Node of Ranvier formation along fibres regenerating through silicone tube implants: a freeze-fracture and thin-section electron microscopic study

Normal molecular repair mechanisms in regenerative peripheral nerve interfaces allow recording of early spike activity despite immature myelination

Clinical use of neurally controlled prosthetics has advanced in recent years, but limitations still remain, including lacking fine motor control and sensory feedback. Indwelling multi-electrode arrays, cuff electrodes, and regenerative sieve electrodes have been reported to serve as peripheral neural interfaces, though long-term stability of the nerve-electrode interface has remained a formidable challenge. We recently developed a regenerative multi-electrode interface (REMI) that is able to record neural activity as early as seven days post-implantation. While this activity might represent normal neural depolarization during axonal regrowth, it can also be the result of altered nerve regeneration around the REMI. This study evaluated high-throughput expression levels of 84 genes involved in nerve injury and repair, and the histological changes that occur in parallel to this early neural activity. Animals exhibiting spike activity increased from 29% to 57% from 7 to 14 days following REMI implantation with a corresponding increase in firing rate of 113%. Two weeks after implantation, numbers of neurofilament-positive axons in the control and REMI implanted nerves were comparable, and in both cases the number of myelinated axons was low. During this time, expression levels of genes related to nerve injury and repair were similar in regenerated nerves, both in the presence or absence of the electrode array. Together, these results indicate that the early neural activity is intrinsic to the regenerating axons, and not induced by the REMI neurointerface.

An experimental study of lingual nerve repair using epineurial sutures or entubulation

The recovery of the mechanosensitive afferent fibres in the lingual branch of the trigeminal nerve has been studied using electrophysiological techniques in cats, after two methods of nerve repair. After nerve section the lingual nerve was repaired by either epineurial sutures or by entubulation. 12 weeks after either method of repair the properties of the regenerated fibres were significantly different from normal, but following repair with epineurial suture they were closer to normal than after repair by entubulation. After entubulation the fibres had a greater reduction in conduction velocity, a greater increase in force threshold, and the adaptation times were faster. The results suggest that epineurial suture should be preferred clinically.

Peripheral nerve repair using a poly(organo)phosphazene tubular prosthesis

Nerve regeneration experiments were carried out using tubular nerve guides of poly[(ethylalanato)1.4(imidazolyl)0.6phosphazene] (PEIP). By means of in vivo tests, this polymer was found to be biodegradable and transformed into harmless products. The tubular nerve guides were prepared by deposition of the dissolved polymer on a glass capillary tube, followed by evaporation of the solvent (methylene dichloride). After transectioning, rat sciatic nerve stumps were immediately sutured into the ends of 10-mm-long polymer tubes. On removal of the prosthesis, after implantation for 45 d, a tissue cable was found bridging the nerve stumps in all cases. Histological analysis revealed that the tissue cable was essentially composed of a regenerated nerve fibre bundle. A parallel series of experiments was undertaken to compare the use of silicone tubes that are not biodegradable and are most frequently used for studies of nerve regeneration with tubulization techniques. The advantages of biodegradable PEIP tubular nerve guides used for peripheral nerve repair are discussed.

Clustering of voltage-dependent sodium channels on axons depends on Schwann cell contact

In myelinated nerves, segregation of voltage-dependent sodium channels to nodes of Ranvier is crucial for saltatory conduction along axons. As sodium channels associate and colocalize with ankyrin at nodes of Ranvier, one possibility is that sodium channels are recruited and immobilized at axonal sites which are specified by the subaxolemmal cytoskeleton, independent of glial cell contact. Alternatively, segregation of channels at distinct sites along the axon may depend on glial cell contact. To resolve this question, we have examined the distribution of sodium channels, ankyrin and spectrin in myelination-competent cocultures of sensory neurons and Schwann cells by immunofluorescence, using sodium channel-, ankyrin- and spectrin-specific antibodies. In the absence of Schwann cells, sodium channels, ankyrin and spectrin are homogeneously distributed on sensory axons. When Schwann cells are introduced into these cultures, the distribution of sodium channels dramatically changes so that channel clusters on axons are abundant, but ankyrin and spectrin remain homogeneously distributed. Addition of latex beads or Schwann cell membranes does not induce channel clustering. Our results suggest that segregation of sodium channels on axons is highly dependent on interactions with active Schwann cells and that continuing axon-glial interactions are necessary to organize and maintain channel distribution during differentiation of myelinated axons.

Growth factor expression in normal and diabetic rats during peripheral nerve regeneration through silicone tubes

The silicone tube model of regeneration has proved to be an invaluable tool for experimental studies aimed at understanding expression of growth factors during normal and abnormal metabolic states of regeneration. Since the morphological parameters of nerve growth and myelination are well-defined and easily identified in this model, the expression of both diffusible and intracellular-acting growth factors can be readily correlated with the occurrence of these cellular events. These studies facilitate the study of the cellular and molecular events that accompany regeneration. Further, because the sciatic nerve can be traced up to its corresponding neurons, growth factor gene expression can also be studied by in situ hybridization and Northern blotting techniques. This is particularly important in defining the cell source of extracellularly released growth factors. Finally, and most importantly, the regeneration process in the normal or diseased metabolic state (such as diabetes) can be manipulated via the administration of adjuncts to the tube that either promote or inhibit regeneration. Further studies in this regard, and in the identification of growth factors involved and their role during regeneration should shed some light on the pathogenesis and possible means of mitigating or reversing diabetic neuropathy.

Ultrastructure of afferent axon endings in a neuroma

Injured sensory axons with endings trapped in a nerve-end neuroma become a source of abnormal impulse discharge and neuropathic pain. We have examined the ultrastructure of such endings anterogradely transported WGA-HRP and freeze-fracture replication, with emphasis on the postinjury period during which the abnormal neural discharge is maximal. Most axons ended in a terminal swelling, depleted of myelin but surrounded by Schwann cell processes. These 'neuroma endbulbs' were richly packed with membrane-bound organelles, and had a smoothly undulating surface with (in neuromas of several weeks standing) a moderate number of short filopodia. Massive sprouting did not occur until several months postinjury. Both p- and e-faces of endbulb axolemma had larger intramembranous particles, on average, than corresponding internodal membrane of control axons. This change, interpreted as indicating remodelling of axolemmal channel (and perhaps receptor) content, may be related to the abnormal electrical behavior of neuroma afferents.

Increased expression of pp60c-src protein-tyrosine kinase during peripheral nerve regeneration

Since little is known about the intracellular changes that take place in response to Schwann cell-neuron interactions that occur during neurite outgrowth and myelination, we investigated the expression of a protein-tyrosine kinase, pp60c-src, during peripheral nerve regeneration through a silicone tube. Segments of regenerated nerve, extracted at various times following nerve-transection, showed an induction of in vitro c-src kinase activity as measured by autophosphorylation of immunoprecipitated pp60c-src. This activity occurred at 7 days following nerve transection coincident with the onset of neurite outgrowth in vivo. This kinase activity, which peaked out between 21 and 35 days and decreased thereafter, appeared to be associated with axonal growth and myelination, but not mitogenesis in the tube. Analysis of c-src proteins levels by Western blot showed a similar expression profile as that of the kinase activity. Qualitatively, the expression of an immunoreactive c-src band, migrating slightly slower than pp60, was detected in extracts of regenerating nerve segments as well as in the corresponding L4 and L5 dorsal root ganglia. This protein may be the CNS neuronal-specific form (pp60+) of the c-src protein. In situ hybridization revealed that Schwann cells and sensory and motor neurons associated with the regenerated sciatic nerve were positive for c-src mRNA during regeneration possibly accounting for the increased src protein expression during regeneration. Since the increased expression of pp60c-src in regenerated nerve segments coincides with both axonal sprouting and myelination, our findings suggest that the c-src protein may play a role in Schwann cell-neuron interactions which facilitate the occurrence of these events during regeneration. In addition, although pp60+ is generally not detectable in the mature PNS, our findings show that this protein may be induced during conditions of PNS differentiation which promote neurite outgrowth.

Differential effects of nerve, muscle, and fat tissue on regenerating nerve fibers in vivo

Axonal regeneration through silicone tubes was studied using distal nerve stumps, denervated, preatrophied muscle tissue, as well as fat tissue as a target. During the first stage of regeneration, i.e., within 2-3 weeks after surgery, a thin, filamentous structure consisting of fibrin and connective tissue was seen bridging the gap in all systems. Thereafter, this cord obviously served as a guideline for the outgrowth of increasing numbers of axons into distal nerve stumps as well as into muscle tissue, but not into fat tissue. These findings confirm that preatrophied muscle tissue has a similar "neurotrophic" effect on regenerating nerve fibers as distal nerve stumps. The ineffectivity of fat tissue in promoting nerve fiber regeneration could be attributed either to the absence of "neurotrophic factors" or even to an inhibitory effect.

Freeze fracture analysis of the axolemma of cultured dorsal root ganglion neurons in the absence of Schwann cells

The distribution of intramembranous particles within the axolemma of cultured dorsal root ganglion neurons was determined by freeze-fracture microscopy. Utilizing culture conditions which eliminate Schwann cells, the particle distribution of the P-face, 735 +/- 119 microns2, and E-face, 100 +/- 39 microns2 resembled that of pre- and non-myelinated axons in vivo and no node-like E-face particle patching was seen. These results indicate that cultured neurite development is similar to that seen in vivo and that axons maintained in a glial-free environment do not develop nodal, E-face membrane specializations.

Extracellular fluid conditioned during peripheral nerve regeneration stimulates Schwann cell adhesion, migration and proliferation

Schwann cell movement and proliferation occur during peripheral nerve regeneration and remyelination. We asked whether soluble factors promoting these activities were present in fluid surrounding rat sciatic nerves regenerating across a 10-mm gap bridged by a silicone tube. In this model, regenerated and remyelinated axons extend across the gap by 28 days following nerve transection and tube implantation. Fluid conditioned by cells participating in nerve regeneration (RCF) was assayed for its ability to promote Schwann cell adhesion, migration and proliferation in vitro. RCFs collected at post-transectional days 1-28 were equally effective in promoting Schwann cell-substratum adhesion. In contrast, the motility-promoting activity of RCF was minimal at 1-2 days following nerve-transection, peaked at 7 days and remained elevated through 21 days. The RCF peak response was 87-fold greater than control. Schwann cell proliferative activity of RCF exhibited peaks of activity at 1 and 14 days post-transection. The biological potency of this fluid for each activity assayed in vitro correlated well with the behavior of Schwann cells chronicled during nerve repair in vivo. These findings suggest that soluble factors promoting Schwann cell adhesion, migration, and proliferation accumulate extracellularly during peripheral nerve regeneration and remyelination.

Ultrastructural and morphometric analysis of long-term peripheral nerve regeneration through silicone tubes

Light and electron microscopy were used to investigate long-term regeneration in peripheral nerves regenerating across a 10 mm gap through silicone tubes. Schwann cells and axons co-migrated behind an advancing front of fibroblasts, bridging the 10 mm gap between 28 and 35 days following nerve transection. Myelination of regenerated fibres started between 14 and 21 days after transection and occurred in a manner similar to that reported during development. Although these early events were successful in producing morphologically normal-appearing regenerated fibres, complete maturation of many of these fibres was never achieved. Axonal distortion by neurofilaments, axonal degeneration and secondary demyelination were seen at 56 days following nerve transection. These changes progressed in severity with time as more axons advanced through the distal stump towards their peripheral target. Since regeneration occurs in the absence of endoneurial tubes, and because constrictive forces act on the nerve during regeneration, we suggest that these extrinsic factors limit the successful advancement of axons through the distal stump to their target organ.

Freeze-fracture studies of reactive myelinated nerve fibres after diffuse axonal injury

We have studied the axonal and myelin sheath response in diffuse axonal injury after angular acceleration using the freeze-fracture and thin section techniques. It was found that the glial-axonal junction was intact until 1 h after injury. But upon loss of the nodal axolemma specialisations, after 3 to 4 h, the dimeric particles of the glial-axonal junction (GAJ) were lost and, by 6 h, the myelin lamellae became separated from the axonal remnant. There was a correlated loss of glial membrane specialisations of the GAJ during this separation. In the internodal region a suggestion of membrane damage occurred after 20 min but discrete myelin dislocations (particle-free areas) were not found until 1-h survival and were extensive by 6 h. Areas of loosely organised myelin occurred between intact axons at 7-28 days after injury. No evidence for growth cone formation was obtained.