Regenerative process of the facial nerve: rate of regeneration of fibers and their bifurcations

Hypoglossal nerve injury with long nerve resection leading to slow motoneuron death

Crush injury to peripheral nerves in adult animals is considered not to trigger retrograde neuronal cell death; however, several studies reported neuronal cell death following severe injuries including nerve transection, resection, or avulsion. However, the rate of neuronal cell death varied among studies. In this study, we evaluated the outcomes of very severe nerve injury by long nerve resection in adult rats. Right hypoglossal (XII) nerve was exposed, and a 9-mm section was resected. At 4, 8, and 12 weeks after the resection, the number of XII neurons were counted in from the rostral to caudal sections. The number of XII neurons in the injured right side was reduced after the XII nerve resection compared with the uninjured left side. The mean rates of surviving neurons at 4, 8, and 12 weeks after the nerve resection were 83.5 %, 73.9 %, and 61.1 %, respectively, which were significantly lower than those of the control. The number of XII neurons after extensive XII nerve resection declined gradually over a relatively long time period, revealing that extensive nerve resection led to slow cell death of the injured neurons.

The effects of venous ensheathment on facial nerve repair in the rat

OBJECTIVE

To investigate the protective effect of autologous venous ensheathment on sutured rat facial nerve and to test whether the ensheathment could improve the functional recovery of repaired nerve and accuracy of axonal growth.

STUDY DESIGN

In vivo study.

METHODS

Forty-six rats were examined, with six rats serving as normal controls and 40 receiving facial nerve transection and suture repair (SR) or transection and suture repair with an additional venous ensheathment (VE). The rats were then subjected to functional testing, histological assessment of nerve specimens, or retrograde tracing, respectively.

RESULTS

At the postoperative day (POD) 60, the venous ensheathment showed no adhesion at the surrounding tissues. No significant difference in neuroma formation was found between the two surgical manipulations (SR and VE groups) (P < 0.05). Retrogradely labeled motoneurons in facial nuclei were extremely disorganized after the facial nerve undertook surgical manipulation. In all manipulated groups, double retrogradely labeled neurons, indicative of aberrant axonal branching during regeneration, could be observed after peripheral manipulation across all time points. With the two facial surgical manipulations, the average count of double-labeled neurons at POD 60 was significantly less than at POD 21 (P < 0.05).

CONCLUSION

Autologous venous ensheathment could not help with the functional recovery of facial nerve or improve the accuracy of axonal regeneration. Further studies are warranted to elucidate the effects of venous ensheathment in other motor and sensory nerve models.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:1558-1564, 2017.

Stereological assessment of the total number of hypoglossal neurons after repeated crush injuries to the hypoglossal nerve in adult rats

OBJECTIVE

Retrograde neuronal cell death does not occur in mature motoneurons following the axonal injury of peripheral nerves. However, a previous study suggested that retrograde neuronal cell death does occur in adult rats after the creation of double lesions on the hypoglossal (XII) nerve based on a substantial decrease in the number of XII neurons. Using stereological methods, we examined neuronal apoptosis in XII neurons and the total number of XII neurons following repeated crush injuries to the XII nerve.

METHODS

The right XII nerve of adult rats was crushed three times at one-week intervals with a brain aneurysm clip. At 4 weeks after the final crush, the total numbers of XII neurons on the injured right and uninjured left sides were estimated stereologically.

RESULTS

After repeated crush injuries, no apoptosis was evident in XII neurons as indicated by immunostaining for cleaved caspase-3. Moreover, immunohistochemistry for the vesicular acetylcholine transporter revealed axonal elongation in the tongue 4 weeks after repeated crush injuries. At 4 weeks, the total numbers of XII neurons were 7800 ± 290 on the injured right side and 8000 ± 230 on the uninjured left side, and no significant difference was evident between the injured and uninjured sides.

CONCLUSION

Neuronal cell death does not occur in XII neurons and the total number of XII neurons does not decrease after repeated crush injuries of the XII nerve in adult rats.

Experimental studies on the recovery processes from severe facial palsy and the development of its sequelae

OBJECTIVE

To experimentally elucidate the pathogenesis of inappropriate co-contraction of facially innervated muscles after severe facial palsy.

METHODS

Twenty-two guinea pigs with severe facial palsy induced by the interruption of the petrosal artery were used to follow up behavioral facial movement, including the degree of facial palsy and abnormal hyperkinetic facial movement of synkinesis and mass contracture. At the end of the follow-up, the evoked facial compound muscle action potential (evoked FCMP) and antidromically evoked facial nerve response (AFNR) were examined in a few typical cases with complete recovery and with incomplete recovery accompanied by synkinesis. After the follow-up, all animals were sacrificed for morphological studies, which consisted of a light-microscopic study (by Luxol fast blue and hematoxylin and eosin staining or toluidine blue staining) and/or an electron-microscopic study.

RESULTS

The initial sign of recovery was mass contracture or spasm. This condition continued for 2 weeks or more. As voluntary facial movement recovered, the mass contracture became unnoticeable. It could not be distinguished when the so-called synkinesis developed. Synkinesis usually developed during the recovery process from severe to moderate palsy, and synkinesis persisted or progressed once it appeared. Histologically, unmyelinated fibers were intermingled with myelinated fibers in an early stage of recovery with mass contracture. In the late stage with the development of synkinesis, however, such an intermingling of unmyelinated and myelinated axons was not observed. In this stage, axons became well myelinated, but they were irregular in shape in cases with synkinesis. Especially, axons irregularly ran at the level of the G1 (at the region of the second genu) segment, and bifurcated axons were sporadically found. The axon count had a tendency to increase toward the periphery. AFNR was not detected, although evoked FCMP could be clearly detected in cases with synkinesis.

CONCLUSION

Misguidance of regenerated axons is an important cause of facial synkinesis in the ischemia-induced facial palsy model. Ephaptic transmission between unmyelinated and myelinated axons is also likely to be responsible for mass contracture manifested in the early stage of the recovery process.

Motor fiber organization in the extratemporal trunk of the facial nerve in rats: A retrograde Fluoro-Gold study

Understanding the microanatomy of the facial nerve is vital to functional restoration of facial nerve injury. This study aimed to locate the spatial orientation of five branches in the extratemporal trunk of the rat facial nerve (ETFN). Fifteen adult Sprague-Dawley albino rats were divided randomly into five groups corresponding to the five facial nerves. Fluoro-Gold(™) (FG) was applied to one branch in all three rats in each group. The trunk of the facial nerve was cut at three points for fluorescence detection. Staining results showed that each branch of the facial motor nerve had a topographical orientation in the distal part of the ETFN. The temporal branch was located in the medial and acroscopic quadrant of the nerve trunk. The zygomatic branch was located in the lateral and acroscopic quadrant. The buccal branch occupied the upper half of the nerve trunk, whereas the mandibular branch occupied the lower half. The cervical branch presented a square-shaped distribution in the lateral nerve trunk. In the middle part of the ETFN, the topographical orientation remained clear, but the FG-labeled zone was extended to some extent. In the stylomastoid foramen region, all branches diffused, thereby blurring the orientation. In conclusion, each branch of the facial motor nerve had a topographical orientation and distribution in the crotch and middle part of the ETFN, but the branches diffused near the stylomastoid foramen.

Effects of repeated crush injuries on motor functional recovery of the sciatic nerve

OBJECTIVES

The present study was conducted to examine whether repeated crush injuries have significant effects on motor functional recovery of peripheral nerves.

METHODS

Repeated crush injuries of the sciatic nerve were inflicted on adult rats at 1-week intervals, and functionality of the sciatic nerve was assessed by the static sciatic index each week for 8 weeks after the final injury. To determine the effects of repeated crush injuries on motor functional recovery of the sciatic nerve, tibialis anterior muscle fibers from single and triple crush injuries were examined, and fiber size and fiber reinnervation during the 2- to 4-week period after the final injury were measured.

RESULTS

Compared to single crush injuries, which completely recovered by post-injury week 4, double crush injuries resulted in retarded, but complete recovery by post-injury week 6, whereas triple crush injuries resulted in marked retardation in the regenerative process with incomplete recovery during week 8 of the experimental period. Muscle fiber size for rats with triple crush did not recover to normal range at post-injury week 4, despite its normal size for rats with single crush. The rate of reinnervation increased prominently between post-injury weeks 2 and 3 in both injuries, but the rate with triple crush was lower than that with single crush at post-injury week 3.

DISCUSSION

These results, which contradict those of a previous study that reported early functional recovery, indicate that repeated crush injuries inhibit motor functional recovery of the damaged sciatic nerve, as evidenced by delayed and incomplete regeneration, atrophied muscle fibers, and delayed reinnervation.

Abnormal motor reflexes and dormant facial motor neurons in rats with facial-facial anastomosis

After facial nerve injury, some post-paralysis sequelae, such as synkinesis, hemispasm and 'crocodile tears' syndrome appear. The psychosocial impact of these sequelae is unavoidable. Despite recent advances and the growing popularity of this field, the pathophysiological mechanisms of facial nerve injury and regeneration are still not well understood. In this report, an abnormal motor reflex coincident with synkinetic facial movement was examined in a rat model using the blink reflex technique. Some dormant facial motor neurons were found which could not innervate through the suture site but remained alive. These results suggest that such dormant neurons might exert roles distinct from those of re-innervated neurons during facial nerve injury and regeneration. Further study is required to elucidate the biomolecular structure and electrophysiological features of such neurons.

T cell memory in the injured facial motor nucleus: relation to functional recovery following facial nerve crush

T cells have the ability to mount a memory response to a previously encountered antigen such that re-exposure to the antigen results in a response that is greater in magnitude and function. Following facial nerve transection, T cells have been shown to traffic to injured motor neurons in the facial motor nucleus (FMN) and may have the ability to promote neuronal survival and functional recovery. Previously, we demonstrated that early exposure to neuronal injury on one side of the brain during young adulthood elicited a T cell response that was greater in magnitude following exposure to the same form of injury on the contralateral side later in adulthood. Whether the T cell memory response to neuronal injury influenced functional recovery following nerve crush injury was unknown. In the current study, we tested the hypotheses that (1) transection of the right facial nerve in sensitized mice would result in faster recovery of the whisker response when the contralateral facial nerve is crushed 10 weeks later, and (2) the early recovery would be associated with an increase in the magnitude of the T cell response in the contralateral FMN following crush injury in sensitized mice. The onset of modest recovery in sensitized mice occurred between 3 and 5 days following crush injury of the contralateral facial nerve, approximately 1.5 days earlier than naïve mice, and was associated with more than a two-fold increase in the magnitude of the T cell response in the contralateral FMN following crush injury. There was no difference between groups in the number of days to full recovery. Further study of how T cell memory influences neuroregeneration may have important implications for translational research.

Motor neurons essential for normal sciatic function in neonatally nerve-injured rats

The present study was aimed to determine neuronal population essential for normal motor function in young adult rats receiving various degrees of crushing to the sciatic nerve at the neonatal stage. Motor function was estimated by the static sciatic index, and a neuronal tracer was applied to the common peroneal nerve. The total numbers of the tracer-labeled neurons of the nerve-crushed rats were 74-383 in the normal function group, 14-61 in the disordered function group, and 0-32 in the severely disordered function group. We conclude that normal motor function can be well preserved by a very small population of motor neurons (approximately 15% of the control value) in the neonatally sciatic nerve-injured rats.

Use of muscle fibrillation for tracking nerve regeneration

Individual muscle fibers in denervated muscle demonstrate repetitive, spontaneous contraction. Such fibrillation activity disappears in denervated muscle if reinnervation occurs, but this relationship has not been formally studied. To test whether the disappearance of fibrillation can be used to track nerve regeneration, we quantified the presence and subsequent disappearance of electromyographic (EMG) fibrillation potentials and fibrillation-related movement in the rat tongue after unilateral hypoglossal nerve crush at two locations. In mice, fibrillation movement of vibrissae were monitored after unilateral facial nerve crush and compared with the return of symmetrical vibrissae sweeping movements. In both of these rodent cranial motor systems, there was a conspicuous loss of fibrillation at a time when reinnervation is known to take place, suggesting that the visual appearance of fibrillation-related movement can be used as a simple, noninvasive means of tracking nerve regeneration in these popular experimental motor systems.

Lack of ultrastructural detrusor changes following endoscopic injection of botulinum toxin type a in overactive neurogenic bladder

INTRODUCTION

Endoscopical injections of Botulinum toxin type A into the detrusor muscle are gaining clinical acceptance in the treatment of neurogenic detrusor overactivity. Structural effects of Botulinum toxin type A are only known from studies on striated muscles, where a widespread nerve sprouting occurs temporarily. The aim of this study was to evaluate the ultrastructural effects of Botulinum toxin type A injections on the human detrusor.

MATERIAL AND METHODS

30 detrusor biopsies were obtained from 24 patients with neurogenic detrusor overactivity. Patients were divided into two groups: Group I included 13 biopsies from patients before the first Botulinum toxin type A injection. Group II included 6 biopsies from patients within 3 months after the first injection and 11 biopsies at the time of decreasing efficacy of Botulinum toxin type A. The biopsies were processed by standard procedure for detailed electron microscopic study and evaluated by 2 examiners without prior knowledge of clinical/urodynamic data.

RESULTS

No statistically significant detrusor changes have been found concerning muscle cell fascicle structure (p = 0.445), width of intercellular space (p = 0.482) and number/kind of muscle cell junctions (p = 0.443). A median of 70% of intrinsic axon terminals presented with signs of degeneration in group I, a median of 66% in group II (p = 0.840). Out of 309 evaluated axon terminals in both groups, 1 sprouting axon was found in group I, 3 sprouting axons in group II (p = 0.864). Specimen from group I and group II showed only limited collagen deposits within the detrusor. No changes in the ultrastructure of the detrusor have been observed in those biopsies obtained before and after the Botulinum toxin type A injection of the same patient.

CONCLUSION

This study verifies our earlier report of severe intrinsic axon degeneration in the detrusor of patients with neurogenic detrusor overactivity. It also shows nearly no structural differences of the detrusor before and after Botulinum toxin type A injections. Contrary to reports of striated muscle, axonal sprouting within the detrusor was very limited after Botulinum toxin type A injections indicating pathophysiologically different reactions to the toxin either between striated muscle and smooth muscle or between different treated diseases.

Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats

Section of motor nerve fibers (axotomy) elicits a variety of morphofunctional responses in the motoneurons in the motor nuclei. Later than the fifth post-operational day after section of the facial nerve, synapse elimination occurs in the facial motoneuron pool, leading to gradual abolishment of synaptic input-driven activities of the axotomized motoneurons. However, it remains unknown how the amount of synaptic input changes during this period between the axotomy and the synaptic elimination. Here we examined a hypothesis that axotomy of the motoneurons itself modifies the synaptic inputs to the motoneurons. One day after axotomy, the postsynaptic currents, mostly mediated by non-N-methyl-D-aspartic acid (non-NMDA) receptors, recorded from the axotomized facial motoneurons in the acute slice preparations of the rats were of higher frequency and larger amplitude than those in the intact motoneurons. This difference was not observed after the third post-operational day and appeared earlier than the changes in the electrophysiological properties and increase in the number of dead neurons in the axotomized motor nucleus. The larger postsynaptic current frequency of the axotomized motoneurons was observed both in the absence and in the presence of tetrodotoxin citrate, suggesting that increased excitability and facilitated release underlie the postsynaptic current frequency increase. These results suggest that synaptic re-organization occurs in the synapses of motoneurons at an early stage following axotomy.

Galectin-1 in regenerating motoneurons

The exogenous application of recombinant galectin-1 has recently been shown to promote the rate of peripheral nerve regeneration. Endogenous neuronal galectin-1 expression has recently been demonstrated to increase after axotomy. Here we demonstrate a significant increase in the endogenous neuronal expression of galectin-1 mRNA in facial motoneurons after either a nerve resection or crush injury in mice. This increase in galectin-1 expression was due in part to the loss of target-derived factor(s) as indicated by both the return of galectin-1 expression to control levels following target re-innervation and the increase in galectin-1 expression after blockade of axonal transport by an interneuronal colchicine injection. Furthermore, interneuronal injections of glial-derived neurotrophic factor into the uninjured nerve also increased galectin-1 mRNA expression within facial motoneurons suggesting that positive signals may also be involved in the regulation of galectin-1 expression. Galectin-1 null mutant mice showed an attenuated rate of functional recovery of whisking movement after a facial nerve crush.