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

Extracellular Environment-Controlled Angiogenesis, and Potential Application for Peripheral Nerve Regeneration

Endothelial cells acquire different phenotypes to establish functional vascular networks. Vascular endothelial growth factor (VEGF) signaling induces endothelial proliferation, migration, and survival to regulate vascular development, which leads to the construction of a vascular plexuses with a regular morphology. The spatiotemporal localization of angiogenic factors and the extracellular matrix play fundamental roles in ensuring the proper regulation of angiogenesis. This review article highlights how and what kinds of extracellular environmental molecules regulate angiogenesis. Close interactions between the vascular and neural systems involve shared molecular mechanisms to coordinate developmental and regenerative processes. This review article focuses on current knowledge about the roles of angiogenesis in peripheral nerve regeneration and the latest therapeutic strategies for the treatment of peripheral nerve injury.

Efficacy of nonviral gene transfer of human hepatocyte growth factor (HGF) against ischemic-reperfusion nerve injury in rats

Ischemic neuropathy is common in subjects with critical limb ischemia, frequently causing chronic neuropathic pain. However, neuropathic pain caused by ischemia is hard to control despite the restoration of an adequate blood flow. Here, we used a rat model of ischemic-reperfusion nerve injury (IRI) to investigate possible effects of hepatocyte growth factor (HGF) against ischemic neuropathy. Hemagglutinating virus of Japan (HVJ) liposomes containing plasmids encoded with HGF was delivered into the peripheral nervous system by retrograde axonal transport following its repeated injections into the tibialis anterior muscle in the right hindlimb. First HGF gene transfer was done immediately after IRI, and repeated at 1, 2 and 3 weeks later. Rats with IRI exhibited pronounced mechanical allodynia and thermal hyperalgesia, decreased blood flow and skin temperature, and lowered thresholds of plantar stimuli in the hind paw. These were all significantly improved by HGF gene transfer, as also were sciatic nerve conduction velocity and muscle action potential amplitudes. Histologically, HGF gene transfer resulted in a significant increase of endoneurial microvessels in sciatic and tibial nerves and promoted nerve regeneration which were confirmed by morphometric analysis. Neovascularization was observed in the contralateral side of peripheral nerves as well. In addition, IRI elevated mRNA levels of P2X3 and P2Y1 receptors, and transient receptor potential vanilloid receptor subtype 1 (TRPV1) in sciatic nerves, dorsal root ganglia and spinal cord, and these elevated levels were inhibited by HGF gene transfer. In conclusion, HGF gene transfer is a potent candidate for treatment of acute ischemic neuropathy caused by reperfusion injury, because of robust angiogenesis and enhanced nerve regeneration.

The role of vascularization in nerve regeneration of nerve graft

Vascularization is an important factor in nerve graft survival and function. The specific molecular regulations and patterns of angiogenesis following peripheral nerve injury are in a broad complex of pathways. This review aims to summarize current knowledge on the role of vascularization in nerve regeneration, including the key regulation molecules, and mechanisms and patterns of revascularization after nerve injury. Angiogenesis, the maturation of pre-existing vessels into new areas, is stimulated through angiogenic factors such as vascular endothelial growth factor and precedes the repair of damaged nerves. Vascular endothelial growth factor administration to nerves has demonstrated to increase revascularization after injury in basic science research. In the clinical setting, vascularized nerve grafts could be used in the reconstruction of large segmental peripheral nerve injuries. Vascularized nerve grafts are postulated to accelerate revascularization and enhance nerve regeneration by providing an optimal nutritional environment, especially in scarred beds, and decrease fibroblast infiltration. This could improve functional recovery after nerve grafting, however, conclusive evidence of the superiority of vascularized nerve grafts is lacking in human studies. A well-designed randomized controlled trial comparing vascularized nerve grafts to non-vascularized nerve grafts involving patients with similar injuries, nerve graft repair and follow-up times is necessary to demonstrate the efficacy of vascularized nerve grafts. Due to technical challenges, composite transfer of a nerve graft along with its adipose tissue has been proposed to provide a healthy tissue bed. Basic science research has shown that a vascularized fascial flap containing adipose tissue and a vascular bundle improves revascularization through excreted angiogenic factors, provided by the stem cells in the adipose tissue as well as by the blood supply and environmental support. While it was previously believed that revascularization occurred from both nerve ends, recent studies propose that revascularization occurs primarily from the proximal nerve coaptation. Fascial flaps or vascularized nerve grafts have limited applicability and future directions could lead towards off-the-shelf alternatives to autografting, such as biodegradable nerve scaffolds which include capillary-like networks to enable vascularization and avoid graft necrosis and ischemia.

Vascularization Strategies for Peripheral Nerve Tissue Engineering

Vascularization plays a significant role in treating nerve injury, especially to avoid the central necrosis observed in nerve grafts for large and long nerve defects. It is known that sufficient vascularization can sustain cell survival and maintain cell integration within tissue‐engineered constructs. Several studies have also shown that vascularization affects nerve regeneration. Motivated by these studies, vascularized nerve grafts have been developed using various different techniques, although donor site morbidity and limited nerve supply remain significant drawbacks. Tissue engineering provides an exciting alternative approach to prefabricate vascularized nerve constructs which could overcome the limitations of grafts. In this review article, we focus on the role of vascularization in nerve regeneration, discussing various approaches to generate vascularized nerve constructs and the contribution of tissue engineering and mathematical modeling to aid in developing vascularized engineered nerve constructs, illustrating these aspects with examples from our research experience. Anat Rec, 301:1657–1667, 2018. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Fetal extracellular matrix nerve wraps locally improve peripheral nerve remodeling after complete transection and direct repair in rat

In peripheral nerve (PN) injuries requiring surgical repair, as in PN transection, cellular and ECM remodeling at PN epineurial repair sites is hypothesized to reduce PN functional outcomes by slowing, misdirecting, or preventing axons from regrowing appropriately across the repair site. Herein this study reports on deriving and analyzing fetal porcine urinary bladder extracellular matrix (fUB-ECM) by vacuum assisted decellularization, fabricating fUBM-ECM nerve wraps, and testing fUB-ECM nerve wrap biocompatibility and bioactivity in a trigeminal, infraorbital nerve (ION) branch transection and direct end-to-end repair model in rat. FUB-ECM nerve wraps significantly improved epi- and endoneurial organization and increased both neovascularization and growth associated protein-43 (GAP-43) expression at PN repair sites, 28-days post surgery. However, the number of neurofilament positive axons, remyelination, and whisker-evoked response properties of ION axons were unaltered, indicating improved tissue remodeling per se does not predict axon regrowth, remyelination, and the return of mechanoreceptor cortical signaling. This study shows fUB-ECM nerve wraps are biocompatible, bioactive, and good experimental and potentially clinical devices for treating epineurial repairs. Moreover, this study highlights the value provided by precise, analytic models, like the ION repair model, in understanding how PN tissue remodeling relates to axonal regrowth, remyelination, and axonal response properties.

Ultrasonographic demonstration of intraneural neovascularization after penetrating nerve injury

INTRODUCTION

Hypervascularization of nerves has been shown to be a pathological sign in some peripheral nerve disorders, but has not been investigated in nerve trauma.

METHODS

An observational cohort study was performed of the intraneural blood flow of 30 patients (34 nerves) with penetrating nerve injuries, before or after nerve reconstruction. All patients underwent electrophysiological assessment, and B-mode and color Doppler ultrasonography.

RESULTS

Intraneural hypervascularization proximal to the site of injury was found in all nerves, which was typically marked and had a longitudinal extension of several centimeters. In 6 nerves, some blood flow was also present within the injury site or immediately distal to the injury. No correlation was found between the degree of vascularization and age, size of the scar / neuroma, or degree of reinnervation.

DISCUSSION

Neovascularization of nerves proximal to injury sites appears to be an essential element of nerve regeneration after penetrating nerve injuries. Muscle Nerve 57: 994-999, 2018.

Changes in nerve microcirculation following peripheral nerve compression

Following peripheral nerve compression, peripheral nerve microcirculation plays important roles in regulating the nerve microenvironment and neurotrophic substances, supplying blood and oxygen and maintaining neural conduction and axonal transport. This paper has retrospectively analyzed the articles published in the past 10 years that addressed the relationship between peripheral nerve compression and changes in intraneural microcirculation. In addition, we describe changes in different peripheral nerves, with the aim of providing help for further studies in peripheral nerve microcirculation and understanding its protective mechanism, and exploring new clinical methods for treating peripheral nerve compression from the perspective of neural microcirculation.

COMP-angiopoietin-1 recovers molecular biomarkers of neuropathy and improves vascularisation in sciatic nerve of ob/ob mice

BACKGROUND

Leptin-deficient ob/ob mice are a model of type 2 diabetes induced peripheral neuropathy. Ob/ob mice exhibit obesity, insulin resistance, hyperglycaemia, and alterations of peripheral nerve fibres and endoneural microvessels. Here we test the hypothesis that cartilage oligomeric matrix protein (COMP)-Ang-1, a soluble and stabile form of Ang-1 which promotes angiogenesis and nerve growth, improves regeneration of nerve fibres and endoneural microvessels in ob/ob mice.

METHODS AND FINDINGS

COMP-Ang-1 (100 ng/ml) or NaCl were intraperitoneally (i.p.) injected into male (N = 184), 3-month old, ob/ob or ob/+ mice for 7 and 21 days. We measured expression of Nf68, GAP43, Cx32, Cx26, Cx43, and TNFα in sciatic nerves using Western blot analysis. To investigate the inflammation in sciatic nerves, numbers of macrophages and T-cells were counted after immunofluorescence staining. In ultrathin section, number of myelinated/non-mylinated nerve fibers, g-ratio, the thickness of Schwann cell basal lamina and microvessel endothelium were investigated. Endoneural microvessels were reconstructed with intracardial FITC injection. Treatment with COMP-Ang-1 over 21 days significantly reduced fasting blood glucose and plasma cholesterol concentrations compared to saline treated ob/ob mice. In addition, COMP-Ang-1 treatment: 1) up-regulated expression of Nf68 and GAP43; 2) improved expression of gap junction proteins including connexin 32 and 26; 3) suppressed the expression of TNFα and Cx43 and 4) led to decreased macrophage and T-cell infiltration in sciatic nerve of ob/ob mice. The significant changes of sciatic nerve ultrastructure were not observed after 21-day long COMP-Ang-1 treatment. COMP-Ang-1 treated ob/ob mice displayed regeneration of small-diameter endoneural microvessels. Effects of COMP-Ang-1 corresponded to increased phosphorylation of Akt and p38 MAPK upon Tie-2 receptor.

CONCLUSIONS

COMP-Ang-1 recovers molecular biomarkers of neuropathy, promotes angiogenesis and suppresses inflammation in sciatic nerves of ob/ob mice suggesting COMP-Ang-1 as novel treatment option to improve morphologic and protein expression changes associated with diabetic neuropathy.

Intramuscular schwannoma arising from the psoas muscle presenting with femoral nerve neuropathy

A spherical mass of the right psoas muscle was found incidentally by magnetic resonance health examination in a 43-year-old woman. The patient had complained of numbness in her right leg over the previous six months and neuralgia at the time of computed tomography-guided core biopsy, which was done to establish the diagnosis. To our knowledge, femoral nerve neuropathy caused by an intramuscular schwannoma arising from the psoas muscle has not been previously published.

Facilitated sprouting in a peripheral nerve injury

During regeneration of injured peripheral nerves, local conditions may influence how regenerative axon sprouts emerge from parent axons. More extensive lesions might be expected to disrupt such growth. In this work, we discovered instead that long segmental crush injuries facilitate the growth and maturation of substantially more axon sprouts than do classical short crush injuries (20 mm length vs. 2 mm). At identical distances from the proximal site of axon interruption there was a 45% rise in the numbers of neurofilament labeled axons extending through a long segmental crush zone by 1 week. By 2 weeks, there was a 35% greater density of regenerating myelinated axons in long compared with short crush injuries just beyond (5 mm) the proximal injury site. Moreover, despite the larger numbers of axons, their maturity was identical and they were regular, parallel, associated with Schwann cells (SCs) and essentially indistinguishable between the injuries. Backlabeling with Fluorogold indicated that despite these differences, the axons arose from similar numbers of parent motor and sensory neurons. Neither injury was associated with ischemia. Both injuries were associated with rises in GFAP (glial acidic fibrillary protein) and p75 mRNAs, markers of SC plasticity but p75, GFAP and brain-derived neurotrophic factor mRNAs did not differ between the injuries. There was a higher local mRNA level of GAP43/B50 at 7 days following injury and a higher sonic hedgehog protein (Shh) mRNA at 24 h in long crush zones. GAP43/B50 protein and SHH protein both had prominent localization within regenerating axons. Long segmental nerve trunk crush injuries do not impair regeneration but instead generate greater axon plasticity that results in larger numbers of mature myelinated axons. The changes occur without apparent change in SC activation, overall nerve architecture or nerve blood flow. While the mechanism is uncertain, the findings indicate that manipulation of the nerve microenvironment can induce substantial changes in regenerative sprouting.

Lesão por esmagamento do nervo isquiático de ratos: estudo da vascularização

Este trabalho teve como objetivo estudar as alterações microvasculares intraneurais aguda em nervo isquiático de rato submetido a esmagamento por diferentes cargas. Foram utilizados 60 ratos machos da linhagem Wistar, distribuídos em grupos experimentais de acordo com a injeção de vasos e com a carga de esmagamento. Os nervos isquiáticos direitos foram isolados e submetidos ao esmagamento com cargas (0,5 Kg, 1 Kg, 5 Kg, 10 kg e 15 kg) por 10 minutos e os nervos isquiáticos esquerdos foram utilizados como controle. Após esmagamento, os animais foram submetidos à cateterização da aorta abdominal e injeção dos vasos, em seguida 30 nervos direitos e esquerdos foram fixados em formol 10%, desidratados e diafanizados para análise longitudinal dos vasos intraneurais e os restantes retirados em toda a sua extensão, cortados em 3 fragmentos, congelados em isopentano em gelo seco e armazenados em freezer -70°C, seccionados transversalmente para análise e contagem dos vasos intraneurais. As análises macroscópica e microscópica mostraram regiões de hematoma endoneural e epineural nas diferentes cargas de esmagamento. A análise morfométrica sugere que a lesão aos vasos intraneurais foi proporcional à carga de esmagamento, causando hematoma endoneural e epineural, que cria microambiente desfavorável para a regeneração das fibras nervosas.

Alleviated tension at the repair site enhances functional regeneration: the effect of full range of motion mobilization on the regeneration of peripheral nerves--histologic, electrophysiologic, and functional results in a rat model

BACKGROUND

In the clinical management of combined tendon and nerve injuries, competing treatment strategies are well known. The effect of mobilization on the functional regeneration of peripheral nerves remains controversial. This study sought to determine the effect of full range of motion mobilization on nerve repair by using tubular segmental nerve splinting to block movement, and thereby variable tension, at the nerve repair site.

METHODS

In 96 rats, the right sciatic nerve was transected midthigh and coapted immediately microsurgically. The groups used in the study were as follows: group N, epineural nerve repair; group T, segmental tubular nerve splinting with fixed in situ tension at the nerve suture site,allowing segmental movement only; group TN, segmental tubular nerve splinting with alleviated in situ tension at the nerve suture site, allowing segmental movement only; and group TM, segmental tubular nerve splinting without fixed in situ tension at the nerve suture site, allowing movement of the nerve suture site. Full range of motion of the lower limbs was ensured by passive motion of hind limbs once a week after functional testing. Blinded histologic, immunohistochemical, and electrophysiologic assessment and 12 postoperative weekly function tests were carried out.

RESULTS

Functional and electrophysiologic results were significantly better in group TN, by segmental tubular nerve splinting with alleviated in situ tension at the nerve repair site, mainly because of less scar formation and enhanced endoneural angiogenesis at the nerve suture segment.

CONCLUSION

Full range of motion mobilization may impede functional nerve recovery by significant endoneural collagenization and decreased angiogenesis at the nerve suture segment. Complete alleviation of in situ (pathophysiologic) tension at the nerve suture site seems to improve functional peripheral nerve regeneration.

Influence of experimental diabetes on the microcirculation of injured peripheral nerve: functional and morphological aspects

Regeneration of diabetic axons has delays in onset, rate, and maturation. It is possible that microangiopathy of vasa nervorum, the vascular supply of the peripheral nerve, may render an unfavorable local environment for nerve regeneration. We examined local nerve blood flow proximal and distal to sciatic nerve transection in rats with long-term (8 month) experimental streptozotocin diabetes using laser Doppler flowmetry and microelectrode hydrogen clearance polarography. We then correlated these findings, using in vivo perfusion of an India ink preparation, by outlining the lumens of microvessels from unfixed nerve sections. There were no differences in baseline nerve blood flow between diabetic and nondiabetic uninjured nerves, and vessel number, density, and area were unaltered. After transection, there were greater rises in blood flow in proximal stumps of nondiabetic nerves than in diabetic animals associated with a higher number, density, and caliber of epineurial vessels. Hyperemia also developed in distal stumps of nondiabetic nerves but did not develop in diabetic nerves. In these stumps, diabetic rats had reduced vessel numbers and smaller mean endoneurial vessel areas. Failed or delayed upregulation of nerve blood flow after peripheral nerve injury in diabetes may create a relatively ischemic regenerative microenvironment.

Revascularization of tissue-engineered nerve grafts and invasion of macrophages

Nonneural derived nerve conduits fail to support regeneration over larger gaps due to lacking viable Schwann cells. Thus, tissue engineering of nerves is focusing on implantation of viable Schwann cells into suitable scaffolds. We established grafts made from acellular muscles and veins, respectively, seeded with cultured Schwann cells. As timing of revascularization is crucial to determine Schwann cell survival and depending axonal regeneration we studied establishment of vascular architecture in a rat sciatic nerve model (2-cm gap) after 3, 5, 7, and 10 days postoperatively, using albumin bound Evans blue. Additionally, macrophage recruitment was immunohistochemically assessed. Engineered grafts showed a delayed revascularization, starting between day 5 and 7 in comparison to normal autografts, that revascularized by day 3. Macrophage recruitment in autologous nerve grafts was evident by day 3. The engineered groups revealed no macrophage invasion until day 7. As Schwann cells survive up to 7 days in autologous grafts without blood supply, depending purely on diffusion, establishment of vascular structure between day 5 and 7 is rapid enough to support Schwann cell survival in engineered grafts. As these grafts are lacking Wallerian degeneration delayed macrophage invasion may not impair degeneration-dependent regeneration, but presence of macrophage derived or induced growth factors may be decreased.

VEGF enhances intraneural angiogenesis and improves nerve regeneration after axotomy

Whilst there is an increased understanding of the cell biology of nerve regeneration, it remains unclear whether there is a direct interrelationship between vascularisation and efficacy of nerve regeneration within a nerve conduit. To establish this is important as in clinical surgery peripheral nerve conduit grafting has been widely investigated as a possible alternative to the use of nerve autografts. The aim of this study was to assess whether vascular endothelial growth factor (VEGF), a highly specific endothelial cell mitogen, can enhance vascularisation and, indirectly, axonal regeneration within a silicone nerve regeneration chamber. Chambers containing VEGF (500-700 ng/ml) in a laminin-based gel (Matrigel) were inserted into 1 cm rat sciatic nerve defects and nerve regeneration examined in relation to angiogenesis between 5 and 180 d. Longitudinal sections were stained with antibodies against endothelial cells (RECA-1), axons (neurofilament) and Schwann cells (S-100) to follow the progression of vascular and neural elements. Computerised image analysis demonstrated that the addition of VEGF significantly increased blood vessel penetration within the chamber from d 5, and by d 10 this correlated with an increase of axonal regeneration and Schwann cell migration. The pattern of increased nerve regeneration due to VEGF administration was maintained up to 180 d, when myelinated axon counts were increased by 78 % compared with plain Matrigel control. Furthermore the dose-response of blood vessel regeneration to VEGF was clearly reflected in the increase of axonal regrowth and Schwann cell proliferation, indicating the close relationship between regenerating nerves and blood vessels within the chamber. Target organ reinnervation was enhanced by VEGF at 180 d as measured through the recovery of gastrocnemius muscle weights and footpad axonal terminal density, the latter showing a significant increase over controls (P < 0.05). The results demonstrate an overall relationship between increased vascularisation and enhanced nerve regeneration within an acellular conduit, and highlight the interdependence of the 2 processes.

Inter-relationships between angiogenesis and nerve regeneration: a histochemical study

Whilst increases in capillary number and permeability occurring during nerve regeneration suggest an interaction between regenerating axons and blood vessels, clinical attempts to improve nerve regeneration by augmenting nerve graft vascularisation have produced conflicting results and the nature of their relationship remains obscure. A better understanding of the process might be exploited in the development of a synthetic alternative to the autologous nerve graft and bring an improvement in the clinical results of nerve repair. To clarify this relationship the growth of axons and blood vessels through mats of orientated fibronectin grafted in rat sciatic nerves was assessed morphologically. Fibronectin, which supports axonal regeneration, is initially acellular, ensuring all vascular and neural elements within the graft are newly formed. To follow the progression of the elements, grafts were harvested between 3 and 30 days and stained with antibodies against endothelial cells (RECA-1), Schwann cells (S-100) and axons (a polyclonal or monoclonal panaxonal marker). Dual fluorescence staining combined with double exposure photography allowed the simultaneous visualisation of these elements and the demonstration of their true relative positions. Graft vascularisation came initially from the adjacent muscle bed. A neovascularisation front preceded axonal regeneration, although vessel and axonal orientation appeared similar. Schwann cells and axons extended together, never exceeding the area of vascularisation and appeared most numerous in well vascularised areas containing longitudinally orientated vessels. These results suggest that provision of a well vascularised, longitudinally orientated conduit may enhance nerve regeneration.

Nerve function and regeneration in diabetic and galactosaemic rats: antioxidant and metal chelator effects

Immature rats were made diabetic with streptozotocin or were fed a 40% galactose diet to stimulate the polyol pathway. Separate diabetic and galactosaemic groups were treated with butylated hydroxytoluene or trientine. After 4 weeks the sciatic nerve was freeze-lesioned. Two weeks later, the degree of myelinated fibre regeneration was assessed electrophysiologically and nerve conduction velocity was measured in the contralateral leg. Similar sciatic motor and saphenous sensory nerve conduction velocity deficits of approximately 18% and 19%, respectively, compared to age-matched control rats were found in both models. They were partially prevented by treatment (approximately 68% for butylated hydroxytoluene and 63% for trientine). There were 12% and 10% deficits in nerve regeneration distance with diabetes and galactosaemia respectively, which were markedly attenuated (approximately 80%) by both treatments. The data emphasise the importance of elevated, free radical activity for the aetiology of neural/neurovascular deficits in experimental diabetes and galactosaemia.

Effects of the sulphydryl donor N-acetyl-L-cysteine on nerve conduction, perfusion, maturation and regeneration following freeze damage in diabetic rats

Peripheral nerve conduction velocity deficits in diabetic rats depend on decreased nerve perfusion, which may be related to increased free radical activity and impaired endogenous protection by the glutathione redox cycle. We studied the effect of treatment with the glutathione precursor N-acetyl-L-cysteine on nerve conduction, blood flow, maturation and regeneration. Two months of diabetes in mature rats caused 20% and 48% deficits in sciatic motor conduction velocity and endoneurial blood flow, respectively, which were largely corrected by N-acetyl-L-cysteine treatment during the second month. In young nondiabetic rats, sciatic motor conduction velocity increased by 31% over 6 weeks. Diabetes halved the conduction velocity maturation rate, however N-acetyl-L-cysteine treatment allowed a normal pattern of development. After 1 month of treated or untreated diabetes, the sciatic nerve was lesioned by a liquid nitrogen-cooled probe. Myelinated fibre regeneration distance, determined electrophysiologically, was reduced by 12.2% with diabetes; this was prevented by N-acetyl-L-cysteine treatment. Thus, the data stress the importance of free radical-mediated changes in the aetiology of experimental diabetic neuropathy.

Microvessel basement membrane reduplication is not associated with repeated nerve fiber degeneration and regeneration

To determine whether repeated nerve fiber degeneration and regeneration can induce reduplication of endoneurial microvessel basement membranes (BMs), typical of such conditions as diabetic polyneuropathy, electronmicrographs of endoneurial microvessels of rat peroneal and tibial nerves were studied in repeatedly crushed (10 x) sciatic nerves and compared to microvessels of sham-operated uncrushed nerves. On average, crushed nerves had 2.6, SE +/- 0.1 BMs, whereas control nerves had 2.7, SE +/- 0.1 (P > 0.05). Microvessel cellular components were significantly increased in both number and size in the crushed nerves. These nerves also demonstrated a trend to increased vessel numbers and density. These results are not in keeping with the hypothesis that BM reduplication of endoneurial microvessels is simply due to repeated fiber degeneration and regeneration.

Impaired myelinated fiber regeneration following freeze-injury in rats with streptozotocin-induced diabetes: involvement of the polyol pathway

This study examined the effects of streptozotocin-induced diabetes mellitus and aldose reductase inhibitor (ZD5522) treatment on myelinated fiber regeneration in rats. After 2 months of diabetes, with or without ZD5522 treatment (10 mg.kg-1.day-1) from induction, sciatic nerve degeneration was initiated by a punctate lesion using a liquid N2 cooled probe. Regeneration was studied over a subsequent 14-day period using in vitro electrophysiological techniques. There was a 21.4% (P < 0.001) deficit in the maximum fiber regeneration distance in diabetic rats, 14 days postlesion. This was partially (64.9%, P < 0.01) prevented by aldose reductase inhibitor treatment, the resultant regeneration distance being not significantly different from that of age-matched nondiabetic control rats. The regeneration rate, assessed from data collected 4, 9 and 14 days postlesion, was 23.7% (P < 0.001) reduced by diabetes and ZD5522 treatment provided 73.1% protection (P < 0.01). We conclude that polyol pathway activity is involved in impaired regeneration in experimental diabetes, potential pathophysiological mechanisms involving a reduction in neurotrophic support and impaired endoneurial blood supply.

Nerve function and regeneration in diabetic rats: effects of ZD-7155, an AT1 receptor antagonist

Effects of the angiotensin II AT1 receptor antagonist ZD-7155 on nerve function, blood flow, capillarization, oxygenation, and regenerative capacity after injury were studied in streptozocin-diabetic rats. Deficits in saphenous sensory and sciatic motor conduction velocity measured after 1 or 2 mo of diabetes in anesthetized rats were prevented and corrected by ZD-7155. Sciatic resistance to hypoxic conduction failure, which was increased by 71% by 2 mo of diabetes, was attenuated by 39% with ZD-7155. Endoneurial capillary density, which was unaffected by diabetes, was increased by 34% with 2 mo of ZD-7155 treatment. Sciatic nutritive endoneurial blood flow, which was reduced by 45% by 2 mo of diabetes, remained in the nondiabetic range with ZD-7155. Mean endoneurial O2 tension was reduced 38% by diabetes, which was attenuated by ZD-7155. Punctate freeze damage of sciatic nerve caused complete fiber degeneration. Fourteen days postlesion, there was a 26% deficit in myelinated fiber regeneration distance after 2 mo of diabetes, which was prevented by ZD-7155 treatment from diabetes induction. Thus alterations in the renin-angiotensin system contribute to the neurovascular etiology of nerve dysfunction in experimental diabetes.

A diffusion-reaction model of nerve regeneration

The process of peripheral nerve regeneration has been modeled using 5 populations of mathematical variables to represent the biological activities of Wallerian degeneration, fibrin matrix development, Schwann cell activity, elongating neurites, and neovascularization. The mathematical model provided simulations of nerve regeneration following transection and crush injuries that correspond with growth behaviors quantified in biological experiments. Neovascularization was spatiotemporally quantified in nerve regeneration chambers and following nerve crush injury in order to test the simulations of the mathematical model. The vasculature in both the chamber and following nerve crush responded as predicted by the model, increasing beyond normal levels to a peak only to decrease back to normal. This behavior appeared as a traveling wave in the proximal-distal direction preceeding the major thrust of neuritic outgrowth suggesting that development of the vasculature is a rate-limiting step in nerve regeneration.

The vascular response to nerve transection: neovascularization in the silicone nerve regeneration chamber

The rat sciatic nerve regeneration chamber was used to study the spatial and temporal response of the endoneurial vasculature during regeneration. Proximal and distal stumps of a transected rat sciatic nerve were placed in opposite ends of a silicone tube and allowed to regenerate for periods of 2, 3, 4 or 52 weeks after the surgery. Serial, transverse sections of nerve were studied at each time-point to quantitate the number of vessels, capillary density and the vessel luminal perimeter per nerve area. The results indicate that the vascular growth relative to the existing tissue in the chamber increases to a peak beyond normal levels and later decreases to values associated with control tissue. While this growth occurred from both the proximal and distal stumps, it appeared predominantly as a traveling wave in the proximal-distal direction preceding the major thrust of neuritic outgrowth from the proximal stump. Morphologic measurements of angiogenesis were paralleled in other animals by measurements of nerve blood flow using laser Doppler flowmetry at corresponding time-points. These data differ somewhat from previous reports of angiogenesis following nerve crush injury and are useful in formulating a general mathematical model of regeneration in the peripheral nervous system.

The susceptibility of rat diabetic nerve to ischemia: increased or decreased?

Diabetic nerve reveals a peculiar paradox between its physiological resistance to ischemia, in conducting impulses for longer than control nerve during ischemia, and its morphological liability to more severe pathological changes of nerve fiber when rendered ischemic. These paradoxical phenomena, however, have never been previously evaluated in the same diabetic rat. In the present study, the effect of ischemia on rat diabetic nerve was assessed both physiologically and morphologically at 2 and 16 weeks after the injection of streptozotocin. At 16 weeks the effects of a rapid normalization of blood glucose by insulin on these phenomena were also evaluated. Two weeks after the induction of diabetes, physiological resistance to ischemia was found, but not morphological vulnerability. After 16 weeks of diabetes, both physiological resistance and morphological vulnerability to ischemia were observed. At this time the administration of insulin had no effect on morphological vulnerability, but shortened the time of preservation of nerve action potentials during ischemia although it was not normalized. These findings indicate that the morphological liability of diabetic nerve to ischemia is most likely due to a combined effect of systemic complications of chronic hyperglycemia. By contrast a substantial component of resistance to ischemic conduction failure appears to be related to rapidly reversible metabolic derangement due to hyperglycemia. The study demonstrates coexistence of physiological resistance and morphological vulnerability to ischemia in rat diabetic nerve, and implies that different factors are involved in these paradoxical phenomena.

Vasodilating effect of dipyridamole on rat endoneurial vessels: morphometric study

Ischaemia plays an important role in the pathogenetic mechanism of various neuropathies. To study the possible vasodilating effects of dipyridamole on peripheral nerve vessels, morphometric analysis of endoneurial vessels was undertaken in rat sciatic and tibial nerves after chronic dipyridamole treatment. Each rat was injected with 4 mg/kg of dipyridamole intraperitoneally twice daily for 5 days/week and once daily during the weekend for a period of 6 weeks. The mean luminal area and perimeter of endoneurial vessels in the sciatic nerve were significantly greater in dipyridamole-treated animals than in controls. The total fascicular area and densities of endoneurial vessels were not significantly different between experimental and control groups. This study demonstrated that daily administration of dipyridamole over a 6 week period caused endoneurial vessels to dilate resulting in increased luminal area and perimeter. Dipyridamole might be a potent therapeutic agent for peripheral neuropathies attributed to nerve ischaemia.

The vascular response to nerve crush: relationship to Wallerian degeneration and regeneration

The response of the endoneurial vasculature in rat sciatic nerve following crush injury was investigated by morphometric analysis of serial nerve transverse sections at the site of injury and in distal segments at 1, 2, 3, 6, and 9 weeks after injury. Quantitative analysis included determination of the number of vessels, vessel radius vessel perimeter, and transfascicular area. The vascular response to crush injury consisted of two phases: an early phase, which peaked at 1 week after crush, consisted of an increase in vessel size but not vessel number. The second phase, which peaked at 6 weeks after crush, consisted of an increase in the number of vessels and in their density. This two-phase response was also evident as a dual peak in the total endoneurial vessel perimeter, a measure of vascular surface area, when this variable was plotted against time. The first phase of the vascular response was temporally related to the recruitment of macrophages and the clearance of degenerating axonal and myelin tissue during the early phase of Wallerian degeneration. The second phase involved an increase in the number of blood vessels and was associated with cellular proliferation, neurite elongation, and myelination during the subsequent period of nerve regeneration.

Photochemically induced experimental ischemic neuropathy: a clinical, electrophysiological and immunohistochemical study

A new experimental model of focal peripheral nerve infarction is presented. Ischemia was produced in 12 rats by intravascular thrombosis induced by the photochemical reaction of systemically injected rose bengal to the local application of light from a cold light source. Clinical, electrophysiological and immunohistochemical techniques were used to monitor the pathology and the time course of experimental ischemic neuropathy (EIN) of the sciatic nerve. Primary axonal neurofilament disintegration was detectable 4-24 h after illumination and was followed by wallerian degeneration within the first week. At 7 days, there was a secondary disruption of myelin sheaths accompanied by massive infiltration of macrophages and phagocytosis of the necrotic debris. The majority of detected macrophages were derived from circulating blood monocytes which had invaded the nerve. Two weeks after the initial lesions, degeneration had advanced without any signs of regeneration or remyelination. Electrophysiological recordings corroborate the findings of primary axonal degeneration and failure of regeneration up to 2 weeks after the lesion.

Total eye transplantation for the blind: a challenge for the future

Blindness is a human and social problem of incalculable weight. In the future, artificial 'bionic' prostheses and retinal grafts could achieve a long-sought cure. Several lines of evidence led to the speculation that a total eye transplantation for the cure of retinal blindness may become feasible in the near future. It is proposed that a brain dead patient's eye, whose retinal viability has been demonstrated with an electroretinogram recording, be transplanted into the blind's voided orbital socket, through a frontoorbitotemporal craniotomy and orbitozygomatic osteotomy. Regenerating optic nerve axons are channeled in a specially constructed guide to the homolateral corpus genicolatum laterale, while the retinal ganglion cells are adequately protected during the regrowth period. Aspects of this paradigm are reviewed and discussed.

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.

Regeneration of perivascular adrenergic innervation in rat tibial nerve after nerve crush

Adrenergic innervation of blood vessels in the rat tibial nerve during degeneration and regeneration was studied using the formaldehyde-induced fluorescence method. The left sciatic nerve was crushed with suture threads to produce a 4-mm length of crushed nerve. At 1, 3, 7, 14, 28, 56 and 84 days after nerve crush, degenerative and regenerative changes in the nerve were verified using light microscopy. At each time point, adrenergic innervation was examined in epi-perineurial whole mount and nerve cross-section preparations. One day after nerve crush, fluorescence of adrenergic nerve fibers in the endoneurium was absent. Fluorescent adrenergic nerve fibers reappeared in the endoneurium at day 56 and reached the control density by 84 days. In the epi-perineurium, adrenergic innervation of small and medium-size arterioles was absent at 3 days, in large arterioles at 7 days. At 56 days, all epi-perineurial arterioles were reinnervated by a faint, sparse adrenergic network, which reached the control density at 84 days. The results suggest that adrenergic innervation in the rat peripheral nerve is lost during nerve degeneration, but recovers when the nerve has regenerated.

Endoneurial microenvironment and acute nerve crush injury in the rat sciatic nerve

In severe peripheral nerve ischemia in the rat, serial nerve blood flow (NBF) measurements have identified evidence of 'no reflow', a mechanism of continued fiber damage during reperfusion. It has been postulated that 'no reflow' also occurs in nerve compression due to direct mechanical or ischemic (if compression is prolonged) injury of microvessels, resulting in continuing nerve fiber damage. To address this question, we measured endoneurial blood flow (NBF), oxygen tension and pH at the site of an acute nerve crush injury. In further sets of experiments, NBF and endoneurial oxygen tension were examined before and after prolonged epochs of crush. NBF and MR (microvascular resistance) were not appreciably different than values obtained in control animals without intervening brief nerve crush. NBF was slightly higher and MR slightly lower 2 h after injury, but the difference was not statistically significant. No evidence of significant endoneurial hypoxia or acidosis was observed. Similarly, after more prolonged crush there was no significant oligemia or hypoxia. The studies provide no evidence that 'no reflow' occurs in crush injury even if the injury is maintained for a period of time known to induce 'no reflow' with severe ischemia. We suggest that nerve damage in crush, and possibly compression, more likely arises from direct mechanical injury of fibers.

Axonal modulation of myelin gene expression in the peripheral nerve

Myelin gene expression (P0, MBP, P2, and MAG) was investigated during Wallerian degeneration and in the presence or absence of subsequent axonal regeneration and remyelination. The steady state levels of mRNA and protein were assessed in the crushed or permanently transected rat sciatic nerve at 0, 1, 4, 7, 10, 12, 14, 21, and 35 days after injury. The mRNA and protein steady state levels of the myelin specific genes, P0 and the MBPs, decreased to low yet detectable levels during Wallerian degeneration and returned to normal levels with subsequent axonal regeneration. The steady state level of P2 protein also followed a similar pattern of expression. The steady state level of MAG mRNA decreased to undetectable levels by 4 days of injury in the permanently transected nerve. After crush injury, re-expression of MAG to levels comparable to those of normal nerves preceded that of P2 by 2 days and that of P0 and the MBPs by 3 weeks during axonal regeneration and remyelination. These results support the proposed roles for MAG in the formation of initial Schwann cell-axonal contact required for myelin assembly, for P2 in fatty acid transport during myelination, and for P0 and the MBPs in the maintenance of the integrity and compactness of the myelin sheath. In addition, these results indicate that the expression of the myelin specific genes, P0 and MBP, is constitutive and that the level of myelin specific mRNAs is modulated by axonal contact and myelin assembly.

Regeneration in the peripheral nervous system

Mammalian peripheral nerve fibres can regenerate after injury. Repair is most likely to succeed if axons are simply crushed or have only a very short (less than 0.5 cm) interstump gap to cross and most likely to fail if the interstump gap is long (greater than 1 cm) and associated with soft tissue damage. Whereas reactive axonal sprouting appears to be an intrinsic neuronal response to injury, the subsequent organization of the axonal sprouts, in particular their orderly outgrowth in minifascicles towards a distant distal stump does not occur unless Schwann cells are present. During the injury response, Schwann cells proliferate; co-migrate with regrowing axons (when the proximal stump is separated from the distal stump); respond to axonal cues by transient upregulation or re-expression of molecules which provide a favourable substrate for axonal extension; and attract bundles of regrowing axons and their associated Schwann cells across interstump gaps up to 1 cm in length. Recruited macrophages remove myelin debris from the Schwann cell tubes; they probably interact with Schwann cells in other ways during the injury response, e.g. by presenting mitogens and cytokines.