Response of axons and glia at the site of anastomosis between the optic nerve and cellular or acellular sciatic nerve grafts

Summary Axonal and glial reactions at the site of optic nerve section and at the junctional zone between optic nerve and normal or acellular peripheral nerve grafts have been studied. Following optic nerve section, no axons grew into the distal optic nerve stump. Similarly, no axons invaded the acellular peripheral nerve grafts, although in both instances fibres did regenerate into the junctional zone and a few remained there at least until 30 days post lesion (dpl, the duration of the experiments). Axons regenerated into normal peripheral nerve grafts by 3–5 dpl and by 10 dpl large numbers had penetrated deeply into the grafts. The glial response to injury appeared similar in both groups of grafted animals. Astrocytes and oligodendrocytes grew out into the junctional zone over the 5–7 day period and invaded the margins of the cellular grafts by 10 dpl. They did not penetrate the acellular nerves or distal optic nerve stumps. We were unable to determine whether Schwann cells invaded the junctional zone from the normal peripheral nerve grafts. Schwann cells are both GFAP⁺ and Vim⁺, especially when reacting after injury, and Lam^– when not associated with axons: it is therefore possible that Schwann cells from the cellular grafts contributed to the population of GFAP⁺, Vim⁺ cells in the junctional zone of the cellular grafts. Anti-laminin immuno-reactivity persisted in the basal lamina tubes of both the normal and acellular peripheral nerve grafts. Thus, the failure of axon regeneration into acellular peripheral nerve grafts can be correlated with the absence of Schwann cells and does not appear to be related to the presence of laminin.     

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.     

Laminin in traumatized peripheral nerve: Basement membrane changes during degeneration and regeneration

Summary The changes in Schwann cell basement membrane associated with degeneration and regeneration during 50 weeks after transection of rat sciatic nerve were studied immunohistochemically with antibodies to laminin. In half of the animals, regeneration was prevented by suturing the nerve stumps aside, whereas in the rest spontaneous regeneration was allowed. Axonal regeneration was monitored with anti-neurofilament protein antibodies.In control nerves, basement membranes surrounding Schwann cells were visualized as circular, laminin-positive structures within the endoneurium. By 8 weeks after transection, Schwann cells had formed columns which were laminin-positive throughout their cross-sectional area and indistinguishable from basement membrane zones in both non-regenerating and regenerating nerves. As axons repopulated the distal stump, the normal shape of Schwann cell basement membrane tubes was slowly restored in freely regenerating nerves. In non-regenerating nerves, however, a striking atrophy of Schwann cell columns was observed. Regenerating axons were only seen inside laminin-positive tubular structures in all phases after 8 weeks in regenerating nerves. On the other hand, restoration of normal shape in laminin-positive basement membrane zones was coincident with appearance of axons in the distal stump, but it did not take place in chronically degenerating nerves.The results show that chronic degeneration leads to an atrophy of Schwann cell columns and results in a decrease in laminin immunoreactivity associated with them.     

An alternative preparation of the acellular muscle graft for reconstruction of the injured nerve--morphological and morphometric analysis.

The application of cutaneous nerve grafts is accompanied by some disadvantages, including insufficient graft material for the reconstruction of a large mixed nerve. Recently, evacuated muscle autografts have been suggested as possible alternatives to cutaneous nerve grafts. In the present paper we have demonstrated a possible preparation of the evacuated muscle graft using an infiltration of Marcaine. The reinnervation of the distal stump of the rat median nerve was evaluated by morphological and morphometric analysis after application of the muscle acellular grafts prepared by three methods: an ordinary freeze-thawed muscle graft, a Marcaine evacuated muscle graft and a Marcaine treated graft with subsequent freezing and thawing. A comparison of the numbers and diameters of the myelinated axons in the distal nerve stumps revealed very similar conditions for axon regrowth and maturation in Marcaine evacuated and freeze-thawed muscle grafts. The best results with myelinated axon numbers and spectrum of their calibres were obtained when the Marcaine treated graft was repeatedly frozen and thawed. The pre-treatment of the muscle graft by Marcaine prevents it from shrinking and fragmenting, the main disadvantage during freeze-thawing of fresh muscle. The present results demonstrate that infiltration of striated muscles with a myotoxic compound, e.g. Marcaine, with subsequent freezing-thawing is the method of choice for the preparation of an acellular muscle graft used in peripheral nerve reconnection in the experimental model.     

An alternative preparation of the acellular muscle graft for reconstruction of the injured nerve — morphological and morphometric analysis

The application of cutaneous nerve grafts is accompanied by some disadvantages, including insufficient graft material for the reconstruction of a large mixed nerve. Recently, evacuated muscle autografts have been suggested as possible alternatives to cutaneous nerve grafts. In the present paper we have demonstrated a possible preparation of the evacuated muscle graft using an infiltration of Marcaine. The reinnervation of the distal stump of the rat median nerve was evaluated by morphological and morphometric analysis after application of the muscle acellular grafts prepared by three methods: an ordinary freeze-thawed muscle graft, a Marcaine evacuated muscle graft and a Marcaine treated graft with subsequent freezing and thawing. A comparison of the numbers and diameters of the myelinated axons in the distal nerve stumps revealed very similar conditions for axon regrowth and maturation in Marcaine evacuated and freeze-thawed muscle grafts. The best results with myelinated axon numbers and spectrum of their calibres were obtained when the Marcaine treated graft was repeatedly frozen and thawed. The pre-treatment of the muscle graft by Marcaine prevents it from shrinking and fragmenting, the main disadvantage during freeze-thawing of fresh muscle. The present results demonstrate that infiltration of striated muscles with a myotoxic compound, e.g. Marcaine, with subsequent freezing-thawing is the method of choice for the preparation of an acellular muscle graft used in peripheral nerve reconnection in the experimental model.     

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

Summary 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.     

Bridging extra large defects of peripheral nerves: possibilities and limitations of alternative biological grafts from acellular muscle and Schwann cells

Defects of peripheral nerves are bridged with autologous nerve grafts. Tissue-engineered nerve grafts offer a laboratory-based alternative to overcome limited donor nerve availability. Our objective was to evaluate whether a graft made from acellular muscle enriched with cultivated Schwann cells can bridge extra large gaps where conventional conduits usually fail. Our well-established rat sciatic nerve model was used with an increased gap length of 50 mm. The conduits consisted of freeze-thawed or chemically extracted homologous acellular rat rectus muscles and implanted Schwann cells. Autologous nerve grafts were used for control purposes. Biocompatibility of the grafts was demonstrated by Schwann cell settlement, revascularization, and macrophage recruitment. After 12 weeks regeneration was assessed clinically, histologically, and morphometrically. The control group showed superior results regarding axon counts, histologic appearance, and functional recovery compared with the muscle grafts. The chemically extracted conduits completely failed to support nerve regeneration. They were not stable enough to bridge longer nerve gaps with an expanded regeneration time. On the basis of morphological parameters freeze-thawed muscle grafts were, however, able to support peripheral nerve regeneration even over the extralong distance of 50 mm, and therefore are of potential benefit for new therapeutic strategies.     

Morphologic and functional evaluation of peripheral nerve fibers regenerated through polyimide sieve electrodes over long-term implantation

We evaluated by morphologic and functional analysis the regeneration of peripheral nerve fibers through polyimide regenerative-type electrodes over long-term implantation. Polyimide sieve electrodes were placed in silicone chambers and implanted between the severed ends of the sciatic nerve in rats. The sieve part had 281 round via holes of 40 microm in diameter, with nine integrated recording-stimulating electrodes arranged around the via holes. The degree of axonal regeneration was examined at 2, 7, and 12 months postimplantation (mpi). Regeneration was successful in 12 of the 13 animals implanted. Reinnervation of distal muscle and nerves increased with time, reaching a plateau about 7 mpi. The number of myelinated fibers increased from 2 to 7 months, at which time it was similar to control values. With time the myelinated fibers matured, with significant increases in axon diameter and myelin thickness. Only 0.6% of the regenerated axons showed evidence of compression near the implanted electrode. The majority of the myelinated fibers that crossed the via holes and had been regenerated through the distal nerve had a normal appearance. Sieve electrodes were useful for nerve stimulation at postimplantation. Stimulation through different active electrodes excited nerve bundles, evoking compound muscle action potentials of varying shape and amplitude, indicative of selective axonal stimulation.     

Highly permeable polylactide-caprolactone nerve guides enhance peripheral nerve regeneration through long gaps.

We compared regeneration and functional reinnervation after sciatic nerve resection and tubulization repair with bioresorbable guides of poly(L-lactide-co-epsilon-caprolactone) (PLC) and permanent guides of polysulfone (POS) with different degrees of permeability, leaving a 6 mm gap in different groups of mice. Functional reinnervation was assessed to determine recovery of motor, sensory and sweating functions in the hindpaw during four months postoperation. Highly permeable PLC guides allowed for faster and higher levels of reinnervation for the four functions tested than impermeable or low-permeable PLC guides, while semipermeable 30 and 100 kDa POS tubes yielded very low levels of reinnervation. The regeneration success rate was higher with PLC than with POS tubes. Morphometrical analysis of cross-sectional nerves under light microscopy showed the highest number of regenerated myelinated fibers at mid tube and distal nerve in high-permeable PLC guides. Impermeable PLC guides allowed slightly worse levels of regeneration, while low-permeable PLC guides promoted neuroma and limited distal regeneration. The lowest number of regenerated fibers were found in POS tubes. In summary, highly permeable bioresorbable PLC guides offer a suitable alternative for repairing long gaps in injured nerves, approaching the success of autologous nerve grafts.     

Reinnervation of fast and slow mammalian muscles by a superfluous number of motor axons

In this study peripheral nerves from flexor digitorum longus, (alien nerve) as well as the deep branch of the muscle's own lateral popliteal nerve were cut and connected to the distal stump of the lateral popliteal nerve. Extensor digitorum longus and tibialis anterior muscles then became reinnervated to a similar extent by either nerve, showing no preference for its own nerve. A significant proportion of the endplates in these muscles remained permanently supplied by more than one axon, and a proportion of the muscle fibres was supplied by both nerves. No ectopic endplates were formed on fast muscle fibres. The same two nerves were also connected to the slow soleus muscle and this muscle became preferentially reinnervated by the nerve to flexor digitorum longus. In contrast to fast muscles, endplates of soleus muscle fibres were only rarely contacted by more than one axon, and ectopic endplates were often found in this muscle. In both types of muscles that had an excess of motor nerves, extensive sprouting persisted for many months. Thus, identical motor nerves induce different patterns of innervation in slow and fast muscles, and muscle fibres do not show a preference for their own nerve.     

Optic nerve regeneration after intravitreal peripheral nerve implants: trajectories of axons regrowing through the optic chiasm into the optic tracts

We have studied axon regeneration through the optic chiasm of adult rats 30 days after prechiasmatic intracranial optic nerve crush and serial intravitreal sciatic nerve grafting on day 0 and 14 post-lesion. The experiments comprised three groups of treated rats and three groups of controls. All treated animals received intravitreal grafts either into the left eye after both left sided (unilateral) and bilateral optic nerve transection, or into both eyes after bilateral optic nerve transection. Control eyes were all sham grafted on day 0 and 14 post-lesion, and the optic nerves either unlesioned, or crushed unilaterally or bilaterally. No regeneration through the chiasm was seen in any of the lesioned control optic nerves. In all experimental groups, large numbers of axons regenerated across the optic nerve lesions ipsilateral to the grafted eyes, traversed the short distal segment of the optic nerve and invaded the chiasm without deflection. Regeneration was correlated with the absence of the mesodermal components in the scar. In all cases, axon regrowth through the chiasm appeared to establish a major crossed and a minor uncrossed projection into both optic tracts, with some aberrant growth into the contralateral optic nerve. Axons preferentially regenerated within the degenerating trajectories from their own eye, through fragmented myelin and axonal debris, and reactive astrocytes, oligodendrocytes, microglia and macrophages. In bilaterally lesioned animals, no regeneration was detected in the optic nerve of the unimplanted eye. Although astrocytes became reactive and their processes proliferated, the architecture of their intrafascicular processes was little perturbed after optic nerve transection within either the distal optic nerve segment or the chiasm. The re-establishment of a comparatively normal pattern of passage through the chiasm by regenerating axons in the adult might therefore be organised by this relatively immutable scaffold of astrocyte processes. Binocular interactions between regenerating axons from both nerves (after bilateral optic nerve transection and intravitreal grafting), and between regenerating axons and the intact transchiasmatic projections from the unlesioned eye (after unilateral optic nerve lesions and after ipsilateral grafting) may not be important in establishing the divergent trajectories, since regenerating axons behave similarly in the presence and absence of an intact projection from the other eye.     

Regeneration and remyelination of Xenopus tadpole optic nerve fibres following transection or crush

Optic nerves of stage 54-56 Xenopus laevis tadpoles were either transected or crushed, and subsequent Wallerian degeneration, regeneration, and remyelination were examined. After 4 days, normal myelinated fibres were no longer present in the distal stump, and only a few unmyelinated fibres remained. After 10-13 days, the distal nerve consisted mainly of a core of reactive astrocytes with enlarged processes and scattered oligodendrocytes which persisted throughout the degenerative period. Regenerating axons traversed the site of the lesion and extended into the distal stump within 13-15 days. As regeneration progressed, astrocytic processes extended radially from the optic nerve's central cellular core and formed longitudinal compartments for regenerating axons. Between 15-19 days, a few regenerating fibres were remyelinated and by 35 days, more axons were surrounded either by thin collars of oligodendrocyte cytoplasm or by 1-3 spiral turns of myelin membrane. By 95 days, the number of myelinated fibres had increased to about 50% of those present in control nerves. Their myelin sheaths were normal in appearance and thickness relative to their respective axon diameters. The largest axons were surrounded by compact sheaths with 4-9 lamellae.     

移植壳聚糖导管修复大鼠坐骨神经缺损

目的:研究移植结合了碱性成纤维细胞生长因子(bFGF)的壳聚糖导管促进周围神经损伤再生的情况。方法:成年Wistar大鼠造成10mm坐骨神经缺损后,以壳聚糖导管(移植组,10只)作桥梁桥接神经两断端,以假手术组和单纯损伤组(造成10mm坐骨神经缺损后,不加以任何干预措施)各10只分别为阳性和阴性对照,术后通过肉眼观察和神经微丝(NF)、乙酰胆碱酯酶(AChE)免疫组织化学染色方法对损伤神经局部及远端靶肌肉运动终板的再生情况进行观察。结果:移植组大鼠术后3个月,新生的神经纤维已越过缺损部位并到达损伤远端。免疫组织化学染色显示:术后3个月,移植组大鼠坐骨神经缺损处再生组织内可观察到均匀、密集分布的NF免疫阳性纤维,且在腓肠肌终板区内可见AChE免疫阳性终末,至5个月时阳性染色明显增强。在术后3个月时可见新生的运动终板,但轮廓不规则、边界不清晰;而在5个月时,新生的运动终板的形态与密度均接近假手术组水平。结论:结合了bFGF的壳聚糖导管对缺损的坐骨神经修复具有良好的桥梁作用和促进神经生长及终板再生的作用。     

Peripheral nerve regeneration using acellular nerve grafts

Long gap peripheral nerve injuries usually require a graft to facilitate axonal regeneration into the distal nerve stump. The use of autografts is often limited because of graft availability and donor-site morbidity. We investigated whether acellular nerve allografts would provide an appropriate channel for the promotion and induction of sciatic nerve regeneration in rats. Axons sprouted from the proximal portion and reached the distal portion in the 1 cm-long grafts by 1 month. The number of axons in the regenerated nerves was similar to that of normal nerves at 1 month. Loading the grafts with betaNGF and VEGF increased the number and mean diameter of axons and neovascularization in the regenerated nerves at 1 month. The motor conduction velocity increased over time and reached 63 +/- 10% of that of normal nerves at 6 months. The nerve injuries treated with the acellular grafts had a significant improvement in motor, nociception, and proprioception function compared to untreated nerves. The results from this study suggest that acellular nerve allografts may be a useful biomaterial for functional peripheral nerve regeneration.     

Peripheral nerve explants grafted into the vitreous body of the eye promote the regeneration of retinal ganglion cell axons severed in the optic nerve

Summary We have conducted experiments in the adult rat visual system to assess the relative importance of an absence of trophic factors versus the presence of putative growth inhibitory molecules for the failure of regeneration of CNS axons after injury. The experiments comprised three groups of animals in which all optic nerves were crushed intra-orbitally: an optic nerve crush group had a sham implant-operation on the eye; the other two groups had peripheral nerve tissue introduced into the vitreous body; in an acellular peripheral nerve group, a frozen/thawed teased sciatic nerve segment was grafted, and in a cellular peripheral nerve group, a predegenerate teased segment of sciatic nerve was implanted. The rats were left for 20 days and their optic nerves and retinae prepared for immunohistochemical examination of both the reaction to injury of axons and glia in the nerve and also the viability of Schwann cells in the grafts. Anterograde axon tracing with rhodamine-B provided unequivocal qualitative evidence of regeneration in each group, and retrograde HRP tracing gave a measure of the numbers of axons growing across the lesion by counting HRP filled retinal ganglion cells in retinal whole mounts after HRP injection into the optic nerve distal to the lesion. No fibres crossed the lesion in the optic nerve crush group and dense scar tissue was formed in the wound site. GAP-43-positive and rhodamine-B filled axons in the acellular peripheral nerve and cellular peripheral nerve groups traversed the lesion and grew distally. There were greater numbers of regenerating fibres in the cellular peripheral nerve compared to the acellular peripheral nerve group. In the former, 0.6–10% of the retinal ganglion cell population regenerated axons at least 3–4 mm into the distal segment. In both the acellular peripheral nerve and cellular peripheral nerve groups, no basal lamina was deposited in the wound. Thus, although astrocyte processes were stacked around the lesion edge, a glia limitans was not formed. These observations suggest that regenerating fibres may interfere with scarring. Viable Schwann cells were found in the vitreal grafts in the cellular peripheral nerve group only, supporting the proposition that Schwann cell derived trophic molecules secreted into the vitreous stimulated retinal ganglion cell axon growth in the severed optic nerve. The regenerative response of acellular peripheral nerve-transplanted animals was probably promoted by residual amounts of these molecules present in the transplants after freezing and thawing. In the optic nerves of all groups the astrocyte, microglia and macrophage reactions were similar. Moreover, oligodendrocytes and myelin debris were also uniformly distributed throughout all nerves. Our results suggest either that none of the above elements inhibit CNS regeneration after perineuronal neurotrophin delivery, or that the latter, in addition to mobilising and maintaining regeneration, also down regulates the expression of axonal growth cone-located receptors, which normally mediate growth arrest by engaging putative growth inhibitory molecules of the CNS neuropil.     

Changes in the extracellular matrix components fibronectin and laminin during immune rejection of skeletal muscle

Extracellular matrix is known to play an important role during development and maintenance of various tissues. In the present study, changes in two extracellular matrix glycoproteins, fibronectin and laminin, were investigated in skeletal muscle undergoing immune rejection. Purified antibodies against fibronectin and laminin were used to analyze the matrix by indirect immunofluorescence at various intervals after transplantation of extensor digitorum longus muscle in rats. Fibronectin and laminin were localized in the pericellular basement membrane zone of the normal myofibers; however, the cytoplasm was devoid of both glycoproteins. Transplanted muscle grafts underwent a process of degeneration and then an initial regeneration during the first 7 days. This regeneration effort ceased with the onset of muscle rejection in 14-day transplants. At this time, fibronectin was seen in the cytoplasmic region as well as the extracellular matrix of myofibers and myotubes. At later time intervals, an increased intensity of staining for fibronectin was seen throughout the rejected muscle. In muscle grafts undergoing regeneration but not rejection (i.e., nonantigenic grafts), such an increase in the presence of fibronectin was not seen ( Gulati et al., 1982). The distribution of laminin did not change during the rejection process and was localized in the basement membrane zone of myofibers and myotubes, although the overall configuration of the basement membranes was deformed and collapsed. It appears that the basement membranes are resistant to degradation, and staining for laminin persists in rejected muscle. These results show marked changes in the extracellular matrix of muscle undergoing rejection. The appearance of fibronectin during the initial stages of muscle rejection may have a causal relationship to the process of immune defence mechanism; however, the exact role of fibronectin remains elusive.     

Bioactive poly(L-lactic acid) conduits seeded with Schwann cells for peripheral nerve regeneration.

This study attempted to enhance the efficacy of peripheral nerve regeneration using our previously tested poly(L-lactic acid) (PLLA) conduits by incorporating them with allogeneic Schwann cells (SCs). The SCs were harvested, cultured to obtain confluent monolayers and two concentrations (1 x 10(4) and 1 x 10(6) SC/ml) were combined with a collagen matrix (Vitrogen) and injected into the PLLA conduits. The conduits were then implanted into a 12 mm right sciatic nerve defect in rats. Three control groups were used: isografts, PLLA conduits filled with collagen alone and empty silicone tubes. The sciatic functional index (SFI) was calculated monthly through four months. At the end of second and fourth months, the gastrocnemius muscle was harvested and weighed for comparison and the graft conduit and distal nerve were harvested for histomorphologic analysis. The mean SFI demonstrated no group differences from isograft control. By four months, there was no significant difference in gastrocnemius muscle weight between the experimental groups compared to isograft controls. At four months, the distal nerve demonstrated a statistically lower number of axons mm2 for the high and low SC density groups and collagen control. The nerve fiber density was significantly lower in all of the groups compared to isograft controls by four months. The development of a "bioactive" nerve conduit using tissue engineering to replace autogenous nerve grafts offers a potential approach to improved patient care. Although equivalent nerve regeneration to autografts was not achieved, this study provides promising results for further investigation.     

Reinnervation of transplanted Pacinian corpuscles by ventral root axons: Ultrastructure of the regenerated nerve terminals

Summary This study addresses two questions. Can mature, denervated and transplanted Pacinian corpuscles accept innervation from motor axons? If so, does the alien target influence the structural characteristics of the regenerated motor axon terminals? Pacinian corpuscles from the hind leg of young rats, together with a segment of the nerve branch through which they receive their sensory innervation, were autotransplanted to the surface of the spinal cord and the nerve stump anastomosed to the central stump of a transected lumbar ventral root. Between 4 and 5 months later the grafts were studied by electron microscopy. Ventral root axons regenerated through the endoneurial tubes of the grafted nerve to reach the corpuscles, most of which became reinnervated by one to three myelinated fibres. The fibres lost their myelin sheaths before entering the inner core, branched, and gave rise to multiple terminals in the inner core. The regenerated terminals were packed with spherical synaptic vesicles and closely resembled normal motor nerve terminals. Thus motor axons are able to reinnervate Pacinian corpuscles but the structural characteristics of the terminals are apparently not modified by the alien target tissue. This finding contrasts with previous studies, in which it was found that terminals of the central axons of large dorsal root ganglion cells, induced to reinnervate Pacinian corpuscles, displayed the structural characteristics of peripheral sensory endings rather than those of dorsal root terminals in the spinal cord.     

Nerve injury, axonal degeneration and neural regeneration: basic insights

Axotomy or crush of a peripheral nerve leads to degeneration of the distal nerve stump referred to as Wallerian degeneration (WD). During WD a microenvironment is created that allows successful regrowth of nerve fibres from the proximal nerve segment. Schwann cells respond to loss of axons by extrusion of their myelin sheaths, downregulation of myelin genes, dedifferentiation and proliferation. They finally aline in tubes (Büngner bands) and express surface molecules that guide regenerating fibres. Hematogenous macrophages are rapidly recruited to the distal stump and remove the vast majority of myelin debris. Molecular changes in the distal stump include upregulation of neurotrophins, neural cell adhesion molecules, cytokines and other soluble factors and their corresponding receptors. Axonal injury not only induces muscle weakness and loss of sensation but also leads to adaptive responses and neuropathic pain. Regrowth of nerve fibres occurs with high specificity with formerly motor fibres preferentially reinnervating muscle. This involves recognition molecules of the L2/HNK-1 family. Nerve regeneration occurs at a rate of 3-4 mm/day after crush and 2-3 mm/day after sectioning a nerve. Nerve regeneration can be fostered pharmacologically. Upon reestablishment of axonal contact Schwann cells remyelinate nerve sprouts and downregulate surface molecules characteristic for precursor/premyelinating or nonmyelinating Schwann cells. At present it is unclear whether axonal regeneration after nerve injury is impeded in neuropathies.     

The neuropathology of DFP at cat soleus neuromuscular junction

Summary The fine structure of the cat soleus neuromuscular junction was studied following a single intra-arterial injection of di-isopropylfluorophosphate (DFP) into the right femoral artery. DFP induced separate subacute and delayed morphologic changes in soleus non-myelinated motor nerve terminals. Three days after DFP administration motor nerve terminals were reduced in number. Subacute DFP damage was also noted in the subneural apparatus and in the immediate subjacent muscle. Both pre- and post-junctional subacute changes were resolved two weeks post-DFP. One week following this initial regeneration, soleus motor nerve terminals underwent a delayed transient degeneration, followed by reinnervation of damaged endplates 6–8 weeks following DFP.Quantitative analysis of methylene blue-stained intramuscular nerves indicated that both subacutely and chronically denervated soleus muscle fibres were reinnervated by regeneration of the original motor axon. Reinnervation by means of collateral sprouting was insignificant. This mechanism of reinnervation and the rapidity with which it occurred suggests that both subacute and delayed soleus motor nerve damage is initiated from local actions of DFP on the non-myelinated terminal. The subacute reaction probably results from a direct cytotoxic action of DFP at pre- and post-junctional sites. The delayed nerve terminal degeneration may also stem from an acute effect not immediately detrimental to nerve function.