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Background: Wilson disease is a rare genetic disorder in which impaired copper excretion results in toxic copper levels and tissue damage. Manifestations are primarily hepatic and/or neuropsychiatric, with a variety of neurological phenotypes. The aim of this study was to characterize neurological signs of Wilson disease in newly diagnosed patients and to determine whether they correlated with disability, liver function, and copper metabolism., Methods: Fifty-three treatment-naïve patients recently diagnosed with Wilson disease who exhibited neurological symptoms were included. Neurological manifestations were characterized by examination in terms of symptom type and degree of neurological impairment (Unified Wilson's Disease Rating Scale [UWDRS] Part III) and correlated with degree of disability (UWDRS Part II), abnormalities in copper parameters and hepatic status., Results: Most patients (62.3%) exhibited tremor and ataxia, whereas 15.1% were dystonic, and 11.3% had parkinsonism. Discrete or unclassified signs only were observed in 11.3% of patients. A good correlation between disability (UWDRS Part II) and neurological impairment (UWDRS Part III) was observed (Pearson r = 0.84). However, there was a lack of correlation when either disability or neurological impairment were analyzed with copper parameters or liver impairment., Conclusions: The predominant neurological manifestations in this cohort of newly diagnosed Wilson disease patients were ataxia and tremor. Neurological impairment measured was highly correlated with the level of disability. However, hepatic manifestations of Wilson disease and copper levels did not appear to be correlated with neurological status and disability. These results highlight the challenges faced when assessing Wilson disease with its highly variable symptomatology.

Background: Wilson's disease is a genetic disorder in which copper accumulates in the liver, brain, and other tissues. Therapies are limited by efficacy, safety concerns, and multiple daily dosing. Bis-choline tetrathiomolybdate (WTX101) is an oral first-in-class copper-protein-binding molecule that targets hepatic intracellular copper and reduces plasma non-ceruloplasmin-bound copper (NCC) by forming tripartite complexes with albumin and increasing biliary copper excretion. We aimed to assess the efficacy and safety of WTX101 in the initial or early treatment of patients with Wilson's disease., Methods: We did this open-label, phase 2 study at 11 hospitals in the USA and Europe. We enrolled patients (≥18 years) with Wilson's disease who were untreated or had received no more than 24 months of treatment with chelators or zinc, had a Leipzig score of 4 or more, and had NCC concentrations above the lower limit of the normal reference range (≥0·8 μmol/L). Eligible patients received WTX101 monotherapy at a starting dose of 15-60 mg/day on the basis of baseline NCC concentrations for the first 4-8 weeks, with response-guided individualised dosing for the remaining weeks up to week 24. Investigators, other hospital personnel, and patients were aware of the identity of the treatment. The primary endpoint was change in baseline NCC concentrations corrected for copper in tetrathiomolybdate-copper-albumin complexes (NCC corrected ) at 24 weeks, with treatment success defined as achievement or maintenance of normalised NCC corrected (≤2·3 μmol/L [upper limit of normal]) or achievement of at least a 25% reduction in NCC corrected from baseline at 24 weeks. This study is registered with ClinicalTrials.gov, number NCT02273596., Findings: Between Nov 24, 2014, and April 27, 2016, 28 patients were enrolled and received WTX101; 22 (79%) patients completed the study up to week 24. At 24 weeks, 20 (71%, 95% CI 51·3-86·8; p<0·0001) of 28 patients met the criteria for treatment success: 16 (57%) treated with WTX101 either achieved or maintained normalised NCC corrected concentrations and 4 (14%) had at least a 25% reduction from baseline NCC corrected . Mean NCC corrected was reduced by 72% from baseline to week 24 (least squares mean difference -2·4 μmol/L [SE 0·4], 95% CI -3·2 to -1·6; p<0·0001). No cases of paradoxical drug-related neurological worsening were recorded. Liver function was stable in all patients, although reversible increased concentrations of asymptomatic alanine or aspartate aminotransferase, or γ-glutamyltransferase, without increased bilirubin, occurred in 11 (39%) of 28 patients who received at least 30 mg/day. 11 serious adverse events were reported in seven (25%) patients and included psychiatric disorders (six events in four patients), gait disturbance (one event), elevated liver aminotransferases (two events in two patients, one with agranulocytosis), and decline in neurological functioning (one event, likely due to natural disease progression although causality could not be ruled out). The seven serious adverse events categorised as psychiatric disorders and as gait disturbance were assessed as unlikely to be related to the study drug, whereas the remaining four events were possibly or probably related., Interpretation: Our findings indicate that WTX101 might be a promising new therapeutic approach for Wilson's disease, with a unique mode of action. In view of its once-daily dose and favourable safety profile, WTX101 could improve the treatment of patients with this debilitating condition., Funding: Wilson Therapeutics AB., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Background: Late-onset Pompe disease is characterized by progressive skeletal myopathy followed by respiratory muscle weakness, typically leading to loss of ambulation and respiratory failure. In this population, enzyme replacement therapy (ERT) with alglucosidase alfa has been shown to stabilize respiratory function and improve mobility and muscle strength. Muscle pathology and glycogen clearance from skeletal muscle in treatment-naïve adults after ERT have not been extensively examined., Methods: This exploratory, open-label, multicenter study evaluated glycogen clearance in muscle tissue samples collected pre- and post- alglucosidase alfa treatment in treatment-naïve adults with late-onset Pompe disease. The primary endpoint was the quantitative reduction in percent tissue area occupied by glycogen in muscle biopsies from baseline to 6months. Secondary endpoints included qualitative histologic assessment of tissue glycogen distribution, secondary pathology changes, assessment of magnetic resonance images (MRIs) for intact muscle and fatty replacement, and functional assessments., Results: Sixteen patients completed the study. After 6months of ERT, the percent tissue area occupied by glycogen in quadriceps and deltoid muscles decreased in 10 and 8 patients, respectively. No changes were detected on MRI from baseline to 6months. A majority of patients showed improvements on functional assessments after 6months of treatment. All treatment-related adverse events were mild or moderate., Conclusions: This exploratory study provides novel insights into the histopathologic effects of ERT in late-onset Pompe disease patients. Ultrastructural examination of muscle biopsies demonstrated reduced lysosomal glycogen after ERT. Findings are consistent with stabilization of disease by ERT in treatment-naïve patients with late-onset Pompe disease., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)

Emerging phenotypes in long-term survivors with Pompe disease on standard enzyme replacement therapy (ERT) (alglucosidase alfa 20 mg/kg/2 weeks) can include patients with worsening motor function. Whether higher doses of ERT improve skeletal function in these patients has not been systematically studied. This exploratory, randomized, open-label, 52-week study examined the safety and efficacy of 2 ERT regimens of alglucosidase alfa (20 mg/kg/week or 40 mg/kg/2 weeks) in 13 patients with Pompe disease and clinical decline or a lack of improvement on standard ERT: late-onset (n = 4), infantile-onset (n = 9). Cross-reactive immunologic material assay-negative patients were excluded. Eleven of 13 patients completed the study. Trends for improvement were seen in total gross motor function, but not mobility; however, 6 (late-onset, 2; infantile-onset, 4) of 11 patients (55%) who met the entry criteria of motor decline (late-onset, 4; infantile-onset, 7) showed improvement in motor and/or mobility skills. No between-regimen differences in efficacy emerged. Two case studies highlight the benefits of increased ERT dose in patients with Pompe disease experiencing clinical decline. Both alternative regimens were generally well tolerated. This study was limited by the small sample size, which is not uncommon for small clinical studies of rare diseases. Additionally, the study did not include direct assessment of muscle pathology, which may have identified potential causes of decreased response to ERT. Results were inconclusive but suggest that increased ERT dose may be beneficial in some patients with Pompe disease experiencing motor decline. Controlled studies are needed to clarify the benefits and risks of this strategy., (Copyright © 2014 The Authors. Published by Elsevier B.V. All rights reserved.)

Oligodendrocyte differentiation and myelination involve dramatic changes in cell signaling pathways, gene expression patterns, cell shape, and cytoskeletal organization. In a pilot study investigating CNS angiogenesis, oligodendrocytes were intensely labeled by antisera directed against the C-terminal of Tie-2, a 140-kDa transmembrane receptor for angiopoietin. Immunoprecipitation of rat brain proteins with Tie-2 C-terminal antisera, however, produced a single spot of approximately 55-kDa pI approximately 5 by two-dimensional (2D) electrophoresis, which was identified as beta-tubulin by mass spectrometry. Isotype-specific antibodies for beta(IV) tubulin selectively labeled oligodendrocytes. First detected in premyelinating oligodendrocytes, beta(IV) tubulin was abundant in myelinating oligodendrocyte perinuclear cytoplasm and processes extending to and along developing myelin internodes. Beta(IV) tubulin-positive MTs were diffusely distributed in oligodendrocyte perinuclear cytoplasm and not organized around the centrosome. Beta(IV) tubulin may play a role in establishing the oligodendrocyte MT network, which is essential for the transport of myelin proteins, lipids, and RNA during myelination.

Accumulating data support axonal degeneration as the major determinant of irreversible neurological disability in patients with multiple sclerosis (MS). The extent of axonal injury correlates with the degree of inflammation in active MS lesions and occurs at early stages of disease, indicating that inflammatory demyelination is an important factor behind axon pathology at the relapsing-remitting stage of MS. Axonal loss from disease onset can remain clinically silent for many years, and permanent neurological disability develops when a threshold of axonal loss is reached and the CNS compensatory resources are exhausted. Lack of myelin-derived trophic support due to long term demyelination may cause continuous axonal degeneration in chronic inactive lesions at the secondary-progressive stage of MS. Axonal pathology is not limited to demyelinated lesions, but also extends into normal appearing white matter. The concept of MS as a neurodegenerative disorder has important clinical implications: First, proactive anti-inflammatory and immunomodulatory treatment should prevent or delay chronic disability since inflammation influences axonal injury. Second, the pathophysiological mechanisms underlying axonal degeneration in MS need to be clarified in order to develop novel neuroprotective therapeutics. Finally, surrogate markers of axonal pathology, such as N-acetyl aspartate, can be used to monitor axonal dysfunction, axonal loss and treatment efficiency in patients with MS.

Previous studies identified the tetraspanin protein CD9 in myelinating oligodendrocytes. The present report extends these observations by identifying CD9 in a subpopulation of oligodendrocyte progenitor cells (OPCs) and in premyelinating oligodendrocytes in rodents. NG2-positive cells expressed CD9 in a temporal and spatial pattern during development that was consistent with CD9 expression in OPCs just prior to their differentiation into premyelinating oligodendrocytes. NG2-positive cells in mature brains were CD9-negative. CD9 expression during oligodendrocyte development in vitro supported this hypothesis, as all CD9-positive cells became O4-positive when switched to oligodendrocyte differentiating media. CD9 immunoreactivity was enriched in myelinating oligodendrocytes and their processes, and the outer aspects of myelin internodes. Immunoprecipitation of CD9 from postnatal rat cerebrum coprecipitated beta1 integrin, CD81, and Tspan-2, another tetraspanin protein recently identified in oligodendrocytes. Following surface biotinylation of oligodendrocytes in vitro, biotinylated beta1 integrin was identified in a CD9 immunoprecipitate. These data support a molecular link between surface integrins and a CD9, Tspan-2 molecular web during the differentiation of oligodendrocytes. Oligodendrocyte production and myelination appears to be normal in CD9-deficient mice. These data support the hypothesis that CD9 helps form the tetraspanin web beneath the plasma membranes of progenitor cells committed to oligodendrogenesis, but that CD9 is not essential for oligodendrogenesis and myelination., (Copyright 2002 Wiley-Liss, Inc.)

Axonal pathology is a major cause of neurological disability in multiple sclerosis. Axonal transection begins at disease onset but remains clinically silent because of compensatory brain mechanisms. Noninvasive surrogate markers for axonal injury are therefore essential to monitor cumulative disease burden in vivo. The neuronal compound N-acetylaspartate, as measured by magnetic resonance spectroscopy, is currently the best and most specific noninvasive marker of axonal pathology in multiple sclerosis. The possibility has been raised, however, that N-acetylaspartate is expressed also by oligodendroglial lineage cells. In order to investigate N-acetylaspartate specificity for white matter axons, transected rat optic nerves were analyzed by high-performance liquid chromatography and immunohistochemistry. In transected adult nerves, N-acetylaspartate and N-acetyl aspartylglutamate decreased in concordance with axonal degeneration and were undetectable 24 days posttransection. Nonproliferating oligodendrocyte progenitor cells, oligodendrocytes, and myelin were abundant in these axon-free nerves. At 24 days posttransection, N-acetylaspartate was increased (42%; p = 0.02) in nontransected contralateral nerves. After transection at postnatal day 4, total N-acetylaspartate decreased by 80% (P14; p = 0.002) and 94% (P20; p = 0.003). In these developing axon-free nerves, 25 to 33% of oligodendrocyte progenitor cells were proliferating. These data validate magnetic resonance spectroscopy measurements of N-acetylaspartate as an axon-specific monitor of central nervous system white matter in vivo. In addition, the results indicate that neuronal adaptation can increase N-acetylaspartate levels, and that 5 to 20% of the N-acetylaspartate in developing white matter is synthesized by proliferating oligodendrocyte progenitor cells.

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). Most patients undergo an initial relapsing-remitting (RR-MS) course that transforms into a relentless neurodegenerative disorder, termed secondary progressive (SP)-MS. Reversible inflammation and demyelination account readily for the pattern of RR-MS but provide an unsatisfactory explanation for irrevocable decline in SP-MS. Axon loss is thought to be responsible for progressive, non-remitting neurological disability during SP-MS. There is considerable potential for neuroprotective therapies in MS, but their application awaits animal models in which axonal loss correlates with permanent neurological disability. In this report, we describe quantitative immunohistochemical methods that correlate inflammation and axonal loss with neurological disability in chronic-relapsing experimental autoimmune encephalomyelitis (EAE). At first attack, CNS inflammation, but not axon loss, correlated with the degree of neurological disability. In contrast, fixed neurological impairment in chronic EAE correlated with axon loss that, in turn, correlated with the number of symptomatic attacks. As proposed for MS, these observations imply a causal relationship between inflammation, axon loss, and irreversible neurological disability. This chronic-relapsing EAE model provides an excellent platform for 2 critical objectives: investigating mechanisms of axon loss and evaluating efficacy of neuroprotective therapies.

N-acetyl aspartate (NAA), a putative marker of neuronal injury, can be measured non-invasively in patients by magnetic resonance spectroscopy (MRS). Interpretation of in vivo MRS data, however, requires neuropathological correlates to NAA alterations using autopsy or biopsy material. Since detailed hydrolysis data is lacking, NAA and the related dipeptide N-acetyl aspartylglutamate (NAAG) were quantified by high performance liquid chromatography (HPLC) in different rat CNS regions over 24 h postmortem. Both molecules decreased rapidly 1-4 h postmortem, and subsequently slower with time. The average reduction at 24 h was 46% and 38% for NAA and NAAG respectively. The NAA reduction was proportionally smaller in cortical areas (34-37%) compared to more caudal regions (54-58%). An exception was the optic nerve, a pure white matter tract, where NAA and NAAG hydrolysis was slower. The NAA/NAAG ratio remained relatively constant, but exhibited marked regional differences. The data show a significant postmortem degradation of NAA and NAAG that needs to be considered when these compounds are studied ex-vivo.

Axonal degeneration has been proposed as a cause of irreversible neurological disability in multiple sclerosis (MS) patients. The purpose of this study was to quantify axonal loss in spinal cord lesions from 5 paralyzed (Expanded Disability Status Scale score > or =7.5) MS patients and to determine if axonal number or volume correlated with levels of the neuronal marker N-acetyl aspartate (NAA). Axonal loss in MS lesions ranged from 45 to 84% and averaged 68%. NAA levels were significantly reduced (>50%) in cross sections of spinal cords containing MS lesions. Reduced NAA correlated with reduced axonal numbers within lesion areas. In addition, NAA levels per axonal volume were significantly reduced in demyelinated axons (42%) and in myelinated axons in normal-appearing white matter (30%). The data support axonal loss as a major cause of irreversible neurological disability in paralyzed MS patients and indicate that reduced NAA as measured by magnetic resonance spectroscopy can reflect axonal loss and reduced NAA levels in demyelinated and myelinated axons.

In mammals, the oligodendrocyte population includes morphological and molecular varieties. We reported previously that an antiserum against the T4-O molecule labels a subgroup of oligodendrocytes related to large myelinated axons in adult chicken white matter. We also reported that T4-O immunoreactive cells first appear in the developing ventral funiculus (VF) at embryonic day (E)15, subsequently increasing rapidly in number. Relevant fine structural data for comparison are not available in the literature. This prompted the present morphological analysis of developing and mature VF white matter in the chicken. The first axon-oligodendrocyte connections were seen at E10 and formation of compact myelin had started at E12. Between E12 and E15 the first myelinating oligodendrocytes attained a Schwann cell-like morphology. At hatching (E21) 60% of all VF axons were myelinated and in the adult this proportion had increased to 85%. The semilunar or polygonal oligodendrocytes associated with adult myelinated axons contained many organelles indicating a vivid metabolic activity. Domeshaped outbulgings with gap junction-like connections to astrocytic profiles were frequent. Oligodendrocytes surrounded by large myelinated axons and those surrounded by small myelinated axons were cytologically similar. But, thick and thin myelin sheaths had dissimilar periodicities and Marchi-positive myelinoid bodies occurred preferentially in relation to large myelinated axons. We conclude that early oligodendrocytes contact axons and form myelin well before the first expression of T4-O and that emergence of a T4-O immunoreactivity coincides in time with development of a Type IV phenotype. Our data also show that oligodendrocytes associated with thick axons are cytologically similar to cells related to thin axons. In addition, the development of chicken VF white matter was found to be similar to the development of mammalian white matter, except for the rapid time course.

Oligodendrocytes provide CNS axons with myelin sheaths through processes of various lengths. The lipid-rich myelin insulates axons electrically, which increases conduction velocity. In addition, oligodendrocytes have trophic effects on axons. During development immature oligodendrocytes undergo controlled migration, proliferation and differentiation, influenced by various growth factors and axons. A number of genetically manipulated animal models have provided insights regarding myelination and the function of myelin components. Current research on myelin related diseases, i.e. multiple sclerosis, focus on novel strategies for remyelination through transplantation of myelinating cells or stimulation of endogenous oligodendrocytes.

Accumulating data suggest that the oligodendrocyte population includes morphological and biochemical subtypes. We recently reported that a polyclonal antiserum against an unknown antigen, the T4-O molecule, labels a subpopulation of chicken oligodendrocytes, obviously representing the type IV variety of Del Rio Hortega. The present study examines the developmental expression of the T4-O molecule in situ and in vitro. The results show that T4-O immunoreactive cells first appear at E15 in the ventral funiculus. But, oligodendrocytes cultured in vitro with or without neurones do not develop a T4-O immunoreactivity. We conclude that oligodendrocytes in the spinal cord of chicken embryos first express the T4-O molecule some time after onset of myelination, and that the T4-O immunoreactive phenotype does not develop in vitro.

The classical studies by Del Rio Hortega (Mem. Real. Soc. Espan. Hist. Nat. 14:40-122, 1928) suggest that the oligodendrocyte population includes four morphological subtypes. Recent data from the cat and the rat show that the anatomy of oligodendrocytes related to early myelinating prospective large fibers differs from that of oligodendrocytes related to late myelinating prospective small fibers. After application of a polyclonal antiserum to cryostat sections from the chicken CNS, we noted that glial cells in the spinal cord white matter had become labeled. Analysis of the occurrence and cellular localization of this immunoreactivity--the T4-O immunoreactivity--in the CNS of the adult chicken showed that T4-O immunoreactive cells are enriched in the ventral funiculus and superficially in the lateral funiculus of the spinal cord, where they are co-localized with large fibers. Double staining with T4-O antiserum and anti-GFAP or the lectin BSI-B4 revealed that T4-O immunoreactive cells are not astrocytes or microglia. Staining with anti-HSP108, a general marker for avian oligodendrocytes, showed that T4-O immunoreactivity defines an oligodendroglial subpopulation. A search for T4-O immunoreactivity in spinal cord white matter of some other vertebrates revealed that T4-O immunoreactive cells are not present in sections from fish, frog, turtle, rat, and rabbit spinal cord white matter. These results suggest the presence of a fiber size-related molecular heterogeneity among chicken white matter oligodendrocytes.

Myelination provides extrinsic trophic signals that influence normal maturation and long-term survival of axons. The extent of axonal involvement in diseases affecting myelin or myelin forming cells has traditionally been underestimated. There are, however, many examples of axon damage as a consequence of dysmyelinating or demyelinating disorders. More than a century ago, Charcot described the pathology of multiple sclerosis (MS) in terms of demyelination and relative sparing of axons. Recent reports demonstrate a strong correlation between inflammatory demyelination in MS lesions and axonal transection, indicating axonal loss at disease onset. Disruption of axons is also observed in experimental allergic encephalomyelitis and in Theiler's murine encephalomyelitis virus disease, two animal models of inflammatory demyelinating CNS disease. A number of dysmyelinating mouse mutants with axonal pathology have provided insights regarding cellular and molecular mechanisms of axon degeneration. For example, the myelin-associated glycoprotein and proteolipid protein have been shown to be essential for mediating myelin-derived trophic signals to axons. Patients with the inherited peripheral neuropathy Charcot-Marie Tooth disease type 1 develop symptomatic progressive axonal loss due to abnormal Schwann cell expression of peripheral myelin protein 22. The data summarized in this review indicate that axonal damage is an integral part of myelin disease, and that loss of axons contributes to the irreversible functional impairment observed in affected individuals. Early neuroprotection should be considered as an additional therapeutic option for these patients.

Oligodendrocytes in different central nervous system regions have dissimilar morphologies. In order to find out if immature oligodendrocytes develop different phenotypes spontaneously, neonatal spinal and cerebral rat oligodendrocytes were cultured in vitro in the absence of neurons. The results show that spinal oligodendrocytes tend to develop a bipolar structure with few processes, while cerebral cells tend to become multipolar with many processes. A similar spinal-cerebral difference was seen in oligodendrocyte-enriched cultures. Hence, spinal and cerebral oligodendrocytes develop partly different morphologies in vitro.

The lengths of uncompacted and partly compacted oligodendroglial sheaths in the developing rat spinal cord (SC) ventral funiculus (ages F19 and F21) and corpus callosum (CC; ages P12 and P17) were studied by serial section electron microscopy. The average newly formed uncompacted sheath is 21 and 33 microns long in the SC (F19) and CC (P12), respectively, many being less than 10 microns. In these early series, approximately 2/3 of the analysed axon length is unensheathed. The average partly compacted sheath is 102 and 69 microns long in the SC (F21) and CC (P17), respectively. Here, about 1/3 of the examined axon length is unensheathed. These results suggest that oligodendroglial sheaths initially are very short, and that they elongate actively before and during compaction. The limited unensheathed space along these axons indicate that some early sheaths must be eliminated.

This study examines the developmental sheath elongation in the rat ventral root L5. Electron microscopic analysis of serial transverse sections through roots from newborn rats shows an average internuclear distance (IND) of 66 microns (calculated fresh length 73 microns). Light microscopic analysis of teased adult roots shows that the largest fibers have an average internodal length of some 1250 microns at 5 months. Hence, large fibers exhibit a developmental sheath elongation of 17 times. The ventral root L5 elongates 11 times only. This mismatch necessitates a myelin sheath remodelling.

Rat CNS nodes of Ranvier were investigated by electron microscopy and immunohistochemistry. Nodes along thin callosal axons possess tiny node gaps containing few or no astrocytic processes. Nodes along thick spinal axons exhibit spatious node gaps containing relatively few irregularly arranged astrocytic processes. Antibodies against HNK-1, chondroitin sulfate, tenascin or NSP-4 do not label small nodes but stain large nodes. We conclude that rat CNS fibers do not exhibit a strict relation between nodal complexity and fiber size comparable to that found in rat PNS fibers.

The microanatomy of ensheathing and early myelinating rat oligodendrocytes was analyzed through electron microscopic examination of serial sections. The study included cells in the spinal cord (SC) ventral funiculus and the corpus callosum (CC), containing early myelinating, prospective large axons and late myelinating, prospective small axons, respectively. The results show that ensheathment commences fetal day (F) 19 in the SC and 12 days postnatally (P12) in the CC. By then, multipolar SC and CC oligodendrocytes provide axons with uncompacted cytoplasmic sheaths. The average number of axons ensheathed by each such cell was 7 in the SC and 13 in the CC. The mean diameter of the ensheathed axons was 0.69 micron in the SC and 0.36 micron in the CC. The formation of compact myelin had clearly been initiated at birth in the SC and at P17 in the CC. At that stage, the mean number of myelinated axons per analyzed oligodendrocyte was 3 in the SC and 15 in the CC. The mean diameter of the myelinated axons was 1.02 micron in the SC and 0.54 micron in the CC. These observations show that myelin-related rat oligodendrocytes are morphologically heterogeneous. It also seems that this heterogeneity is related to time of onset of myelination and prospective axon diameter. Further, the data suggest that some oligodendrocytes reduce the number of sheaths initially elaborated before formation of compact myelin.

(1) Lamellated glial sheaths surrounding axons, and electrogenetically active axolemmal foci have evolved independently in widely different phyla. In addition to endowing the axons to conduct trains of impulses at a high speed, myelination and node formation results in a remarkable saving of space and energy. This is particularly important in the CNS, where space is restricted. Unlike the PNS, most CNS axons are myelinated, and several axons may be myelinated by a single cell. This adds further economy of space and energy. On the other hand the high level of complexity of the CNS white matter makes it vulnerable. There are several different kinds of disease affecting myelinated fibre tracts, particularly with respect to CNS white matter. (2) The CNS node of Ranvier presents a more complex structure the larger the fibre. The constricted nodal axon is encircled by perinodal astrocytic processes which contain large gliosomes and emit delicate processes towards the nodal axolemma. One astrocyte may project to several nodes. The node gap contains a polyanionic extracellular material. (3) Lamellated myelinoid bodies are frequent along paranodes of large myelinated CNS fibres. These bodies probably form through budding off from the paranodal myelin sheath. Similar bodies are seen inside astrocytes and microglia. The observation that these bodies are Marchi-positive and argyrophilic, and the presence of acid phosphatase activity around myelinoid bodies inside microglia suggests that they might represent degenerating myelin quanta, involved in the turnover of large myelin sheaths. This putative quantal release and breakdown of myelin material must be compensated for by a production of new myelin at other sites. Therefore, myelination may be viewed as a process that continues throughout life. (4) Biochemical analysis of a sub-cellular fraction enriched in myelinoid bodies shows that these bodies have a composition basically similar to that of myelin. However, breakdown products of myelin constituents, as well as exotic high molecular substances, not present in conventional myelin, can also be found. In addition, the myelinoid body fraction contains proteolytic activity. Studies using isotope labelling of myelin proteins show a source-product relation between myelin and myelinoid bodies. Altogether these data strongly support the hypothesis that myelinoid bodies reflect the catabolic side of myelin turnover. (5) Axons in the nerve fibre layer of the adult rat retina are all unmyelinated, although their diameters range up to over 2 microns. These axons exhibit focally differentiated axolemmal areas. At these sites the axolemma presents a dense undercoating with externally associated Müller cell processes or astrocytic processes.(ABSTRACT TRUNCATED AT 400 WORDS)