Axon numbers and landmarks of trigeminal donor nerves for corneal neurotization

Purpose

Corneal anesthesia leads to chronic corneal injury. This anatomical study characterizes the donor nerve branches of the supratrochlear and supraorbital nerves used for corneal neurotization.

Methods

In 13 non-embalmed cadavers, the supratrochlear and supraorbital nerves were dissected and distances to anatomical landmarks measured. Cross-sections of supratrochlear and supraorbital donor nerves were harvested and histomorphometrically analyzed to assess the number of myelinated axons.

Results

The donor axon counts were 3146 ± 1069.9 for the supratrochlear and 1882 ± 903 for the supraorbital nerve distal to the supraorbital notch. The supratrochlear nerve was dissected on the medial upper eyelid 2 cm lateral to the facial midline and the branch of the supraorbital nerve 1 cm medial to the mid-pupillary line.

Conclusion

The supraorbital and supratrochlear branches of the trigeminal nerve are potent donor nerves for corneal neurotization in the treatment of neuropathic keratopathy and can be reliably dissected using anatomical landmarks.

Lamina propria and olfactory bulb ensheathing cells exhibit differential integration and migration and promote differential axon sprouting in the lesioned spinal cord

Olfactory bulb-derived (central) ensheathing cell (OB OEC) transplants have shown significant promise in rat models of spinal cord injury, prompting the use of lamina propria-derived (peripheral) olfactory ensheathing cells (LP OECs) in both experimental and clinical trials. Although derived from a common embryonic precursor, both sources of OECs reside in different nervous system compartments postnatally, and their ability to promote regeneration and efficacy after transplantation may differ depending on both their source and mode of transplantation. Here, we have purified green fluorescent protein-expressing LP and OB OECs, assayed their biological differences in vitro, and transplanted them acutely either directly into or rostral and caudal to a dorsolateral funiculus crush. LP and OB OECs exhibit multiple morphological and antigenic similarities in vitro, and, after transplantation, they both attenuate lesion and cavity formation and promote angiogenesis, endogenous Schwann cell infiltration, and axonal sprouting. However, an increased mitotic rate and migratory ability of LP OECs in vitro was reflected in vivo by their superior ability to migrate within the spinal cord, reduce cavity formation and lesion size, and differentially stimulate outgrowth of axonal subpopulations compared with OB OECs. An undesired behavior (autotomy) was also significantly enhanced by LP OEC, over OB OEC, transplantation. These results suggest that LP and OB OECs exhibit intrinsic biological differences that, after transplantation into the lesioned CNS, result in differences in postlesion spinal cord neuropathology and anatomical and behavioral regeneration outcomes that also vary depending on direct versus rostrocaudal transplantation.

Cross-reinnervation changes the expression patterns of the monocarboxylate transporters 1 and 4: An experimental study in slow and fast rat skeletal muscle

The monocarboxylate transporters 1 and 4 are expressed in brain as well as in skeletal muscle and play important roles in the energy metabolism of both tissues. In brain, monocarboxylate transporter 1 occurs in astrocytes, ependymocytes, and endothelial cells while monocarboxylate transporter 4 appears to be restricted to astrocytes. In muscle, monocarboxylate transporter 1 is enriched in oxidative muscle fibers whereas monocarboxylate transporter 4 is expressed in all fibers, with the lowest levels in oxidative fiber types. The mechanisms regulating monocarboxylate transporter 1 and monocarboxylate transporter 4 expression are not known. We hypothesized that the expression of these transporters would be sensitive to long term changes in metabolic activity level. This hypothesis can be tested in rat skeletal muscle, where permanent changes in activity level can be induced by cross-reinnervation. We transplanted motor axons originally innervating the fast-twitch extensor digitorum longus muscle to the slow-twitch soleus muscle and vice versa. Four months later, microscopic analysis revealed transformation of muscle fiber types in the cross-reinnervated muscles. Western blot analysis showed that monocarboxylate transporter 1 was increased by 140% in extensor digitorum longus muscle and decreased by 30% in soleus muscle after cross-reinnervation. In contrast, cross-reinnervation induced a 62% decrease of monocarboxylate transporter 4 in extensor digitorum longus muscle and a 1300% increase in soleus muscle. Our findings show that cross-reinnervation causes pronounced changes in the expression levels of monocarboxylate transporter 1 and monocarboxylate transporter 4, probably as a direct consequence of the new pattern of nerve impulses. The data indicate that the mode of innervation dictates the expression of monocarboxylate transporter proteins in the target cells and that the change in monocarboxylate transporter isoform profile is an integral part of the muscle fiber transformation that occurs after cross-reinnervation. Our findings support the hypothesis that the expression of monocarboxylate transporter 1 and monocarboxylate transporter 4 in excitable tissues is regulated by activity.

Development of transplantable nervous tissue constructs comprised of stretch-grown axons

Pursuing a new approach to nervous system repair, fasciculated axon tracts grown in vitro were developed into nervous tissue constructs designed to span peripheral nerve or spinal cord lesions. We optimized the newfound process of extreme axon stretch growth to maximize the number and length of axon tracts, reach an unprecedented axon growth-rate of 1cm/day, and create 5cm long axon tracts in 8 days to serve as the core component of a living nervous tissue construct. Immunocytochemical analysis confirmed that elongating fibers were axons, and that all major cytoskeletal constituents were present across the stretch-growth regions. We formed a transplantable nervous tissue construct by encasing the elongated cells in an 80% collagen hydrogel, removing them from culture, and inserting them into a synthetic conduit. Alternatively, we induced axon stretch growth directly on a surgical membrane that could be removed from the elongation device, and formed into a cylindrical construct suitable for transplant. The ability to rapidly create living nervous tissue constructs that recapitulates the uniaxial orientations of the original nerve offers an unexplored and potentially complimentary direction in nerve repair. Ideally, bridging nerve damage with living axon tracts may serve to establish or promote new functional connections.

Upregulation of activating transcription factor 3 (ATF3) by intrinsic CNS neurons regenerating axons into peripheral nerve grafts

The expression of the transcription factor ATF3 in the brain was examined by immunohistochemistry during axonal regeneration induced by the implantation of pieces of peripheral nerve into the thalamus of adult rats. After 3 days, ATF3 immunoreactivity was present in many cells within approximately 500 mum of the graft. In addition, ATF3-positive cell nuclei were found in the thalamic reticular nucleus (TRN) and medial geniculate nuclear complex (MGN), from which most regenerating axons originate. CNS cells with ATF3-positive nuclei were predominantly neurons and did not show signs of apoptosis. The number of ATF3-positive cells had declined by 7 days and further by 1 month after grafting when most ATF3-positive cells were found in the TRN and MGN. 14 days or more after grafting, some ATF3-positive nuclei were distorted and may have been apoptotic. In some experiments of 1 month duration, neurons which had regenerated axons to the distal ends of grafts were retrogradely labeled with DiAsp. ATF3-positive neurons in these animals were located in regions of the TRN and MGN containing retrogradely labeled neurons and the great majority were also labeled with DiAsp. SCG10 and c-Jun were found in neurons in the same regions as retrogradely labeled and ATF3-positive cells. Thus, ATF3 is transiently upregulated by injured CNS neurons, but prolonged expression is part of the pattern of gene expression associated with axonal regeneration. The co-expression of ATF3 with c-jun suggests that interactions between these transcription factors may be important for controlling the program of gene expression necessary for regeneration.

The astrocytic barrier to axonal regeneration at the dorsal root entry zone is induced by rhizotomy

After dorsal rhizotomy, sensory axons fail to regenerate beyond the astrocytic glia limitans at the dorsal root entry zone (DREZ) but this inhibition can be overcome with the delivery of exogenous neurotrophin-3. We investigated whether axonal inhibition at the DREZ is constitutive or induced after dorsal rhizotomy. Primary afferent neurones from enhanced green fluorescent protein-expressing mice were transplanted into adult rat dorsal root ganglia in the presence or absence of dorsal rhizotomy. In the absence of dorsal rhizotomy mouse axons freely extended into the rat central nervous system. After host dorsal rhizotomy, mouse axons were unable to cross the DREZ. However, in rats that received a dorsal rhizotomy concomitant with intrathecal neurotrophin-3, the mouse axons were able to cross the DREZ. These results indicate that, under normal circumstances, the adult DREZ is permissive to the regeneration of adult sensory axons and that it only becomes inhibitory once dorsal root axons have been injured and astrocytes at the DREZ have become reactive.

Transplantation of Bone Marrow Stromal Cell-Derived Schwann Cells Promotes Axonal Regeneration and Functional Recovery after Complete Transection of Adult Rat Spinal Cord

The aim of this study was to evaluate whether transplantation of Schwann cells derived from bone marrow stromal cells (BMSC-SCs) promotes axonal regeneration and functional recovery in completely transected spinal cord in adult rats. Bone marrow stromal cells (BMSCs) were induced to differentiate into Schwann cells in vitro. A 4-mm segment of rat spinal cord was removed completely at the T7 level. An ultra-filtration membrane tube, filled with a mixture of Matrigel (MG) and BMSC-SCs (BMSC-SC group) or Matrigel alone (MG group), was grafted into the gap. In the BMSC-SC group, the number of neurofilament- and tyrosine hydroxylase-immunoreactive nerve fibers was significantly higher compared to the MG group, although 5-hydroxytryptamine- or calcitonin gene-related peptide-immunoreactive fibers were rarely detectable in both groups. In the BMSC-SC group, significant recovery of the hindlimb function was recognized, which was abolished by retransection of the graft 6 weeks after transplantation. These results demonstrate that transplantation of BMSC-SCs promotes axonal regeneration of lesioned spinal cord, resulting in recovery of hindlimb function in rats. Transplantation of BMSC-SCs is a potentially useful treatment for spinal cord injury.

Bridge over troubled waters

Spinal cord injury interrupts connections between the brain and spinal cord, rather than producing large-scale damage. Reconnecting severed axons with their prior targets is a primary objective of spinal cord repair. Despite progress, this goal will probably not be attained soon because many problems remain to be solved. We discuss an alternative for promoting motor function after spinal damage by bridging the injury. We highlight a novel spinal injury bridge that we have developed to reconnect spinal motor circuits below the injury with the brain. A spinal nerve that exits above the injury is disconnected and inserted into the cord caudal to injury. Motor axons in the inserted nerve regenerate into the cord and synapse on neurons producing a novel circuit to bypass the injury.

Coculture of elongated neuron axon with poly (D,L-lactide-co-glycolide) biomembrane in vitro

OBJECTIVE

To elongate human nerve axon in culture and search for suitable support matrices for peripheral nervous system transplantation.

METHODS

Human embryo cortical neuronal cells, seeded on poly (D,L-lactide-co-glycolide) (PLGA) membrane scaffolds, were elongated with a self-made neuro-axon extending device. The growth and morphological changes of neuron axons were observed to measure axolemmal permeability after elongation. Neurofilament protein was stained by immunohistochemical technique.

RESULTS

Human embryo neuron axon could be elongated and cultured on the PLGA membrane and retain their normal form and function.

CONCLUSIONS

Three dimensional scaffolds with elongated neuron axon have the basic characteristics of artificial nerves, indicating a fundemental theory of nerve repair with elongated neuron axon.

Engineering novel spinal circuits to promote recovery after spinal injury

We have developed an innovative way to establish a functional bridge around a spinal lesion. We disconnected the T13 nerve from its muscle targets, leaving the proximal end intact. The cut end was inserted either into an intact spinal cord, to assess regeneration of T13 axons into the cord and synapse formation with spinal neurons, or caudal to a hemisection at L2/3, to assess restoration of function below the injury. Four to 28 weeks later, anterograde tracers indicated that axons from the inserted T13 nerve regenerated into the ventral horn, the intermediate zone, and dorsal horn base, both in intact and hemisected animals. Antibodies to cholinergic markers showed that many regenerating axons were from T13 motoneurons. Electrical stimulation of the T13 nerve proximal to the insertion site 4 weeks or more after insertion into the intact cord evoked local field potentials in the intermediate zone and ventral horn, which is where T13 axons terminated. Stimulation of T13 in 71% of the animals (8 hemisected, 7 intact) evoked contraction of the back or leg muscles, depending on the level of insertion. Animals in which T13 was inserted caudal to hemisection had significantly less spasticity and muscle wasting and greater mobility at the hip, knee, ankle, and digits in the ipsilateral hindlimb than did animals with a hemisection only. Thus, T13 motor axons form novel synapses with lumbosacral motor circuits. Because the T13 motor neurons retain their connections to the brain, these novel circuits might restore voluntary control to muscles paralyzed below a spinal lesion.

A novel DNA enzyme reduces glycosaminoglycan chains in the glial scar and allows microtransplanted dorsal root ganglia axons to regenerate beyond lesions in the spinal cord

CNS lesions induce production of ECM molecules that inhibit axon regeneration. One major inhibitory family is the chondroitin sulfate proteoglycans (CSPGs). Reduction of their glycosaminoglycan (GAG) chains with chondroitinase ABC leads to increased axon regeneration that does not extend well past the lesion. Chondroitinase ABC, however, is unable to completely digest the GAG chains from the protein core, leaving an inhibitory "stub" carbohydrate behind. We used a newly designed DNA enzyme, which targets the mRNA of a critical enzyme that initiates glycosylation of the protein backbone of PGs, xylosyltransferase-1. DNA enzyme administration to TGF-beta-stimulated astrocytes in culture reduced specific GAG chains. The same DNA enzyme applied to the injured spinal cord led to a strong reduction of the GAG chains in the lesion penumbra and allowed axons to regenerate around the core of the lesion. Our experiments demonstrate the critical role of PGs, and particularly those in the penumbra, in causing regeneration failure in the adult spinal cord.

CNS axons retain their competence for myelination throughout life

An important question relevant to developing remyelination therapies is whether axons that remain without myelin sheaths after an episode of demyelination retain myelination competence. To resolve this, we have developed a model of transplantation into the nerve fibre layer of the adult rat retina, where the axons are unmyelinated. In the adult, these axons can be myelinated by transplantation of both the oligodendrocyte progenitor cells (OPCs) and an OPC line (CG4). The extent of myelination achieved following transplantation of OPCs is the same in young adult recipients (2 months old) as that which occurs in old adult recipients (12-18 months old), indicating that there are no changes in axons remaining unmyelinated for many months that would prevent effective remyelination. This finding suggests that chronically demyelinated regions of axons such as those in seen in multiple sclerosis are likely to remain competent to be remyelinated.

Peripheral olfactory ensheathing cells reduce scar and cavity formation and promote regeneration after spinal cord injury

Bridging of a lesion site and minimizing local damage to create an environment permissive for regeneration are both primary components of a successful strategy to repair spinal cord injury (SCI). Olfactory ensheathing cells (OECs) are prime candidates for autologous transplantation to bridge this gap, but little is known currently about their mechanism of action. In addition, OECs from the accessible lamina propria (LP) of the olfactory mucosa are a more viable source in humans but have yet to be tested for their ability to promote regeneration in established SCI models. Here, mouse LP-OECs expressing green fluorescent protein (GFP) transplanted directly into both rat and mouse dorsolateral spinal cord lesion sites demonstrate limited migration but interact with host astrocytes to develop a new transitional zone at the lesion border. LP-OECs also promote extensive migration of host Schwann cells into the central nervous system repair zone and stimulate angiogenesis to provide a biological scaffold for repair. This novel environment created by transplanted and host glia within the spinal cord inhibits cavity and scar formation and promotes extensive sprouting of multiple sensory and motor axons into and through the lesion site. Sixty days after rat SCI, serotonin- and tyrosine hydroxylase-positive axons sprouted across the lesion into the distal cord, although axotomized rubrospinal axons did not. Thus, even in a xenotransplant paradigm, LP-OECs work collaboratively with host glial cells to create an environment to ameliorate local damage and simultaneously promote a regenerative response in multiple axonal populations.

Effect of number of fascicle on axonal regeneration in cable grafts

The effect of number of fascicles on axonal regeneration in cable grafts was examined in the rat cable graft model. The study comprised three experimental groups: the 5f-group, which received 5 fascicles, larger than the host; the 3f-group, in which the total area of the graft fascicles was similar to that of the host; and the 1f-group, which received one fascicle cable graft, smaller in diameter than the host nerve. At the graft segment, well-myelinated fibers were observed both inside and outside the graft fascicles. The three groups showed no difference in morphometric and functional assessment, suggesting that the fibers which regenerated through the outside of the graft might be effectively induced into the distal host. The disproportionate enlargement of the graft fascicle of the 1f-group also increased the fibers passing through it. These findings suggest that a small number of fascicles can induce a larger population of regenerated fibers in the 20-mm cable graft model.

Grafted swine neuroepithelial stem cells can form myelinated axons and both efferent and afferent synapses with xenogeneic rat neurons

Neuroepithelial stem cells derived from the swine mesencephalic neural tube were examined regarding their eligibility for neural xenografting as a donor material, with the aim of evaluating myelinated axon formation and both types of synaptic formation with xenogeneic host neurons as part of possible neural circuit reconstruction. The mesencephalic neural tube tissues were dissected out from swine embryos at embryonic days 17 and 18 and were implanted immediately into the striatum of the Parkinsonian model rat. The swine-derived grafts had many nestin-positive rosette-forming, neurofilament-positive, and tyrosine hydroxylase-positive cells in the rat striatum. Electron microscopic study revealed both efferent and afferent synaptic formations in the donor-derived immature neurons or tyrosine hydroxylase-positive donor cells in the grafts. Myelinated axons, both positive and negative for swine-specific neurofilament antibody, were mingled together in the graft. These results indicated that implanted neuroepithelial stem cells could survive well and divide asymmetrically into both nestin-expressing precursors and differentiated neurochemical marker-expressing neurons in the xenogeneic rat striatum, with the help of an immunosuppressant. Donor-derived immature neurons formed both efferent and afferent synapses with xenogeneic host neurons, and donor-derived axons were myelinated, which suggests that implanted swine neuroepithelial stem cells could possibly restore damaged neuronal circuitry in the diseased brain.

Evidence for impaired axonal regeneration in PMP22 duplication: studies in nerve xenografts

Whether axonal regeneration in Charcot-Marie-Tooth (CMT) neuropathies is impaired has not been addressed in detail. Our studies in nude mice harboring xenografts from patients with different primary Schwann cell (SC) genetic defects suggested an intimate association between the onset of myelination and impairment in the growth capacity of nude mice axons engulfed by the mutant SCs. To assess the effects of peripheral myelin protein 22 (PMP22) gene duplication on the regeneration process, we conducted morphometric studies to generate temporal growth profiles of myelinated axons within the xenografts obtained from CMT1A patients and from healthy controls. Axon size distribution histograms in controls at different time intervals revealed that size differentiation of myelinated fibers within the grafts is established as early as 2 weeks, and that the temporal pattern of myelination of different sized axons has striking similarities to myelination during development. In PMP22 duplication grafts, the onset of myelination is delayed and the regeneration capacity of all fiber sizes is impaired. This defect, however, is most pronounced for the large diameter axons. In addition, significant large fiber loss occurred after 12 weeks with a concomitant new cycle of regeneration of small size axons. These studies show that the PMP22 duplication in SCs have profound effects on the regeneration process, which might be a contributing factor to preferential distal axonal loss.

Transplantation of olfactory ensheathing cells fails to promote significant axonal regeneration from dorsal roots into the rat cervical cord

The olfactory ensheathing cell (OEC) is a class of glial cell that has been reported to support regeneration in the central nervous system after various types of lesions, including rhizotomy of spinal dorsal roots at thoracic, lumbar and sacral levels. We have therefore carried out a detailed anatomical analysis to assess the efficacy of dorsal horn OEC transplants at promoting regeneration of primary afferents across the dorsal root entry zone (DREZ) at the cervical level in the adult rat. OECs were cultured from adult rat olfactory bulb and immunopurified (90% purity). Regeneration by large diameter afferents and by both peptidergic and non-peptidergic small diameter afferents was assessed using respectively cholera toxin B (CTB) labelling and immunocytochemistry for calcitonin gene-related peptide (CGRP) and the purinoceptor P2X3. Following an extensive (C3-T3) rhizotomy, CGRP and P2X3 immunoreactive axons regenerated across the rhizotomy site as far as the DREZ but there was no evidence of regeneration across the DREZ, except through sites where the OEC transplant was directly grafted into the DREZ. No evidence of regeneration into the dorsal horn by CTB-labelled axons was obtained. In addition, there was little sign of sprouting by intact axons in the vicinity of OEC transplant sites. In contrast to these results in vivo, cocultures of OECs and adult dorsal root ganglion cells showed that OECs stimulate extensive neurite outgrowth. The failure of the OECs to promote regeneration in vivo following cervical rhizotomy is therefore most likely due to factors in the environment of the graft site and/or the method of transplantation.

Ectopic endplates induce localized changes in skeletal muscle ultrastructure

To investigate the processes by which motoneurons control protein synthesis, and thus the ultrastructure of the muscle fibers they innervate, ectopic endplates were induced to form on adult mouse skeletal muscle fibers by transplantation of a foreign nerve onto the muscle. In the dually innervated muscle fibers thus created, we examined two ultrastructural parameters that correlate with the expression of distinct isoforms of the myofibrillar proteins alpha-actinin and titin, specifically, Z-line width and sarcomere length. It was found that Z-lines were significantly thinner (98 vs. 128 nm) and sarcomeres were significantly shorter (1.69 vs. 2.06 microm) near the ectopic than near the original endplates. Thus, ectopic endplate formation on adult skeletal muscle fibers induces a localized alteration in myofibrillar morphology. These results may help to elucidate the role played by motoneurons in the determination and maintenance of muscle fiber properties and the processes that occur following muscle reinnervation after nerve injury.

Identification of regenerative tissue cables using in vivo MRI after spinal cord hemisection and schwann cell bridging transplantation

The purpose of this study was to examine the feasibility of a non-invasive in vivo magnetic resonance imaging (MRI) procedure, performed at 1.5 T, to detect regenerative tissue cables in a rat spinal cord hemisection and Schwann cell (SC) bridging transplantation paradigm. Two months after implantation of a SC-seeded guidance channel (1.25 mm in diameter and 3.0 mm in length) into a T8 spinal cord hemisection-gap lesion, axial fast-spin echo (FSE) T2-weighted MR imaging (T2WI) was performed. Axial T2WI through the graft identified a circular area of low intensity surrounded by high-intensity signal within the guidance channel lumen. Correlative histological assessments of Toluidine blue-stained sections confirmed that the low-intensity signal represented a tissue cable, which, in most cases, contained a substantial number of myelinated axons oriented along the rostro-caudal axis of the spinal cord. The percentage of guidance channel cross-sectional area occupied by the tissue cable, expressed as the tissue cable index (TCI), was also determined from histological sections. Linear regression analysis of the TCI plotted relative to the number of myelinated axons revealed a strong positive correlation (r(2) = 0.85) between these two outcome measures. In addition, the sensitivity of MRI to detect regenerative tissue cables within guidance channels was 86%. These results demonstrate that (1). 1.5 T MR imaging performed 2 months after spinal cord hemisection and SC bridging transplantation is sensitive in detecting low-intensity regenerative tissue cables, and (2). the TCI strongly correlates with the extent of axonal regeneration into implanted SC-seeded guidance channels.

Grafts of BDNF-producing fibroblasts rescue axotomized rubrospinal neurons and prevent their atrophy

We have reported that intraspinal transplants of fibroblasts genetically modified to express brain-derived neurotrophic factor (BDNF) promote rubrospinal axon regeneration and functional recovery following subtotal cervical hemisection that completely ablated the rubrospinal tract. In the present study we examined whether these transplants could prevent cell loss and/or atrophy of axotomized Red nucleus neurons. Adult rats received a subtotal spinal cord cervical hemisection followed by a graft of unmodified fibroblasts or fibroblasts producing BDNF into the lesion cavity. One or 2 months later, fluorogold was injected several segments caudal to the lesion-transplant site to retrogradely label those Red nucleus neurons whose axons have regenerated. Unmodified fibroblasts failed to protect against either cell loss or atrophy. Neuron counts and soma-size measurements in Nissl-stained preparations showed a 45% loss of recognizable neurons and 40% atrophy of the surviving neurons in the injured Red nucleus. Grafts of BDNF-producing fibroblasts reduced neuron loss to less than 15% and surviving neurons showed only a 20% decrease in mean soma size. Soma size analysis of fluorogold-labeled Red nucleus neurons indicated that the Red nucleus neurons whose axons regenerated caudal to the graft did not atrophy. We conclude that fibroblasts engineered ex vivo to secrete BDNF and grafted into a partial cervical hemisection promote axon regeneration while reducing cell loss and atrophy of neurons in the Red nucleus. These results suggest that transplants of genetically engineered cells could be an important tool for delivery of therapeutic factors that contribute to the repair of spinal cord injury.

Functional impact of axonal misdirection after peripheral nerve injuries followed by graft or tube repair

Accuracy of reinnervation is one of the main factors conditioning functional recovery after brain, spinal, or peripheral axonal damage. Using the peripheral nerve as an experimental model, we studied the amount of inaccurate muscle reinnervation and its consequences on walking. Adult rats were submitted to an 8-mm resection of the sciatic nerve repaired by autograft (AG, n = 9), silicone (SIL, n = 13) or poly-L-lactate-epsilon-caprolactone (PLC, n = 11) single guides, and fascicular tubulization of peroneal and tibial branches with a dual silicone tube (FSIL, n = 9). At the end of follow-up, the sciatic nerve and its tibial and peroneal fascicles were dissected and stimulated by means of a suction electrode. In control rats, gastrocnemius and plantar muscles are fully innervated by the tibial fascicle and the tibialis anterior muscle by the peroneal nerve. None of the groups had noticeable recovery of locomotion assessed by the walking track index (SFI around -70 in all groups). After resection, all animals of groups AG, SIL, and PLC showed aberrant muscle reinnervation by axons from a non-corresponding fascicle, whereas in group FSIL only one of six regenerated animals showed misdirected activity. The proportion of inaccurate muscle activation was similar in group AG (47% for gastrocnemius, 54% for tibialis anterior, and 44% for plantar muscles) and in group SIL (42%, 42%, and 42%), and reduced in group PLC (26%, 38%, and 27%). In conclusion, fascicular silicone tubulization allowed the highest degree of accuracy but the lowest recovery, whereas resorbable PLC guides provided for the best balance between amount and accuracy of reinnervation after nerve resection.

Migration of LHRH neurons into the spinal cord: evidence for axon-dependent migration from the transplanted chick olfactory placode

In the chick embryo, luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and migrate along the olfactory nerve to the forebrain. In previous studies, we demonstrated that LHRH neurons followed the trigeminal nerve when the olfactory nerve was physically interrupted. To examine whether LHRH neurons possess the capacity to migrate along the different type of axons, the olfactory placode was transplanted into the base of the forelimb. Three to five days after the transplantation, LHRH neurons were detectable in the spinal nerve, the dorsal root ganglion, the sympathetic ganglion and the spinal cord. Double or triple labelling studies for LHRH, somatostatin and/or axonin-1 showed that LHRH neurons entered the spinal nerve in contact with the olfactory axons, which are specifically immunoreactive to somatostatin. Migrating LHRH neurons continued to associate closely with the olfactory axons in the spinal nerve. However, some LHRH neurons often migrated along with the axonin-1 positive spinal sensory axons, maintaining a distance from the olfactory axons. Furthermore, a few LHRH neurons were observed in the ventral root and the ventral funiculus independent of olfactory axons. As LHRH neurons were observed in the motor component of the spinal nerve, it is probable that LHRH neurons also invaded the spinal cord using the motor axons as a guiding substrate for their migration. These results suggest that the migration mode of LHRH neurons is axon dependent in the peripheral region, however, chemical identity with regard to axonal substrate choice for migration was not specified in the present study.

Transplants of fibroblasts genetically modified to express BDNF promote axonal regeneration from supraspinal neurons following chronic spinal cord injury

Transplants of fibroblasts genetically modified to express BDNF (Fb/BDNF) have been shown to promote regeneration of rubrospinal axons and recovery of forelimb function when placed acutely into the injured cervical spinal cord of adult rats. Here we investigated whether Fb/BDNF cells could stimulate supraspinal axon regeneration and recovery after chronic (4 week) injury. Adult female Sprague-Dawley rats received a complete unilateral hemisection injury at the third cervical spinal cord segment (C3). Four-five weeks later the injury site was exposed and rats received transplants of unmodified fibroblasts (Fb/UM) or Fb/BDNF. Four-five weeks after transplantation, locomotor recovery was examined on a test of forelimb usage and regeneration of supraspinal axons was studied following injection of the anterograde tracer biotin dextran amine (BDA). Rubrospinal tract (RST), reticulospinal tract (ReST), and vestibulospinal tract (VST) axons regenerated into transplants of either Fb/UM or Fb/BDNF but the length of axonal growth was significantly different in the two groups. The absolute distance of ReST growth was 1.8-fold greater in Fb/BDNF than in Fb/UM and the absolute distance of growth of RST and VST axons showed a statistically significant 4-fold increase. All three types of regenerated axons occupied a greater proportional length of Fb/BDNF transplants than of Fb/UM transplants. Only VST axons extended into the host spinal cord caudal to the Fb/BDNF grafts, but these axons were sparse. Rats receiving Fb/BDNF used both forelimbs together to explore walls of a cylinder more often than rats receiving Fb/UM, indicating partial recovery of forelimb usage. These results demonstrate that fibroblasts genetically modified to express BDNF promote axon regeneration from supraspinal neurons in the chronically injured spinal cord with accompanying partial recovery of locomotor performance.

Development of a bioartificial nerve graft. II. Nerve regeneration in vitro

A promising alternative for the repair of peripheral nerve injuries is the bioartificial nerve graft, or BNG, comprised of a tubular conduit preseeded with Schwann cells, which are an effective substrate for enhancing nerve regeneration. The physical properties of the conduit, porosity and wall thickness, as well as the Schwann cell seeding density, were tested for their effect on axon growth using rat dorsal root ganglia. These parameters can influence the amount of nutrients and growth factors made available to the neural tissue. Results show that a greater wall thickness and lower porosities have a detrimental effect on the growth of the axons. Over a four week period, axons extended 3.2 mm for the optimum case (DeltaR = 0.82 mm, epsilon = 0.75) compared to 1.8 and 1.6 mm for a lower porosity (0.55) and a greater wall thickness (1.4 mm), respectively. A maximum in the growth rate occurs at a porosity of 75% for Schwann cell seeded conduits but not for unseeded ones. When compared to mass transfer predictions, the results suggest that, at higher porosities, more growth factors diffuse out of the conduit, while at low porosities there is competition for nutrients. Increasing the Schwann cell seeding density enhances growth but also leads to an increase in the number of axons along the length of the conduit. This is indicative of branching of the axons, which requires additional resources to maintain and can lead to painful neuroma formation. Wall thickness and porosity were found not to have any significant effect on the axon number sprouting from the dorsal root ganglia and the mean diameter (p > 0.05). Considerations need to be made, not just on the polymer used, but also on its porosity, wall thickness, and Schwann cell seeding density. These parameters can be adjusted to create a bioartificial nerve graft that provides the optimal environment for nerve growth.

Clinical use of nerve conduits in peripheral-nerve repair: review of the literature

The use of nerve conduits has evolved from a previous experimental idea to a clinical reality over the last ten years. An overview of the literature on the clinical use of nerve conduits in peripheral-nerve repair is presented.

Spinal cord injury repair research: a new combination treatment strategy

The optimism that a cure will soon be found for paraplegia and quadriplegia is strongly founded on the series of discoveries in the last two decades which showed that adult mammalian spinal cord axons can be made to regenerate given appropriate conditions and microenvironment. But then, why no cure yet in sight? Why is the delay? Spinal cord scientists are encountering a newfound obstacle in regeneration research. While axons do regenerate up and down through a graft/transplant placed at the injury site, they fail to regenerate further on once they reach healthy cord tissue beyond the injury zone. Research from our laboratory since the 1980s found that the principal reason for this failure of long distance regeneration is that the neural circuitry these axons have to traverse through are in a well-stabilized state which is unreceptive and refractory to new growth. Successful long distance regeneration is possible only within labilized (destabilized) neural tissues. We have shown simple and reliable methods of inducing labile state in adult spinal cord neural circuitry. This is achieved by inducing polyneuronal spinal motor control in the paralyzed limb muscles. We had predicted (Krishnan, 1991, 1983) two outcomes of inductive lability in paraplegia. One is partial revival of functions in the paralyzed limbs. The second outcome addresses effective relinking of the severed cord ends. Our preliminary results from adult paraplegic frogs convince us that inductive lability in these animals is capable of generating new growth and new connections in the distal isolated cord. Locomotor rhythm and function reappeared in the hind limbs, which enabled these animals to swim and progress on surface for long periods of observation up to 120 days. Based on these results we now recommend that inductive lability should be included as an essential component in the treatment strategy for spinal cord injury repair for effective relinking of the severed cord ends.

Microglial changes accompanying the promotion of retinal ganglion cell axonal regeneration into peripheral nerve grafts

Intravitreal injection of the microglia inhibitor tuftsin 1-3 leads to an increase in retinal ganglion cell axonal regeneration into peripheral nerve grafts and a decrease in phagocytic cells in the retina. However, the relation of phagocytic cells and particularly microglia towards axonal regeneration remains unclear. Initially, to assess this, tuftsin 1-3's effect on axonal regeneration was reexamined by doing a dose-response study. Optimal doses were found to be 2.5 microg/ml and 250 microg/ml in rats and hamsters respectively. We then studied retinal phagocytic cells in rats. Microglial cells were classified as resting or activated based on their morphology following OX42 immunolabelling. In controls, most microglial cells were in the resting state. Optic nerve cut led to an increase in the total number of microglia and a ten-fold elevation in the proportion of activated cells; changes were more pronounced at the optic nerve stump. Anastomosis of an autologous segment of sciatic nerve to the stump of the freshly cut optic nerve minimized the overall increase in microglia, and combined with 2.5 microg/ml tuftsin 1-3, lead to a marked blunting of activation. Preservation within the retina of a higher proportion of resting over active form of microglia, and not the prevention of microglial proliferation per se, may be a crucial factor in allowing additional retinal ganglion cell axons to regenerate into peripheral nerve grafts.

Non-neuronal cells are not the limiting factor for the low axonal regeneration in C57BL/6J mice

Peripheral axonal regeneration was investigated in adult male mice of the C57BL/6J (C), BALB/cJ (B) and A/J (A) strains and in their F1 descendants using a predegenerated nerve transplantation model. Four types of transplants were performed: 1) isotransplants between animals of the C, B and A strains; 2) donors of the C strain and recipients of the C x B and C x A breeding; 3) donors of the B strain and recipients of the C x B breeding, and 4) donors of the A strain and recipients of the C x A breeding. Donors had the left sciatic nerve transected and two weeks later a segment of the distal stump was transplanted into the recipient. Four weeks after transplantation the regenerated nerves were used to determine the total number of regenerated myelinated fibers (TMF), diameter of myelinated fibers (FD) and myelin thickness (MT). The highest TMF values were obtained in the groups where C57BL/6J mice were the donors (C to F1 (C x B) = 4658 +/- 304; C to F1 (C x A) = 3899 +/- 198). Also, A/J grafts led to a significantly higher TMF (A to F1 (C x A) = 3933 +/- 565). Additionally, isotransplant experiments showed that when the nerve is previously degenerated, C57BL/6J mice display the largest number of myelinated fibers (C to C = 3136 +/- 287; B to B = 2759 +/- 170, and A to A = 2835 +/- 239). We also observed that when C57BL/6J was the graft donor, FD was the highest and MT did not differ significantly when compared with the other groups. These morphometric results reinforce the idea that Schwann cells and the nerve environment of C57BL/6J provide enough support to the regenerative process. In this respect, the present results support the hypothesis that the non-neuronal cells, mainly Schwann cells, present in the sciatic nerve of C57BL/6J mice are not the main limiting factor responsible for low axonal regeneration.

Regenerated synaptic terminals on a crayfish slow muscle identify with transplanted phasic or tonic axons

Phasic or tonic nerves transplanted onto a denervated slow superficial flexor muscle in adult crayfish regenerated synaptic connections that displayed large or small excitatory postsynaptic potentials (EPSPs), respectively, suggesting that the neuron specifies the type of synapse that forms (Krause et al., J Neurophysiol 80:994-997, 1998). To test the hypothesis that such neuronal specification would extend to the synaptic structure as well, we examined the regenerated synaptic terminals with thin serial section electron microscopy. There are distinct differences in structure between regenerated phasic and tonic innervation. The phasic nerve provides more profuse innervation because innervation sites occurred more frequently and contained larger numbers of synaptic terminals than the tonic nerve. Preterminal axons of the phasic nerve also had many more sprouts than those of the tonic nerve. Phasic terminals were thinner and had a lower mitochondrial volume than their tonic counterparts. Phasic synapses were half the size of tonic ones, although their active zone-dense bars were similar in length. The density of active zones was higher in the phasic compared with the tonic innervation, based on estimates of the number of dense bars per synapse, per synaptic area, and per nerve terminal volume. Because these differences mirror those seen between phasic and tonic axons in crayfish muscle in situ, we conclude that the structure of the regenerated synaptic terminals identify with their transplanted axons rather than with their target muscle. Therefore, during neuromuscular regeneration in adult crayfish, the motoneuron appears to specify the identity of synaptic connections.

Olfactory ensheathing cells: bridging the gap in spinal cord injury

Spinal cord injury (SCI) continues to be an insidious and challenging problem for scientists and clinicians. Recent neuroscientific advances have changed the pessimistic notion that axons are not capable of significant extension after transection. The challenges of recovering from SCI have been broadly divided into four areas: 1) cell survival; 2) axon regeneration (growth); 3) correct targeting by growing axons; and 4) establishment of correct and functional synaptic appositions. After acute SCI, there seems to be a therapeutic window of opportunity within which the devastating consequences of the secondary injury can be ameliorated. This is supported by several observations in which apoptotic glial cells have been identified up to 1 week after acute SCI. Moreover, autopsy studies have identified anatomically preserved but unmyelinated axons that could potentially subserve normal physiological properties. These observations suggest that therapeutic strategies after SCI can be directed into two broad modalities: 1) prevention or amelioration of the secondary injury, and 2) restorative or regenerative interventions. Intraspinal transplants have been used after SCI as a means for restoring the severed neuraxis. Fetal cell transplants and, more recently, progenitor cells have been used to restore intraspinal circuitry or to serve as relay for damaged axons. In an attempt to remyelinate anatomically preserved but physiologically disrupted axons, newer therapeutic interventions have incorporated the transplantation of myelinating cells, such as Schwann cells, oligodendrocytes, and olfactory ensheathing cells. Of these cells, the olfactory ensheathing cells have become a more favorable candidate for extensive remyelination and axonal regeneration. Olfactory ensheathing cells are found along the full length of the olfactory nerve, from the basal lamina of the epithelium to the olfactory bulb, crossing the peripheral nervous system-central nervous system junction. In vitro, these cells promote robust axonal growth, in part through cell adhesion molecules and possibly by secretion of neurotrophic growth factors that support axonal elongation and extension. In animal models of SCI, transplantation of ensheathing cells supports axonal remyelination and extensive migration throughout the length of the spinal cord. Although the specific properties of these cells that govern enhanced axon regeneration remain to be elucidated, it seems certain that they will contribute to the establishment of new horizons in SCI research.

Axons of olfactory receptor cells of transsexually grafted antennae induce development of sexually dimorphic glomeruli in Manduca sexta

The influence of olfactory receptor cell (ORC) axons from transsexually grafted antennae on the development of glomeruli in the antennal lobes (ALs), the primary olfactory centers, was studied in the moth Manduca sexta. Normally during metamorphic adult development, the pheromone-specific macroglomerular complex (MGC) forms only in the ALs of males, whereas two lateral female-specific glomeruli (LFGs) develop exclusively in females. A female AL innervated by ORC axons from a grafted male antenna developed an MGC with three glomeruli, like the MGC of a normal male AL. Conversely, a male AL innervated by ORC axons from a grafted female antenna lacked the MGC but exhibited LFGs. ORC axons from grafted male antenna terminated in the MGC-specific target area, even in cases when the antennal nerve (AN) entered the AL via an abnormal route. Within ectopic neuromas formed by ANs that had become misrouted and failed to enter the brain, male-specific axons were not organized and formed terminal branches in many areas. The results suggest the presence of guidance cues within the AL for male-specific ORC axons. Depending on the sex of the antennal innervation, glial borders formed in a pattern characteristic of the MGC or LFGs. The sex-specific number of projection neurons (PNs) in the medial group of AL neurons remained unaffected by the antennal graft, but significant changes occurred in the organization of PN arborizations. In gynandromorphic females, LFG-specific PNs extended processes into the induced MGC, whereas in gynandromorphic males, PNs became restricted to the LFGs. The results indicate that male-and female-specific ORC axons play important roles in determining the position, anatomical features, and innervation of sexually dimorphic glomeruli.

Regulation of the size and distribution of ectopic neuromuscular junctions in adult skeletal muscle by nerve-derived trophic factor and electrical muscle activity

Transplanted axons induced multiple, irregularly distributed acetylcholine receptor (AChR) aggregates on muscle fibers at early stages of ectopic neuromuscular junction formation in denervated adult rat soleus muscles. Subsequently, most AChR aggregates disappeared (the losers). A few aggregates survived (the winners) and, as part of the surviving junctions, reached a certain size and spatial separation along the fibers. This elimination of losers and development of winners occurred only in electrically active muscles whether the activity was elicited by intact axons or by electrical muscle stimulation after the axons had been cut early. We conclude that electrical muscle activity regulates the size and distribution of ectopic neuromuscular junctions by acting in conjunction with a nerve-derived priming influence that does not require the continued presence of nerve terminals.

Trophic and tropic effects of striatal astrocytes on cografted mesencephalic dopamine neurons and their axons

Astrocytes from the ventral mesencephalon and from the striatum respectively promote the dendritic and axonal arborization of dopamine (DA) neurons in vitro. To test this response in vivo, astrocytes in primary cultures from the neonatal cerebral cortex, ventral mesencephalon, or striatum were coimplanted with fetal ventral mesencephalic tissue into the intact or DA-denervated striatum of adult rats and these cografts examined after 3-6 months by tyrosine hydroxylase (TH) immunohistochemistry (intact recipients) or after 5-6 months by in vitro [3H]DA-uptake autoradiography (DA-denervated recipients). In contrast with single ventral mesencephalic grafts, all types of cograft displayed a rather uniform distribution of TH-immunoreactive perikarya. The average size of TH-immunoreactive cell bodies was not significantly different in cografts containing cortical or mesencephalic astrocytes and in single ventral mesencephalic grafts, but it was significantly larger in cografts containing striatal astrocytes. Nevertheless, the number of [3H]DA-labeled terminals in the DA-lesioned host striatum was clearly smaller with cografts of striatal astrocytes than with single mesencephalic grafts or with cografts containing cortical astrocytes. On the other hand, cografts of striatal astrocytes contained much higher numbers of [3H]DA-labeled terminals than the other types of graft or cograft. Thus, while cografted astrocytes in general influence the distribution of DA neurons within the graft, astrocytes from the neonatal striatum have a trophic effect on DA perikarya and a tropic effect on DA axons, keeping the latter within the graft.

Nerve anastomoses with human fibrin. Preliminary clinical report (56 cases)

Since 1980, 56 peripheral nerve repairs have been done with fibrin. For technical reasons, combined anastomoses have been chosen in brachial plexus repairs (23 cases), fibrin alone being used in most other cases (8 free flaps, 17 main trunks, 8 digital nerves). As a whole, results compare evenly with the so-called classical repair methods using stitches. The adhesive method's main advantage is the gain in operative time, without impairing precision. Secondary benefits, such as hemostasis and easier stabilization of small grafts, can be achieved. One question remains: what becomes of fibrin? The survey of present cases would tend to prove that axonal growth through the second anastomosis is impeded proportionally to the length of the graft. The possible action of fibrin in the alteration process leading to a sclerotic diaphragm is not elucidated to this day. Experimental as well as clinical research must be carried on, in order to improve this new way of repairing nerves.

Axonal growth within poly (2-hydroxyethyl methacrylate) sponges infiltrated with Schwann cells and implanted into the lesioned rat optic tract

Porous hydrophilic sponges made from 2-hydroxyethyl methacrylate (HEMA) have a number of possible biomedical applications. We have investigated whether these poly(HEMA) hydrogels, when coated with collagen and infiltrated in vitro with cultured Schwann cells, can be implanted into the lesioned optic tract and act as prosthetic bridges to promote axonal regeneration. Nineteen rats (20-21 days old) were given hydrogel/Schwann cell implants. No obvious toxic effects were seen, either to the transplanted glia or in the adjacent host tissue. Schwann cells survived the implantation technique and were immunopositive for the low affinity nerve growth factor receptor, S100 and laminin. Immunohistochemical studies showed that host non-neuronal cells (astrocytes, oligodendroglia and macrophages) migrated into the implanted hydrogels. Astrocytes were the most frequently observed host cell in the polymer bridges. RT97-positive axons were seen in about two thirds of the implants. The axons were closely associated with transplanted Schwann cells and, in some cases, host glia (astrocytes). Individual axons regrowing within the implanted hydrogels could be traced for up to 900 microns, showing that there was continuity in the network of channels within the polymer scaffold. Axons did not appear to be myelinated by either Schwann cells or by migrated host oligodendroglia. In three rats, anterograde tracing with WGA/HRP failed to demonstrate the presence of retinal axons within the hydrogels. The data indicate that poly(HEMA) hydrogels containing Schwann cells have the potential to provide a stable three-dimensional scaffold which is capable of supporting axonal regeneration in the damaged CNS.

Regeneration of adult dorsal root axons into transplants of dorsal or ventral half of foetal spinal cord

Several dorsal root axons regenerate into the transplants of foetal spinal cord (FSC) and form synapses there. It is unknown whether the growth is specific to transplants of dorsal half FSC, a normal target of most dorsal root axons, or whether it is due to properties shared by transplants of ventral half FSC. We used calcitonin gene-related peptide immunohistochemistry to label subsets of regenerated host dorsal root axons, and morphometric analysis to compared neuronal populations within both transplants. Adult Sprague-Dawley rats received intraspinal grafts of dorsal or ventral half FSC (E14), and the L4 or L5 dorsal root was cut and juxtaposed to the grafts. Three months later sagittal sections were prepared for immunohistochemistry and Nissl-Myelin stain. Histograms of the perikaryal area showed that the transplants of dorsal half FSC consisted of small neurons predominantly, whereas transplants of ventral half FSC consisted of neurons of variable sizes. Dorsal root axons regenerated into both transplants, but growth into dorsal half FSC was more robust. These results indicate that both transplants provide an environment that supports dorsal root regeneration, but that the environment provided by dorsal half FSC is more favorable. Transplants of dorsal half FSC may offer advantages for the long-term goal of repairing of damaged spinal cord circuits.

An aberrant retinal pathway and visual centers in Xenopus tadpoles share a common cell surface molecule, A5 antigen

A monoclonal antibody A5 (MAb-A5), which was raised against Xenopus tadpole tectal cells, recognizes a cell surface-related protein molecule (A5 antigen) expressed on the visual centers of Xenopus tadpoles (S. Takagi, T. Tsuji, T. Amagai, T. Takamatsu, and H. Fujisawa, 1987, Dev. Biol. 122, 90-100). The present immunohistochemistry using MAb-A5 indicated that, in addition to the visual centers, A5 antigen was expressed on the general somatic sensory tract in the medulla and spinal cord of Xenopus tadpoles. As the general somatic sensory tract has been shown to be a pathway for ectopically transplanted retinal axons (M. Constantine-Paton and R. R. Capranica, 1976, J. Comp. Neurol. 170, 17-32; M. J. Katz and R. J. Lasek, 1979, J. Comp. Neurol. 183, 817-832), we examined whether retinal axons transplanted close to the spinal cord or medulla preferentially grow into the A5 antigen-positive general somatic sensory tract. We performed eye transplantation at embryonic stages and detected precise locations and trajectories of transplanted retinal axons within the medulla and spinal cord in tadpoles after filling retinal axons with horseradish peroxidase (HRP). HRP histochemistry in combination with MAb-A5 immunohistochemistry indicated that almost all HRP-filled transplanted retinal axons joined the A5 antigen-positive general somatic sensory tract. These findings suggest the involvement of A5 antigen in specific cell-cell recognition between retinal axons and their targets.

Axonal regeneration from GABAergic neurons in the adult rat thalamus

Peripheral nerve grafts were inserted into the thalamus in 27 Sprague-Dawley rats. From 6 weeks to 15 months later, horseradish peroxidase (HRP) was applied to the extracranial end of each graft and sections of the brains reacted for peroxidase histochemistry. Of the thalamic neurons that were retrogradely labelled with HRP, more than 80% were located in the reticular nucleus of the thalamus (RNT), a distinct group of nerve cells that contain glutamic acid decarboxylase (GAD)-like immunoreactivity and are presumably GABAergic. By combining immunocytochemistry with HRP histochemistry, it was possible to confirm that the RNT neurons that had grown axons into the peripheral nerves grafts retained their GAD-like immunoreactivity. The apparent selectivity in their regenerative responses of RNT neurons to peripheral nerve grafts may relate to special properties of the neurons that did and did not grow into the grafts.

Long-term survival of peripheral axons that have reinnervated the spinal cord

Adult cats received grafts after the fashion of Barnes and Worall by anastomosing the central stump of a ventral L7 root to the central stump of a dorsal L6 root. After regeneration periods from 4 weeks to 316 weeks (79 months) the regenerated axons were identified by the horseradish peroxidase technique. In some animals, regenerated boutons were identified using electron microscopy. With shorter regeneration times, most of the labeled axons were seen in the white matter of the dorsal funiculus. In the longest surviving cat, labeled axons were seen in the entire medial half of the dorsal horn grey matter. Boutons derived from regenerated axons appeared typical of CNS boutons, showing none of the morphologic characteristics of the motor end plate.

Common mechanisms in vertebrate axonal navigation: retinal transplants between distantly related amphibia

Embryonic eye primordia were transplanted from the anuran, Xenopus laevis, to the urodele, Ambystoma mexicanum, in order to assess whether retinal axons of one species could grow to appropriate central targets in a distantly related species. To trace the early retinal projections, Xenopus primordia were removed, incubated for 20 min in a solution containing [3H]proline, washed, and then transplanted to host axolotl embryos. Genetically eyeless hosts were used in some of the experiments so that the Xenopus optic fibers would not be guided to their targets by the host's own optic nerve projections. Xenopus eyes were usually able to differentiate into small eyes in the axolotl host. Shortly after primary differentiation, the development of these eyes seemed to arrest. Autoradiography on paraffin sections of the central nervous system of the host revealed that in some cases Xenopus retinal ganglion cells were indeed able to send axons to the tectum of the axolotl host. This result suggests that the mechanisms of axonal navigation for this particular central projection are evolutionarily conserved.

Transplantation of central nervous tissue

The inability of the central nervous system (CNS) to regenerate after injury has long been known. However, extensive research into regeneration during the past 2 decades has demonstrated a degree of CNS plasticity that was heretofore thought impossible. Subsequently, research into CNS transplantation has documented the ability of transplanted CNS tissues to survive, develop processes, elongate, organize into anatomically correct configurations, and establish electrophysiologically and secretorly functional synaptic connections with the host tissue. This review focuses on current trends and techniques in CNS transplantation, with special emphasis on target tissue transplantation, peripheral nerve-axonal bridge transplantation, and neuronal replacement by transplantation. Although the majority of CNS transplantation research remains in the domain of the neurobiologists, a substantial amount is beginning to reach the level of practical clinical application. The capability of transplanting neurons to correct CNS abnormalities, whether due to disease, trauma, or genetic defects, has enormous neurosurgical implications.

Neurotization or nerve transfer for brachial plexus lesions

Neurotization is one of the methods available for treating avulsive lesions of the brachial plexus. It has to be inserted into a series of other procedures when all the roots are not avulsed when all of them are avulsed but represents the only chance. It can only hope to restore some simple function to replace the very complex ones which have been lost. It has limited applications but is fully justified when one is dealing with a totally paralyzed limb. Finally, it can improve the trophicity of the limb and relieve the severe pain which is present in some 40% of cases of avulsive lesions, a problem which cannot be developed here. It is necessary that the patients and the surgeons realize its limits in order to avoid disappointment.

Some metabolic responses of axotomized neurones to contact between their axons and denervated muscle

1. The nucleolar and cell body dry mass and nucleic acid content of hypoglossal neurones were measured in adult rats using interference microscopy and ultra-violet absorption microspectrography.2. The left hypoglossal nerve was transplanted into the ipsilateral sternomastoid. Seventy days later the sternomastoid was denervated by dividing the ipsilateral spinal accessory nerve. This was followed by metabolic changes in hypoglossal nerve cells.3. The changes induced in hypoglossal neurones by division of the ipsilateral accessory nerve did not occur if botulinum toxin was injected locally at the same time.4. In other rats the left hypoglossal nerve was anastomosed to the proximal stump of the ipsilateral median nerve simultaneously divided at the level of the wrist. Seventy days later this median nerve was divided in the axilla. This was followed by metabolic changes in hypoglossal nerve cells.5. These results are discussed in relation to the possible roles of reacting Schwann cells, degenerating axoplasm and denervated muscle in maintaining aspects of the metabolic response of nerve cells to injury.6. It is suggested that the synthesis of acetylcholine by an axonal ending, or its release, is dependent upon the presence of an adjacent membrane which can respond to it, and that the metabolic changes measured in the nerve cell body are secondary to this response of the axon terminal.