The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.     

Schwann cells, neurotrophic factors, and peripheral nerve regeneration

The peripheral nervous system retains a considerable capacity for regeneration. However, functional recovery rarely returns to the preinjury level no matter how accurate the nerve repair is, and the more proximal the injury the worse the recovery. Among a variety of approaches being used to enhance peripheral nerve regeneration are the manipulation of Schwann cells and the use of neurotrophic factors. Such factors include, first, nerve growth factor (NGF) and the other recently identified members of the neurotrophin family, namely, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4/5); second, the neurokines ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF); and third, the transforming growth factors (TGFs)-beta and their distant relative, glial cell line-derived neurotrophic factor (GDNF). In this review article we focus on the roles in peripheral nerve regeneration of Schwann cells and of the neurotrophin family, CNTF and GDNF, and the relationship between these. Finally, we discuss what remains to be understood about the possible clinical use of neurotrophic factors.     

Myelinated sensory and alpha motor axon regeneration in peripheral nerve neuromas

Histochemical staining for carbonic anhydrase and cholinesterase (CE) activities was used to analyze sensory and motor axon regeneration, respectively, during neuroma formation in transected and tube-encapsulated peripheral nerves. Median-ulnar and sciatic nerves in the rodent model permitted testing whether a 4 cm greater distance of the motor neuron soma from axotomy site or intrinsic differences between motor and sensory neurons influenced regeneration and neuroma formation 10, 30, and 90 days later. Ventral root radiculotomy confirmed that CE-stained axons were 97% alpha motor axons. Distance significantly delayed axon regeneration. When distance was negligible, sensory axons grew out sooner than motor axons, but motor axons regenerated to a greater quantity. These results indicate regeneration differences between axon subtypes and suggest more extensive branching of motor axons within the neuroma. Thus, both distance from injury site to soma and inherent motor and sensory differences should be considered in peripheral nerve repair strategies.     

Expression of three forms of nitric oxide synthase in peripheral nerve regeneration

Nitric oxide (NO) is a short-lived molecule with messenger and cytotoxic functions in nervous, cardiovascular, and immune systems. Nitric oxide synthase (NOS), the enzyme responsible for NO synthesis, exists in three different forms: the neuronal (nNOS), present in discrete neuronal populations; the endothelial (eNOS), present in vascular endotheliun, and the inducible isoform (iNOS), expressed in various cell types when activated, including macrophages and glial cells. In this study, we have investigated the possible involvement of NO in Wallerian degeneration and the subsequent regeneration occurring after sciatic nerve ligature, using histochemistry and immunocytochemistry for the three NOS isoforms, at different postinjury periods. Two days after lesion, the three NOS isoforms are overexpressed, reaching their greatest expression during the second week. nNOS is upregulated in dorsal root ganglion neurons, centrifugally transported and accumulated in growing axons. eNOS is overexpressed in vasa nervorum of the distal stump and around ligature, and iNOS is induced in recruited macrophages. These findings indicate that different cellular sources contribute to maintain high levels of NO at the lesion site. The parallelism between NOS inductions and well-known repair phenomena suggests that NO, acting in different ways, may exert a beneficial effect on nerve regeneration.     

Ascending collaterals of cutaneous neurons in the fasciculus gracilis of the cat during peripheral nerve regeneration

The present investigation was undertaken to determine some morphological characteristics of Japanese strain of Schistosoma japonicum, the results are summarized as follows: (1) The pattern of testes in male worms can be divided into two types: the tandem (37.6%) and non-tandem (62.4%). (2) The number of testes varied from 1 to 13. The percentage of specimens with 7 testes was much higher than of others with both in tandem (70% against 30%) and non-tandem (86% against 14%) arrangement. (3) The unusual/irregular sets of testes including 2-3 additional sets were found in 158 (or 4.5%) of 3,498 male worms. Specimens with 7 testes in the unusual/irregular location numbered the highest (69.6%); the next with 8 (14.6%) and 6 (12%); and the lowest with 5 (3.8%). (4) Forty or 2.8% of 1,417 male worms were found to have 2-4 cecal loops. The cecal union located in 71-80% of worm bodys from the anterior end of male worm was found in highest number; the next in 61-70%; and the lowest in 51-60% and 81-90%. The location of united ceca in female worms was found all in the posterior part of body. (5) The ovary in all 125 females was located in the posterior portion of worm body except in 4 specimens. (6) The number of eggs in the uterus of female worms varied from 50 to 300. The mean number was 103.     

Degeneration and regeneration in rabbit peripheral nerve with long-term nerve cuff electrode implant: a stereological study of myelinated and unmyelinated axons

Spatio-temporal patterns of neuronal activity after the induction of long-term potentiation (LTP) in mouse hippocampal slices were studied with the use of a real-time high-resolution optical recording system. The slices were stained with voltage-sensitive dye and then high-frequency pulses (tetanus) were delivered to Schaffer collaterals of CA3 at the stratum radiatum of CA1. Optical signals as well as field potential in response to test pulses were potentiated after tetanus. The area of response measured by optical recording was slightly but significantly enlarged after tetanus, suggesting that the propagation of optical responses from CA1 towards the subiculum was enlarged. It was suggested that a great increase in neuronal activity was elicited at CA1 and the subiculum after LTP. These changes of spatio-temporal patterns of neuronal activity may contribute to learning and memory. The effect of trichloroethylene (TCE) on LTP was studied with the use of both electrical and optical and recording. The hippocampus from mice injected with 300 mg/kg or 1000 mg/kg TCE was sliced 24 hours after TCE-administration. Test pulses were delivered to Schaffer collaterals every 30 sec and the field potential from the stratum pyramidale of CA1 was recorded. At 40 min after the application of tetanus, population spikes (PS) were potentiated in all groups, but the post/pre ratio of PS was smaller in TCE groups than in the control. Optical recording was also carried out in 1000 mg/kg TCE-injected mice. At 40 min after tetanus, the optical signal response to the test pulse was potentiated in both TCE and control groups, but the post/pre ratio of the optical signals was smaller in TCE than in the control. No significant difference was detected in the increase in the neuronal response area between TCE and the control. It was suggested that TCE depressed the LTP, whereas the increase in the area after tetanus meant that a good amount of neuronal activity was still potentiated.     

Two modes of microtubule-associated protein 1B phosphorylation are differentially regulated during peripheral nerve regeneration

Two major modes of MAP1B phosphorylation (I and II), respectively recognized by monoclonal antibodies 150 and 125, have been related to remodeling and formation of processes in the mature nervous system. To gain insight into the cytoskeletal modifications underlying peripheral nerve regeneration, the pattern of expression of both MAP1B phosphorylated modes was studied during this process. Sciatic nerves from adult Wistar rats were crushed and animals allowed to survive for 5, 7, 10 or 14 days. After those survival periods, damaged and undamaged sciatic nerves, dorsal root ganglia (DRG), and spinal cords, were subjected to immunohistochemistry and Western blot, using antibodies 150 and 125. At all survival periods analysed, MAP1B phosphorylated at mode I was concentrated at the distal region of regenerating nerves whereas mode II phosphorylation underwent an overall decrease in regenerating axons that was less evident in more proximal nerve regions. Very high levels of MAP1B phosphorylated at mode II were detected in the bodies of DRG neurons and in bodies and dendrites of spinal motor neurons. This phosphorylation mode was also encountered in some Schwann cells and oligodendroglia associated with more proximal regions of regenerating axons. In this study we conclude that MAP1B was differentially phosphorylated depending on the cell type, subcellular compartment and stage of the regenerative process and discuss the possible functional implications that differential expression of each MAP1B phosphorylation mode might have during nerve regeneration.     

Perceptual processing of partially and fully assimilated words in French.

Models of speech perception attribute a different role to contextual information in the processing of assimilated speech. This study concerned perceptual processing of regressive voice assimilation in French. This phonological variation is asymmetric in that assimilation is partial for voiced stops and nearly complete for voiceless stops. Two auditory-visual cross-modal form priming experiments were used to examine perceptual compensation for assimilation in French words with voiceless versus voiced stop offsets. The results show that, for the former segments, assimilating context enhances underlying form recovery, whereas it does not for the latter. These results suggest that two sources of information--contextual information and bottom-up information from the assimilated forms themselves--are complementary and both come into play during the processing of fully or partially assimilated word forms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Does the cortical representation of body parts follow both injury to the related sensory peripheral nerve and its regeneration?

A study was made of the borderline between the physiological representations of the digits (D2, D3 and D4) and sinus whiskers in the rat primary somatosensory cortex after a contralateral infraorbital nerve crush. Following the injury, the physiological representation of the digits of the contralateral forepaw extended posterolaterally, occupying the anterolateral part of the whisker region (posteromedial barrel subfield). The extended physiological representation of the digits, though somewhat shrunken, remained after the reappearance of whisker-evoked responses, forming an overlapping area between the obligate digit and whisker representations. The findings emphasize the importance of afferent inputs in modulating cortical organization, but show that a reversible change in a sensory input (nerve damage) does not result in a perfectly reversible change in cortical representation.     

The influence of periodontitis on the subgingival flora around implants in partially edentulous patients

The hypothesis that teeth act as reservoirs of micro-organisms for the colonization of oral implants has recently been stated several times. The present study aimed at examining, in partially edentulous patients with severe periodontitis, whether pockets around teeth and implants harbored a comparable micro-flora. In 6 patients (3 with refractory periodontitis and 3 with advanced chronic adult periodontitis), plaque samples were taken from a deep and shallow pocket around both teeth and implants for differential phase contrast microscopy and DNA probe analysis. The results showed important differences in the sub-gingival flora between the 2 disease groups, as well as between deep and shallow pockets, around both implants and teeth. On the other hand, when pockets around teeth and implants with equal depths were compared a striking similarity was observed in the microbial composition. These observations confirm the hypothesis that pockets around teeth act as a reservoir and highlight the importance of periodontal health when oral implants are planned.     

p75 neurotrophin receptor induction and macrophage infiltration in peripheral nerve during experimental diabetic neuropathy: possible relevance on regeneration

In this study we examined the expression of the neurotrophin receptor p75 (p75NTR) and the activation of macrophages in the sciatic nerve of rats at different time points after the induction of diabetes with streptozotocin (STZ). Northern blot and immunocytochemical analysis showed that p75NTR was not detectable in the sciatic nerve by Week 2 after STZ treatment. At this time, single nerve fiber immunostaining using ED1 monoclonal antibody revealed that active macrophages were infiltrating the endoneurium, which had a normal morphological aspect. By Weeks 5 and 15 p75NTR mRNA and protein were induced in the endoneurium of diabetic animals. Immunocytochemical analysis of teased single nerve fibers showed that p75NTR protein was distributed uniformly along isolated fibers with no pathological evidence of axonal degeneration or myelin disruption. At this time, cells of the phagocyte lineage had already disappeared from the nerve. These data show that during experimental diabetic neuropathy, the endoneurial induction of p75NTR is localized along isolated nerve fibers showing no morphological alterations, and in time, follows the recruitment of active macrophages in the nerve, suggesting that these cells, directly or through their products, can influence p75NTR induction. This process might play an important role in STZ diabetic neuropathy, as a response to decreased levels of neurotrophins such as NGF and promoting nerve regeneration in the early phases of the disease.     

A magnetic evaluation of peripheral nerve regeneration: I. The discrepancy between magnetic and histologic data from the proximal segment

Histologic techniques can quantify the number of axons in a nerve, but give no information about electrical conductibility. The number of functional myelinated neuronal units in a nerve can be quantified based on a magnetic recording technique. When studying reconstructed peripheral nerves a significant difference between the results found with these two techniques can be observed. A comparison was made between the long-term changes in the number of histologically and magnetoneurophysiologically measured neuronal units proximal to a nerve reconstruction. This study was performed on 6 New Zealand White rabbits, 20 weeks after the peroneal nerve had been reconstructed. The contralateral nerves were used as a control. Histologic examination demonstrates a statistically significant decrease of approximately 5% in the number of myelinated fibers. The magnetoneurophysiological results demonstrate a decrease which is estimated to be caused by the loss of approximately 50% of the functional myelinated neuronal units in the nerve. Therefore we conclude that of the initially available myelinated neuronal units, 5% degenerate completely, 45% are vital but lose their signal conducting capability, and the remaining 50% are vital and continue to conduct signals. Apparently, only this latter group of 50% of the initially available functional neuronal units appears to remain available for functional recovery.     

Effects of guided bone regeneration around commercially pure titanium and hydroxyapatite-coated dental implants. II. Histologic analysis

The purpose of this study was to determine which treatment of a large osseous defect adjacent to an endosseous dental implant would produce the greatest regeneration of bone and degree of osseointegration: barrier membrane therapy plus demineralized freeze-dried bone allograft (DFDBA), membrane therapy alone, or no treatment. The current study histologically assessed changes in bone within the healed peri-implant osseous defect. In a split-mouth design, 6 implants were placed in edentulous mandibular ridges of 10 mongrel dogs after preparation of 6 cylindrical mid-crestal defects, 5 mm in depth, and 9.525 mm in diameter. An implant site was then prepared in the center of each defect to a depth of 5 mm beyond the apical extent of the defect. One mandibular quadrant received three commercially pure titanium (Ti) screw implants (3.75 x 10 mm), while the contralateral side received three hydroxyapatite (HA) coated root-form implants (3.3 x 10 mm). Consequently, the coronal 5 mm of each implant was surrounded by a circumferential defect approximately 3 mm wide and 5 mm deep. The three dental implants in each quadrant received either DFDBA (canine source) and an expanded polytetrafluoroethylene membrane (ePTFE), ePTFE membrane alone, or no treatment which served as the control. Clinically, the greatest increase in ridge height and width was seen with DFDBA/ePTFE. Histologically, statistically significant differences in defect osseointegration were seen between treatment groups (P < 0.0001: DFDBA/ePTFE > ePTFE alone > control). HA-coated implants had significantly greater osseointegration within the defect than Ti implants (P < 0.0001). Average trabeculation of newly formed bone in the defect after healing was significantly greater for HA-coated implants than for titanium (P < 0.0001), while the effect on trabeculation between treatments was not significantly different (P = 0.14). Finally, there were significantly less residual allograft particles in defect areas adjacent to HA-coated implants than Ti implants (P = 0.0355). The use of HA-coated implants in large size defects with DFDBA and ePTFE membranes produced significantly more osseointegration histologically than other treatment options and more than Ti implants with the same treatment combinations. The results of this study indicate that, although the implants appeared osseointegrated clinically after 4 months of healing, histologic data suggest that selection of both the implant type and the treatment modality is important in obtaining optimum osseointegration in large size defects.     

Peripheral nerve regeneration: the effects of postoperative irradiation

Our purpose is to review recent data and provide a clinical opinion on the use of antibiotics to prevent preterm birth or related maternal-neonatal complications. A literature review and a synthesis of opinion are provided. During prenatal care, standard practices should be applied regarding Neisseria gonorrhoeae, Chlamydia trachomatis, and bacteriuria. In addition, screen for and treat bacterial vaginosis in patients at high risk for preterm birth but do not treat Ureaplasma urealyticum or group B streptococci genital colonization. With preterm labor and intact membranes, standard practices should be applied regarding group B streptococci prophylaxis. Do not give antibiotics routinely to prolong pregnancy, but in patients with bacterial vaginosis and Trichomonas vaginalis specific treatment should be given. With preterm premature rupture of membranes, standard practices should be applied regarding group B streptococci prophylaxis, but additional antibiotics should also be given to prolong pregnancies at 24 to 32 weeks' gestation. Reported adverse effects have been few to date. However, increased diligence is needed for resistant organisms. In selected clinical settings antibiotic therapy is now indicated to prolong pregnancy and prevent maternal-neonatal complications associated with preterm birth.     

An electron microscopic study of the processes of nerve regeneration after lyophilized nerve homografting

The objective of the present study was to investigate the role of the Schwann cell basal lamina in nerve regeneration. To achieve this goal, we observed the process of axonal regeneration within a lyophilized nerve graft, in which only the basal lamina of the Schwann cell persisted. Sciatic nerves were removed from rats and lyophilized to kill the Schwann cells and other components. These grafts were transplanted to rat sciatic nerve defects. The rats were then killed after lapses of time. We observed the processes of axonal regeneration using a transmission electron microscope. Regeneration of axons along the inner surface of the Schwann cell basal lamina was clearly seen. These results suggest that, if tubular basal laminae persist, Schwann cells are not always necessary, and axonal regeneration can be induced in the direction toward the basal lamina.     

Transient neovascularisation of the frog retina during optic nerve regeneration

In conditions such as diabetic retinopathy, degenerative events in the retina are associated with neovascularisation. It is well established that a proportion of retinal ganglion cells die during optic nerve regeneration in the frog. The present study has determined whether neovascularisation takes place during this regenerative process. To do so, the pattern of blood vessels overlying the retinal ganglion cell layer was analysed in the frog Litoria (Hyla) moorei. We examined normal animals and those undergoing optic nerve regeneration following nerve crush. Blood vessels were visualised by perfusion with Indian ink and retinae were prepared as wholeamounts. In normal animals, the vascular tree was found to lie superficial to the nerve fibre layer and was more complex in regions overlying the area centralis and visual streak. After nerve crush, abnormal blood vessels transiently formed between the existing branches of the vascular tree. The new vessels were concentrated as an annulus centred on the optic nerve head and over the area centralis in midtemporal retina. The neovascularisation became most extensive between 6 and 10 weeks postcrush and disappeared by 12 weeks. The spatiotemporal sequence of neovascularisation suggests that it is triggered by accumulations of degenerating material formed as a proportion of the ganglion cells die during optic nerve regeneration.