THE DEPOSITION OF COLLAGEN IN RELATION TO SCHWANN CELL BASEMENT MEMBRANE DURING PERIPHERAL NERVE REGENERATION

Proteomic analysis of peripheral nerve myelin during murine aging

Aging of the peripheral nervous system (PNS) is associated with structural and functional changes that lead to a reduction in regenerative capacity and the development of age-related peripheral neuropathy. Myelin is central to maintaining physiological peripheral nerve function and differences in myelin maintenance, degradation, formation and clearance have been suggested to contribute to age-related PNS changes. Recent proteomic studies have elucidated the complex composition of the total myelin proteome in health and its changes in neuropathy models. However, changes in the myelin proteome of peripheral nerves during aging have not been investigated. Here we show that the proteomes of myelin fractions isolated from young and old nerves show only subtle changes. In particular, we found that the three most abundant peripheral myelin proteins (MPZ, MBP, and PRX) do not change in old myelin fractions. We also show a tendency for high-abundance myelin proteins other than these three to be downregulated, with only a small number of ribosome-related proteins significantly downregulated and extracellular matrix proteins such as collagens upregulated. In addition, we illustrate that the peripheral nerve myelin proteome reported in this study is suitable for assessing myelin degradation and renewal during peripheral nerve degeneration and regeneration. Our results suggest that the peripheral nerve myelin proteome is relatively stable and undergoes only subtle changes in composition during mouse aging. We proffer the resultant dataset as a resource and starting point for future studies aimed at investigating peripheral nerve myelin during aging. Said datasets are available in the PRIDE archive under the identifier PXD040719 (aging myelin proteome) and PXD041026 (sciatic nerve injury proteome).

Three-layer microfibrous peripheral nerve guide conduit composed of elastin-laminin mimetic artificial protein and poly(L-lactic acid)

We developed a microfibrous poly(L-lactic acid) (PLLA) nerve conduit with a three-layered structure to simultaneously enhance nerve regeneration and prevent adhesion of surrounding tissue. The inner layer was composed of PLLA microfiber containing 25% elastin-laminin mimetic protein (AG73-(VPGIG)30) that promotes neurite outgrowth. The thickest middle layer was constructed of pure PLLA microfibers that impart the large mechanical strength to the conduit. A 10% poly(ethylene glycol) was added to the outer layer to prevent the adhesion with the surrounding tissue. The AG73-(VPGIG)30 compositing of an elastin-like repetitive sequence (VPGIG)30 and a laminin-derived sequence (RKRLQVQLSIRT: AG73) was biosynthesized using Escherichia coli. The PLLA microfibrous conduits were fabricated using an electrospinning procedure. AG73-(VPGIG)30 was successfully mixed in the PLLA microfibers, and the PLLA/AG73-(VPGIG)30 microfibers were stable under physiological conditions. The PLLA/AG73-(VPGIG)30 microfibers enhanced adhesion and neurite outgrowth of PC12 cells. The electrospun microfibrous conduit with a three-layered structure was implanted for bridging a 2.0-cm gap in the tibial nerve of a rabbit. Two months after implantation, no adhesion of surrounding tissue was observed, and the action potential was slightly improved in the nerve conduit with the PLLA/AG73-(VPGIG)30 inner layer.

Colony-stimulating factor-1 mediates macrophage-related neural damage in a model for Charcot-Marie-Tooth disease type 1X

Previous studies in our laboratory have shown that in models for three distinct forms of the inherited and incurable nerve disorder, Charcot-Marie-Tooth neuropathy, low-grade inflammation implicating phagocytosing macrophages mediates demyelination and perturbation of axons. In the present study, we focus on colony-stimulating factor-1, a cytokine implicated in macrophage differentiation, activation and proliferation and fostering neural damage in a model for Charcot-Marie-Tooth neuropathy 1B. By crossbreeding a model for the X-linked form of Charcot-Marie-Tooth neuropathy with osteopetrotic mice, a spontaneous null mutant for colony-stimulating factor-1, we demonstrate a robust and persistent amelioration of demyelination and axon perturbation. Furthermore, functionally important domains of the peripheral nervous system, such as juxtaparanodes and presynaptic terminals, were preserved in the absence of colony-stimulating factor-1-dependent macrophage activation. As opposed to other Schwann cell-derived cytokines, colony-stimulating factor-1 is expressed by endoneurial fibroblasts, as revealed by in situ hybridization, immunocytochemistry and detection of β-galactosidase expression driven by the colony-stimulating factor-1 promoter. By both light and electron microscopic studies, we detected extended cell-cell contacts between the colony-stimulating factor-1-expressing fibroblasts and endoneurial macrophages as a putative prerequisite for the effective and constant activation of macrophages by fibroblasts in the chronically diseased nerve. Interestingly, in human biopsies from patients with Charcot-Marie-Tooth type 1, we also found frequent cell-cell contacts between macrophages and endoneurial fibroblasts and identified the latter as main source for colony-stimulating factor-1. Therefore, our study provides strong evidence for a similarly pathogenic role of colony-stimulating factor-1 in genetically mediated demyelination in mice and Charcot-Marie-Tooth type 1 disease in humans. Thus, colony-stimulating factor-1 or its cognate receptor are promising target molecules for treating the detrimental, low-grade inflammation of several inherited neuropathies in humans.

Nerve fibroblast impact on Schwann cell behavior

In order to reveal non-neuronal cell interactions after peripheral nerve lesions, we began to analyze the impact of sciatic nerve fibroblasts on Schwann cells in vitro. Both cell types are considered to have opposite effects on axonal regeneration. Few data are available on how repulsive nerve fibroblasts affect neuritotrophic Schwann cells and thus might indirectly influence axonal regrowth. Using different culture systems in conjunction with time-lapse video recording, metabolic labeling, pharmacological intervention, RNAi knockdown, Western blotting and RT-PCR analysis, we found that nerve fibroblasts differentially modify the various responses of Schwann cells. In the presence of collagen type IV and heparan sulfate proteoglycan but not of laminin, diffusible fibroblast factors slow down Schwann cell proliferation. In contrast, fibroblast factors increase the migratory activity of Schwann cells without being chemoattractive. One pro-migratory fibroblast factor turned out to be neuregulin. The pro-migratory activity of nerve fibroblasts and of recombinant neuregulin-1beta1 can be counteracted by neuregulin-specific pharmacological intervention and by neuregulin RNA interference. We show for the first time that nerve fibroblasts play antagonistic and agonistic roles for Schwann cells in a context-dependent manner. The data shed light on cellular mechanisms and have implications for some neuro-tissue engineering strategies.

Comparison of matrix metalloproteinase expression during Wallerian degeneration in the central and peripheral nervous systems

The matrix metalloproteinases (MMPs) are a large family of zinc-dependent enzymes which are able to degrade the protein components of the extracellular matrix. They can be placed into subgroups based on structural similarities and substrate specificity. Aberrant expression of these destructive enzymes has been implicated in the pathogenesis of immune-mediated neuroinflammatory disorders. In this study we investigate the involvement of MMPs, from each subgroup, in Wallerian degeneration in both the central and peripheral nervous systems. Wallerian degeneration describes the process initiated by transection of a nerve fibre and entails the degradation and removal of the axon and myelin from the distal stump. A similar degenerative process occurs as the final shared pathway contributing to most common neuropathies. MMP expression and localisation in the peripheral nervous system are compared with events in the CNS during Wallerian degeneration. Within 3 days after axotomy in the peripheral nervous system, MMP-9, MMP-7 and MMP-12 are elevated. These MMPs are produced by Schwann cells, endothelial cells and macrophages. The temporospatial expression of activated MMP-9 correlates with breakdown of the blood-nerve barrier. In the CNS, 1 week after optic nerve crush, four MMPs are induced and primarily localised to astrocytes, not microglia or oligodendrocytes. In the degenerating optic nerve, examined at later time points (4, 8, 12 and 18 weeks), MMP expression was down-regulated. The absence of MMPs in oligodendrocytes and mononuclear phagocytes during Wallerian degeneration may contribute to the slower removal of myelin debris observed in the CNS. The low level of the inactive pro-form of MMP-9 in the degenerating optic nerve may explain why the blood-brain barrier remains intact, while the blood-nerve barrier is rapidly broken down. We conclude that the difference in the level of expression, activation state and cellular distribution of MMPs may contribute to the different sequence of events observed during Wallerian degeneration in the peripheral compared to the CNS.

Mutual attraction between emigrant cells from transected denervated nerve

It is known that regenerating axons emerging from the proximal stump of a transected nerve are attracted towards the distal stump. It is not certain whether this neurotropic effect is on the axons themselves or whether it is on supporting cells such as Schwann cells that the axons then follow. In order to investigate this question in rats, segments of the sciatic nerve were either isolated or removed and reinserted as grafts, and then sutured into the opposing ends of double-Y silicone tubes. In these tubes, a central conduit was formed by connecting the centrally facing limb of each Y tube. The nerve segments were sutured into one of the limbs at either end. The third limbs of the Y tubes formed side arms, one of which was left open; a plug of mobilised fatty connective tissue was sutured into the other. A gap of 6 mm was left between the cut ends and the fat pads (or openings from the side arms). After 2-3 wk a significantly greater outgrowth (P < 0.001) was found to link the nerve segments than to invade the side arms. The major cell component in the outgrowth was Schwann cells, supported by fibroblasts and capillaries and surrounded by a lamellated layer of flattened fibroblasts. The growth into the side arms had a looser cellular architecture and contained considerably fewer Schwann cells. The results strongly suggest the existence of mutual attraction between emigrant Schwann cells, or possibly endoneurial fibroblasts, from the 2 cut ends of transected nerves. This conclusion has implications for the guidance of axons across gaps in nerves. It does not exclude an additional neurotropic effect from the distal stump on axons.

Different conduits in peripheral nerve surgery

A considerable amount of research is being undertaken regarding the possibility of bridging loss of nerve substance with different guiding tubes, in order to improve functional outcome, reduce the surgical time, and reduce damage at donor nerve sites. A review of the literature and personal research allows us to state that: for short gaps, biological tubes (autologous veins) may give good results and also allow chemotactic attraction with selective arrangements of motor and sensory axons. Gaps longer than 1 cm do not allow tropism and are associated with failure to support axonal regrowth. Artificial biodegradable conduits still show results that are controversial; they may give good results provided that the material of which they are made is perfectly tolerated. Empty tubes, longer than 8-10 mm, besides being deprived of the chemotactic attraction, may collapse or be partially reabsorbed and replaced by scar. Probably in the near future biological or biodegradable tubes, containing laminin-like substances or muscle scaffold, will allow us to bridge increasingly large defects in nerves.

Repair of mixed peripheral nerves using muscle autografts: a preliminary communication

Twelve patients were reviewed following the reconstruction of mixed motor/sensory nerves using freeze-thawed skeletal muscle autografts. A total of 13 nerves were reconstructed-6 median and 7 ulnar. All the repairs were secondary procedures, with a mean time from injury to grafting of 17 months. Sensory and motor evaluation was carried out and the patients asked to complete a pain questionnaire. Five patients achieved an S3+ level of sensory recovery. Two underwent revision of their muscle grafts to nerve grafts. Motor recovery was universally poor.

Bridging nerve defects with combined skeletal muscle and vein conduits

The use of vein or muscle grafts to bridge nerve defects longer than 1-1.5 cm gives poor results. Veins collapse and in muscle grafts axons may regrow outside the graft. We used veins (to guide regeneration) filled with muscle (to avoid vein collapse). Nerve regeneration through 1 and 2 cm grafts made of vein plus muscle was compared with similarly long traditional nerve grafts, free fresh muscle grafts, and empty vein grafts. Regeneration was assessed clinically and histologically (qualitative and quantitative evaluation) in the graft and distal nerve stumps. Vein plus muscle grafts were superior to vein and fresh muscle grafts both functionally and histologically. Functional results were similar to those found in traditional nerve grafts, but axon number was superior in the veins filled with muscle. This suggests that vein filled with muscle might serve as a grafting conduit for the repair of peripheral nerve injuries and could give better results than traditional nerve grafting.

Effects of advancing age on peripheral nerve regeneration

Following axotomy, the regrowth of peripheral axons takes longer in older individuals than in young ones. The present study compares the crush-induced process of degeneration and regeneration in the buccal branch of the facial motor nerve in groups of rats aged 3 months and 15 months. Observations are based on qualitative and quantitative analyses of the nerve 20 mm from the site of injury in rats 1, 2, 4, 16, 21, 28, and 56 days after crush. The buccal branch is purely motor and contains a unimodal population of about 1,600 axons commonly in a single fascicle. During the first 28 days post crush (dpc) in the 3-month animals, the progression of myelin and axon degeneration, myelin clearance, regrowth of axon sprouts, and axon maturation are relatively synchronized and uniform. In the older rats, the degeneration of myelin and axons, myelin clearance, and the appearance of axon sprouts at the site of sample are all delayed. In the younger animals, axon sprouts increase in numbers from their first appearance at 4 dpc through the 2 weeks examined following the restoration of whisking behavior. The numbers of regenerating older axons increase at a rate comparable to that in the younger animals through the time that bilaterally symmetrical whisking behavior is evident, but afterwards the number of axon sprouts decreases. At 2 months after crush the young animals have 30% more fibers in the buccal branch than control nerves, while the older animals have fewer than control numbers. In the 3-month regenerated nerve, 2 months post crush, 30% of the regenerated fibers are of very small caliber, less than 3 microns2 in cross sectional area, and typically these small axons have unusually thick myelin sheaths; the older nerves do not have such a skewed distribution of axon areas. The older regenerated axons at 2 months post crush have an unusually high density of microtubules compared to the younger regenerated ones (and controls), and the ratio of neurofilaments to microtubules is very low. The conclusions are that motor neurons in older animals regenerate damaged axons after a delay not apparent in the young; the strong regenerative response apparent initially in animals of both age groups is not maintained in the older animals; and the relationship between the numerical density of cytoskeletal elements and the axon cross-sectional area deviates from normal in the regenerated axons of the older animals.

Role of macrophages in peripheral nerve degeneration and repair

A cut or crush injury to a peripheral nerve results in the degeneration of that portion of the axon isolated from the cell body. The rapid degeneration of this distal segment was for many years believed to be a process intrinsic to the nerve. It was believed that Schwann cells both phagocytosed degenerating axons and myelin sheaths and also provided growth factors to promote regeneration of the damaged axons. In recent years, it has become apparent that the degenerating distal segment is invaded by monocytes from the blood. We will review the evidence that these recruited macrophages play a role in both degeneration and regeneration of peripheral nerve axons after injury and consider whether the slow degeneration and poor monocyte recruitment in the central nervous system may contribute to the poor regeneration there.

Allogeneic nerve grafts in the rat, with special reference to the role of Schwann cell basal laminae in nerve regeneration

The role of basal laminae as conduits for regenerating axons in an allogeneic graft was examined by transplanting a 3 cm long segment of the sciatic nerve from the Brown Norway to the Fischer 344 strain of rat. These strains are not histocompatible with each other. In order to compare the nerve regeneration in variously treated grafts, three different types of graft were employed: non-treated (NT), predenervated (PD), and predenervated plus freeze-treated (PDC) grafts. The cytology of nerve regeneration through these grafts was examined by electron microscopy at four, seven, 14, 30 and 60 days after grafting. In the PDC graft, in which Schwann cells were dead on grafting, basal laminae were well preserved in the form of tubes after Schwann cells and myelin sheaths had been removed at seven days after grafting. Regenerating axons accompanied by immature host Schwann cells grew out through such basal lamina tubes in the same fashion as observed in our previous studies. By day 14, axons extended as far as the middle of the graft. In the proximal part they were separated into individual fibres and even thinly myelinated by Schwann cells. On the other hand, in the NT and PD grafts in which Schwann cells were alive on grafting, most Schwann cells and myelin sheaths appeared to undergo autolytic degeneration by day 14, while Schwann cell basal laminae were left almost intact in the form of tubes. A few regenerating axons were seen associated with Schwann cells in the proximal portion by day seven. It is probable that host Schwann cells moved into the graft after donor cells had been degraded. Schwann cell basal laminae tended to be damaged at the site of extensive lymphoid cell infiltration. By day 30, regenerating axons had arrived at the distal end of the graft in all three types of graft: in the PDC graft thick axons were fully myelinated, whereas in the PD graft they were only occasionally myelinated and in the NT graft most axons were still surrounded by common Schwann cells. By 60 days after grafting, regenerating axons were well myelinated in the host nerve as observed 1 cm distal to the apposition site in all the three types of graft. These findings show that Schwann cell basal laminae can serve as pathways (most efficiently in the PDC graft) for regenerating axons in a 3 cm long allograft in the rat.

Diabetic neuropathy: abnormalities of Schwann cell and perineurial basal laminae. Implications for diabetic vasculopathy

During Wallerian degeneration, the Schwann cell basal laminal ensheathment around myelinated nerve fibres remains after the removal of myelin and axonal debris, forming a corrugated tube within which Schwann cell proliferation takes place. In nerve biopsies from patients with diabetic neuropathy, such residual basal laminal tubes tend to be circular rather than corrugated and appear to be more persistent during regeneration; this suggests increased rigidity and durability. These changes could be the result of increased cross-linkage of type IV collagen or alterations to other components of the basal lamina. A similar mechanism may be responsible for the thickening of perineurial basal laminae and the reduplication of basal laminae around endoneurial capillaries in diabetic patients; such reduplication may lead to reduced compliance of the vessel walls and impaired vascular perfusion.

Radial and tibial nerve pathology of two lactating ewes with "kangaroo gait"

Radial and tibial nerves of two ewes with clinical signs of chronic "kangaroo gait" were examined by qualitative and quantitative techniques and compared to the same nerves of a clinically normal ewe in late lactation. In affected ewes, there was extensive axonal degeneration of myelinated fibres in the radial nerve. Large and small myelinated fibres were affected equally and unmyelinated fibres were normal. Nerve fibre regeneration was present. In contrast, tibial nerve changes in the "kangaroo gait" ewes were minimal. The chronic nature of the radial nerve pathology was consistent with the clinical time course of "kangaroo gait". Regeneration may account for gradual improvement with eventual recovery in most chronically affected ewes. An episode of bilateral severe compression of a proximal radial nerve site is proposed as an explanation for the neuropathy, although the specific mechanism of this trauma is not known.

Post-traumatic endoneurial neovascularization and nerve regeneration: a morphometric study

Neovascularization would be expected to play an important role in regeneration after nerve injury, but its mechanism is poorly understood. Quantitative investigations of endoneurial capillaries and myelinated fibers 5 and 15 mm distal to different types of nerve injury have therefore been performed. This study demonstrated that numbers of endoneurial capillaries were significantly increased at the 5 mm level 2, 4, 6 and 8 weeks after crush, transection and ischemic lesions, but not following permanent axotomy. Late neovascularization associated with delayed nerve regeneration was found following nerve ischemia. These results suggest that neovascularization following nerve injury is dependent on two variables, the degree of nerve regeneration and the severity of ischemia. Axonal outgrowth appears to be an important determinant of post-traumatic new capillary formation, while nerve ischemia causes both delayed neovascularization and nerve regeneration.

Schwann cell responses during regeneration after one or more crush injuries to myelinated nerve fibres

The responses of Schwann cells during regeneration of myelinated nerve fibres were studied ultrastructurally in the distal segment of mouse phrenic nerve after a single or repeated localized crush injury. Chronological observations on nerves after a single crush confirmed the occurrence of myelination of only single regenerating axons among many that appeared in individual Büngner bands. The redundant axon sprouts often showed the structural features of degeneration and decreased in number with time. During the process, supernumerary Schwann cells not related to myelin formation were produced. They commonly failed not only to make a one-to-one relationship with an axon, but they also failed to acquire a new basal lamina of their own. With time, they showed shrinkage of their cytoplasm and became arranged circumferentially around the myelinating axon with unipolar or bipolar cytoplasmic processes. Electron microscopic, quantitative assessment of the nuclear population of Schwann cells following repeated crushes up to four times, clearly indicated a progressive and predominant increase in the number of the supernumerary Schwann cells with the number of crushes. Also, they were found to form separate concentric cytoplasmic lamellae around the myelinating axons, developing structures resembling onion-bulbs. It was concluded that essentially the same regenerating process as that observed after a single crush was repeated following re-crush, thereby resulting in the successive accumulation of supernumerary Schwann cells around a myelinating axon.

Schwann cells and collagen synthesis in taxol-treated nerve crush. An electron microscopic study

The effect of nerve crush on collagen synthesis in rat sciatic nerve was studied by electron microscope. The crushed nerves were treated with taxol which is known to increase the amount of cytoplasmic microtubules at the expense of other cell organelles such as rough endoplasmic reticulum and Golgi complexes. The results were compared to those seen in crushed nerves without taxol treatment. After the injury the amount of collagen fibrils increased at the site of the trauma in both groups when compared to intact controls. Thin (30 mn in diameter) collagen fibrils were often arranged closely to the Schwann cell surface and were connected to deep invaginations in areas where the basal lamina had lost its typical integrity. This was concluded to indicate a probable site of collagen secretion and it provides further evidence that an adult injured nerve Schwann cell is capable of synthesizing fibrous collagen. In taxol-treated nerves additional, abnormally close connection between thin microfibrils of about 10 nm and thin 20-30 nm collagen fibrils appeared in an end-to-end fashion. The microfibrils showed occasional collagenous transverse band like structures. The rough endoplasmic reticulum and Golgi complex play an important role in the posttranslational modifications of the procollagen molecule. Taxol-induced degeneration of cell organelles such as the Golgi complex, which is also essential in the secretion of proteins may thus lead to defective maturation of collagen and may explain partly the altered collagen fibril formation.

Long-term endoneurial changes after nerve transection

Long-term endoneurial changes in the distal stump of transected rat sciatic nerve were examined from 8 to 50 weeks after nerve transection. The morphological alterations were followed both in nerves which were allowed to regenerate and in nerves in which regeneration was prevented by suturing. The nerves prevented from regenerating showed markedly atrophied Schwann cell columns after 20 weeks and a disappearance of some Schwann cell columns after 30 weeks. The surrounding endoneurial fibroblast-like cells gradually lost their delicate cytoplasmic extensions and formed rough fascicles around numerous shrunken Schwann cell columns or around areas from which Schwann cells had apparently disappeared. Inside the fascicles, the Schwann cell loss was replaced by collagen fibrils or occasionally, by a dense accumulation of microfibrils. The loss of endoneurial cytoplasmic processes continued up to 50 weeks, leaving behind patches of thin fibrils around numerous shrunken Schwann cell columns or around collagenous areas where Schwann cells were lost. The endoneurial matrix showed presence of thin 25- to 30-nm collagen fibrils close to shrunken Schwann cell columns up to 50 weeks but in areas with advanced degeneration a shift towards regular 50- to 60-nm collagen fibrils occurred. The degenerated areas resembled those described in Renaut bodies and neurofibromas. Despite suturing of transected nerves to prevent sprouting, occasional regenerating sprouts were noted in the Schwann cell columns. These axons were surrounded in a sheath-like fashion by pre-existing endoneurial cell fascicles covered by a basal lamina. In the reinnervating nerves the endoneurial space gradually lost its compartmentized structures consisting of collagen fibrils and endoneurial fibroblast-like cells. After 20 weeks the endoneurial cells were inconspicuous and the extracellular matrix consisted mainly of 50- to 60-nm collagen fibrils. During axonal growth and maturation, Schwann cells containing unmyelinated axons surrounded large, myelinated axons in a collar-like fashion. Close to these collars of Schwann cells, thin 25- to 30-nm collagen fibrils were noted in focal areas, even after 50 weeks. Occasionally, numerous clusters of regenerating axonal sprouts were noted in the perineurium. These were surrounded by multiple layers of cells possessing basal lamina. The present results show that after nerve transection the distal stump of the severed nerve shows dynamic changes in the endoneurial space, especially in nerves where reinnervation is prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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.

Collagens in neurofibromas and neurofibroma cell cultures

Neurofibromas contain approximately 30-50% collagen of their lipid-free dry weight, which is about half of the value of skin but approximately twice that described for peripheral nerve endoneurium. Immunohistochemical stainings indicate that neurofibromas contain types I, III, IV, and V collagens and fibronectin. Most of the neurofibroma cells are type IV collagen and S-100 protein positive, which provides immunohistochemical evidence that neurofibromas are mostly composed of Schwann cell-like cells. The proteoglycan/collagen ratio is 4 to 10 times higher in the neurofibromas than in the surrounding dermal tissue. This would explain the typical soft consistency of the neurofibromas and may contribute to a favorable milieu for tumor growth. Pure fibroblastic cell cultures are obtained from neurofibromas after repeated passages. The cultured cells synthesized type I and III collagens and fibronectin, indicating that these cells are important in the production of the fibrous connective tissue proteins in neurofibromas.

The dependence of nerve regeneration through muscle grafts in the rat on the availability and orientation of basement membrane

Nerve regeneration through grafts of basement membrane matrix, prepared by freezing of autogenous muscle followed by thawing in distilled water, was investigated in Sprague-Dawley rats. Electrophysiological evidence of recovery in distal nerve was observed at 51 days after implantation of treated grafts whose basement membrane tubes were coaxial with the proximal and distal ends of the transected sciatic nerve. This correlated with histological findings of well-developed myelinated nerve fibres within both grafts and distal nerve. However, whereas normal axon numbers were achieved in the grafts by 3 months, the regenerating nerve in these muscle grafts took 6 months to 1 year to recover normal axon diameter and myelination. Recovery was delayed through grafts whose basement membrane tubes were at right angles to the nerve fibres and through grafts of untreated muscle coaxially aligned. It is concluded that successful repopulation of the distal stump and functional recovery can follow nerve regeneration through treated muscle autografts. The rate of regeneration is dependent on the availability of empty basement membrane tubes. If these are unavailable or inappropriately orientated, regeneration can still occur but is significantly delayed.

Proximal and distal changes in collagen content of peripheral nerve that follow transection and crush lesions

Collagen content of rat sciatic nerve was measured 10 weeks after either nerve transection or nerve crush. Nerve transection led to a significant increase in fascicular collagen in nerve segments 2.5 mm proximal and distal to the injury site. Remote from the transection, fascicular collagen was also significantly increased, this effect being most marked distally. Nerve crush by comparison resulted in only a small increase in fascicular collagen, significantly less than after transection. The greater amount of fascicular collagen far distal to the nerve injury could relate to a predominantly caudal endoneurial flow of inflammatory or growth factors. Differences in the amount of fascicular collagen formed after nerve transection compared with nerve crush are clearly due to factors other than axonal degeneration, and may relate to collagen synthesis by denervated Schwann cells or to the severity of the nerve injury.

Axonal regeneration through a peripheral nerve implanted into a brain cavity

A cavity was prepared in the rat parietal cortex by suction, filled with gel foam and left for 3 weeks during which time it became highly vascularised. Into this 3-week-old capillary bed a 5 mm length of autologous common peroneal nerve was implanted. Animals were killed at various time intervals up to 7 months after implantation of the nerve segment. The ultrastructural features of the vascular bed before and after implantation of the nerve segment were compared. In the absence of a peripheral nerve implant no axons were found within the cavity. However, at 5 weeks after implantation numerous axon-like profiles and capillaries containing fenestrations were observed within the implant. Eight weeks after implantation of the peripheral nerve both myelinated and non-myelinated axons were observed within the implant and in the surrounding capillary bed. No obvious increase in the number of axons was observed with increasing time periods. To investigate the origin of the axons within the vascular bed and/or implant the fluorochrome true blue was injected into the cavity 7 months after implantation of the nerve. Three days later selected areas of the brain, the trigeminal, superior cervical and otic ganglia were examined for retrogradely labelled fluorescent cells. Labelled cells were found adjacent to the cavity and in the ipsilateral trigeminal and superior cervical ganglia. The significance of these results in relation to the enhancement of axonal regeneration from the damaged central nervous system (CNS) is discussed.

Regeneration of the perineurium across a surgically induced gap in a nerve encased in a plastic tube

Sciatic nerves of mice were cut and the early regenerative stages were studied after the stumps had been encased within plastic tubes and kept separate by a gap of 5 mm. Only isolated cells were seen inside the tube after 7 days; after 12 days active regeneration and myelination were seen proximally; more distally, cells with long processes formed large spaces filled with collagen and less numerous Schwann cells. Zonulae occludentes and segments of basal lamina became more evident at a later stage. One month after the operation an almost complete regeneration of the nerve had taken place and perineurial cells were lined by a continuous basal lamina. The regeneration of the perineurium seemed to take place from fibroblasts; their cytoplasm as well as that of Schwann cells contained fibrillary material at this stage, sometimes in relation to segments of basal lamina. The results of this study indicate that both types of cells take part in the formation of endoneurial structures and that the early arrangement of fibroblasts contributes to the orderly longitudinal alignment of collagen fibrils.

An ultrastructural study of the non-neuronal cells in the cardiac ganglia of the monkey (Macaca fascicularis) following unilateral vagotomy

Non-neuronal cells in the intracardiac ganglia of the monkey (Macaca fascicularis) showed rapid responses to neuronal damage following unilateral vagotomy. Most of the satellite cells appeared normal at one day after operation. However, from three to seven days after operation they were transformed into phagocytic cells, thereby engulfing the transneuronally affected dendrites of the neurons. By seven days after operation, most of the affected dendrites from the neuronal soma were engulfed by the satellite cells. Few macrophages were seen amongst the neurons of the intracardiac ganglia at one day. In monkeys at three to seven days after operation there was a massive infiltration of macrophages into the vicinity of the affected neurons. Several of these macrophages were filled with numerous phagosomes containing partially digested debris. Macrophages also accumulated perivascularly and some intravascular monocytes lay in conformations suggesting that they were undergoing diapedesis. Therefore, perivascular macrophages are probably derived from the circulating monocytes, though the possibility of them being transformed fibroblasts should not be excluded. Schwann cells appeared to be the main scavengers in removing the degenerating myelinated and unmyelinated axons in the intracardiac ganglia after unilateral vagotomy.

An Experimental Study of the Effects of Pulsed Electromagnetic Field (Diapulse) on Nerve Repair

This study investigates the effects of a pulsed electromagnetic field (PEMF) (Diapulse) on experimentally divided and sutured common peroneal nerves in rats. Evidence is presented to show that PEMF accelerates recovery of use of the injured limb and enhances regeneration of damaged nerves.

Schwann cell basal lamina and nerve regeneration

Nerve segments approximately 7 mm long were excised from the predegenerated sciatic nerves of mice, and treated 5 times by repetitive freezing and thawing to kill the Schwann cells. Such treated nerve segments were grafted into the original places so as to be in contact with the proximal stumps. The animals were sacrificed 1, 2, 3, 5, 7 and 10 days after the grafting. The grafts were examined by electron microscopy in the middle part of the graft, i.e. 3-4 mm distal to the proximal end and/or near the proximal and distal ends of the graft. In other instances, the predegenerated nerve segments were minced with a razor blade after repetitive freezing and thawing. Such minced nerves were placed in contact with the proximal stumps of the same nerves. The animals were sacrificed 10 days after the grafting. Within 1-2 days after grafting, the dead Schwann cells had disintegrated into fragments. They were then gradually phagocytosed by macrophages. The basal laminae of Schwann cells, which were not attacked by macrophages, remained as empty tubes (basal lamina scaffolds). In the grafts we examined, no Schwann cells survived the freezing and thawing process. The regenerating axons always grew out through such basal lamina scaffolds, being in contact with the inner surface of the basal lamina (i.e. the side originally facing the Schwann cell plasma membrane). No axons were found outside of the scaffolds. One to two days after grafting, the regenerating axons were not associated with Schwann cells, but after 5-7 days they were accompanied by Schwann cells which were presumed to be migrating along axons from the proximal stumps. Ten days after grafting, proliferating Schwann cells observed in the middle part of the grafts had begun to sort out axons. In the grafts of minced nerves, the fragmented basal laminae of the Schwann cells re-arranged themselves into thicker strands or small aggregations of basal laminae. The regenerating axons, without exception, attached to one side of such modified basal laminae. Collagen fibrils were in contact with the other side, indicating that these modified basal laminae had the same polarity in terms of cell attachment as seen in the ordinary basal laminae of the scaffolds.(ABSTRACT TRUNCATED AT 400 WORDS)

A fine-structural study of the termination of intrafusal muscle fibres in the Chinese hamster

The termination of intrafusal muscle fibres was studied in muscle spindles of the tenuissimus muscle of the Chinese hamster. Most nuclear chain fibres terminated within the spindle capsule. The nuclear bag- and nuclear chain fibres that extended beyond the limit of the spindle capsule terminated in the endomysial connective tissue of the adjacent extrafusal fibres. Three types of intracapsular terminations were found: (1) termination on the neighbouring nuclear bag fibre, (2) termination in the periaxial space without any obvious attachment site, or (3) termination on the spindle capsule. The intrafusal fibres were tapered and longitudinal sarcolemmal invaginations were poorly developed or were completely lacking. The fibre ends were characterized by their folded basal lamina, which appeared to be mechanically reinforced by associated collagen fibrils and elastic fibres.

Production of laminin and fibronectin by Schwannoma cells: cell-protein interactions in vitro and protein localization in peripheral nerve in vivo

We studied a rat Schwannoma cell line (RN22F) to determine if it produced the basement membrane glycoproteins laminin and fibronectin, and how it interacted with these proteins in vitro. We used antisera to laminin and fibronectin for immunoprecipitation experiments and immunocytochemical localization at the electron microscope level. Polyacrylamide gels of antilaminin immunoprecipitates of conditioned medium and solubilized Schwannoma cells contained bands of reduced Mr 200,000 and 150,000. Antilaminin immunoprecipitates of conditioned medium contained nonreduced bands of 850,000 daltons and 150,000, and immunoprecipitates of solubilized cells contained nonreduced bands of 850,000, 400,000, 200,000, and 150,000 daltons. Antifibronectin immunoprecipitates of conditioned medium contained a reduced band of 220,000 daltons, and nonreduced bands of 440,000 and 220,000 daltons. Radio-labeled protein was not detected in antifibronectin immunoprecipitates of solubilized cells. By immunocytochemistry, laminin was found along the cell surface in a continuous band, whereas fibronectin was only sparsely distributed along the cell surface. In cell adhesion assays, Schwannoma cells bound preferentially to laminin-coated substrates as compared to fibronectin or noncoated substrates. A number of Schwannoma cells displayed a curved and elongated morphology on laminin substrates, as compared to a uniformly spread morphology on fibronectin, and a round, nonspread morphology on noncoated substrates. Immunofluorescent staining showed laminin in the endoneurium and perineurium and fibronectin predominantly in the perineurium of mouse sciatic nerve in vivo. The production of laminin and fibronectin by Schwann cells may be important in the development and myelination of peripheral nerves, and the proper regeneration of axons following nerve injury.

[Fine structure of intrafusal and extrafusal nerve segments of the rat muscle spindle after sciatic nerve transsection]

Intrafusal and extrafusal nerve segments in muscle spindles from lumbricalis muscles of the hindpaw of the rat were studied by electron microscopy from 10 h to 5 days after severance of the sciatic nerve. In several spindles examined, nerve fascicles piercing the spindle capsule in the equatorial region contained a large myelinated, a smaller myelinated, and an unmyelinated fiber. Unmyelinated fibers were not present in small fascicles leading to the polar region. The changes in the extrafusal nerve segments followed the pattern of Wallerian degeneration. Intra-axonal glycogen deposits were prominent in sensory fibers. The unmyelinated fibers were the earliest to degenerate, the large myelinated ones the latest. Differences between motor and sensory fiber degeneration emerged in their preterminal intrafusal segments and were analogous to those of the nerve endings. Terminal nerve fibers in the spindle equator succumbed to attack of mesenchymal cells, leaving extensive basement membrane reduplications around myelin debris-laden Schwann cells, while polar fibers were engulfed by Schwann cell processes, leaving regular bands of Büngner.

Peripheral neuropathy in the course of progressive systemic sclerosis: light and ultrastructural study

We present the case of a woman with progressive systemic sclerosis (PSS) in whom the usual symptoms were preceded by a rapidly progressive peripheral neuropathy. Few cases of peripheral nerve involvement have been described. For the first time we report an ultrastructural study of an affected peripheral nerve and muscle. In the sural nerve we found an almost complete loss of myelinated fibers. Schwann cells showed an abnormal hyperplasia of their basal membranes and structural signs of denervation. Spindle-shaped banded structures were seen in the cytoplasm of Schwann cells and in the endoneurium. On the basis of these ultrastructural data some hypotheses on the pathogenetic mechanism of this neuropathy are discussed.

Reinnervation of denervated parasympathetic neurones in cardiac ganglia from Rana pipiens

1. The sequence of events during reinnervation of the cardiac ganglion in the frog following interruption of the vagosympathetic nerve supply was studied with both electrophysiological and morphological techniques. 2. When cardiac ganglia were denervated by crushing the vagosympathetic nerve supply to the heart all synaptic endings on parasympathetic ganglion cells degenerated. Vacated post-synaptic densities were detected on denervated neurones for periods of at least 7 weeks. 3. The earliest signs of reinnervation were subthreshold responses evoked by stimulating the regenerating vagosympathetic trunks 2 1/2-3 weeks after crushing the cardiac branches of the vagus nerves. Analysis of the reversal potentials of these responses indicated that these synapses were distant from the cell body. 4. At slightly longer times (4-5 weeks), regenerating synapses could be recognized on post-ganglionic axons; no synapses were detected on the neuronal perikarya at these times. 5. By 6-7 weeks following denervation, vagal synapses reinnervated neuronal perikarya as well as post-ganglionic axons. At the same time, vacated post-synaptic densities declined in number. Furthermore, vagal stimulation at this stage evoked large, suprathreshold post-synaptic potentials. 6. These studies indicate that post-ganglionic axons are the initial sites for reinnervation of parasympathetic neurones in the heart. Only some time later are neuronal perikarya reinnervated and ganglionic transmission completely restored.

An electron-microscopical study of the developing transitional region in feline S1 dorsal rootlets

The organization of the PNS-CNS transitional region in S1 dorsal roots was studied electron-microscopically in cat foetuses and in kittens. The adult organization pattern was recognized first during the 5th-6th postnatal week. Before this date the transitional region underwent a period of conspicuous remodelling. In 25- to 47-days-old foetuses the transitional region was characterized by large clusters of Schwann cells clinging to bundles of unmyelinated axons. This part of the root then remained unmyelinated after the more distal PNS part and the more proximal CNS part had acquired myelin. Axons of the transitional region started to myelinate first around the 60th day after conception. At this stage the transitional region was characterized by its cellularity: Schwann cells, glioblasts and fibroblasts were abundant. The CNS compartment started to grow out into the root during the 1st postnatal week. Concomitant with the distal expansion of the CNS compartment - calculated to be about 5 micrometer/day during the 1st month - there appeared in the adjacent PNS compartment numerous extraordinarily short internodes carrying myelin sheaths. A glial fringe began to develop and encapsulate PNS-borderline paranodes. The observations are discussed with respect to the adult ultrastructure. It is suggested that there is a conspicuous reorganization of the proximal part of the root. The need for supplementary quantitative data is emphasized.

Basal lamina glycoproteins are produced by neuroblastoma cells

Murine neuroblastoma cells have been widely used as a model system for neuronal cells as they can be induced to differentiate in culture by various stimuli, such as dibutyryl cyclic AMP (dbcAMP), prostaglandin, and serum starvation. The cells respond with assembly of microtubules, leading to neurite outgrowth, with increased activity of neuronal-specific enzymes, tyrosine hydroxylase, choline acetyltransferase and acetylcholine-esterase, and synthesis of neurotransmitters. The differentiated cells lose tumorigenicity. Cell-to-substratum adhesion is evidently crucial for neurone extension in vitro. Neurite outgrowth is induced by treatments that increase cell-to-substratum adhesion in some neuronal cell cultures. We have now identified the major high molecular weight proteins synthesized and secreted by murine C1300 neuroblastoma cells as fibronectin, laminin and type IV procollagen, of which the latter two were also found to be deposited in pericellular matrix form.

Comparison of nerve cell and nerve cell plus Schwann cell cultures, with particular emphasis on basal lamina and collagen formation

The availability of cultures of normal cells (NCs) and Schwann cells (SCs) with and without fibroblasts has allowed us to investigate the sources of endoneurial and perineurial constituents of peripheral nerve. NCs cultured alone, devoid of ensheathment but healthy in appearance, lack basal lamina and extracellular fibrils. In contrast, when SCs accompany NCs, basal lamina and extracellular fibrils are consistently visible around SCs in outgrowth areas formed de novo in culture. These fibrils average 18 nm in diameter, exhibit a repeating banding pattern, and are trypsin-resistant and collagenase-sensitive. Collagen synthesis is also indicated by the incorporation of [14C]proline into peptide-bound hydroxy-proline in NC + SC or SC cultures. That the [14C]hydroxyproline polypeptides formed in NC + SC cultures are collagenous was determined in part by pepsin digestion-ammonium sulfate precipitation-polyacrylamide gel electrophoresis techniques; the 14C-polypeptides migrate to the positions of alpha 1 (I), alpha 2, alpha 1 (III), and alpha B chains of type I, type III, and A-B collagens. Also formed are thin, ruthenium red-preserved strands interconnecting basal laminae. SC ensheathment of axons is similar to that found in the animal; one SC is related to a number of unmyelinated axons or a single myelinated axon. This proclivity to ensheathe and myelinate axons indicates that SC function is not lost during the preparative procedures or after lengthy isolation in culture and provides the most reliable means for SC identification. Perineurial ensheathment and macrophages are lacking in NC + SC culture preparations divested of fibroblasts. We conclude that SCs do not form perineurium or the larger diameter collagen fibrils typical of endoneurium but that in combination with neurons they generate biochemically detectable collagens and morphologically visible basal lamina and thin collagenous fibrils.

The fate of Schwann cells isolated from axonal contact

Chronically denervated rat and rabbit tibial nerve distal stumps were studied 3-58 weeks following nerve transection. Schwann cells, macrophages and possibly fibroblasts participated in myelin removal which was largely complete by seven weeks. Degenerating myelinated and unmyelinated fibres developed respectively into circular and flattened columns of Schwann cell processes each delimited by a basal lamina. Schwann cell columns became encircled by fibroblasts and later by cells of perineurial type, underwent shrinkage with time and eventually were replaced by connective tissue. In another experiment, endoneurial tissue was removed from rabbit tibial nerve stumps seven weeks after transection and transplanted between the corneal stroma of the same animal for 2-6 weeks. In this locus, Schwann cells developed a thickened basal lamina and then underwent necrosis. It was concluded that the maintenance of Schwann cells in bands of Büngner is in part dependent on axonal contact and that failure of reinnervation eventually causes the columns of Schwann cells to disappear.

Observations on the development of the connective tissues of developing human nerve

Trochlear nerves from two human fetuses, and digital nerves from a third, have been examined by electron microscopy. Very marked differences in maturation were found between trochlear nerves of fetuses of ages differing only by 2--3 weeks, and between proximal and distal parts of the same trochlear nerve. Immaturity was reflected in paucity of endoneurial space and collagen and in the rarity, or virtual absence, of endoneurial fibroblasts. Circumstantial evidence of collagen formation by Schwann cells has been presented and discussed.

Study on the ultrastructure and acetylcholinesterase activity in von Recklinghausen's neurofibromatosis

The cutaneous nodules obtained from seven patients with von Recklinghausen's neurofibromatosis were investigated by electron microscopy, and ultrastructural localization of acetylcholinesterase activity was demonstrated in the nerve fibers of this tumor for the first time using Karnovsky's thiocholine method. The enzymatic activity was mainly found in unmyelinated fibers, exactly associated with their axonal membranes, the interspace between the apposing axonal and Schwann cell membrane, and some different mesaxons, which indicated their cholinergic nature. Almost all myelinated fibers and some unmyelinated fibers did not possess the activity. The relationship between axon and Schwann cell was quite similar to that of normal peripheral nervous system, but two striking alterations of the nerves existed: One is the dissociation of unmyelinated fibers, and the other is the degenerative changes of the axon and the myelin sheath. As the evidence of schwannian proliferation, onion bulb formations and collagen pockets were observed. Some signs of fibroblastic proliferation were also found. From the present study and the review of the literature, the probable histogenesis of this disease was discussed.

Ultrastructure of perineurial cells during peripheral nerve regeneration. Electron microscopical investigations on the so-called amputation neuroma

The ultrastructural morphology of perineurial cells was examined after transsection of the sciatic nerve of rats. The material prepared for investigation was taken from the proximal nerve stump and the so-called amputation neuroma at different times after operation. We found that perineurial cells begin to activate in the first week, and in the further course of regeneration they modulate their cytoplasmic structure corresponding to the function of proliferation or synthesis. Besides the reconstitution of a diffusion barrier the perineurial cells may exert a stabilizing function in regenerating nerve tissue comparable to myofibroblasts in granulation tissue, and may be engaged in remodeling of connective tissue fibers. The relations of perineurial cells to other cells of peripheral nerves under the condition of regeneration are discussed.

Ultrastructural relations of macrophages to Schwann cells. An experimental study on regenerating peripheral nerves

The correlations between macrophages and Schwann cells are studied on transsectional sciatic nerves from rats using the electron microscope. As early as 1 day after operation typical macrophages can be found within the endoneurial space of the proximal stump. On the other hand, 1 and 3 days as well as 4 weeks after operation Schwann cells may show cytoplasmic features comparable to that of macrophages. Our findings suggest that (1) macrophages are not able to transform into Schwann cells and (2) Schwann cells can act as phagocytosing cells exhibiting characteristics of macrophages. But in this functional state they can only be diagnosed as Schwann cells if a basement membrane and/or included axons with mesaxons are present.

The characterization of type I and type III collagens from human peripheral nerve

The normal chemical features of peripheral nerve collagens were determined on postmortem, histologically normal adult human femoral nerve. 1. Genetically distinct type I, [alpha1(I)2]alpha2, and type III, [alpha1(III)]3, were isolated by differential salt precipitation and the component subunit chains, alphal(I), alpha2 and alphal(III) were obtained by ion-exchange chromatography and gel filtration. 2. The molecular weight of alphal(I) and alpha2 of type I collagen was 95 000 and that for type III was 280 000. Reduction of type III with dithiothreitol yielded expected alpha1(III) chains of 95 000 molecular weight. 3. The amino acid composition of the three collagen chains, alpha1(I), alpha2, and alpha1(III), was the same as previously reported values for the corresponding chains from human skin except for slightly elevated hydroxylysine content. 4. Peripheral nerve collagen was found to contain 81% type I collagen and 19% type III. These results indicate that peripheral nerve collagen characteristics closely simulate that of human skin and differ from that of human aorta and other parenchymal organs. These data will permit a chemical analysis for possible abnormalities of peripheral nerve collagen in various neurogenic disorders.