Ultrastructural studies of the dying-back process. II. The sequestration and removal by Schwann cells and oligodendrocytes of organelles from normal and diseases axons

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

The therapeutic potential of berberine chloride against SARM1-dependent axon degeneration in acrylamide-induced neuropathy

Chronic acrylamide (ACR) intoxication causes typical pathology of axon degeneration. Moreover, sterile-α and toll/interleukin 1 receptor motif-containing protein 1 (SARM1), the central executioner of the programmed axonal destruction process under various insults, is up-regulated in ACR neuropathy. However, it remains unclear whether inhibitors targeting SARM1 are effective or not. Among all the pharmacological antagonists, berberine chloride (BBE), a natural phytochemical and the first identified non-competitive inhibitor of SARM1, attracts tremendous attention. Here, we observed the protection of 100 μM BBE against ACR-induced neurites injury (2 mM ACR, 24 hr) in vitro, and further evaluated the neuroprotective effect of BBE (100 mg/kg p.o. three times a week for 4 weeks) in ACR-intoxicated rats (40 mg/kg i.p. three times a week for 4 weeks). The expression of SARM1 was also detected. BBE intervention significantly inhibited the overexpression of SARM1, ameliorated axonal degeneration, alleviated pathological changes in the sciatic nerve and spinal cord, and improved neurobehavioral symptoms in ACR-poisoned rats. Thus, BBE exhibits a strong neuroprotective effect against the SARM1-dependent axon destruction in ACR neuropathy. Meanwhile, our study underscores the need for appropriate inhibitor selection in diverse situations that would benefit from blocking the SARM1-dependent axonal destruction pathway.

2,5-Hexanedione induced apoptosis in rat spinal cord neurons and VSC4.1 cells via the proNGF/p75NTR and JNK pathways

Increasing evidence suggests that n-hexane induces nerve injury via neuronal apoptosis induced by its active metabolite 2,5-hexanedione (HD). However, the underlying mechanism remains unknown. Studies have confirmed that pro-nerve growth factor (proNGF), a precursor of mature nerve growth factor (mNGF), might activate apoptotic signaling by binding to p75 neurotrophin receptor (p75NTR) in neurons. Therefore, we studied the mechanism of the proNGF/p75NTR pathway in HD-induced neuronal apoptosis. Sprague-Dawley (SD) rats were injected with 400 mg/kg HD once a day for 5 weeks, and VSC4.1 cells were treated with 10, 20, and 40 mM HD in vitro. Results showed that HD effectively induced neuronal apoptosis. Moreover, it up-regulated proNGF and p75NTR levels, activated c-Jun N-terminal kinase (JNK) and c-Jun, and disrupted the balance between B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). Our findings revealed that the proNGF/p75NTR signaling pathway was involved in HD-induced neuronal apoptosis; it can serve as a theoretical basis for further exploration of the neurotoxic mechanisms of HD.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

Axonal degeneration in Guillain-Barré syndrome: a reappraisal

The aim of this review was to analyse the pathophysiology of axonal degeneration in Guillain-Barré syndrome (GBS) with emphasis on early stages (≤ 10 days after onset). An overview of experimental autoimmune neuritis (EAN) models is provided. Originally GBS and acute inflammatory demyelinating polyneuropathy were equated, presence of axonal degeneration being attributed to a "bystander" effect. Afterwards, primary axonal GBS forms were reported, designated as acute motor axonal neuropathy/acute motor-sensory axonal neuropathy. Revision of the first pathological description of axonal GBS indicates the coexistence of active axonal degeneration and demyelination in spinal roots, and pure Wallerian-like degeneration in peripheral nerve trunks. Nerve conduction studies are essential for syndrome subtyping, though their sensitivity is scanty in early GBS. Serum markers of axonal degeneration include increased levels of neurofilament light chain and presence of anti-ganglioside reactivity. According to nerve ultrasonographic features and autopsy studies, ventral rami of spinal nerves are a hotspot in early GBS. In P₂-induced EAN models, the initial pathogenic change is inflammatory oedema of spinal roots and sciatic nerve, which is followed by demyelination, and Wallerian-like degeneration in nerve trunks possessing epi-perineurium; a critical elevation of endoneurial fluid pressure is a pre-requisite for inducing ischemic axonal degeneration. Similar lesion topography may occur in GBS. The repairing role of adaxonal Schwann cytoplasm in axonal degeneration is analysed. A novel pathophysiological mechanism for nerve trunk pain in GBS, including pure motor forms, is provided. The potential therapeutic role of intravenous boluses of methylprednisolone for early severe GBS and intractable pain is argued.

Neuroprotein Targets of γ-Diketone Metabolites of Aliphatic and Aromatic Solvents That Induce Central-Peripheral Axonopathy

Peripheral neuropathy associated with chronic occupational and deliberate overexposure to neurotoxic organic solvents results from axonal degeneration in the central and peripheral nervous system. Human and experimental studies show that axonopathy is triggered by the action of neuroprotein-reactive γ-diketone metabolites formed from exposure to certain aliphatic solvents (n-hexane, 2-hexanone) and aromatic compounds (1,2-diethylbenzene, 1,2-4-triethylbenzene, 6-acetyl-1,1,4,4-tetramethyl-7-ethyl-1,2,3,4-tetralin). Neuroprotein susceptibility is related primarily to their differential content of lysine, the ∊-amino group of which is targeted by γ-diketones. Specific neuroprotein targets have been identified, and the sequence of molecular mechanisms leading to axonal pathology has been illuminated. While occupational n-hexane neuropathy continues to be reported, lessons learned from its experimental study may have relevance to other causes of peripheral neuropathy, including those associated with aging and diabetes mellitus.

Incidental Ultrastructural Findings in the Sural Nerve and Dorsal Root Ganglion of Aged Control Sprague Dawley Rats in a Nonclinical Carcinogenicity Study

Xenobiotic-induced peripheral nerve damage is a growing concern. Identifying relative risks that a new drug may cause peripheral nerve injury over long periods of administration is gathering importance in the evaluation of animal models. Separating out age-related changes in peripheral nerves of rats caused by compression injury from drug-induced effects has been difficult. Biopsy of the sural nerve is utilized in humans for investigations of peripheral neuropathy, because it is largely removed from the effects of nerve compression. This study used transmission electron microscopy to identify incidental findings in the sural nerves and dorsal root ganglia of aged control rats over time. The goal was to establish a baseline understanding of the range of possible changes that could be noted in controls compared to rats treated with any new investigative drug. In this evaluation, most sural nerve fibers from aged control rats had few ultrastructural abnormalities of pathologic significance. However, glycogenosomes, polyglucosan bodies, swollen mitochondria, autolysosomes, split myelin, Schwann cell processes, and endoneural macrophages with phagocytosed debris (considered an indication of ongoing degenerative changes) were occasionally noted.

Essential References for Structural Analysis of the Peripheral Nervous System for Pathologists and Toxicologists

Toxicologic neuropathology for the peripheral nervous system (PNS) is a vital but often underappreciated element of basic translational research and safety assessment. Evaluation of the PNS may be complicated by unfamiliarity with normal nerve and ganglion biology, which differs to some degree among species; the presence of confounding artifacts related to suboptimal sampling and processing; and limited experience with differentiating such artifacts from genuine disease manifestations and incidental background changes. This compilation of key PNS neurobiology, neuropathology, and neurotoxicology references is designed to allow pathologists and toxicologists to readily access essential information that is needed to enhance their proficiency in evaluating and interpreting toxic changes in PNS tissues from many species.

Mitochondrial Dynamics in Physiology and Pathology of Myelinated Axons

Mitochondria play essential roles in neurons and abnormal functions of mitochondria have been implicated in neurological disorders including myelin diseases. Since mitochondrial functions are regulated and maintained by their dynamic behavior involving localization, transport, and fusion/fission, modulation of mitochondrial dynamics would be involved in physiology and pathology of myelinated axons. In fact, the integration of multimodal imaging in vivo and in vitro revealed that mitochondrial localization and transport are differentially regulated in nodal and internodal regions in response to the changes of metabolic demand in myelinated axons. In addition, the mitochondrial behavior in axons is modulated as adaptive responses to demyelination irrespective of the cause of myelin loss, and the behavioral modulation is partly through interactions with cytoskeletons and closely associated with the pathophysiology of demyelinating diseases. Furthermore, the behavior and functions of axonal mitochondria are modulated in congenital myelin disorders involving impaired interactions between axons and myelin-forming cells, and, together with the inflammatory environment, implicated in axonal degeneration and disease phenotypes. Further studies on the regulatory mechanisms of the mitochondrial dynamics in myelinated axons would provide deeper insights into axo-glial interactions mediated through myelin ensheathment, and effective manipulations of the dynamics may lead to novel therapeutic strategies protecting axonal and neuronal functions and survival in primary diseases of myelin.

Invaginating Structures in Mammalian Synapses

Invaginating structures at chemical synapses in the mammalian nervous system exist in presynaptic axon terminals, postsynaptic spines or dendrites, and glial processes. These invaginating structures can be divided into three categories. The first category includes slender protrusions invaginating into axonal terminals, postsynaptic spines, or glial processes. Best known examples of this category are spinules extending from postsynaptic spines into presynaptic terminals in forebrain synapses. Another example of this category are protrusions from inhibitory presynaptic terminals invaginating into postsynaptic neuronal somas. Regardless of the direction and location, the invaginating structures of the first category do not have synaptic active zones within the invagination. The second category includes postsynaptic spines invaginating into presynaptic terminals, whereas the third category includes presynaptic terminals invaginating into postsynaptic spines or dendrites. Unlike the first category, the second and third categories have active zones within the invagination. An example of the second category are mossy terminal synapses of the hippocampal CA3 region, in which enlarged spine-like structures invaginate partly or entirely into mossy terminals. An example of the third category is the neuromuscular junction (NMJ) where substantial invaginations of the presynaptic terminals invaginate into the muscle fibers. In the retina, rod and cone synapses have invaginating processes from horizontal and bipolar cells. Because horizontal cells act both as post and presynaptic structures, their invaginating processes represent both the second and third category. These invaginating structures likely play broad yet specialized roles in modulating neuronal cell signaling.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Pathogenesis of autosomal dominant hereditary spastic paraplegia (SPG6) revealed by a rat model

Supplemental Digital Content is available in the text.

Hereditary spastic paraplegias (HSPs) are characterized by progressive spasticity and weakness in the lower extremities that result from length-dependent central to peripheral axonal degeneration. Mutations in the non-imprinted Prader-Willi/Angelman syndrome locus 1 (NIPA1) transmembrane protein cause an autosomal dominant form of HSP (SPG6). Here, we report that transgenic (Tg) rats expressing a human NIPA1/SPG6 mutation in neurons (Thy1.2-hNIPA1^G106R) show marked early onset behavioral and electrophysiologic abnormalities. Detailed morphologic analyses reveal unique histopathologic findings, including the accumulation of tubulovesicular organelles with endosomal features that start at axonal and dendritic terminals, followed by multifocal vacuolar degeneration in both the CNS and peripheral nerves. In addition, the NIPA1^G106R mutation in the spinal cord from older Tg rats results in an increase in bone morphogenetic protein type II receptor expression, suggesting that its degradation is impaired. This Thy1.2-hNIPA1^G106R Tg rat model may serve as a valuable tool for understanding endosomal trafficking in the pathogenesis of a subgroup of HSP with an abnormal interaction with bone morphogenetic protein type II receptor, as well as for developing potential therapeutic strategies for diseases with axonal degeneration and similar pathogenetic mechanisms.

Myelin peroxisomes - essential organelles for the maintenance of white matter in the nervous system

Peroxisomes are cellular compartments primarily associated with lipid metabolism. Most cell types, including nervous system cells, harbor several hundred of these organelles. The importance of peroxisomes for central nervous system white matter is evidenced by a variety of human peroxisomal disorders with neurological impairment frequently involving the white matter. Moreover, the most frequent childhood white matter disease, X-linked adrenoleukodystrophy, is a peroxisomal disorder. During the past decade advances in imaging techniques have enabled the identification of peroxisomes within the myelin sheath, especially close to nodes of Ranvier. Although the function of myelin peroxisomes is not solved yet on molecular level, recently acquired knowledge suggests a central role for these organelles in axo-glial metabolism. This review focuses on the biology of myelin peroxisomes as well as on the pathology of myelin and myelinated axons that is observed as a consequence of partial or complete peroxisomal dysfunction in the brain.

Neuroprotection in multiple sclerosis: a therapeutic approach

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that is pathologically characterized by inflammatory demyelination and neurodegeneration. Axonal damage, along with neuronal loss, occurs from disease onset and may lead to progressive and permanent disability. In contrast with the inflammatory pathways, the molecular mechanisms leading to MS neurodegeneration remain largely elusive. With improved understanding of these mechanisms, new potential therapeutic targets for neuroprotection have emerged. We review the current understanding of neurodegenerative processes at play in MS and discuss potential outcome measures and targets for neuroprotection trials.

Biology of Schwann cells

The fundamental roles of Schwann cells during peripheral nerve formation and regeneration have been recognized for more than 100 years, but the cellular and molecular mechanisms that integrate Schwann cell and axonal functions continue to be elucidated. Derived from the embryonic neural crest, Schwann cells differentiate into myelinating cells or bundle multiple unmyelinated axons into Remak fibers. Axons dictate which differentiation path Schwann cells follow, and recent studies have established that axonal neuregulin1 signaling via ErbB2/B3 receptors on Schwann cells is essential for Schwann cell myelination. Extracellular matrix production and interactions mediated by specific integrin and dystroglycan complexes are also critical requisites for Schwann cell-axon interactions. Myelination entails expansion and specialization of the Schwann cell plasma membrane over millimeter distances. Many of the myelin-specific proteins have been identified, and transgenic manipulation of myelin genes have provided novel insights into myelin protein function, including maintenance of axonal integrity and survival. Cellular events that facilitate myelination, including microtubule-based protein and mRNA targeting, and actin based locomotion, have also begun to be understood. Arguably, the most remarkable facet of Schwann cell biology, however, is their vigorous response to axonal damage. Degradation of myelin, dedifferentiation, division, production of axonotrophic factors, and remyelination all underpin the substantial regenerative capacity of the Schwann cells and peripheral nerves. Many of these properties are not shared by CNS fibers, which are myelinated by oligodendrocytes. Dissecting the molecular mechanisms responsible for the complex biology of Schwann cells continues to have practical benefits in identifying novel therapeutic targets not only for Schwann cell-specific diseases but other disorders in which axons degenerate.

Neuroprotection and repair in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease that is considered by many people to have an autoimmune aetiology. In recent years, new data emerging from histopathology, imaging and other studies have expanded our understanding of the disease and may change the way in which it is treated. Conceptual shifts have included: first, an appreciation of the extent to which the neuron and its axon are affected in MS, and second, elucidation of how the neurobiology of axon-glial and, particularly, axon-myelin interaction may influence disease progression. In this article, we review advances in both areas, focusing on the molecular mechanisms underlying axonal loss in acute inflammation and in chronic demyelination, and discussing how the restoration of myelin sheaths via the regenerative process of remyelination might prevent axon degeneration. An understanding of these processes could lead to better strategies for the prevention and treatment of axonal loss, which will ultimately benefit patients with MS.

Interrelationships of undernutrition and neurotoxicity: food for thought and research attention

The neurotoxic actions of chemical agents on humans and animals are usually studied with little consideration of the subject's nutritional status. States of protein-calorie, vitamin and/or mineral undernutrition are associated with a range of neurodevelopmental, neurological and psychiatric disorders, commonly with involvement of both the central and the peripheral nervous system. Undernutrition can modify risk for certain chemical-induced neurologic diseases, and in some cases undernutrition may be a prerequisite for neurotoxicity to surface. In addition, neurologic disease associated with undernutrition or neurotoxicity may show similarities in clinical and neuropathological expression, especially in the peripheral nervous system. The combined effects of undernutrition and chemical neurotoxicity are most relevant to people with low incomes who experience chronic hunger, parasitism and infectious disease, monotonous diets of plants with neurotoxic potential (notably cassava), environmental pollution from rapid industrial development, chronic alcohol abuse, or prolonged treatment with certain therapeutic drugs. Undernutrition alone or in combination with chemical exposure is also important in high-income societies in the setting of drug and alcohol abuse, old age, food faddism, post-bariatric surgery, and drug treatment for certain medical conditions, including cancer and tuberculosis. The nutritional demands of pregnancy and lactation increase the risk of fetal and infant undernutrition and chemical interactions therewith.

A role for myelin-associated peroxisomes in maintaining paranodal loops and axonal integrity

Demyelinating diseases of the nervous system cause axon loss but the underlying mechanisms are not well understood. Here we show by confocal and electron microscopy that in myelin-forming glia peroxisomes are associated with myelin membranes. When peroxisome biogenesis is experimentally perturbed in Pex5 conditional mouse mutants, myelination by Schwann cells appears initially normal. However, in nerves of older mice paranodal loops become physically unstable and develop swellings filled with vesicles and electron-dense material. This novel model of a demyelinating neuropathy demonstrates that peroxisomes serve an important function in the peripheral myelin compartment, required for long-term axonal integrity.

Axon-glial interaction in the CNS: what we have learned from mouse models of Pelizaeus-Merzbacher disease

In the central nervous system (CNS) the majority of axons are surrounded by a myelin sheath, which is produced by oligodendrocytes. Myelin is a lipid-rich insulating material that facilitates the rapid conduction of electrical impulses along the myelinated nerve fibre. Proteolipid protein and its isoform DM20 constitute the most abundant protein component of CNS myelin. Mutations in the PLP1 gene encoding these myelin proteins cause Pelizaeus-Merzbacher disease and the related allelic disorder, spastic paraplegia type 2. Animal models of these diseases, particularly models lacking or overexpressing Plp1, have shed light on the interplay between axons and oligodendrocytes, and how one component influences the other.

Perspectives on past practices and future trends in toxicologic neuropathology: notes from a keynote address by Dr. Peter S. Spencer

Dr. Peter S. Spencer, a pioneering neurotoxicologist of international renown, delivered the keynote address at the 2010 Joint Scientific Symposium of the Society of Toxicologic Pathology (STP) and the International Federation of Societies of Toxicologic Pathologists (IFSTP). He has made many landmark discoveries during his four-decade career. Dr. Spencer's address communicated several fundamental principles of past and present toxicologic neuropathology research, and he also predicted future trends in the field. First, classic approaches to toxicologic neuropathology emphasized morphologic techniques such as light microscopic and ultrastructural assessment. However, neuropathology methods alone rarely reveal the mechanism(s) and etiology of neurotoxic conditions, so neurotoxicity problems are now being investigated using a multidisciplinary approach in which neuropathologic assessment is but one component of the analysis. The two primary trends for future toxicologic neuropathology investigations, in both animals and humans, will be an increased use of noninvasive neural imaging and greater preference for in situ molecular ("omic") methods, which provide functional information in a structural context. These trends will significantly enhance the ability of scientists to translate animal data to human situations, thereby improving our understanding of disease mechanisms and facilitating efforts to design new therapies for neural diseases.

Axl-/- mice have delayed recovery and prolonged axonal damage following cuprizone toxicity

Activation of the receptor tyrosine kinase Axl recruits signaling molecules that regulate cell growth and survival. To evaluate Axl's role in brain during cuprizone toxicity, mice were fed cuprizone and evaluated at 3-, 4-, and 6-week cuprizone treatment and 3- and 5-week post-cuprizone withdrawal. At 4-week cuprizone treatment, the corpora callosa of wildtype (WT) mice had robust Oil Red O+ staining indicative of ongoing phagocytosis. Axl-/- mice had minimal Oil Red O+ staining, fewer microglia, and significantly more TUNEL+/ASPA+ mature oligodendrocytes than the WT. At 6-week cuprizone treatment, there was significantly more Oil Red O+ staining in the Axl-/- corpora callosa than in the WT indicating a lag in the clearance of cellular and myelin debris. Relative to WT mice, there were fewer mature oligodendrocytes and significantly more SMI-32+ axons at 3-week post-cuprizone withdrawal, indicative of axonal damage in the Axl-/- corpora callosa. Electron microscopy determined that at 3-week post-cuprizone withdrawal the number of dystrophic axons and axons containing autophagosome-like vacuoles/mouse was increased in the Axl-/- mice relative to the WT mice. In Axl-/- corpora callosa, 5-week post-cuprizone withdrawal, the number of mature oligodendrocytes was comparable to the WT mice, but axons in the Axl-/- mice were SMI-32+, suggesting that Axl-/- mice have delayed clearance of apoptotic oligodendrocytes and myelin debris resulting in prolonged axonal damage and recovery from cuprizone toxicity.

Acrylamide-induced nerve terminal damage: relevance to neurotoxic and neurodegenerative mechanisms

Acrylamide (ACR) has demonstrable neurotoxic effects in animals and humans that stem from its chemical behavior as a soft electrophilic alpha,beta-unsaturated carbonyl compound. Evidence is presented that the nerve terminal is a primary site of ACR action and that inhibition of neurotransmission mediates the development of neurological deficits. At the mechanistic level, recent proteomic, neurochemical, and kinetic data are considered, which suggest that ACR inhibits neurotransmission by disrupting presynaptic nitric oxide (NO) signaling. Nerve-terminal damage likely mediates the neurological complications that accompany the occupational exposure of humans to ACR. In addition, the proposed molecular mechanism of synaptotoxicity has substantial implications for the pathogenesis of Alzheimer's disease and other neurodegenerative conditions that involve neuronal oxidative stress and the secondary endogenous generation of acrolein and other conjugated carbonyl chemicals.

Axon myelin transfer of a non-enveloped virus

We showed previously that Theiler's virus, a neurotropic non-enveloped picornavirus of mouse, traffics from the axon of infected neurons into the surrounding myelin. When this traffic is interrupted, as in the shiverer mouse which bears a mutation in the myelin basic protein gene, the virus is unable to persist in the central nervous system. In the present work, we used the Wld(s) mutant mouse, a strain in which axonal degeneration is considerably slowed down, to show that axon to myelin traffic takes place in the absence of axon degeneration. Our results suggest the existence of a mechanism of transfer of axonal cytoplasm into the myelin which Theiler's virus might exploit to ensure its persistence.

Comparable myelinated nerve pathology in feline and human diabetes mellitus

The occurrence of diabetic neuropathy in cats provides an opportunity to study the development and treatment of neurological complications not present in diabetic rodent models, where few pathological alterations are evident. The present study further defines pathological alterations in nerve biopsies from 12 cats with spontaneously occurring diabetes mellitus. Peroneal nerve biopsies displayed concurrent injury to both Schwann cells and axons of myelinated fibers that was remarkably similar to that present in human diabetic neuropathy. In addition to demyelination, remyelination (constituting 20-84% of the total myelinated fiber population) was indicated by fibers with inappropriately thin myelin sheaths. Unlike our previous investigations, striking axonal injury was apparent, and consisted of dystrophic accumulations of membranous debris or neurofilaments, as well as degenerative fiber loss resulting in a 50% decrease in myelinated fiber density. In spite of extensive fiber loss, regenerative clusters were apparent, suggesting that axonal regeneration was not completely frustrated. These data highlight the potential utility of feline diabetic neuropathy as a model that faithfully replicates the nerve injury in human diabetes mellitus.

Axonal degeneration with unusual lesions of the myelin in an occupational neuropathy

A 35-year-old man had prolonged occupational exposure to lead carboxylate, triethylbenzene, xylene, and dichloromethane, when he developed a subacute predominantly sensory neuropathy. Ultrastructural examination of a peripheral nerve biopsy showed axonal degeneration and unusual lesions of the myelin, with Schwann cell sequestration of vesicular and lamellar debris. Biochemical analysis of lead in a frozen peripheral nerve specimen revealed no significant difference between the propositus and a control. The authors were unable to find any similar peripheral nerve lesions in the literature dealing with neurotoxic chemicals. Any of the several organic solvents could have equally caused the neuropathy and may have been potentialized by the other chemicals.

Subsurface cisterna-lined axonal invaginations and double-walled vesicles at the axonal–myelin sheath interface

The axonal-myelin sheath interface of vertebrate myelinated axons possesses special structural complexities, and there may be an intercellular macromolecular traffic transversing the periaxonal cleft that spans the internodal axon. By conventional electron microscopy and serial sectioning, we observed a category of double-walled vesicles at the axonal-myelin sheath interface, which often contained ribosome-like particles or endoplasmic reticulum. Some of them were demonstrated to continue with the subjacent axon with a thin stalk. In addition, we described a special category of axonal invaginations, probably mediated by subsurface cisternae. The functional implications of these specialized structures were discussed.

The Changing View of Acrylamide Neurotoxicity

Acrylamide (ACR) is a water-soluble, vinyl monomer that has multiple chemical and industrial applications: e.g., waste water management, ore processing. In addition, ACR is used extensively in molecular laboratories for gel chromatography and is present in certain foods that have been prepared at very high temperatures. Extensive studies in rodents and other laboratory animals have provided evidence that exposure to monomeric ACR causes cellular damage in both the nervous and reproductive systems, and produces tumors in certain hormonally responsive tissues. Whereas human epidemiological studies have demonstrated a significantly elevated incidence of neurotoxicity in occupationally exposed populations, such research has not, to date, revealed a corresponding increase in cancer risk. Since the announcement by a Swedish research group in April 2002 [J. Ag. Food Chem. 50 (2002) 4998] regarding the presence of ACR in potato and grain-based foods, there has been a renewed interest in the toxic actions of this chemical. Therefore, in this review, we consider the different toxic effects of ACR. The neurotoxic actions of ACR will be the focal point since neurotoxicity is a consequence of both human and laboratory animal exposure and since this area of investigation has received considerable attention over the past 30 years. As will be discussed, a growing body of evidence now indicates that the nerve terminal is a primary site of ACR action and that inhibition of corresponding membrane-fusion processes impairs neurotransmitter release and promotes eventual degeneration. The electrophilic nature of ACR suggests that this neurotoxicant adducts nucleophilic sulfhydryl groups on certain proteins that are critically involved in membrane fusion. Adduction of thiol groups also might be common to the reproductive and carcinogenic effects of ACR. A final goal of this review is to identify data gaps that retard a comprehensive understanding of ACR pathophysiological processes.

Transforming growth factor beta 1 may regulate the stability of mature myelin sheaths

The molecular mechanisms underlying peripheral neuropathies have only been partially elucidated. In particular, the regulatory factors that control the stability and turnover of mature myelin are largely unknown. Transforming growth factor beta 1 (TGF-beta1), and its associated receptors, are expressed by mature Schwann cells. On this basis, we postulated that TGF-beta1 may be an autocrine regulator of mature myelin. This hypothesis was tested by examining the ultrastructure of myelin in adult mice that have a null mutation of their TGF-beta1 gene. We report here that the myelin of these mice is grossly abnormal. At the nodes of Ranvier, the cytoplasmic collars of the Schwann cells were expanded and the myelin had a honeycomb appearance. Focal (tomacula-like) hypermyelin structures were observed in the internodal regions of a significant number of axons in mutant nerve, and were not observed in littermate controls. Axon diameters were within the normal range and no axonal pathology was evident in mutant nerve and macrophages were absent. Results imply that lack of TGF-beta1 may have a direct effect on Schwann cells. We suggest that TGF-beta1 may stabilise compact myelin via an autocrine mechanism.

Early onset of degenerative changes at nodes of Ranvier in alpha-motor axons of Cntf null (-/-) mutant mice

The nodes of Ranvier are sites of specific interaction between Schwann cells and axons. Besides their crucial role in transmission of action potentials, the nodes of Ranvier and in particular the paranodal axon-Schwann cell networks (ASNs) are thought to function as local centers in large motor axons for removal, degradation, and disposal of organelles. In order to test whether ciliary neurotrophic factor (CNTF), which is present at high levels in the Schwann cell cytoplasm, is involved in the maintenance of these structures, we have examined lumbar ventral root nerve fibers of alpha-motor neurons by electron microscopy in 3- and 9-month-old Cntf null ((-/-)) mutant mice. Nerve fibers and nodes of Ranvier in 3-month-old Cntf(-/-) mutants appeared morphologically normal, except that ASNs were more voluminous in the mutants than in wild-type control animals at this age. In 9-month-old Cntf(-/-) animals, morphological changes, such as reduction in nerve fiber and axon diameter, myelin sheath disruption, and loss of ASNs at nodes of Ranvier, were observed. These findings suggest that endogenous CNTF, in addition to its role in promoting motor neuron survival and regeneration, is needed for long-term maintenance of alpha-motor nerve fibers. The premature loss of paranodal ASNs in animals lacking CNTF, which seems to be a defect related to a disturbed interaction in the nodal region between the axon and its myelinating Schwann cells, could impede the maintenance of a normal milieu in the motor axon, preceding more general neuronal damage.

Nerve terminals as the primary site of acrylamide action: a hypothesis

Acrylamide (ACR) is considered to be prototypical among chemicals that cause a central-peripheral distal axonopathy. Multifocal neurofilamentous swellings and eventual degeneration of distal axon regions in the CNS and PNS have been traditionally considered the hallmark morphological features of this axonopathy. However, ACR has also been shown to produce early nerve terminal degeneration of somatosensory, somatomotor and autonomic nerve fibers under a variety of dosing conditions. Recent research from our laboratory has demonstrated that terminal degeneration precedes axonopathy during low-dose subchronic induction of neurotoxicity and occurs in the absence of axonopathy during higher-dose subacute intoxication. This relationship suggests that nerve terminal degeneration, and not axonopathy, is the primary or most important pathophysiologic lesion produced by ACR. In this hypothesis paper, we review evidence suggesting that nerve terminal degeneration is the hallmark lesion of ACR neurotoxicity, and we propose that this effect is mediated by the direct actions of ACR at nerve terminal sites. ACR is an electrophile and, therefore, sulfhydryl groups on presynaptic proteins represent rational molecular targets. Several presynaptic thiol-containing proteins (e.g. SNAP-25, NSF) are critically involved in formation of SNARE (soluble N-ethylmaleimide (NEM)-sensitive fusion protein receptor) complexes that mediate membrane fusion processes such as exocytosis and turnover of plasmalemmal proteins and other constituents. We hypothesize that ACR adduction of SNARE proteins disrupts assembly of fusion core complexes and thereby interferes with neurotransmission and presynaptic membrane turnover. General retardation of membrane turnover and accumulation of unincorporated materials could result in nerve terminal swelling and degeneration. A similar mechanism involving the long-term consequences of defective SNARE-based turnover of Na+/K(+)-ATPase and other axolemmal constituents might explain subchronic induction of axon degeneration. The ACR literature occupies a prominent position in neurotoxicology and has significantly influenced development of mechanistic hypotheses and classification schemes for neurotoxicants. Our proposal suggests a reevaluation of current classification schemes and mechanistic hypotheses that regard ACR axonopathy as a primary lesion.

Ultrastructural changes in peripheral nerve in hereditary motor and sensory neuropathy-Lom

Ultrastructural observations have been made on nerve biopsy specimens from five cases of hereditary motor and sensory neuropathy-Lom (HMSNL). A number of features that distinguish it from other hereditary demyelinating neuropathies were identified. Teased fibre studies were not feasible but examination of longitudinal sections by electron microscopy demonstrated demyelination/remyelination. Severe progressive axonal loss was a conspicuous feature. There was no indication of axonal atrophy. Hypertrophic onion bulb changes were present in the younger patients which later regressed, probably secondary to axonal loss. Myelin thickness was generally reduced in relation to axon diameter, indicating hypomyelination, and partial ensheathment of axons by Schwann cells was observed. The Schmidt-Lanterman incisures were atypical in extending for long lengths along the internode. Uncompacted myelin with a periodicity greater than that observed in other neuropathies in which it occurs was a feature, as was the accumulation of pleomorphic material in the adaxonal Schwann cell cytoplasm. An unusual finding was the presence of intra-axonal accumulations of irregularly arranged curvilinear profiles. These resemble those that have been described in experimental vitamin E deficiency. The amount of endoneurial collagen was markedly increased and some endoneurial blood vessels showed a non-specific basal laminal reduplication.

Corpora-amylacea and the family of polyglucosan diseases

The history, characters, composition and topography of corpora amylacea (CA) in man and the analogous polyglucosan bodies (PGB) in other species are documented, noting particularly the wide variation in the numbers found with age and in neurological disease. Their origins from both neurons and glia and their probable migrations and ultimate fate are discussed. Their presence is also noted in other organs, particularly in the heart. The occurrence in isolated cases of occasional 'massive' usually focal accumulations of similar polyglucosan bodies in association with certain chronic neurological diseases is noted and the specific conditions Adult Polyglucosan body disease and type IV glycogenosis where they are found throughout the nervous system in great excess is discussed. The distinctive differences of CA from the PGB of Lafora body disease and Bielschowsky body disease are emphasised. When considering their functional roles, a parallel is briefly drawn on the one hand between normal CA and the bodies in the polyglucosan disorders and on the other with the lysosomal system and its associated storage diseases. It is suggested that these two systems are complementary ways by which large, metabolically active cells such as neurons, astrocytes, cardiac myocytes and probably many other cell types, dispose of the products of stressful metabolic events throughout life and the continuing underlying process of aging and degradation of long lived cellular proteins. Each debris disposal system must be regulated in its own way and must inevitably, a priori, be heir to metabolic defects that give rise in each to its own set of metabolic disorders.

Effects of steroid hormones on gene expression of glial markers in the central and peripheral nervous system: variations induced by aging

The present article summarizes our data regarding: (a) the effect of sex steroids on the expression of a specific astrocytic marker in glial cell cultures (GFAP); (b) the effects of aging on two markers of the peripheral myelin (glycoprotein Po and the myelin basic protein, MBP); (c) the possible modification of the damaging effects of aging on these two markers by the in vivo administration of progesterone and its derivatives; and, finally, (d) the effect of progesterone derivatives on the gene expression of Po in cultures of rat Schwann cells. The data obtained have indicated that progesterone and its 5 alpha-reduced metabolites may play an important role in the control of gene expression of GFAP and Po, respectively, in type 1 astrocytes and Schwann cells. It has also been found that the gene expression of Po and MBP is dramatically decreased in the myelin of the sciatic nerve of aged male rats and that the aged-linked decrease of the gene expression of Po is partially reversible with steroid treatment.

Spinal corpora amylacea and motor neuron disease: a quantitative study

OBJECTIVE

To test the hypothesis that as there is growing evidence that corpora amylacea, or amyloid bodies, in the CNS are derived primarily from neurons, it might be expected that their numbers in the spinal cord would decline with loss of neurons in motor neuron degeneration as they do in the retina on destruction of ganglion cells by glaucoma.

METHODS

The numbers of corpora amylacea were counted in PAS stained transverse sections of the lumbar cord from 27 patients with motor neuron disease and 21 control subjects of similar age and sex mix. The numbers and sizes of corpora amylacea were determined both in the anterior horn grey matter and in the submeningeal white matter regions in each case.

RESULTS

In both groups the total numbers in the white matter and submeningeal regions ranged from 160 to more than 5000/section and there was minimal significant difference between the two groups. No relation with age was found in this narrow age range. The mean diameters of the corpora amylacea were significantly less in the grey matter of both groups than in the submeningeal regions. However, their densities in the grey matter of the anterior horn were significantly reduced in the spinal cord sections in the motor neuron disease group, but only where few motor neurons remained.

CONCLUSIONS

These findings support the view that corpora amylacea may arise from neurons, and suggest that that there may be two compartments, one mobile and one static, the second most likely remaining in the periphery of the spinal cord for prolonged periods.

Age-induced decrease of glycoprotein Po and myelin basic protein gene expression in the rat sciatic nerve. Repair by steroid derivatives

The data here reported show that the gene expression of the glycoprotein Po and of the myelin basic protein, the major components of myelin in the peripheral nervous system, dramatically decreases with ageing in the sciatic nerve of normal male rats. A one-month treatment with dihydroprogesterone, the 5alpha-reduced derivative of progesterone, is able to partially restore the fall in Po gene expression occurring in the sciatic nerve of aged male rats, without significantly modifying the gene expression of the myelin basic protein. In cultures of neonatal Schwann cells (the peripheral nervous system elements involved in the synthesis of myelin), the addition of progesterone and of dihydroprogesterone significantly increases Po gene expression; the 3alpha-reduced metabolite of dihydroprogesterone, tetrahydroprogesterone proved to be even more effective. These data suggest that the effect of progesterone is linked to its conversion into dihydroprogesterone and especially into tetrahydroprogesterone, since Schwann cells possess the 5alpha-reductase-3alpha-hydroxysteroid dehydrogenase system. The data provide the first demonstration that ageing decreases the gene expression of two major components of the peripheral myelin in the sciatic nerve; they also show that this phenomenon may be partially reversed by progesterone derivatives, which might act by stimulating Po gene expression in the Schwann cells.

Removal of retrogradely transported material from rat lumbosacral alpha-motor axons by paranodal axon-Schwann cell networks

The aim of this study was to investigate the potential ability of Schwann cells to sequester axonally transported material via so called axon-Schwann cell networks (ASNs). These are entities consisting of sheets of Schwann cell adaxonal plasma membrane that invade the axon and segregate portions of axoplasm in paranodes of large myelinated mammalian nerve fibres. Rat hindlimb alpha-motor axons were examined in the L4-S1 ventral roots using light/fluorescence, confocal laser, and electron microscopy for detection of retrogradely transported red-fluorescent latex nanospheres taken up at a sciatic nerve crush, and intramuscularly injected horseradish peroxidase endocytosed by intact synaptic terminals. Survival times after tracer administration ranged from 27 hours to 4 weeks. During their retrograde transport toward the motor neuron perikarya, organelles carrying nanospheres/peroxidase accumulated at nodes of Ranvier, where they often appeared in close association with the paranodal myelin sheath. Serial section electron microscopy showed that many of the tracer-containing bodies were situated within ASN complexes, thereby being segregated from the main axon. Four weeks after nanosphere administration, several node-paranode regions still showed ASN-associated aggregations of spheres, some of which were situated in the adaxonal Schwann cell cytoplasm. The data establish the ability of Schwann cells to segregate material from motor axons with intact myelin sheaths, using the ASN as mediator. Taken together with our earlier observations that ASNs in alpha-motor axons are also rich in lysosomes, this process would allow a local elimination and secluded degradation of retrogradely transported foreign substances and degenerate organelles before reaching the motor neuron perikarya. In addition, ASNs may serve as sites for disposal of indigestable material.

Node-paranode regions as local degradative centres in alpha-motor axons

Lysosomes play an important role for the maintenance of a normal internal milieu in the cell. In neurons lysosomes are abundant in the perikaryon and dendrites, but have been observed to a much lesser degree in the axon. A general opinion has therefore formed among biologists interested in the nervous system that axonal material destined for degradation has to be transported to the neuronal perikaryon. The lysosomal occurrence and distribution at the level of the axon have, however, not been investigated systematically. This review summarizes recent morphological data based on light, fluorescence, and electron microscopic observations in peripheral nerve fibres of cats and rats providing evidence that node-paranode regions mainly along the peripheral parts of alpha motor axons, where the axon cross-section area decreases to 10-25% of internodal values, can control the passage and participate in a lysosome-mediated degradation of axonally transported materials directed towards the neuronal perikaryon. An important role is played by the paranodal axon-Schwann cell networks, which are lysosome-rich entities whereby the Schwann cells can sequester material from the axoplasm of large myelinated peripheral nerve fibres. The networks also seem to serve as depots for axonal waste products. The degradative ability of node-paranode regions in alpha-motor axons could be of some significance for the protection of the motor neuron perikarya from being flooded with and perhaps injured by indigestible materials as well as potentially deleterious, exogenous substances imbibed by the axon terminals in the muscle. A similar degradative capacity may not be needed in nerve fibres with synaptic terminals in the CNS where the local environment is regulated by the blood-brain barrier.

Transfer of horseradish peroxidase from oligodendrocyte to axon in the myelinating neonatal rat optic nerve: artefact or transcellular exchange?

In this paper we make the surprising observation that intracellular injection of horseradish peroxidase (HRP) into a single myelinating oligodendrocyte also resulted in localised HRP labelling at the nodes of Ranvier of some axons of the unit. It appeared that HRP had been transferred to the nodal axoplasm from the paranodal loops of the HRP-filled oligodendrocyte. Three HRP-filled oligodendrocytes from isolated optic nerves of 14-day-old rats were analysed by serial section electron microscopy, and HRP was observed in the axonal cytoplasm at three of the nodes of Ranvier delineated by one of the cells. At labelled nodes, HRP was of a uniform intensity throughout the nodal axoplasm. Axonal labelling gradually diminished along the paranodal regions and was not evident in the contiguous internodal axoplasm beyond 20 microns from the node. The myelin sheaths, paranodal loops, and axons appeared normal at labelled nodes, and the paranodal loops and astrocyte perinodal processes adjacent to those of the HRP-filled oligodendrocyte unit did not contain HRP. There was no evidence of extracellular HRP or tissue damage in the surrounding neuropil, and axons neighbouring those enwrapped by the HRP-filled oligodendrocyte did not contain HRP. The possibility that axonal labelling was an artefact of either iontophoretic injection or tissue preparation is discussed. This provocative finding is not definite proof of exchange, but the balance of evidence supports the possibility that there was transcellular exchange of HRP at paranodes between the labelled oligodendrocyte and some of the axons in the unit. The rarity of HRP transfer to axons suggests that it may be a transient or labile event. It is not clear whether oligodendrocyte to axon macromolecular exchange has real physiological and/or pathological significance.

Development of nodes of Ranvier in feline nerves: an ultrastructural presentation

The ultrastructure of developing nodes of Ranvier and adjacent paranodes of future large myelinated fibers in feline lumbar spinal roots is described. The development starts before birth concurrent with myelination and is finished at the end of the first postnatal month when the nodal regions of future large fibers, now 4-5 microns of diameter, for the first time appear like miniatures of those of their 4 times thicker and fully mature counterparts. At this stage the fibers also begin to show mature functional properties. The latent maturation process is denoted "nodalization" and includes two major events: (1) the formation of a narrow node gap bordered by compact myelin segments and filled with Schwann cell microvilli that interconnect an undercoated nodal axolemma with rapidly increasing accumulations of mitochondria lodging in the longitudinal cords of Schwann cell cytoplasm that is distributed outside a more and more crenated paranodal myelin sheath; (2) the setting of a fixed number of nodes along the axons; an event that includes segmental axonal and myelin sheath degeneration and is concluded by the elimination of supernumerary Schwann cells.

Early nodal changes in the acute motor axonal neuropathy pattern of the Guillain-Barré syndrome

The axonal patterns of Guillain-Barré syndrome, associated in many cases with antecedent Campylobacter jejuni infection, are now recognized as frequent causes of acute flaccid paralysis in some regions of the world. This study examined ultrastructurally the PNS of seven cases of the acute motor axonal neuropathy form of Guillain-Barré syndrome. In this disorder previous studies of advanced cases have found Wallerian-like degeneration of motor fibres in the spinal roots and peripheral nerves, with little lymphocytic inflammation or demyelination. The present study was focused on identifying early changes and establishing the sequence of changes. By electron microscopy the earliest and mildest changes consisted of lengthening of the node of Ranvier with distortion of the paranodal myelin, and in some instances with breakdown of the outermost myelin terminal loops. At this stage many nodes had overlying macrophages which extended their processes through the Schwann cell basal lamina covering the node and apposed the axolemma. Macrophage processes then extended beneath the myelin terminal loops, and the whole macrophage entered the periaxonal space at the paranode. Macrophage processes dissected the axon from the adaxonal Schwann cell plasmalemma and the macrophages advanced into the internodal periaxonal space, where they typically surrounded a condensed-appearing axon. At this stage the adaxonal Schwann cell cytoplasm regularly degenerated and disappeared, so that the periaxonal space was bounded by the innermost myelin lamella, and the axolemma of many fibres could not be seen. The internodal myelin sheath and the abaxonal Schwann cell cytoplasm remained normal. This arrangement appeared to be stable for some time, but in many fibres the axon subsequently underwent Wallerian-like degeneration. By interfering with impulse conduction, these nodal and periaxonal changes may explain paralysis in some pathologically mild cases. In addition, at early stages, these changes may be reversible, thus explaining the rapid recovery of some patients who become paralysed with acute motor axonal neuropathy. These observations, taken together with previous studies, suggest that acute motor axonal neuropathy is an antibody- and complement-mediated disorder in which the relevant epitopes are present on the nodal and internodal axolemma.

Distribution of retrogradely transported fluorescent latex microspheres in rat lumbosacral ventral root axons following peripheral crush injury: a light and electron microscopic study

The retrograde axonal transport of fluorescent latex microspheres, which are tracers extensively used for studying the neuronal connectivity in the CNS, was investigated in large myelinated lumbosacral ventral root nerve fibres of adult rats following peripheral crush injury. After crushing the sciatic nerve, a suspension of 30 nm red-fluorescent latex beads was injected in the crush region. Following postoperative survival times of 24, 48, 72 and 120 h, the animals were fixed by vascular perfusion using different types of paraformaldehyde-based fixatives. At shorter survival times, red-fluorescent granules were seen distributed mainly internodally in several axons, while at longer times (> 48 h) an accumulation at nodes of Ranvier, close to the paranodal myelin sheath, predominated. Photoconversion of the fluorescent labelling into a stable, highly electron dense reaction product was performed using diaminobenzidine, permitting ultrastructural observations. The electron dense material that formed over the fluorescent granules appeared in association with membrane-delimited bodies. In some bodies the electron dense material formed well-defined, solitary spheres of sizes corresponding to those of the latex beads. When located close to the paranodal myelin sheath, the bodies were often situated within larger membranous structures, which sometimes were partly engulfed by protrusions of the so called axon-Schwann cell network. At longer survival times, some bodies containing photoconversion reaction product appeared within the axon-Schwann cell network, thereby being segregated from the main axoplasm. The study introduces a new application for fluorescent latex microspheres. The used approach, combining light/fluorescence and electron microscopy, should be suitable for long term investigations of the fate of axonally transported non-neuronal substances.

Axoplasmic organelles at nodes of Ranvier. I. Occurrence and distribution in large myelinated spinal root axons of the adult cat

Using light microscopy (LM) and electron microscopy (EM) we have examined the occurrence and distribution of axoplasmic organelles in large myelinated nerve fibres of the L7 ventral and dorsal spinal roots of the cat with special reference to the paranode-node-paranode (pnp)-regions. Ninety-eight percent of the 550 Toluidine Blue-stained paranode-node-paranode-regions examined in the light microscope contained dark-blue bodies accumulated distal to the midlevel of the paranode-node-paranode-region. Further, a veil of Toluidine Blue positive material was observed in about 50% of the paranode-node paranode-regions. In about 25% of these paranode-node-paranode-regions the veil lay distal to the midlevel of the paranode-node-paranode-region and in the remainder it lay proximally. Electron microscopy suggested that the ultrastructural equivalents of the dark-blue bodies and of the veil were dense lamellar bodies and a diffuse granular material, respectively. Our calculations indicate that from 70% to more than 90% of some organelles (dense lamellar bodies, multivesicular bodies and vesiculo-tubular membranous organelles) present in an axon are accumulated in the paranode-node-paranode-regions. The occurrence of these organelles in the individual paranode-node-paranode-regions varied within wide limits also in adjacent fibres. The dense lamellar and multivesicular bodies dominated the distal part of the paranode-node-paranode-regions while the vesiculo-tubular membranous organelles dominated the proximal part, i.e. the organelles showed a mutual proximo-distal segregation with reference to the midlevel of the paranode-node-paranode-region. Of seventeen paranode-node-paranode-regions analyzed ultrastructurally, seven were classified as 'fully segregated', that is 67% or more of the lamellar and multivescular bodies, present in the whole paranode-node-paranode-region, lay distal to the mid-level, and 67% or more of the vesiculo-tubular membranous organelles lay proximal to it.

Morphometric study of the sensory neuron and peripheral nerve changes induced by chronic cisplatin (DDP) administration in rats

We performed a morphological, morphometric and toxicological study on the spinal ganglia and peripheral nerves of the rat after chronic administration of cisplatin (cis-dichlorodiammineplatinum II; DDP) with two different schedules. Severe damage of the spinal ganglia neurons was demonstrated with predominant involvement of the nucleus and nucleolus associated with a decrease in the cell size. Morphological and morphometric changes also occurred in the sciatic and peroneal nerves with the features of axonopathy. All these changes were more marked in the group of rats which underwent the most intense DDP treatment and the tissue platinum concentrations were also higher in this group. This experimental model is the first available for chronic DDP administration in which concomitant spinal ganglia and peripheral nerve damage has been confirmed pathologically. Our study supports the hypothesis that DDP-induced peripheral nerve fiber degeneration may result from nuclear and nucleolar changes in the sensory ganglion cell perikaryon.