EDITORIAL REVIEW
Report on the 1977 Meeting of the Peripheral Nerve Study Group Sponsored by the Muscular Dystrophy Association of America and the National Institute of Neurological and Communicative Disorders and Stroke, 103 investigators from 11 countries met at Airlie House, Airlie, Virginia, on August 11-14, 1977, to discuss recent research relevant to the pathogenesis of peripheral neuropathy. There were five half-day sessions: (1) new methods for old probiems; (2) normal and pathological nerve development; (3) mechanisms of peripheral neuropathy; ( 4 ) diabetic and metabolic neuropathies; and (5) biology of nerve: mechanisms of degeneration and repair. The meeting proved to be an effective forum for investigators representing many fields of basic and clinical research to evaluate techniques, discuss data, and exchange ideas. The organizers were H. Webster (Chairman),A. Aguayo, A. K. Asbury, and P. J. Dyck.
New Methods for Old Problems The first session focused on some recently developed techniques to study the axon membrane, axonal transport, peripheral nerve myelin, myelinated nerve fibers, and other aspects of peripheral nerve tissue. Methods that deserve wider application in peripheral nerve research were also included in each of the other sessions. The first group of papers concerned properties of axon membranes. Recent electron microscopy studies have shown that the axolemma at nodes of Ranvier can be selectively stained by ferric ion and ferrocyanide. In freeze-fracture replicas, cytoplasmic faces of nodal plasma membranes contain many more large particles than do internodal regions. Voltage clamp studies on frog Ranvier nodes demonstrated that two scorpion neurotoxins selectively alter sodium ion activation and inactivation during nerve impulse conduction. An axolemma-enriched fraction has been isolated from central nervous system white matter and characterized biochemically. The same method is now being applied to the peripheral nervous system with modifications to reduce contamination by connective tissue elements. The observations presented suggest that all these techniques can be used to investigate alterations associated with abnormal conduction found in experimental and human neuropathies. Only three studies of axonal transport could be included in this session. Normal values are now available for the redistribution of acetykholinesterase and dopamine-/3-hydroxylase activities in human sural
384
nerves during in vitro incubation; in patients with neuropathy studied to date, abnormally reduced transport is observed most consistently in DejerineSottas neuropathy. In another in vitro study, dependence of axonal transport on calcium can be demonstrated by using desheathed nerve preparations. When transverse sections of nerves are studied by electron microscopy, axons projecting to a particular muscle can be identified by observing intraaxonal horseradish peroxidase that had been transported retrograde after intramuscular injection and uptake into nerve fiber endings. Biochemical and immunocytochemical methods are also being used to characterize axon filament proteins. The freeze-fracture technique was discussed again in the group of papers devoted to myelin. Although preparative techniques can produce many variations in particle appearance, present evidence suggests that particles associated with the cytoplasmic membrane faces (dense lines) of PNS myelin are less uniform than in CNS myelin. An important biochemical approach currently being used to characterize the protein-protein and protein-lipid associations in these particles is the use of bifunctional reagents to crosslink them in their native state. The protein-protein or protein-lipid products can then be identified. With this approach, it may also be possible to characterize pathological changes in the molecular structure of PNS myelin. That approach is being used to study the localization of proteins in myelin lamellar itructure. Other methods suitable for this type of study are immunocytochemicd techniques and the use of nonpenetrating probes or solubilizing agents. Presentations later in the session showed that elegant techniques are available for investigating physiological properties of single mammalian myelinated nerve fibers. Long-term recordings from sciatic nerves of cats can now be made during normal activity and during nerve degeneration and regeneration. Another study demonstrated that spontaneous activity originating in lumbosacral roots can be observed in root fibers of dystrophic mice, trembler mice, and rats with demyelination induced by diphtheria toxin. In vivo recordings from intact spinal roots can also be used to analyze membrane current distribution along single fibers. A film slowed down 20,000 to 80,000 times showed the nerve impulse jumping from node to node in a normal region, crawling slowly along a demyelinated segment, and then resuming saltation.
0364-5134/78/0003-0502$01.00 @ 1978 by the Peripheral Nerve Study Group
Other observations on the spatial distribution of membrane current demonstrated that conduction along regenerating sprouts distal to a crush lesion is continuous initially and later becomes saltatory as the sprouts become myelinated. Of interest also is evidence obtained by intracellular recordings that prevertebral ganglia in the autonomic nervous system act as neural integrators. Finally, scanning electron microscopy and electron microprobe analysis are proving to be useful in exploring the structure of normal and diseased nerves. N o r m a l and Pathological Development in Peripheral Nerves The session on normal and abnormal nerve development was devoted mainly to discussion of cell interactions that play a role in neurogenesis and regeneration. These interactions were examined by a variety of techniques including the making of chimeras by fusion of two embryos, the separate culture of neurons and Schwann cells, the characterization of axonal membrane components by immunohistochemical techniques, and ultrastructural electrophysiological and radioautographic methods. Evidence provided at this session served to stress the close interdependence between the neurons and sheath cells of peripheral nerves. Results from in vitro studies indicate that axons may have a mitogenic effect on Schwann cells while, on the other hand, these cells support the survival of developing axons and may substitute for substances such as nerve growth factor. Amino acids, macromolecules, and other substances appear to pass from glia into axons, and there is evidence of exogenous tracers being picked up from the endoneurial fluid by Schwann cells and transported into axons. Certain neurotransmitters were reported to be found in high concentrations in Schwann cells; in squid nerves, for instance, the amount of acetylcholine has been found to be 80 times greater than in axons. This transmitter can be released in response to axonal mediators such as glutamate. In addition to these interdependencies between axons and glia, Schwann cells and fibroblasts interact in the formation of collagen, and there seem to be communications between developing fibers. Studies in mutant mice provided information on disorders of axon-Schwann cell interaction in abnormal developing nerves; several strains of mice (trembler, dystrophic, and quaking) have been used to define and characterize the role played by different cell components of peripheral nerves in the pathogenesis of some inherited neuropathies. Another group of papers presented at the session referred to influences of peripheral fields of innervation on development and regeneration of nerves. From quantitative histological studies and tracer tech-
niques using horseradish peroxidase in chick embryos, it was postulated that neuronal populations in developing nerves also depend for survival on their ability to compete effectively for synaptic sites or trophic factors in target cells. Another example was provided by the possibility that trophic interactions may exist between certain end-organs (Merkel cells) and regenerating nerve sprouts: Merkel cells appear to regulate certain aspects of axon growth in the salamander. Mechanisms involved in the regulation of axonal sprouting at the neuromuscular junction were also investigated. In skeletal muscles, presynaptic blockade with botulinum toxin as well as disuse atrophy resulted in increased sprouting as measured by end-plate length and terminal branching. Postsynaptic blockade with a-bungarotoxin, however, failed to induce sprouting; and, interestingly, bungarotoxin inhibited the sprout-inducing effect of botulinum toxin, a finding that could be interpreted as indicating that the sprouting response of the terminal axon may be influenced by postsynaptic factors in the muscle. Mechanisms of Peripheral N e u r o p a t h y Since the main purpose of the Peripheral Nerve Study Group is to investigate and define mechanisms underlying peripheral neuropathy and subsequent recovery of function, the last three sessions were devoted to this subject. With the use of radioactive precursors, increased synthesis of deoxyribonucleic acid and of various ribonucleic acid components was observed in nodose ganglion neurons after the vagus nerve was crushed. The RNA response was biphasic at approximately 2 and 14 days. The mechanisms underlying the neuronal response to axotomy still are not well understood and deserve further study. In the late nineteenth century it was proposed that tetanus toxin passes up the axon to perikarya of motor neurons to induce the clinical syndrome. Evidence was presented demonstrating uptake by nerve endings and retrograde transport of tetanus toxin’s “injurious macromolecules” to the CNS, where glycinergic synapses appeared to be selectively affected. Myelin of muscle nerves was found to have more glycoprotein than that of cutaneous nerves. In discussion regarding this finding, the view was expressed that this difference might be even greater if it were possible to evaluate only motor fibers, since approximately 50% of muscle nerve fibers are afferent. The role of axonal transport of labeled protein in the function of the neuron and its target tissue, whether and how it is altered by disease, seems still to be unclear. No changes were found in fast axonal transport during nerve development and aging. However, alterations are found during nerve regeneration.
Editorial Review: Peripheral Nerve Study Group
385
Fast axonal transport was reported to be abnormal in acrylamide and 2,5-hexanedione neuropathy also. An increased fast axonal transport was reported for some radiolabeled precursors and not for others in dystrophic hamsters. These data suggest that the substances are probably incorporated into different macromolecules with various rates of transport. Slow axonal transport of actin, tubulin, and neurofilament proteins may be important in the formation of axonal growth cones. The session on the role of lipid disturbances in neuropathy began with a theoretical consideration of the role of glycoproteins in the immunological and biochemical derangement of peripheral neuropathies. In recessively inherited hypertrophic neuropathy (Dejerine-Sottas disease or HMSN 111) there are several plasma glycolipid abnormalities. This finding appears to strengthen the case for an underlying abnormality of lipid metabolism in the disorder. Following focal application of Jysophosphatidylcholine to produce segmental demyelination, the proliferative response of Schwann cells could be inhibited in the promyelin state by 5-bromodeoxyuridine, an agent which selectively interferes with nucleic acid metabolism. This further supports the idea of degenerative and regenerative phases of repair, the latter involving repriming of the Schwann cell genome for myelination. Results of transplanting nerve allografts and xenografts to T-cell-suppressed mice were presented and discussed. Human Schwann cells can be transplanted and express their genetic characteristics. The technique permits evaluation of whether it is the axon or the Schwann cell which is primarily responsible for abnormalities of myelination. Schwann cells of trembler mouse express the abnormality of hypomyelination while those from human metachromatic leukodystrophy (MLD) express the formation of MLD granules in the graft. Another investigator studied human surd nerve xenografts transplanted from patients with inherited hypertrophic neuropathy and with Friedreich’s ataxia to nude and T-cellsuppressed mice. He agreed that the cellular nature of abnormalities of myelination could be studied by these techniques, but voiced caution in interpretation of the results because myelination of transplanted healthy human nerve is delayed-in addition to the genetic factor there may be a vascular or size-oftransplant factor-and some fibers grow outside the transplant, making comparison of the same fiber population uncertain. When a recently described technique to measure endoneurial fluid pressure was used, an increase was observed prior to the onset of segmental demyelination in lead neuropathy. This is the first demonstration
386 Annals of Neurology Vol 3 No 5
May 1978
of such an increase in neuropathy. Although the magnitude of the pressure is insufficient to collapse capillaries, it may be a factor in the development of this neuropathy. The fact that increased pressure could be related to transverse fascicular area suggests that increased pressure may be found in other human and experimental neuropathies. The final paper of the session provided evidence from experiments using horseradish peroxidase that the blood-nerve barrier extends even to the muscle spindle. Diabetic and Metabolic Neuropathies The session on diabetic and metabolic disorders opened with a review of current understanding of the various neuropathies encountered in human diabetes mellirus, including distal symmetrical polyneuropathy, proximal primarily motor neuropathy, autonomic neuropathy, and cranial mononeuropathy. A quantitative biopsy study of polyneuropathies encountered in diabetes, hypothyroidism, and Cushing syndrome was reported from Japan, confirming previous writings. In a morphometric autopsy study, the lumbar ventral horn and ventral roots of patients with diabetic neuropathy were compared to those of patients with longstanding limb amputations. Comparable degrees of motor neuron cell body loss were encountered in the lateral ventral horn of the lumbar spinal cord in both conditions, leaving unanswered the question of the primary site of neuropathic damage in diabetes mellitus. Finally, a striking associadon was demonstrated between diabetes and thickening of perineurial cell basement membrane in human surd nerve, although little correlation with the presence or severity of neuropathy was found. The second part of the session comprised three reports dealing in the main with ultrastructural pathology of experimental diabetic neuropathy. Distinctive features, which nonetheless are not specific, include heavy glycogen deposition in many nerve structures and accumulation of polyglucosan bodies. Neuropathic changes generally are mild. Increased water content of nerve in experimental diabetes was shown by one investigator to be interstitial (extracellular) within the endoneurial spaces. Studies of nerve lipids and nerve metabolism in both human and experimental diabetes made up the final portion of this session. Axoplasmic flow, as measured by accumulation of enzymatic markers proximal to ligature of nerve, is mildly impaired in experimental diabetes and is not reversed by the feeding of myo-inositol. In a correlated morphometriclipid neurochemical study of biopsied desheathed human sural nerve, myelin lipids were generally re-
duced in concordance with loss of myelinated fibers. There was also the suggestion of selective reduction of the phosphatidyl serine-phosphatidyl inositol fraction and a shift in fatty acid pattern to shorter chain, more highly saturated compounds. Ultrastructural radioautographic studies of inositol metabolism in normal nerve demonstrated high turnover rates and an axon-axolemmal localization. In contrast to previous reports, no effect ofmyo-inositol feeding on nerve conduction velocity was found in experimental diabetes in rats. In vitro studies of glucose and insulin metabolism of nerve with and without intact perineurium illustrated the importance of perineurium as a diffusion barrier and cast considerable doubt on the interpretation of previous studies.
Biology of Nerve: Mechanisms of Degeneration and Repair The final session was devoted primarily to mechanisms of nerve degeneration and repair and to observations on experimental and genetic neuropathies in animals as well as human neuropathies. Wallerian degeneration has long been the main experimental model and still presents a number of unsolved problems. Topics considered included local recycling of metabolic products derived from regeneration. Thyroxine, despite recent claims to the contrary, has little effect on the rate of regeneration. Dying-back neuropathy constitutes a second type of process in which the disturbance primarily affects the axon. However, the earlier concept of axonal degeneration that spreads proximally from the periphery now requires modification. It is evident that the dying-back process involves a multifocal disturbance affecting distal portions of nerve fibers. In giant axonal neuropathies characterized by focal swellings containing neurofi brillar accumulations, axonal enlargements begin at the proximal side of a branching node, distal to which axonal degeneration occurs. This successively involves more proximal paranodes. It is also clear that dying-back neuropathies involve not only the distal portion of the motor and sensory axons in the peripheral nerve, but also the distal portions of the central processes of the primary sensory neurons in the dorsal columns of the spinal cord, constituting a
“central-peripheral distal axonopathy.” The pathological changes in the terminal portions of the peripheral and central processes are similar. Although the mechanisms for reduction of conduction velocity in demyelinating neuropathies are beginning to be understood in some detail, the same is not true of neuropathies that primarily affect axons. In the neuropathy due to acrylamide intoxication, the major factor appears to be a selective loss of the larger and faster conducting fibers rather than any disturbance of the conducting mechanism in individual fibers. In neuropathies that primarily affect Schwann cells, the detailed ultrastructural changes in those cells during demyelination and remyelination and their significance are still not fully defined. In acute inflammatory polyneuritis (Guillain-BarrC syndrome) it is clear that the predominant pathological change is selective demyelination. Although some axonal degeneration occurs in such patients, others show predominantly axonal degeneration. The reasons are uncertain, although perhaps the immunological attack is directed against peripheral axons rather than myelin. An interesting model of a demyelinating neuropathy is that which results from a mutant gene in the hamster, in which focal filamentous accumulations appear in the adaxonal Schwann cell cytoplasm. Demyelination appears to result from progressive accumulation of such filaments. T h e adaxonal region is the portion of Schwann cell cytoplasm that is most peripheral to the nucleus. Possibly this disorder represents a “dying-back’ of the Schwann cell. Focal peripheral nerve lesions were also considered. T h e relative roles of pressure and ischemia in tourniquet paralysis have long been argued, but the importance of myelin damage from pressure has been established in recent years. A report given in this session documented that ischemia has no effect in experimental localized pressure lesions. Localized nerve injury may give rise to persistent pain, the explanation for which is not understood. Clearly, correlation between the occurrence of pain and morphological change can only be performed in man, and the application of ultrastructural methods to the question has been undertaken by studying plantar neuromas (Morton’s metatarsalgia).
Editorial Review: Peripheral Nerve Study Group
387
Report on the 1977 Meeting of the Peripheral Nerve Study Group Sponsored by the Muscular Dystrophy Association of America and the National Institute of Neurological and Communicative Disorders and Stroke, 103 investigators from 11 countries met at Airlie House, Airlie, Virginia, on August 11-14, 1977, to discuss recent research relevant to the pathogenesis of peripheral neuropathy. There were five half-day sessions: (1) new methods for old probiems; (2) normal and pathological nerve development; (3) mechanisms of peripheral neuropathy; ( 4 ) diabetic and metabolic neuropathies; and (5) biology of nerve: mechanisms of degeneration and repair. The meeting proved to be an effective forum for investigators representing many fields of basic and clinical research to evaluate techniques, discuss data, and exchange ideas. The organizers were H. Webster (Chairman),A. Aguayo, A. K. Asbury, and P. J. Dyck.
New Methods for Old Problems The first session focused on some recently developed techniques to study the axon membrane, axonal transport, peripheral nerve myelin, myelinated nerve fibers, and other aspects of peripheral nerve tissue. Methods that deserve wider application in peripheral nerve research were also included in each of the other sessions. The first group of papers concerned properties of axon membranes. Recent electron microscopy studies have shown that the axolemma at nodes of Ranvier can be selectively stained by ferric ion and ferrocyanide. In freeze-fracture replicas, cytoplasmic faces of nodal plasma membranes contain many more large particles than do internodal regions. Voltage clamp studies on frog Ranvier nodes demonstrated that two scorpion neurotoxins selectively alter sodium ion activation and inactivation during nerve impulse conduction. An axolemma-enriched fraction has been isolated from central nervous system white matter and characterized biochemically. The same method is now being applied to the peripheral nervous system with modifications to reduce contamination by connective tissue elements. The observations presented suggest that all these techniques can be used to investigate alterations associated with abnormal conduction found in experimental and human neuropathies. Only three studies of axonal transport could be included in this session. Normal values are now available for the redistribution of acetykholinesterase and dopamine-/3-hydroxylase activities in human sural
384
nerves during in vitro incubation; in patients with neuropathy studied to date, abnormally reduced transport is observed most consistently in DejerineSottas neuropathy. In another in vitro study, dependence of axonal transport on calcium can be demonstrated by using desheathed nerve preparations. When transverse sections of nerves are studied by electron microscopy, axons projecting to a particular muscle can be identified by observing intraaxonal horseradish peroxidase that had been transported retrograde after intramuscular injection and uptake into nerve fiber endings. Biochemical and immunocytochemical methods are also being used to characterize axon filament proteins. The freeze-fracture technique was discussed again in the group of papers devoted to myelin. Although preparative techniques can produce many variations in particle appearance, present evidence suggests that particles associated with the cytoplasmic membrane faces (dense lines) of PNS myelin are less uniform than in CNS myelin. An important biochemical approach currently being used to characterize the protein-protein and protein-lipid associations in these particles is the use of bifunctional reagents to crosslink them in their native state. The protein-protein or protein-lipid products can then be identified. With this approach, it may also be possible to characterize pathological changes in the molecular structure of PNS myelin. That approach is being used to study the localization of proteins in myelin lamellar itructure. Other methods suitable for this type of study are immunocytochemicd techniques and the use of nonpenetrating probes or solubilizing agents. Presentations later in the session showed that elegant techniques are available for investigating physiological properties of single mammalian myelinated nerve fibers. Long-term recordings from sciatic nerves of cats can now be made during normal activity and during nerve degeneration and regeneration. Another study demonstrated that spontaneous activity originating in lumbosacral roots can be observed in root fibers of dystrophic mice, trembler mice, and rats with demyelination induced by diphtheria toxin. In vivo recordings from intact spinal roots can also be used to analyze membrane current distribution along single fibers. A film slowed down 20,000 to 80,000 times showed the nerve impulse jumping from node to node in a normal region, crawling slowly along a demyelinated segment, and then resuming saltation.
0364-5134/78/0003-0502$01.00 @ 1978 by the Peripheral Nerve Study Group
Other observations on the spatial distribution of membrane current demonstrated that conduction along regenerating sprouts distal to a crush lesion is continuous initially and later becomes saltatory as the sprouts become myelinated. Of interest also is evidence obtained by intracellular recordings that prevertebral ganglia in the autonomic nervous system act as neural integrators. Finally, scanning electron microscopy and electron microprobe analysis are proving to be useful in exploring the structure of normal and diseased nerves. N o r m a l and Pathological Development in Peripheral Nerves The session on normal and abnormal nerve development was devoted mainly to discussion of cell interactions that play a role in neurogenesis and regeneration. These interactions were examined by a variety of techniques including the making of chimeras by fusion of two embryos, the separate culture of neurons and Schwann cells, the characterization of axonal membrane components by immunohistochemical techniques, and ultrastructural electrophysiological and radioautographic methods. Evidence provided at this session served to stress the close interdependence between the neurons and sheath cells of peripheral nerves. Results from in vitro studies indicate that axons may have a mitogenic effect on Schwann cells while, on the other hand, these cells support the survival of developing axons and may substitute for substances such as nerve growth factor. Amino acids, macromolecules, and other substances appear to pass from glia into axons, and there is evidence of exogenous tracers being picked up from the endoneurial fluid by Schwann cells and transported into axons. Certain neurotransmitters were reported to be found in high concentrations in Schwann cells; in squid nerves, for instance, the amount of acetylcholine has been found to be 80 times greater than in axons. This transmitter can be released in response to axonal mediators such as glutamate. In addition to these interdependencies between axons and glia, Schwann cells and fibroblasts interact in the formation of collagen, and there seem to be communications between developing fibers. Studies in mutant mice provided information on disorders of axon-Schwann cell interaction in abnormal developing nerves; several strains of mice (trembler, dystrophic, and quaking) have been used to define and characterize the role played by different cell components of peripheral nerves in the pathogenesis of some inherited neuropathies. Another group of papers presented at the session referred to influences of peripheral fields of innervation on development and regeneration of nerves. From quantitative histological studies and tracer tech-
niques using horseradish peroxidase in chick embryos, it was postulated that neuronal populations in developing nerves also depend for survival on their ability to compete effectively for synaptic sites or trophic factors in target cells. Another example was provided by the possibility that trophic interactions may exist between certain end-organs (Merkel cells) and regenerating nerve sprouts: Merkel cells appear to regulate certain aspects of axon growth in the salamander. Mechanisms involved in the regulation of axonal sprouting at the neuromuscular junction were also investigated. In skeletal muscles, presynaptic blockade with botulinum toxin as well as disuse atrophy resulted in increased sprouting as measured by end-plate length and terminal branching. Postsynaptic blockade with a-bungarotoxin, however, failed to induce sprouting; and, interestingly, bungarotoxin inhibited the sprout-inducing effect of botulinum toxin, a finding that could be interpreted as indicating that the sprouting response of the terminal axon may be influenced by postsynaptic factors in the muscle. Mechanisms of Peripheral N e u r o p a t h y Since the main purpose of the Peripheral Nerve Study Group is to investigate and define mechanisms underlying peripheral neuropathy and subsequent recovery of function, the last three sessions were devoted to this subject. With the use of radioactive precursors, increased synthesis of deoxyribonucleic acid and of various ribonucleic acid components was observed in nodose ganglion neurons after the vagus nerve was crushed. The RNA response was biphasic at approximately 2 and 14 days. The mechanisms underlying the neuronal response to axotomy still are not well understood and deserve further study. In the late nineteenth century it was proposed that tetanus toxin passes up the axon to perikarya of motor neurons to induce the clinical syndrome. Evidence was presented demonstrating uptake by nerve endings and retrograde transport of tetanus toxin’s “injurious macromolecules” to the CNS, where glycinergic synapses appeared to be selectively affected. Myelin of muscle nerves was found to have more glycoprotein than that of cutaneous nerves. In discussion regarding this finding, the view was expressed that this difference might be even greater if it were possible to evaluate only motor fibers, since approximately 50% of muscle nerve fibers are afferent. The role of axonal transport of labeled protein in the function of the neuron and its target tissue, whether and how it is altered by disease, seems still to be unclear. No changes were found in fast axonal transport during nerve development and aging. However, alterations are found during nerve regeneration.
Editorial Review: Peripheral Nerve Study Group
385
Fast axonal transport was reported to be abnormal in acrylamide and 2,5-hexanedione neuropathy also. An increased fast axonal transport was reported for some radiolabeled precursors and not for others in dystrophic hamsters. These data suggest that the substances are probably incorporated into different macromolecules with various rates of transport. Slow axonal transport of actin, tubulin, and neurofilament proteins may be important in the formation of axonal growth cones. The session on the role of lipid disturbances in neuropathy began with a theoretical consideration of the role of glycoproteins in the immunological and biochemical derangement of peripheral neuropathies. In recessively inherited hypertrophic neuropathy (Dejerine-Sottas disease or HMSN 111) there are several plasma glycolipid abnormalities. This finding appears to strengthen the case for an underlying abnormality of lipid metabolism in the disorder. Following focal application of Jysophosphatidylcholine to produce segmental demyelination, the proliferative response of Schwann cells could be inhibited in the promyelin state by 5-bromodeoxyuridine, an agent which selectively interferes with nucleic acid metabolism. This further supports the idea of degenerative and regenerative phases of repair, the latter involving repriming of the Schwann cell genome for myelination. Results of transplanting nerve allografts and xenografts to T-cell-suppressed mice were presented and discussed. Human Schwann cells can be transplanted and express their genetic characteristics. The technique permits evaluation of whether it is the axon or the Schwann cell which is primarily responsible for abnormalities of myelination. Schwann cells of trembler mouse express the abnormality of hypomyelination while those from human metachromatic leukodystrophy (MLD) express the formation of MLD granules in the graft. Another investigator studied human surd nerve xenografts transplanted from patients with inherited hypertrophic neuropathy and with Friedreich’s ataxia to nude and T-cellsuppressed mice. He agreed that the cellular nature of abnormalities of myelination could be studied by these techniques, but voiced caution in interpretation of the results because myelination of transplanted healthy human nerve is delayed-in addition to the genetic factor there may be a vascular or size-oftransplant factor-and some fibers grow outside the transplant, making comparison of the same fiber population uncertain. When a recently described technique to measure endoneurial fluid pressure was used, an increase was observed prior to the onset of segmental demyelination in lead neuropathy. This is the first demonstration
386 Annals of Neurology Vol 3 No 5
May 1978
of such an increase in neuropathy. Although the magnitude of the pressure is insufficient to collapse capillaries, it may be a factor in the development of this neuropathy. The fact that increased pressure could be related to transverse fascicular area suggests that increased pressure may be found in other human and experimental neuropathies. The final paper of the session provided evidence from experiments using horseradish peroxidase that the blood-nerve barrier extends even to the muscle spindle. Diabetic and Metabolic Neuropathies The session on diabetic and metabolic disorders opened with a review of current understanding of the various neuropathies encountered in human diabetes mellirus, including distal symmetrical polyneuropathy, proximal primarily motor neuropathy, autonomic neuropathy, and cranial mononeuropathy. A quantitative biopsy study of polyneuropathies encountered in diabetes, hypothyroidism, and Cushing syndrome was reported from Japan, confirming previous writings. In a morphometric autopsy study, the lumbar ventral horn and ventral roots of patients with diabetic neuropathy were compared to those of patients with longstanding limb amputations. Comparable degrees of motor neuron cell body loss were encountered in the lateral ventral horn of the lumbar spinal cord in both conditions, leaving unanswered the question of the primary site of neuropathic damage in diabetes mellitus. Finally, a striking associadon was demonstrated between diabetes and thickening of perineurial cell basement membrane in human surd nerve, although little correlation with the presence or severity of neuropathy was found. The second part of the session comprised three reports dealing in the main with ultrastructural pathology of experimental diabetic neuropathy. Distinctive features, which nonetheless are not specific, include heavy glycogen deposition in many nerve structures and accumulation of polyglucosan bodies. Neuropathic changes generally are mild. Increased water content of nerve in experimental diabetes was shown by one investigator to be interstitial (extracellular) within the endoneurial spaces. Studies of nerve lipids and nerve metabolism in both human and experimental diabetes made up the final portion of this session. Axoplasmic flow, as measured by accumulation of enzymatic markers proximal to ligature of nerve, is mildly impaired in experimental diabetes and is not reversed by the feeding of myo-inositol. In a correlated morphometriclipid neurochemical study of biopsied desheathed human sural nerve, myelin lipids were generally re-
duced in concordance with loss of myelinated fibers. There was also the suggestion of selective reduction of the phosphatidyl serine-phosphatidyl inositol fraction and a shift in fatty acid pattern to shorter chain, more highly saturated compounds. Ultrastructural radioautographic studies of inositol metabolism in normal nerve demonstrated high turnover rates and an axon-axolemmal localization. In contrast to previous reports, no effect ofmyo-inositol feeding on nerve conduction velocity was found in experimental diabetes in rats. In vitro studies of glucose and insulin metabolism of nerve with and without intact perineurium illustrated the importance of perineurium as a diffusion barrier and cast considerable doubt on the interpretation of previous studies.
Biology of Nerve: Mechanisms of Degeneration and Repair The final session was devoted primarily to mechanisms of nerve degeneration and repair and to observations on experimental and genetic neuropathies in animals as well as human neuropathies. Wallerian degeneration has long been the main experimental model and still presents a number of unsolved problems. Topics considered included local recycling of metabolic products derived from regeneration. Thyroxine, despite recent claims to the contrary, has little effect on the rate of regeneration. Dying-back neuropathy constitutes a second type of process in which the disturbance primarily affects the axon. However, the earlier concept of axonal degeneration that spreads proximally from the periphery now requires modification. It is evident that the dying-back process involves a multifocal disturbance affecting distal portions of nerve fibers. In giant axonal neuropathies characterized by focal swellings containing neurofi brillar accumulations, axonal enlargements begin at the proximal side of a branching node, distal to which axonal degeneration occurs. This successively involves more proximal paranodes. It is also clear that dying-back neuropathies involve not only the distal portion of the motor and sensory axons in the peripheral nerve, but also the distal portions of the central processes of the primary sensory neurons in the dorsal columns of the spinal cord, constituting a
“central-peripheral distal axonopathy.” The pathological changes in the terminal portions of the peripheral and central processes are similar. Although the mechanisms for reduction of conduction velocity in demyelinating neuropathies are beginning to be understood in some detail, the same is not true of neuropathies that primarily affect axons. In the neuropathy due to acrylamide intoxication, the major factor appears to be a selective loss of the larger and faster conducting fibers rather than any disturbance of the conducting mechanism in individual fibers. In neuropathies that primarily affect Schwann cells, the detailed ultrastructural changes in those cells during demyelination and remyelination and their significance are still not fully defined. In acute inflammatory polyneuritis (Guillain-BarrC syndrome) it is clear that the predominant pathological change is selective demyelination. Although some axonal degeneration occurs in such patients, others show predominantly axonal degeneration. The reasons are uncertain, although perhaps the immunological attack is directed against peripheral axons rather than myelin. An interesting model of a demyelinating neuropathy is that which results from a mutant gene in the hamster, in which focal filamentous accumulations appear in the adaxonal Schwann cell cytoplasm. Demyelination appears to result from progressive accumulation of such filaments. T h e adaxonal region is the portion of Schwann cell cytoplasm that is most peripheral to the nucleus. Possibly this disorder represents a “dying-back’ of the Schwann cell. Focal peripheral nerve lesions were also considered. T h e relative roles of pressure and ischemia in tourniquet paralysis have long been argued, but the importance of myelin damage from pressure has been established in recent years. A report given in this session documented that ischemia has no effect in experimental localized pressure lesions. Localized nerve injury may give rise to persistent pain, the explanation for which is not understood. Clearly, correlation between the occurrence of pain and morphological change can only be performed in man, and the application of ultrastructural methods to the question has been undertaken by studying plantar neuromas (Morton’s metatarsalgia).
Editorial Review: Peripheral Nerve Study Group
387
![[Report on the meeting of the Working Group on Hematopathology].](/assets/img/hold.png)
