Observations on a transient phase of focal swelling in degenerating unmyelinated nerve fibers

Acute anoxic changes in peripheral nerve: anatomic and physiologic correlations

INTRODUCTION

The response of the peripheral nerve to anoxia is modulated by many factors including glucose and temperature. The purposes of this article are to demonstrate the effects of these factors on the pathological changes induced by anoxia and to compare the electrophysiologic changes and pathological changes in the same nerves.

METHODS

Sciatic nerves were harvested from rats and placed in a perfusion apparatus where neurophysiologic responses could be recorded continuously during a 16 h experiment. After the experiment, light microscopy and electron microscopy were performed.

RESULTS

Light microscopic images showed mild changes from anoxia at normoglycemia. Hypoglycemic anoxia produced massive axonal swelling while hyperglycemic anoxia produced apparent changes in the myelin. Anoxic changes were not uniform in all axons. Electron microscopy showed only minor disruptions of the cytoskeleton with anoxia during normoglycemia. At the extremes of glucose concentration especially with hyperglycemia, there was a more severe disruption of intermediate filaments and loss of axonal structure with anoxia. Hypothermia protected axons from the effect of anoxia and produced peak axonal swelling in the 17-30°C range.

CONCLUSIONS

The combination of hyperglycemia or hypoglycemia and anoxia produces extremely severe axonal disruption. Changes in axonal diameter are complex and are influenced by many factors.

Sciatic nerve block with resiniferatoxin: an electron microscopic study of unmyelinated fibers in the rat

BACKGROUND

Perineural administration of the naturally occurring vanilloids (capsaicin, resiniferatoxin [RTX]) produces selective nociceptive blockade. Studies using perineural vanilloids in high concentrations suggest that they can cause a degeneration of unmyelinated fibers. However, electron microscopic studies of local vanilloid toxicity produced conflicting outcomes. In the present study, we sought to determine whether RTX-induced reversible sciatic nerve block results in the degenerative changes of unmyelinated fibers.

METHODS

In rat experiments, RTX was administered percutaneously at the sciatic nerve. The effect of RTX was monitored by measuring the rat's response to noxious heat. The sciatic nerves were removed 48 h after the blockade initiation. Quantitative electron microscopic evaluation of the unmyelinated fibers was performed in three groups of animals: RTX 0.0001% (0.1 microg), RTX 0.001% (1 microg), and control (RTX vehicle, 0.1 mL).

RESULTS

Cross-sections of the sciatic nerve 48 h after the initiation of RTX-induced reversible nerve blockade appeared essentially normal. One rarely observed finding was the irregularly compacted membranous deposits in the unmyelinated axons. The frequency of this finding was approximately one per thousand fibers with both concentrations of RTX.

CONCLUSIONS

The results of the study suggest that a selective and long-lasting sciatic nerve block (up to 2 wk) can be provided by RTX without any significant damage to the unmyelinated nerve fibers.

Sequence of cellular changes following localized axotomy to cortical neurons in glia-free culture

This study utilizes an in vitro model of localized physical injury to axons to examine the specific responses of neocortical neurons to trauma in isolation from glia cell types. The neuronal response to axotomy was closely linked with nerve cell maturity. Cultures grown for 14 days in vitro showed no accumulation of either neurofilaments or, the axonal sprouting marker, GAP43, within injured axons following injury. In older cultures (21 days in vitro), however, temporally distinct axonal changes were evident following transection of axonal bundles. At 12 h postinjury, these included extensive accumulation of neurofilaments into ring-like structures within the cut stumps and an increase in punctate GAP43 labelling throughout the damaged area. At 24 h postinjury, bulb-like accumulations of neurofilaments were also present within the transected axons. Finally at 3 days postinjury, distinct GAP43 and neurofilament immunolabeled axons, and GAP43 immunopositive growth cones, emanated from the cut stump. These results indicate that injured axons of mature neurons undergo a defined series of reactive changes, ultimately culminating in a sprouting response, which occur independently of the presence or effects of glial cell populations.

In vitro central nervous system models of mechanically induced trauma: a review

Injury is one of the leading causes of death among all people below the age of 45 years. In the United States, traumatic brain injury (TBI) and spinal cord injury (SCI) together are responsible for an estimated 90,000 disabled persons annually. To improve treatment of the patient and thereby decrease the associated mortality, morbidity, and cost, several in vivo models of central nervous system (CNS) injury have been developed and characterized over the past two decades. To complement the ability of these in vivo models to reproduce the sequelae of human CNS injury, in vitro models of neuronal injury have also been developed. Despite the inherent simplifications of these in vitro systems, many aspects of the posttraumatic sequelae are faithfully reproduced in cultured cells, including ultrastructural changes, ionic derangements, alterations in electrophysiology, and free radical generation. This review presents a number of these in vitro systems, detailing the mechanical stimuli, the types of tissue injured, and the in vivo injury conditions most closely reproduced by the models. The data generated with these systems is then compared and contrasted with data from in vivo models of CNS injury. We believe that in vitro models of mechanical injury will continue to be a valuable tool to study the cellular consequences and evaluate the potential therapeutic strategies for the treatment of traumatic injury of the CNS.

The origin and nature of beading: a reversible transformation of the shape of nerve fibers

Nerve fibers which appear beaded (varicose, spindle-shaped, etc.) are often considered the result of pathology, or a preparation artifact. However, beading can be promptly elicited in fresh normal nerve by a mild stretch and revealed by fast-freezing and freeze-substitution, or by aldehyde fixating at a temperature near 0 degree C (cold-fixation). The key change in beading are the constrictions, wherein the axon is much reduced in diameter. Axoplasmic fluid and soluble components are shifted from the constrictions into the expansions leaving behind compacted microtubules and neurofilaments. Labeled cytoskeletal proteins carried down by slow axonal transport are seen to move with the soluble components and not to have been incorporated into and remain with, the cytoskeletal organelles on beading the fibers. Lipids and other components of the myelin sheath are also shifted from the constrictions into the expansions, with preservation of its fine structure and thickness. Additionally, myelin intrusions into the axons are produced and a localized bulging into the axon termed "leafing". The beading constrictions do not arise from the myelin sheath: beading occurs in the axons of unmyelinated fibers. It does not depend on the axonal cytoskeleton: exposure of nerves in vitro to beta, beta'-iminodipropionitrile (IDPN) disaggregates the cytoskeletal organelles and even augments beading. The hypothesis advanced was that the beading constrictions are due to the membrane skeleton; the subaxolemmal network comprised of spectrin/fodrin, actin, ankyrin, integrins and other transmembrane proteins. The mechanism can be activated directly by neurotoxins, metabolic changes, and by an interruption of axoplasmic transport producing Wallerian degeneration.

Axon overproduction and elimination in the anterior commissure of the developing rhesus monkey

We have analyzed axon overproduction and elimination in the anterior commissure (AC) of 16 fetal, neonatal, and juvenile rhesus monkeys. Axons are added to the AC at an average rate of 115,000/day during the last two-thirds of gestation, and growth cones are present in a constant proportion to AC axons throughout this period. The peak number of approximately 11 million axons in the AC is reached at birth. Thereafter, axons are eliminated at a net rate of approximately 1 axon/sec during the first 3 postnatal months until the adult number of approximately 3.3 +/- 0.5 million axons is reached. Although there is considerable variability in AC axon number during the period of axon loss, the adult number of AC axons is relatively invariant among the eight adult rhesus monkeys examined. Increase in axon diameter and myelination begins before the major phase of axon elimination and is completed long after the adult number of axons is reached. Apparently, myelinated axons are not eliminated from the AC. Quantitative differences in the magnitude and timing of axon overproduction and elimination in the AC versus that in the corpus callosum (LaMantia and Rakic [1990] J. Neurosci. 10:2156) indicate specific modulation of the development of each commissure, perhaps reflecting differences in the developmental history and functional identity of the distinct cortical regions that give rise to them. This process of overproduction and elimination of AC axons during postnatal development in primates might contribute to individual variations in AC size correlated with a wide range of physical and behavioral differences.

In vitro studies of multiple impact injury to mammalian CNS neurons: prevention of perikaryal damage and death by ketamine

We have developed an in vitro model of rapid acceleration injury (RAI) to study the effects of multiple impact (220 g/impact, 3-5 s intervals) trauma on cultures of mammalian CNS cells. Our initial investigations have shown that: (1) multiple impacts delivered tangential to the plane of growth caused neuronal death while normal impacts did not; (2) glia were not affected by tangential or normal RAI; (3) most neuronal death occurred within 15 min; (4) the threshold for neuronal death was above 440 g (cumulative); (5) neuronal death reached a maximum of 50% at cumulative accelerations greater than or equal to 1100 g; (6) somal swelling and increased nuclear prominence were often observed after tangential RAI, and the frequency of these changes increased with the cumulative acceleration; and (7) ketamine prevented neuronal death and morphological changes during tangential RAI. We hypothesize that neuronal sensitivity to multiple impact RAI depends on the density of N-methyl-D-aspartate (NMDA) complexes in the dendrosomatic membranes. We also hypothesize that the events leading to neuronal death during multiple impact injury are: (1) calcium leakage through NMDA channels causes weakening of the cytoskeleton; (2) loss of cytoskeletal integrity allows nuclear shifting during impact; and (3) nuclear pressure disrupts the plasmalemma causing a lethal influx of calcium.

Retinotectal reorganization in goldfish--IV. Effects of retinal ganglion cells after half tectal ablation

During compression of the entire retinotectal projection into the rostral half of the tectum after ablation of the caudal half there is widespread sprouting of ganglion cell axons, not only those cut during the operation but also those left intact. However, unlike cut axons those left intact sprout without their cell bodies showing chromatolysis or swelling. Chromatolysis and swelling of the cell bodies of cut axons are more prolonged than after optic nerve section and resolve in more central regions of retina first. The cut axons of cell bodies in these regions tend to be the first to form terminal arborizations during the compression process as judged electrophysiologically. However, there is no clear correlation in individual fish between these measures and the state of compression assessed electrophysiologically. Large areas of retina may contain chromatolysed cells even after compression has occurred. Electrophysiological mapping alone may give a misleading picture of the interactions occurring between retinal and tectal cells during reorganization.

Neuronal survival or death after dendrite transection close to the perikaryon: correlation with electrophysiologic, morphologic, and ultrastructural changes

We investigated the probability of survival of mouse spinal neurons in monolayer cultures after transection lesions of dendrites made within 400 microns of the perikarya. Based on a total of 650 lesioned neurons, the following observations were made. First, neuronal survival is a function of lesion distance from the perikaryon and of process diameter at the lesion site. For an average lesion diameter of 3 microns, dendrite transections at 50 microns, 100 microns, and 150 microns were associated with survival probabilities of 30%, 53%, and 70%, respectively. Second, the fate of the injured cells was definitely established 24 hours after injury and very likely was determined as early as 2 hours. Third, early stages of deterioration leading to cell death were associated with cytoplasmic phase brightness on light microscopy, correlating with an appearance of numerous, small, electron-lucent vacuoles and swollen mitochondria on electron microscopy. The cytoplasm of these moribund cells stained darkly and contained no visible microtubules or neurofilaments. Fourth, the magnitude and time course of injury potentials recorded at the somata were a function of the lesion distance and did not return to prelesion levels within 30 minutes after transection. Fifth, at 24 hours after injury, the average membrane potential of lesioned neurons was 8% below that of control neurons. Sixth, at a lesion distance of approximately 300 microns both the injury potential and the probability of cell death approach zero. We conclude that, in the model system used, neuronal survival after dendrite amputation depends on physical parameters of the lesion that determine the magnitude of the injury current reaching the soma. Survival is not assured if the injury is inflicted within 250 microns of the cell body, and cell death is likely for lesions within 50 microns of the soma. The below-normal membrane potentials at 24 hours after injury suggest a possible greater vulnerability of recovering neurons to secondary insults. The characteristic mitochondrial disruption and loss of microtubules implies that the calcium component of the injury current contributes to cell death.

An ultrastructural study of intrarenal catecholamine-containing elements

Histochemical visualization of catecholamines and electron microscopy in the same tissue sample were used to localize and study catecholamine-containing nerve enlargements or swellings in male Wistar and Sprague-Dawley rat kidneys. These swellings lie in the perivascular nerve plexuses of arcuate and interlobular arteries near the points of origin of arterioles, and are composed of modified axons and associated Schwann cells. Transverse sections of the enlarged nerves reveal that individual axons are also enlarged, have processes or folds and make contact with one another. The axonal enlargements contain small mitochondria with a dense matrix and clusters of small vesicles, many of which are associated with an organelle composed of parallel cisternae of smooth membranes.

Degenerative and regenerative changes in the trochlear nerve of goldfish

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Axonal swellings in jimpy mice: does lack of myelin cause neuronal abnormalities?

We have observed focal axonal enlargements in jimpy, a myelin deficient mutant mouse. Similar axonal swellings have also been found in other studies, in two other myelin deficient mutant mice and in a myelin deficient mutant rat. We suggest that this axonal abnormality represents a common secondary reaction to lack of myelin. Such a secondary reaction might also occur in other species including human, in response to deficient myelin or to loss of myelin due to disease.

Microsurgical transection of small nerve fibre bundles in vitro. Effects on axons, growth cones and glial cells

Fibres growing from neurons of explanted dorsal root ganglia from 10 day chick embryos were transected and subsequently observed by light and electron microscopy after periods of a few to fifty minutes. Changes immediately proximal and distal to the cut together with alterations further away from the site of injury on both sides of the cut were recorded. Observations were also made on the growth cones of damaged axons and on changes in associated glial cells. Reactive and degenerative changes including the rotation, retraction and swelling of cutaxons occurred rapidly. Electron microscopy revealed tracts of filamentous material close to the sealed-off ends of axons, swollen organelles such as mitochondria, and lamellar bodies of varying dimensions. Proximal to the injury and closer to the explant, damaged and degenerating axons mingled with normal processes. Many contained only a fine granular material, others clumps of organelles, particularly mitochondria. Distal to the cut, microspikes were lost from some growth cones. The dense granular material filling microspikes and growth cones remained unchanged. Clumps of large clear vesicles, lamellar bodies and swollen degenerating mitochondria were present, not only within growth cones, but also in all parts of the axon distal to the cut. Glial cells associated with transected axons soon developed an electron dense cytoplasm containing swollen organelles. Large numbers of vesicles filled with a particulate substance were also found. The possible significance of the changes observed after transection are considered and discussed.

Anatomic effect of distention therapy in unstable bladder: new approach

The recent clinical success of distention therapy in the treatment of the unstable bladder is reviewed. Bladder stability and increased capacity as measured by cystometry following distention therapy as well as relief of symptoms have prompted this anatomic study. The neuromuscular pathways of conduction in rat and rabbit bladder wall were examined following short-term (two-hour) and prolonged (six-hour) distention. Treated and control animals were studied at fixed intervals for four months. Prolonged distention did not alter either smooth muscle cell architecture or intercellular junctions. It did produce a transient phase of degeneration among the unmyelinated nerve fibers in the bladder wall consisting of axonal swelling and lysis of organelles. A quantitative estimate of nerve injury was compiled using pooled histograms. These results suggest that bladder stability following distention therapy may be related to nerve degeneration in the bladder wall.

An electron-microscopic analysis of axonal alterations following blunt contusion of the spinal cord of the rhesus monkey (Macaca mulatta)

Following contusion (500 g-cm) at upper thoracic levels, sections from the spinal cords of 13 rhesus monkeys were examined with the electron microscope. Survival times ranged from 4 hr to 10 weeks. Samples were taken from the lesion site, from areas 3 and 10 mm rostral and caudal to the lesion center, and from the lumbosacral cord. Four hours postoperatively, several small axons located close to the grey matter at the lesion site exhibit abnormal accumulations of organelles including mitochondria, dense bodies, vesicular structures, and multivesicular bodies. By 12 hr postoperatively many axons at the lesion site appear to be swollen with organelles and exhibit thinning of their myelin sheath. Some organelle-rich profiles lack a myelin sheath altogether. At this time dark axons are present, and myelin sheaths which appear to be empty or to contain small amounts of flocculent material. By 18 hr the first signs of axonal changes appear in the tissue taken 3 mm from the center of the lesion, both swollen and pyknotic axons being present. The axonal pathology spreads from the central part of the cord to the periphery at the impact site, and from the impact site rostrally and caudally, beginning at 18 hr and continuing for the duration of the study. Small fibers degenerate first and large fibers later. The axonal changes observed appear to be comparable to those reported for the central and peripheral nervous systems in other species.

Optic nerve axoplasm and papilledema

A detailed review of optic nerve axoplasm is presented. A number of hypotheses have been postulated for the pathogenesis of papilledema associated with increased intracranial pressure. These hypotheses, mechanical and nonmechanical, are critically evaluated in relation to five essential features of papilledema. Theories, as well as clinical and experimental studies, of axonal transport are reviewed, and a new hypothesis is proposed: Papilledema is primarily a mechanical, nonvascular phenomenon in which an excess amount of extracellular fluid is present in the prelaminar region of the optic disc and the accumulation of that fluid results from the leakage of axoplasm from optic nerve fibers which are compressed posterior to the lamina cribrosa of the optic disc. The authors believe that this is the only existing hypothesis consistent with all the known facts about papilledema. Discussions by Drs. J. Terry Ernest, Thomas R. Hedges, and S. S. Hayreh follow the review.

Retrograde axonal transport of horseradish peroxidase in peripheral autonomic nerves

An exogeneous marker protein, horseradish peroxidase (HRP) was used to race peripheral autonomic pathways in adult guinea pigs and cats. Small doses of HRP were injected into various organs and after a brief survival period, HRP activity appeared in the perikarya of autonomic neurons that supplied each injection site. After injection of HRP into the anterior chamber of the eye, reaction product was detected in the postganglionic sympathetic neurons of the superior cervical sympathetic ganglion. In another experiment, HRP reaction product was found in the cell bodies of the preganglionic sympathetic neurons that supply the adrenal medulla. These were located in the lateral gray column of the spinal cord at T6 and T7 segmental levels. Reaction product appeared in intramural postganglionic parasympathetic neurons close to an injection site in the wall of the urinary bladder and in a similiar situation in Meissner's ganglia of the ileum. Following injection into the walls of the stomach and ileum, HRP labelled cells were detected in the nodose ganglion of the vagus and in preganglionic parasympathetic neurons in the dorsal motor nucleus of this nerve. After injection into the subepicardial tissue of the heart, reaction product appeared in the stellate ganglion and also in an upper thoracic dorsal root ganglion. These data suggest that HRP is taken up by peripheral autonomic nerves of all types, and then undergoes rapid retrograde axonal transport to the perikaryon. It appears, therefore, that HRP may be useful in tracing both motor and sensory peripheral autonomic pathways.

Association of nerve fibers and plasma cells in abnormal human nasal respiratory mucosa

Degeneration of sensory and autonomic nerve endings and intimate association of preterminal nerve fibers with plasma cells are described in eight patients suffering from nasal congestion. Two of the patients also suffered from nasal polyps. The association between nerve fibers and plasma cells is characterized by membrane to membrane apposition, absence of the nerve basement membrane in the contact area, and by extension of the nerve basement membrane from the free surface of the nerve fiber over the free surface of the plasma cell. It is suggested that the degeneration of nerves may be caused by autoimmune reaction as a result of tissue exposure to infection or drugs, and that the development of polyps may be the consequence of chronic tissue denervation.

Effects of calcium ion concentration on the degeneration of amputated axons in tissue culture

Light and electron microscope studies were conducted on the nature of the degenerative changes in amputated nerve fibers of cultured rat sensory ganglia and on the effects of media with differing calcium concentrations upon these changes. With glucose-enriched Eagle's media (MEM) containing 1.6 mM calcium, the amputated myelinated and unmyelinated axons undergo a progressive granular disintegration of their axoplasm with collapse and fragmentation of myelin sheaths between 6 and 24 h after transection. With MEM containing only 25-50 microM calcium, the granular axoplasmic degeneration does not occur in transected fibers and they retain their longitudinal continuity and segmental myelin ensheathment for at least 48 h. Addition of 6 mM EGTA to MEM (reducing the estimated Ca(++) below 0.3 microM) results in the structural preservation of both microtubules and neurofilaments within transected axons. A transient focal swelling of amputated axons occurs, however, in cultures with normal and reduced calcium. These observations suggest that an alteration in the permeability of the axolemma is a crucial initiating event leading to axonal degenerative changes distal to nerve transection. The loss of microtubules and neurofilaments and the associated granular alterations of the axoplasm in transected fibers appears to result from the influx of calcium into the axoplasm.