Laser doppler perfusion imaging of skin territory to reflect autonomic functional recovery following sciatic nerve autografting repair in rats

Chitosan Nerve Grafts Incorporated with SKP-SC-EVs Induce Peripheral Nerve Regeneration

BACKGROUND

Repair of long-distance peripheral nerve defects remains an important clinical problem. Nerve grafts incorporated with extracellular vesicles (EVs) from various cell types have been developed to bridge peripheral nerve defects. In our previous research, EVs obtained from skin-derived precursor Schwann cells (SKP-SC-EVs) were demonstrated to promote neurite outgrowth in cultured cells and facilitate nerve regeneration in animal studies.

METHODS

To further assess the functions of SKP-SC-EVs in nerve repair, we incorporated SKP-SC-EVs and Matrigel into chitosan nerve conduits (EV-NG) for repairing a 15-mm long-distance sciatic nerve defect in a rat model. Behavioral analysis, electrophysiological recording, histological investigation, molecular analysis, and morphometric assessment were carried out.

RESULTS

The results revealed EV-NG significantly improved motor and sensory function recovery compared with nerve conduits (NG) without EVs incorporation. The outgrowth and myelination of regenerated axons were improved, while the atrophy of target muscles induced by denervation was alleviated after EVs addition.

CONCLUSION

Our data indicated SKP-SC-EVs incorporation into nerve grafts represents a promising method for extended peripheral nerve damage repair.

Repair of peripheral nerve defects by nerve grafts incorporated with extracellular vesicles from skin-derived precursor Schwann cells

Our previous studies have shown that extracellular vesicles from skin-derived precursor Schwann cells (SKP-SC-EVs) promote neurite outgrowth of sensory and motor neurons in vitro. This study was aimed at generating an artificial nerve graft incorporated with SKP-SC-EVs to examine in vivo effects of SKP-SC-EVs on peripheral nerve regeneration. Here SKP-SC-EVs were isolated and then identified by morphological observation and phenotypic marker expression. Following co-culture with SCs or motoneurons, SKP-SC-EVs were internalized, showing the capability to enhance SC viability or motoneuron neurite outgrowth. In vitro, SKP-SC-EVs released from Matrigel could maintain cellular uptake property and neural activity. Nerve grafts were developed by incorporating Matrigel-encapsulated SKP-SC-EVs into silicone conduits. Functional evaluation, histological investigation, and morphometric analysis were performed to compare the nerve regenerative outcome after bridging the 10-mm long sciatic nerve defect in rats with our developed nerve grafts, silicone conduits (filled with vehicle), and autografts respectively. Our developed nerve grafts significantly accelerated the recovery of motor, sensory, and electrophysiological functions of rats, facilitated outgrowth and myelination of regenerated axons, and alleviated denervation-induced atrophy of target muscles. Collectively, our findings suggested that incorporation of SKP-SC-EVs into nerve grafts might represent a promising paradigm for peripheral nerve injury repair. STATEMENT OF SIGNIFICANCE: Nerve grafts have been progressively developed to meet the increasing requirements for peripheral nerve injury repair. Here we reported a design of nerve grafts featured by incorporation of Matrigel-encapsulated extracellular vesicles from skin-derived precursor Schwann cells (SKP-SC-EVs), because SKP-SC-EVs were found to possess in vitro neural activity, thus raising the possibility of cell-free therapy. Our developed nerve grafts yielded the satisfactory outcome of nerve grafting in rats with a 10-mm long sciatic nerve defect, as evaluated by functional and morphological assessments. The promoting effects of SKP-SC-EVs-incorporating nerve grafts on peripheral nerve regeneration might benefit from in vivo biological cues afforded by SKP-SC-EVs, which had been released from Matrigel and then internalized by residual neural cells in sciatic nerve stumps.

Longitudinal Study of Functional Reinnervation of the Denervated Skin by Collateral Sprouting of Peptidergic Nociceptive Nerves Utilizing Laser Doppler Imaging

Restitution of cutaneous sensory function is accomplished by neural regenerative processes of distinct mechanisms following peripheral nerve lesions. Although methods available for the study of functional cutaneous nerve regeneration are specific and accurate, they are unsuitable for the longitudinal follow-up of the temporal and spatial aspects of the reinnervation process. Therefore, the aim of this study was to develop a new, non-invasive approach for the longitudinal examination of cutaneous nerve regeneration utilizing the determination of changes in the sensory neurogenic vasodilatatory response, a salient feature of calcitonin gene-related peptide-containing nociceptive afferent nerves, with scanning laser Doppler flowmetry. Scanning laser Doppler imaging was applied to measure the intensity and spatial extent of sensory neurogenic vasodilatation elicited by the application of mustard oil onto the dorsal skin of the rat hindpaw. Mustard oil induced reproducible and uniform increases in skin perfusion reaching maximum values at 2-4 min after application whereafter the blood flow gradually returned to control level after about 8-10 min. Transection and ligation of the saphenous nerve largely eliminated the vasodilatatory response in the medial aspect of the dorsal skin of the hindpaw. In the 2 nd to 4 th weeks after injury, the mustard oil-induced vasodilatatory reaction gradually recovered. Since regeneration of the saphenous nerve was prevented, the recovery of the vasodilatatory response may be accounted for by the collateral sprouting of neighboring intact sciatic afferent nerve fibers. This was supported by the elimination of the vasodilatatory response in both the saphenous and sciatic innervation territories following local treatment of the sciatic nerve with capsaicin to defunctionalize nociceptive afferent fibers. The present findings demonstrate that this novel technique utilizing scanning laser Doppler flowmetry to quantitatively measure cutaneous sensory neurogenic vasodilatation, a vascular response mediated by peptidergic nociceptive nerves, is a reliable non-invasive approach for the longitudinal study of nerve regeneration in the skin.

The role of C-afferents in mediating neurogenic vasodilatation in plantar skin after acute sciatic nerve injury in rats

Background

Vasomotor regulation of dermal blood vessels, which are critical in the function of the skin in thermoregulatory control, involves both neural and non-neural mechanisms. Whereas the role of sympathetic nerves in regulating vasomotor activities is comprehensively studied and well recognized, that of sensory nerves is underappreciated. Studies in rodents have shown that severance of the sciatic nerve leads to vasodilatation in the foot, but whether sympathetic or sensory nerve fibers or both are responsible for the neurogenic vasodilatation remains unknown.

Results

In adult Sprague–Dawley rats, vasodilatation after transection of the sciatic nerve gradually diminished to normal within 3–4 days. The neurotmesis-induced neurogenic vasodilatation was not detectable when the sciatic nerve was chronically deafferentated by selective resection of the dorsal root ganglia (DRGs) that supply the nerve. Specific activation of C-afferents by intra-neural injection of capsaicin resulted in neurogenic vasodilatation to a magnitude comparable to that by neurotmesis, and transection of the sciatic nerve pre-injected with capsaicin did not induce further vasodilatation.

Conclusions

Our results collectively indicate that vasodilatation after traumatic nerve injury in rats is predominantly mediated by C-fiber afferents.

Axonotmesis-evoked plantar vasodilatation as a novel assessment of C-fiber afferent function after sciatic nerve injury in rats

Quantitative assessment of the recovery of nerve function, especially sensory and autonomic nerve function, remains a challenge in the field of nerve regeneration research. We previously found that neural control of vasomotor activity could be potentially harnessed to evaluate nerve function. In the present study, five different models of left sciatic nerve injury in rats were established: nerve crush injury, nerve transection/suturing, nerve defect/autografting, nerve defect/conduit repair, and nerve defect/non-regeneration. Laser Doppler perfusion imaging was used to analyze blood perfusion of the hind feet. The toe pinch test and walking track analysis were used to assess sensory and motor functions of the rat hind limb, respectively. Transmission electron microscopy was used to observe the density of unmyelinated axons in the injured sciatic nerve. Our results showed that axonotmesis-evoked vasodilatation in the foot 6 months after nerve injury/repair recovered to normal levels in the nerve crush injury group and partially in the other three repair groups; whereas the nerve defect/non-regeneration group exhibited no recovery in vasodilatation. Furthermore, the recovery index of axonotmesis-evoked vasodilatation was positively correlated with toe pinch reflex scores and the density of unmyelinated nerve fibers in the regenerated nerve. As C-fiber afferents are predominantly responsible for dilatation of the superficial vasculature in the glabrous skin in rats, the present findings indicate that axonotmesis-evoked vasodilatation can be used as a novel way to assess C-afferent function recovery after peripheral nerve injury. This study was approved by the Ethics Committee for Laboratory Animals of Nantong University of China (approval No. 20130410-006) on April 10, 2013.

Injection of Fluoro-Gold into the tibial nerve leads to prolonged but reversible functional deficits in rats

Tract tracing with neuronal tracers not only represents a straightforward approach to identify axonal projection connection between regions of the nervous system at distance but also provides compelling evidence for axonal regeneration. An ideal neuronal tracer meets certain criteria including high labeling efficacy, minimal neurotoxicity, rapid labeling, suitable stability in vivo, and compatibility to tissue processing for histological/immunohistochemical staining. Although labeling efficacy of commonly used fluorescent tracers has been studied extensively, neurotoxicity and their effect on neural functions remains poorly understood. In the present study, we comprehensively evaluated motor and sensory nerve function 2-24 weeks after injection of retrograde tracer Fluoro-Gold (FG), True Blue (TB) or Fluoro-Ruby (FR) in the tibial nerve in adult Spague-Dawley rats. We found that motor and sensory nerve functions were completely recovered by 24 weeks after tracer exposure, and that FG lead to a more prolonged delay in functional recovery than TB. These findings shed light on the long-term effect of tracers on nerve function and peripheral axonal regeneration, and therefore have implications in selection of appropriate tracers in relevant studies.

Tubulation repair mitigates misdirection of regenerating motor axons across a sciatic nerve gap in rats

The repair of peripheral nerve laceration injury to obtain optimal function recovery remains a big challenge in the clinic. Misdirection of regenerating axons to inappropriate target, as a result of forced mismatch of endoneurial sheaths in the case of end-to-end nerve anastomosis or nerve autografting, represents one major drawback that limits nerve function recovery. Here we tested whether tubulation repair of a nerve defect could be beneficial in terms of nerve regeneration accuracy and nerve function. We employed sequential retrograde neuronal tracing to assess the accuracy of motor axon regeneration into the tibial nerve after sciatic nerve laceration and entubulation in adult Sprague-Dawley rats. In a separate cohort of rats with the same sciatic nerve injury/repair protocols, we evaluated nerve function recovery behaviorally and electrophysiologically. The results showed that tubulation repair of the lacerated sciatic nerve using a 3-6-mm-long bioabsorbable guidance conduit significantly reduced the misdirection of motor axons into the tibial nerve as compared to nerve autografting. In addition, tubulation repair ameliorated chronic flexion contracture. This study suggests that tubulation repair of a nerve laceration injury by utilizing a bioresorbable nerve guidance conduit represents a potential substitute for end-to-end epineurial suturing and nerve autografting.

Functional and structural microanatomy of the fetal sciatic nerve

INTRODUCTION

The ultrastructure of a nerve has implications for surgical nerve repair. The aim of our study was to characterize the fascicular versus fibrillar anatomy and the autonomic versus somatic nature of the fetal sciatic nerve (SN).

METHODS

Immunohistochemistry for vesicular acetylcholine transporter, tyrosine hydroxylase, and peripheral myelin protein 22 was performed to identify cholinergic, adrenergic, and somatic axons, respectively, in the human fetal SN. Two-dimensional (2D) analysis and 3D reconstructions were performed.

RESULTS

The fetal SN is composed of one-third stromal tissue and two-thirds neural tissue. Autonomic fibers are predominant over somatic fibers within the neural tissue. The distribution of somatic fibers is initially random, but then become topographically organized after intra- and interfascicular rearrangements have occurred within the nerve.

CONCLUSIONS

The fetal model presents limitations but enables illustration of the nature of the nerve fibers and the 3D fascicular anatomy of the SN. Muscle Nerve 56: 787-796, 2017.

Intra and postoperative evaluations of microcirculation and micro-rheological parameters in a rat model of musculocutaneous flap ischemia-reperfusion

PURPOSE

To examine how the ischemia-reperfusion injury of latissimus dorsi-cutaneous maximus (LDCM) musculocutaneous flap affects the microcirculatory (flap's skin surface) and hemorheological parameters, and whether an intraoperative deterioration would predictively suggest flap failure in the postoperative period.

METHODS

Ten healthy male rats were subjected to the study. In Group I the left flap was sutured back after 2-hour, while the contralateral side was right after its elevation. In Group II the same technique was applied, but the pedicle of the left flap was atraumatically clamped for 2-hour. The contralateral side was left intact. On the flap skin surface laser Doppler tissue flowmetry measurements were done before and after and during the protocols applied in the groups. Microcirculatory and hemorheological examinations were done postoperatively.

RESULTS

The microcirculatory parameters significantly decreased during immobilization and ischemia. Afterwards, all the regions showed normalization. In the retrospective analysis there was a prominent difference between the microcirculatory parameters of necrotic and survived flap during the early postoperative days (1-3) in Group II. Erythrocyte aggregation and deformability showed only slight differences.

CONCLUSIONS

Two-hour ischemia and reperfusion caused deterioration in latissimus dorsi-cutaneous maximus flap microcirculation. Predicting the possible postoperative complication, the intraoperative laser Doppler measurement can be informative.

Active skin perfusion and thermoregulatory response in the hand following nerve injury and repair in human upper extremities

Cutaneous vasoconstriction/vasodilatation occurs in response to whole body and local cooling/heating, and the vasomotor activities play a pivotal role in thermal control of the human body. The mechanisms underlying regulation of skin blood flow involve both neurogenic and humeral/local chemical influence, contributing to the initial response to thermal stimuli and the prolonged phase of response, respectively. Previous studies have suggested the impairment of cutaneous thermal regulation after nerve injury. However, the evidence regarding how the skin perfusion and thermoregulatory response evolve after nerve injury and repair remains limited. Here we observed, by utilizing laser-Doppler perfusion imaging, baseline skin perfusion and perfusion change in response to thermal stimuli after median and ulnar nerve injury, and the results showed that baseline perfusion in autonomous skin area profoundly decreased and active rewarming after clod stress dramatically diminished before sensory recovery of the skin became detectable. In addition, baseline cutaneous perfusion was recovered as the skin regained touch sensation, and exhibited positive correlation to touch sensibility of the skin. These data indicate that both active perfusion and thermoregulatory response of the skin are markedly compromised during skin denervation and can be recovered by re-innervation. This suggests the importance of timely repair of injured nerve, especially in the practice of replantation.

Neurological function following intra-neural injection of fluorescent neuronal tracers in rats

Fluorescent neuronal tracers should not be toxic to the nervous system when used in long-term labeling. Previous studies have addressed tracer toxicity, but whether tracers injected into an intact nerve result in functional impairment remains to be elucidated. In the present study, we examined the functions of motor, sensory and autonomic nerves following the application of 5% Fluoro-Gold, 4% True Blue and 10% Fluoro-Ruby (5 μL) to rat tibial nerves via pressure injection. A set of evaluation methods including walking track analysis, plantar test and laser Doppler perfusion imaging was used to determine the action of the fluorescent neuronal tracers. Additionally, nerve pathology and ratio of muscle wet weight were also observed. Results showed that injection of Fluoro-Gold significantly resulted in loss of motor nerve function, lower plantar sensibility, increasing blood flow volume and higher neurogenic vasodilatation. Myelinated nerve fiber degeneration, unclear boundaries in nerve fibers and high retrograde labeling efficacy were observed in the Fluoro-Gold group. The True Blue group also showed obvious neurogenic vasodilatation, but less severe loss of motor function and degeneration, and fewer labeled motor neurons were found compared with the Fluoro-Gold group. No anomalies of motor and sensory nerve function and no myelinated nerve fiber degeneration were observed in the Fluoro-Ruby group. Experimental findings indicate that Fluoro-Gold tracing could lead to significant functional impairment of motor, sensory and autonomic nerves, while functional impairment was less severe following True Blue tracing. Fluoro-Ruby injection appears to have no effect on neurological function.

Cold-induced vasodilatation in cold-intolerant rats after nerve injury

PURPOSE

Cold-induced vasodilatation (CIVD) is a cyclic regulation of blood flow during prolonged cooling of protruding body parts. It is generally considered to be a protective mechanism against local cold injuries and cold intolerance after peripheral nerve injury. The aim of this study was to determine the role of the sympathetic system in initiating a CIVD response.

METHODS

Eight rats were operated according to the spared nerve injury (SNI) model, eight underwent a complete sciatic lesion (CSL) and six underwent a sham operation. Prior to operation, 3, 6 and 9 weeks postoperatively, both hind limbs were cooled and the skin temperature was recorded to evaluate the presence of CIVD reactions. Cold intolerance was determined using the cold plate test and mechanical hypersensitivity measured using the Von Frey test.

RESULTS

No significant difference in CIVD was found comparing the lateral operated hind limb for time (preoperatively and 3, 6 and 9 weeks postoperatively; p = 0.397) and for group (SNI, CSL and Sham; p = 0.695). SNI and CSL rats developed cold intolerance and mechanical hypersensitivity.

CONCLUSION

Our data show that the underlying mechanisms that initiate a CIVD reaction are not affected by damage to a peripheral nerve that includes the sympathetic fibres. We conclude that the sympathetic system does not play a major role in the initiation of CIVD in the hind limb of a rat.

CLINICAL RELEVANCE

No substantial changes in the CIVD reaction after peripheral nerve injury imply that the origin of cold intolerance after a traumatic nerve injury is initiated by local factors and has a more neurological cause. This is an important finding for future developing treatments for this common problem, as treatment focussing on vaso-regulation may not help diminish symptoms of cold-intolerant patients.