tissue-engineered skin by linkage of neurotrophic factors in the sponge before transplantation.

Stem cells in the teleost retina: persistent neurogenesis and injury-induced regeneration

The retina of the adult teleost fish is an important model for studying persistent and injury-induced neurogenesis in the vertebrate central nervous system. All neurons, with the exception of rod photoreceptors, are continually appended to the extant retina from an annulus of progenitors at the margin. Rod photoreceptors, in contrast, are added to differentiated retina only from a lineage of progenitors dedicated to making rods. Further, when the retina is lesioned, the lineage that produces only rods ceases this activity and regenerates retinal neurons of all types. The progenitors that supply neurons at the retinal margin and rod photoreceptors and regenerated neurons in the mature tissue originate from multipotent stem cells. Recent data suggest that the growth-associated neurogenic activity in the retina is regulated as part of the growth hormone/insulin-like growth factor-I axis. This paper reviews recent evidence for the presence of stem cells in the teleost retina and the molecular regulation of neurogenesis and presents a consensus cellular model that describes persistent and injury-induced neurogenesis in the retinas of teleost fish.

HSP27 is markedly induced in Schwann cell columns and associated regenerating axons

It is well known that regenerating axons enter Schwann cell (SC) columns, within which they grow to reinnervate the appropriate targets. The current study detected a marked induction of a 27-kDa heat shock protein (HSP27) in the SC columns of crush-injured rat sciatic nerves. Immunohistochemical studies showed the first appearance of strong HSP27-immunoreactive linear structures in the proximal stump near an injury site 7 h after an operation. The HSP27-immunoreactive linear structures crossed the injury site to the distal stump 2 days after the operation. They then extended in a more proximal and more distal direction and were found to have propagated through the entire length of the nerve 1 week after the operation. This pattern of expression was maintained until 3 weeks after the operation. Double-immunofluorescent labeling and confocal laser microscopy confirmed that the linear structures consisted of SC columns and associated multiple axons. The HSP27-immunoreactive SC columns expressed glial fibrillary acidic protein, but not S-100 protein. Electron microscopy and immunoelectron microscopy demonstrated that reactive Schwann cells (SCs) and the associated axons with an outgrowing profile exhibited a strong immunoreactivity to HSP27, with the former containing a greater number of bundles of intermediate filaments. It is suggested that HSP27 may play an essential role in axonal outgrowth, especially by contributing to cytoskeletal dynamics in SCs.

Functional peptide sequences derived from extracellular matrix glycoproteins and their receptors: strategies to improve neuronal regeneration

Peptides derived from extracellular matrix proteins have the potential to function as potent therapeutic reagents to increase neuronal regeneration following central nervous system (CNS) injury, yet their efficacy as pharmaceutical reagents is dependent upon the expression of cognate receptors in the target tissue. This type of codependency is clearly observed in successful models of axonal regeneration in the peripheral nervous system, but not in the normally nonregenerating adult CNS. Successful regeneration is most closely correlated with the induction of integrins on the surface of peripheral neurons. This suggests that in order to achieve optimal neurite regrowth in the injured adult CNS, therapeutic strategies must include approaches that increase the number of integrins and other key receptors in damaged central neurons, as well as provide the appropriate growth-promoting peptides in a "regeneration cocktail." In this review, we describe the ability of peptides derived from tenascin- C, fibronectin, and laminin-1 to influence neuronal growth. In addition, we also discuss the implications of peptide/receptor interactions for strategies to improve neuronal regeneration.

Contralateral reinnervation of midline muscles in nonidiopathic facial palsy

The purpose of this study was to analyze contralateral reinnervation of the facial nerve in eight patients with complete facial palsy after surgery or trauma and seven healthy volunteers. All patients had contralateral reinnervation of facial muscles as demonstrated by electrical nerve stimulation versus none of the control subjects. Four patients had facial muscle movements at the site of the damaged nerve. In one patient this was entirely the result of contralateral reinnervation, whereas the other three patients had innervation both ipsilaterally and contralaterally. This implies that renewed facial muscle activity should be examined considering the origin of the reinnervation, either contralateral or ipsilateral. Contralateral reinnervation is a common phenomenon after total facial palsy and can occur alongside ipsilateral reinnervation. It can be mistaken for adequate reinnervation of the damaged nerve, causing postponement of dynamic reconstruction therapy.

[Regeneration of vertebrate appendage: an old experimental model to study stem cells in the adult]

The application of stem cell therapy to cure degenerative diseases offers immense possibilities, but the research in this field is the subject of ethical debates raised by the question of destructive research on early human embryos. Stem cells taken in the adult constitute an alternative to human embryonic stem cells, but our knowledge on totipotent or pluripotent cells is currently insufficient. Furthermore, many questions must be solved before selection and differentiation of these cells in a given cellular type can be controlled on a routine basis. What are the molecular characteristics of an adult stem cell? What are the mechanisms involved in cell reprogramming? Which signals control stem cell replication and differentiation? Basic research activities must be carried out in order to clarify all these points. In this context, the regeneration of vertebrate appendages provides a model for this type of research. The regeneration process is defined by both the morphological and functional reconstruction of a part of a living organism, which has previously been destroyed. But why are some vertebrates able to regenerate complex structures and others apparently not? Among most vertebrates, the capacity to regenerate is limited to some tissues. It is however possible to observe the regeneration of appendages (limb, tail, fin, jaw, etc.) among several amphibians and fish. This regeneration leads to re-forming of the amputated part with a complete restoration of its shape, segmentation and function. Why is the amputation of limbs not followed by regeneration in mammals and birds: absence of stem cells, absence of recruitment signals for these cells, or absence of signal receptivity? This review constitutes a report on the current understanding of the basis of on regeneration of legs in tetrapods and of fins in fish with an emphasis in the role of the nervous system in this process.

Axonal regeneration of descending and ascending spinal projection neurons in spinal cord-transected larval lamprey

The distributions of descending and ascending spinal projection neurons (i.e., spinal neurons with moderate to long axons) were compared in normal larval lamprey and in animals that had recovered for 8 weeks following a complete spinal cord transection at 50% body length (BL, normalized distance from the anterior head). In normal animals, application of HRP to the spinal cord at 60% BL (40% BL) labeled an average of 713.8 +/- 143.2 descending spinal projection neurons (718.4 +/- 108.0 ascending spinal projection neurons) along the rostral (caudal) spinal cord, most of which were unidentified neurons. Some of these neurons project for at least approximately 50-60 spinal cord segments (approximately 36-47 mm in animals with an average length of approximately 90 mm used in the present study). At 8 weeks posttransection, the numbers of HRP-labeled descending or ascending spinal neurons that extended their axons through the transection were about 40% of those in similar areas of the spinal cord in normal animals. Thus, in larval lamprey, axonal regeneration of descending and ascending spinal projection neurons is incomplete, similar to that found for descending brain neurons. The majority of restored projections were from unidentified spinal neurons that have not been documented previously. In contrast to results from several other lower vertebrates, in the lamprey ascending spinal neurons exhibited substantial axonal regeneration. Identified descending spinal neurons, such as lateral interneurons and crossed contralateral interneurons, and identified ascending spinal neurons, such as giant interneurons and edge cells, regenerated their axons at least 9 mm beyond the transection site in animals with an average length of approximately 90 mm, which is appreciably farther than previously reported. In contrast, most dorsal cells, which are centrally located sensory neurons, exhibited very little axonal regeneration.

Pathophysiology of nerve injury

The response to nerve injury is a complex and often poorly understood mechanism. An in-depth and current command of the relevant neuroanatomy, classifications systems, and responses to injury and regeneration are critical to current clinical success. Continued progress must be made in our current understanding of these varied physiologic mechanisms of neuro-regeneration if any significant progress in clinical treatments or outcome is to be expected in the future. Reconstructive surgeons have in many ways maximized the technical aspects of peripheral nerve repair. However, advances in functional recovery may be seen with improvements in sensory and motor rehabilitation after peripheral nerve surgery and with a combined understanding of the neurobiology and neurophysiology of nerve injury and regeneration.