Cell loss in sensory ganglia after peripheral nerve injury. An anatomical tracer study using lectin-coupled horseradish peroxidase in cats

The histomorphological and stereological assessment of rat dorsal root ganglion tissues after various types of sciatic nerve injury

Peripheral nerve injuries lead to significant changes in the dorsal root ganglia, where the cell bodies of the damaged axons are located. The sensory neurons and the surrounding satellite cells rearrange the composition of the intracellular organelles to enhance their plasticity for adaptation to changing conditions and response to injury. Meanwhile, satellite cells acquire phagocytic properties and work with macrophages to eliminate degenerated neurons. These structural and functional changes are not identical in all injury types. Understanding the cellular response, which varies according to the type of injury involved, is essential in determining the optimal method of treatment. In this research, we investigated the numerical and morphological changes in primary sensory neurons and satellite cells in the dorsal root ganglion 30 days following chronic compression, crush, and transection injuries using stereology, high-resolution light microscopy, immunohistochemistry, and behavioral analysis techniques. Electron microscopic methods were employed to evaluate fine structural alterations in cells. Stereological evaluations revealed no statistically significant difference in terms of mean sensory neuron numbers (p > 0.05), although a significant decrease was observed in sensory neuron volumes in the transection and crush injury groups (p < 0.05). Active caspase-3 immunopositivity increased in the injury groups compared to the sham group (p < 0.05). While crush injury led to desensitization, chronic compression injury caused thermal hyperalgesia. Macrophage infiltrations were observed in all injury types. Electron microscopic results revealed that the chromatolysis response was triggered in the sensory neuron bodies from the transection injury group. An increase in organelle density was observed in the perikaryon of sensory neurons after crush-type injury. This indicates the presence of a more active regeneration process in crush-type injury than in other types. The effect of chronic compression injury is more devastating than that of crush-type injury, and the edema caused by compression significantly inhibits the regeneration process.

Injury to the radial nerve caused by fracture of the humeral shaft:Timing and neurobiological aspects related to treatment and diagnosis

The radial nerve may not function in association with fractures of the humeral shaft. There are various opinions about the causes and treatment. We report a case of complete rupture of the radial nerve after a fracture of the proximal shaft of the humerus. The nerve injury was treated with grafting and TENDON transfer. Here we discuss diagnoses and treatments including neurobiological aspects of nervous regeneration. We suggest that electrodiagnostic examination after a radial nerve palsy caused by a humeral fracture is done 5-6 weeks after injury and that nerve repair and reconstruction should be done within two, and not later than three, months after injury.

Does the physical disector method provide an accurate estimation of sensory neuron number in rat dorsal root ganglia?

The physical disector is a method of choice for estimating unbiased neuron numbers; nevertheless, calibration is needed to evaluate each counting method. The validity of this method can be assessed by comparing the estimated cell number with the true number determined by a direct counting method in serial sections. We reconstructed a 1/5 of rat lumbar dorsal root ganglia taken from two experimental conditions. From each ganglion, images of 200 adjacent semi-thin sections were used to reconstruct a volumetric dataset (stack of voxels). On these stacks the number of sensory neurons was estimated and counted respectively by physical disector and direct counting methods. Also, using the coordinates of nuclei from the direct counting, we simulate, by a Matlab program, disector pairs separated by increasing distances in a ganglion model. The comparison between the results of these approaches clearly demonstrates that the physical disector method provides a valid and reliable estimate of the number of sensory neurons only when the distance between the consecutive disector pairs is 60 microm or smaller. In these conditions the size of error between the results of physical disector and direct counting does not exceed 6%. In contrast when the distance between two pairs is larger than 60 microm (70-200 microm) the size of error increases rapidly to 27%. We conclude that the physical dissector method provides a reliable estimate of the number of rat sensory neurons only when the separating distance between the consecutive dissector pairs is no larger than 60 microm.

Dynamics of repair regeneration of rat cutaneous nerves after traumas of different severity

Studies on 163 mongrels with experimental crush trauma to cutaneous nerves addressed the dynamics of regeneration over a period of 10-50 days post-trauma. Two series of experiments were performed, in which a cutaneous nerve (the saphenous nerve) was crushed with a hemostatic clamp over lengths of 2 and 4 mm, respectively. Destructive processes in the L3 and L4 spinal ganglia, increases in the numbers of myelin fibers in the nerve distal to the trauma site and the rate of growth of the damaged nerve fibers towards the skin after nerve traumas of different lengths, recorded at 10-50 days, were identical in the two series. The rate of myelinization of regenerating fibers after 2-mm crush lesions was greater than that seen after 4-mm crush injuries only in the period up to 30 days post-trauma.

Phrenic nerve transfer in the treatment of brachial plexus avulsion: An experimental study of nerve regeneration and muscle morphology in rats

The regeneration of motor and sensory neurons and the morphological changes of the target muscle after phrenic nerve transfer were investigated in adult rats. Six months following nerve transfer, 326.0 +/- 16.31 phrenic motoneurons regenerated into musculocutaneous nerve, which is not different from the normal number of phrenic motoneurons. The regenerated motoneurons exhibited a 14% nonsignificant hypertrophy. Of the dorsal root ganglia (DRG) neurons, 255.8 +/- 45.26 regenerated, which was significantly lower than the number of normal phrenic DRG neurons. The regenerated phrenic DRG neurons showed a 24% close-to-significant atrophy. The target muscle fiber morphology changed considerably after reinnervation. The present results suggest that the phrenic nerve has very good regenerative ability in terms of its motoneurons and a relatively insufficient sensory neuronal regeneration.

Primary sensory neuronal rescue with systemic acetyl-l-carnitine following peripheral axotomy. A dose-response analysis

The loss of a large proportion of primary sensory neurons after peripheral nerve axotomy is well documented. As a consequence of this loss, the innervation density attained on completion of regeneration will never be normal, regardless of how well the individual surviving neurons regenerate. Acetyl-L-carnitine (ALCAR), an endogenous peptide in man, has been demonstrated to protect sensory neurons, thereby avoiding loss after peripheral nerve injury. In this study we examined the dose-response effect of ALCAR on the primary sensory neurons in the rat dorsal root ganglia (DRG) 2 weeks after sciatic nerve axotomy. Six groups of adult rats (n=5) underwent unilateral sciatic nerve axotomy, without repair, followed by 2 weeks systemic treatment with one of five doses of ALCAR (range 0.5-50 mg/kg/day), or normal saline. L4 and L5 dorsal root ganglia were then harvested bilaterally and sensory neuronal cell counts obtained using the optical disector technique. ALCAR eliminated neuronal loss at higher doses (50 and 10 mg/kg/day), while lower doses did result in loss (12% at 5 mg/kg/day, p<0.05; 19% at 1 mg/kg/day, p<0.001; 23% at 0.5 mg/kg/day, p<0.001) compared to contralateral control ganglia. Treatment with normal saline resulted in a 25% (p<0.001) loss, demonstrating no protective effect in accordance with previous studies.ALCAR preserves the sensory neuronal cell population after axotomy in a dose-responsive manner and as such, has potential for improving the clinical outcome following peripheral nerve trauma when doses in excess of 10 mg/kg/day are employed.

Thyroid hormone reduces the loss of axotomized sensory neurons in dorsal root ganglia after sciatic nerve transection in adult rat

We have shown that a local administration of thyroid hormones (T3) at the level of transected rat sciatic nerve induced a significant increase in the number of regenerated axons. To address the question of whether local administration of T3 rescues the axotomized sensory neurons from death, in the present study we estimated the total number of surviving neurons per dorsal root ganglion (DRG) in three experimental group animals. Forty-five days following rat sciatic nerve transection, the lumbar (L4 and L5) DRG were removed from PBS-control, T3-treated as well as from unoperated rats, and serial sections (1 microm) were cut. The physical dissector method was used to estimate the total number of sensory neurons in the DRGs. Our results revealed that in PBS-control rats transection of sciatic nerve leads to a significant (P < 0.001) decrease in the mean number of sensory neurons (8743.8 +/- 748.6) compared with the number of neurons in nontransected ganglion (mean 13,293.7 +/- 1368.4). However, administration of T3 immediately after sciatic nerve transection rescues a great number of axotomized neurons so that their mean neuron number (12,045.8 +/- 929.8) is not significantly different from the mean number of neurons in the nontransected ganglion. In addition, the volume of ganglia showed a similar tendency. These results suggest that T3 rescues a high number of axotomized sensory neurons from death and allows these cells to grow new axons. We believe that the relative preservation of neurons is important in considering future therapeutic approaches of human peripheral nerve lesion and sensory neuropathy.

Neuronal loss after transsection of the facial nerve: a morphological and neurophysiological study in monkeys

Functional recovery after nerve lesions seems to depend on peripheral as well as central factors. To investigate the central neuronal loss after transsection of a pure motor nerve, the middle branch of the facial nerve on one side was transsected and immediately repaired microsurgically by epineural suturing. After a period of 6-15 months, a quantitative neurophysiological recording was made to estimate muscle response. A nerve tracer was injected into the mimic muscles innervated by the nerve to label the surviving motor neurons within the facial nucleus. The opposite side was used as the control in all cases. After the regenerative period, a mean loss of 15% of the total cell number was observed within the facial nucleus compared with the opposite side. The cell loss comprised all types of neurons. This amount of neuronal loss was followed by an even greater loss of muscle response when a quantitative neurophysiological recording was made after nerve regeneration. The results are discussed in relation to loss of nerve elements after nerve lesions and its effect on functional recovery.

A 25-year perspective of peripheral nerve surgery: evolving neuroscientific concepts and clinical significance

In spite of an enormous amount of new experimental laboratory data based on evolving neuroscientific concepts during the last 25 years peripheral nerve injuries still belong to the most challenging and difficult surgical reconstructive problems. Our understanding of biological mechanisms regulating posttraumatic nerve regeneration has increased substantially with respect to the role of neurotrophic and neurite-outgrowth promoting substances, but new molecular biological knowledge has so far gained very limited clinical applications. Techniques for clinical approximation of severed nerve ends have reached an optimal technical refinement and new concepts are needed to further increase the results from nerve repair. For bridging gaps in nerve continuity little has changed during the last 25 years. However, evolving principles for immunosuppression may open new perspectives regarding the use of nerve allografts, and various types of tissue engineering combined by bioartificial conduits may also be important. Posttraumatic functional reorganizations occurring in brain cortex are key phenomena explaining much of the inferior functional outcome following nerve repair, and increased knowledge regarding factors involved in brain plasticity may help to further improve the results. Implantation of microchips in the nervous system may provide a new interface between biology and technology and developing gene technology may introduce new possibilities in the manipulation of nerve degeneration and regeneration.

Primary sensory neuron survival following targeted administration of nerve growth factor to an injured nerve

Nerve injuries induce neurochemical changes within primary sensory neurons, including expression of neuropeptides, and a loss of a substantial proportion of the neurons may possibly be caused by a lack of neurotrophic support. In the present study the role of nerve growth factor (NGF) in preventing these changes was investigated in monkeys by giving NGF peripherally through a fibronectin (Fn) conduit. A sensory nerve (superficial radial) was transected and a gap of 5 mm was bridged with either autologous sural nerve graft (SNG), Fn, or Fn impregnated with NGF (Fn-NGF). After four months the dorsal root ganglia, that received the cutaneous afferents of the nerve, were removed and analysed by quantitative immunohistochemistry using antibodies to calcitonin gene related polypeptide (CGRP) and substance P. The percentage of immunostained cells was taken as an indication of neuronal survival. The results showed that SNG and Fn-NGF reduced the loss of CGRP positive sensory neurons compared with Fn alone. For substance P-positive neurons the differences were small with only a tendency towards reduction of neuronal death after NGF had been given, suggesting that NGF might act preferentially on a subpopulation of CGRP immunoreactive sensory neurons that do not coexist with substance P.

Neuronal survival using a resorbable synthetic conduit as an alternative to primary nerve repair

Clinically optimal situations for primary nerve repair are rarely observed. Crushed nerve ends result in either suboptimal repair or a need for nerve grafting. Functional results after nerve surgery are relatively poor, including major sensory deficits, which may be due to the death of primary sensory neurons that follows the nerve injury. The aim of this study was to determine if using polyhydroxybutyrate (PHB), a resorbable nerve conduit, could be an alternative to primary nerve repair in reducing loss of neurons. The superficial radial nerves in 20 cats were sectioned bilaterally and primarily repaired microsurgically by the use of two different strategies; either wrapping the nerve ends in sheets of PHB or epineurally suturing of the nerve. After 6 or 12 months, the surviving neurons within the dorsal root ganglia [C5-T1] were counted. No statistically significant differences were found between the two methods. This may imply a future possibility of using PHB as a synthetic nerve graft in situations where suboptimal primary repair or nerve grafts are the alternatives.

The neurotrophins NGF and NT-3 reduce sensory neuronal loss in adult rat after peripheral nerve lesion

The effect of three different neurotrophins on axotomy-induced death of adult rat sensory neurons was examined. The ventral branch of the 13th spinal nerve was transected and the corresponding neurons in the 13th thoracic (T13) dorsal root ganglion (DRG) were pre-labelled with Fast Blue (FB). For a period of 4 weeks, animals received either no treatment, continuous intrathecal infusion of phosphate buffer, nerve growth factor (NGF), neurotrophin-3 (NT-3), or brain-derived neurotrophic factor (BDNF). Labelled neurons remaining after this period were counted. Inert, or no treatment, resulted in extensive loss of the DRG neurons. BDNF application was virtually non-effective, while NGF or NT-3 resulted in a greater number of FB-labelled neurons compared to normal controls. This suggests that NGF and NT-3 are survival factors for adult sensory neurons with a therapeutic potential in peripheral nerve injuries.

Peripheral nerve regeneration and neurotrophic factors

The role of neurotrophic factors in the maintenance and survival of peripheral neuronal cells has been the subject of numerous studies. Administration of exogenous neurotrophic factors after nerve injury has been shown to mimic the effect of target organ-derived trophic factors on neuronal cells. After axotomy and during peripheral nerve regeneration, the neurotrophins NGF, NT-3 and BDNF show a well defined and selective beneficial effect on the survival and phenotypic expression of primary sensory neurons in dorsal root ganglia and of motoneurons in spinal cord. Other neurotrophic factors such as CNTF, GDNF and LIF also exert a variety of actions on neuronal cells, which appear to overlap and complement those of the neurotrophins. In addition, there is an indirect contribution of GGF to nerve regeneration. GGF is produced by neurons and stimulates proliferation of Schwann cells, underlining the close interaction between neuronal and glial cells during peripheral nerve regeneration. Different possibilities have been investigated for the delivery of growth factors to the injured neurons, in search of a suitable system for clinical applications. The studies reviewed in this article show the therapeutic potential of neurotrophic factors for the treatment of peripheral nerve injury and for neuropathies.

Loss of nerve endings in the spinal dorsal horn after a peripheral nerve injury. An anatomical study in Macaca fascicularis monkeys

In patients, the long-term outcome of injuries to sensory nerves is poor. This is only partly due to mismatching of regenerating axons at the transection site. We found in the macaque monkey that 70% of the transganglionic labelling in the spinal dorsal horn was still significantly reduced 21 months after transection and suturing of the sensory radial nerve. The reduction was evenly distributed throughout the terminal field of nerve endings, which were labelled with a mixture of the intra-axonal nerve tracer wheat germ agglutinin-horseradish peroxidase conjugate and pure horseradish peroxidase.

Loss of neurons in the dorsal root ganglia after transection of a peripheral sensory nerve. An anatomical study in monkeys

Injury to a sensory nerve often results in a poor long term outcome, partly because of sensory motor mismatch of regenerating axons at the transection site. We studied nine macaque monkeys and found that 27% of nerve cells in the projecting dorsal root ganglia had been lost 21 months after transection and suturing of the radial sensory nerve. No specific cell sizes were lost and the reduction was evenly distributed in the affected ganglia in which neurons had been labelled with a mixture of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) and HRP alone.

Reorganisation of primary afferent nerve terminals in the brainstem after peripheral nerve injury. An anatomical study in cats

A pure sensory nerve (the superficial branch of the radial nerve) in adult cats was cut to investigate the changes in the nerve endings (terminals) on the neurons of the nucleus cuneatus of the brainstem. In one group of cats (n = 22) the ends of the cut nerve were approximated immediately by epineural suturing to promote optimum regeneration. In another group (n = 11) the proximals tump of the nerve was enclosed in a capsule to prevent regeneration. Four to 17 months later the same nerve was re-exposed. The sutured nerves were cut and nerve-tracer was exhibited to the proximal end of the cut nerves and to the proximal stump of the nerves which had been encapsulated. The purpose was to investigate the labelling of nerve terminals in the cuneate nucleus, because it receives an input of primary afferents from the front leg. The nerve and the cuneate nucleus of the opposite side served as controls. Labelled terminals were distributed throughout the dorsal part of the entire rostrocaudal extent of the cuneate nucleus. The distribution was patchy and was superimposed on clusters of nerve cells. The quantity of labelled nerve terminals on the experimental and control sides was compared: 60% of the labelling observed on the control side was in the sutured nerves while the encapsulated nerves exhibited only 32%. This difference was apparent 4 months after transection of the nerve. Up to 17 months after the nerve was cut, however, there was some increase in the quantity of labelled nerve terminals and this was most apparent in cats in which the nerves had been sutured.

Changes in the spinal terminal pattern of the superficial radial nerve after a peripheral nerve injury. An anatomical study in cats

The occurrence of changes within the spinal cord over a long period after a peripheral nerve injury was investigated in adult cats. The lateral superficial branch of the radial nerve was exposed and transsected unilaterally. In one group the nerve endings were re-approximated with epineural sutures and in the other group the proximal nerve stump was enclosed to prevent regeneration. After a survival period of 4-17 months the same nerve on both sides was exposed to an intra-axonal nerve tracer, lectin-conjugated horseradish peroxidase, to label the nerve terminals within the spinal gray matter by transganglionic transport. The general distribution of the terminal field was almost the same after suturing as after encapsulation of the transsected nerve, except for a slightly more cranial location of the terminal area after suturing compared with the control side. The terminal area comprised laminae I-IV of the fifth cervical to the first thoracic spinal segment, concentrated towards the sixth to eighth cervical segments. This distribution was the same as on the control side, but the experimental and control sides differed in intensity of terminals. There was a loss of terminals throughout the terminal field in both operated groups, but after nerve suture there was some recovery of terminal intensity between 4 and 17 months after the injury.