A structural analysis of the myelin sheath in the central nervous system

Central nervous system demyelinating diseases: glial cells at the hub of pathology

Inflammatory demyelinating diseases (IDDs) are among the main causes of inflammatory and neurodegenerative injury of the central nervous system (CNS) in young adult patients. Of these, multiple sclerosis (MS) is the most frequent and studied, as it affects about a million people in the USA alone. The understanding of the mechanisms underlying their pathology has been advancing, although there are still no highly effective disease-modifying treatments for the progressive symptoms and disability in the late stages of disease. Among these mechanisms, the action of glial cells upon lesion and regeneration has become a prominent research topic, helped not only by the discovery of glia as targets of autoantibodies, but also by their role on CNS homeostasis and neuroinflammation. In the present article, we discuss the participation of glial cells in IDDs, as well as their association with demyelination and synaptic dysfunction throughout the course of the disease and in experimental models, with a focus on MS phenotypes. Further, we discuss the involvement of microglia and astrocytes in lesion formation and organization, remyelination, synaptic induction and pruning through different signaling pathways. We argue that evidence of the several glia-mediated mechanisms in the course of CNS demyelinating diseases supports glial cells as viable targets for therapy development.

Río-Hortega's drawings revisited with fluorescent protein defines a cytoplasm-filled channel system of CNS myelin

A century ago this year, Pío del Río-Hortega (1921) coined the term 'oligodendroglia' for the 'interfascicular glia' with very few processes, launching an extensive discovery effort on his new cell type. One hundred years later, we review his original contributions to our understanding of the system of cytoplasmic channels within myelin in the context of what we observe today using light and electron microscopy of genetically encoded fluorescent reporters and immunostaining. We use the term myelinic channel system to describe the cytoplasm-delimited spaces associated with myelin; being the paranodal loops, inner and outer tongues, cytoplasm-filled spaces through compact myelin and further complex motifs associated to the sheath. Using a central nervous system myelinating cell culture model that contains all major neural cell types and produces compact myelin, we find that td-tomato fluorescent protein delineates the myelinic channel system in a manner reminiscent of the drawings of adult white matter by Río-Hortega, despite that he questioned whether some cytoplasmic figures he observed represented artefact. Together, these data lead us to propose a slightly revised model of the 'unrolled' sheath. Further, we show that the myelinic channel system, while relatively stable, can undergo subtle dynamic shape changes over days. Importantly, we capture an under-appreciated complexity of the myelinic channel system in mature myelin sheaths.

Myelin structure in unfixed, single nerve fibers: Scanning X-ray microdiffraction with a beam size of 200nm

Previous raster-scanning with a 1μm X-ray beam of individual, myelinated fibers from glutaraldehyde-fixed rat sciatic nerve revealed a spatially-dependent variation in the diffraction patterns from single fibers. Analysis indicated differences in the myelin periodicity, membrane separations, distribution of proteins, and orientation of membrane lamellae. As chemical fixation is known to produce structural artifacts, we sought to determine in the current study whether the structural heterogeneity is intrinsic to unfixed myelin. Using a 200nm-beam that was about five-fold smaller than before, we raster-scanned individual myelinated fibers from both the peripheral (PNS; mouse and rat sciatic nerves) and central (CNS; rat corpus callosum) nervous systems. As expected, the membrane stacking in the internodal region was nearly parallel to the fiber axis and in the paranodal region it was perpendicular to the axis. A myelin lattice was also frequently observed when the incident beam was injected en face to the sheath. Myelin periodicity and diffracted intensity varied with axial position along the fiber, as did the calculated membrane profiles. Raster-scanning with an X-ray beam at sub-micron resolution revealed for the first time that the individual myelin sheaths in unfixed nerve are heterogeneous in both membrane structure and packing.

Role of Oligodendrocyte Dysfunction in Demyelination, Remyelination and Neurodegeneration in Multiple Sclerosis

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS) during development and throughout adulthood. They result from a complex and well controlled process of activation, proliferation, migration and differentiation of oligodendrocyte progenitor cells (OPCs) from the germinative niches of the CNS. In multiple sclerosis (MS), the complex pathological process produces dysfunction and apoptosis of OLs leading to demyelination and neurodegeneration. This review attempts to describe the patterns of demyelination in MS, the steps involved in oligodendrogenesis and myelination in healthy CNS, the different pathways leading to OLs and myelin loss in MS, as well as principles involved in restoration of myelin sheaths. Environmental factors and their impact on OLs and pathological mechanisms of MS are also discussed. Finally, we will present evidence about the potential therapeutic targets in re-myelination processes that can be accessed in order to develop regenerative therapies for MS.

Chemically Induced Myelinopathies

The diverse, structurally unrelated chemicals that cause toxic myelinopathies have been investigated and can be categorized into two types of primary demyelinators. Some demyelinating chemicals seem to leave intact the myeli-nating cells (oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system), while others damage the myelinating cells as well as the myelin. The significance between the two is that with the myelinating cells still in tact, repair of the myelin sheath can occur. However, if the myelinating cells are destroyed, repair and reversal of the neuropathy may not occur. Histologically, these chemicals produce an edema of the white matter of the brain, and in some cases the peripheral nervous system, that appears spongy by light microscopy. By electron microscopy, vacuoles can be seen in the myelin surrounding axons. These vacuoles are characterized as fluid-filled separations (splitting) of myelin lamellae at the intraperiod line. In some cases these vacuoles can degenerate further to full demyelination, affecting conduction through those axons. Regeneration of the myelin layers can occur, and in some cases occurs at the same time other axons are undergoing toxic demyelination. Several of these chemicals, however, have been shown to increase cerebrospinal fluid pressure in the brain, optic nerve, and spinal cord, and/or intraneuronal pressure in the perineurium surrounding the axons in the peripheral nervous system. This increased pressure has been correlated with decreased conduction capacity through the axon, ischemia to the neuronal tissue from decreased blood flow because of pressure against the blood vessels, and, if unrelieved, permanent axonal damage. Several of these chemicals havebeen shown to inhibit oxidative phosphorylation, while others uncouple oxidative phosphorylation. One chemical appears to inhibit an enzyme critical to cholesterol synthesis, thus destabilizing myelin. Another hypothesis for a mechanism of action may be in the ability of these compounds to alter membrane permeability.

Myelination at a glance

The myelin sheath is a plasma membrane extension that is laid down in regularly spaced segments along axons of the nervous system. This process involves extensive changes in oligodendrocyte cell shape and membrane architecture. In this Cell Science at a Glance article and accompanying poster, we provide a model of how myelin of the central nervous system is wrapped around axons to form a tightly compacted, multilayered membrane structure. This model may not only explain how myelin is generated during brain development, but could also help us to understand myelin remodeling in adult life, which might serve as a form of plasticity for the fine-tuning of neuronal networks.

CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process

Myelin - the multilayer membrane that envelops axons - is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into "myelinophore organelles" in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.

Label-free real-time imaging of myelination in the Xenopus laevis tadpole by in vivo stimulated Raman scattering microscopy

The myelin sheath plays an important role as the axon in the functioning of the neural system, and myelin degradation is a hallmark pathology of multiple sclerosis and spinal cord injury. Electron microscopy, fluorescent microscopy, and magnetic resonance imaging are three major techniques used for myelin visualization. However, microscopic observation of myelin in living organisms remains a challenge. Using a newly developed stimulated Raman scattering microscopy approach, we report noninvasive, label-free, real-time in vivo imaging of myelination by a single-Schwann cell, maturation of a single node of Ranvier, and myelin degradation in the transparent body of the Xenopus laevis tadpole.

Time-lapse imaging of the dynamics of CNS glial-axonal interactions in vitro and ex vivo

Background

Myelination is an exquisite and dynamic example of heterologous cell-cell interaction, which consists of the concentric wrapping of multiple layers of oligodendrocyte membrane around neuronal axons. Understanding the mechanism by which oligodendrocytes ensheath axons may bring us closer to designing strategies to promote remyelination in demyelinating diseases. The main aim of this study was to follow glial-axonal interactions over time both in vitro and ex vivo to visualize the various stages of myelination.

Methodology/Principal Findings

We took two approaches to follow myelination over time: i) time-lapse imaging of mixed CNS myelinating cultures generated from mouse spinal cord to which exogenous GFP-labelled murine cells were added, and ii) ex vivo imaging of the spinal cord of shiverer (Mbp mutant) mice, transplanted with GFP-labelled murine neurospheres. We demonstrate that oligodendrocyte-axonal interactions are dynamic events with continuous retraction and extension of oligodendroglial processes. Using cytoplasmic and membrane-GFP labelled cells to examine different components of the myelin-like sheath, we provide evidence from time-lapse fluorescence microscopy and confocal microscopy that the oligodendrocytes' cytoplasm-filled processes initially spiral around the axon in a corkscrew-like manner. This is followed subsequently by focal expansion of the corkscrew process to form short cuffs, which then extend longitudinally along the axons. We predict from this model that these spiral cuffs must extend over each other first before extending to form internodes of myelin.

Conclusion

These experiments show the feasibility of visualizing the dynamics of glial-axonal interaction during myelination over time. Moreover, these approaches complement each other with the in vitro approach allowing visualization of an entire internodal length of myelin and the ex vivo approach validating the in vitro data.

Electron tomography of paranodal septate-like junctions and the associated axonal and glial cytoskeletons in the central nervous system

The polarized domains of myelinated axons are specifically organized to maximize the efficiency of saltatory conduction. The paranodal region is directly adjacent to the node of Ranvier and contains specialized septate-like junctions that provide adhesion between axons and glial cells and that constitute a lateral diffusion barrier for nodal components. To complement and extend earlier studies on the peripheral nervous system, electron tomography was used to image paranodal regions from the central nervous system (CNS). Our three-dimensional reconstructions revealed short filamentous linkers running directly from the septate-like junctions to neurofilaments, microfilaments, and organelles within the axon. The intercellular spacing between axons and glia was measured to be 7.4 ± 0.6 nm, over twice the value previously reported in the literature (2.5-3.0 nm). Averaging of individual junctions revealed a bifurcated structure in the intercellular space that is consistent with a dimeric complex of cell adhesion molecules composing the septate-like junction. Taken together, these findings provide new insight into the structural organization of CNS paranodes and suggest that, in addition to providing axo-glial adhesion, cytoskeletal linkage to the septate-like junctions may be required to maintain axonal domains and to regulate organelle transport in myelinated axons.

Visualization of cytoplasmic diffusion within living myelin sheaths of CNS white matter axons using microinjection of the fluorescent dye Lucifer Yellow

The compactness of myelin allows for efficient insulation defining rapid propagation of action potentials, but also raises questions about how cytoplasmic access to its membranes is achieved, which is critical for physiological activity. Understanding the organization of cytoplasmic ('water') spaces of myelin is also important for diffusion MRI studies of CNS white matter. Using longitudinal slices of mature rat spinal cord, we monitored the diffusion of the water-soluble fluorescent dye Lucifer Yellow injected into individual oligodendrocytes or internodal myelin. We show that living myelin sheaths on CNS axons are fenestrated by a network of diffusionally interconnected cytoplasmic 'pockets' (1.9 ± 0.2 pockets per 10μm sheath length, n=58) that included Schmidt-Lanterman clefts (SLCs) and numerous smaller compartments. 3-D reconstructions of these cytoplasmic networks show that the outer cytoplasmic layer of CNS myelin is cylindrically 'encuffing', which differs from EM studies using fixed tissue. SLCs were found in different 'open states' and remained stable within a 1-2hour observation period. Unlike the peripheral nervous system, where similarly small (<500Da) molecules diffuse along the whole myelin segment within a few minutes, in mature CNS this takes more than one hour. The slower cytoplasmic diffusion in CNS myelin possibly contributes to its known vulnerability to injury and limited capacity for repair. Our findings point to an elaborate cytoplasmic access to compact CNS myelin. These results could be of relevance to MRI studies of CNS white matter and to CNS repair/regeneration strategies.

Glycosphingolipid synthesis in cerebellar Purkinje neurons: roles in myelin formation and axonal homeostasis

Glycosphingolipids (GSLs) occur in all mammalian plasma membranes. They are most abundant in neuronal cells and have essential roles in brain development. Glucosylceramide (GlcCer) synthase, which is encoded by the Ugcg gene, is the key enzyme driving the synthesis of most neuronal GSLs. Experiments using conditional Nestin-Cre Ugcg knockout mice have shown that GSL synthesis in vivo is essential, especially for brain maturation. However, the roles of GSL synthesis in mature neurons remain elusive, since Nestin-Cre is expressed in neural precursors as well as in postmitotic neurons. To address this problem, we generated Purkinje cell-specific Ugcg knockout mice using mice that express Cre under the control of the L7 promoter. In these mice, Purkinje cells survived for at least 10-18 weeks after Ugcg deletion. We observed apparent axonal degeneration characterized by the accumulation of axonal transport cargos and aberrant membrane structures. Dendrites, however, were not affected. In addition, loss of GSLs disrupted myelin sheaths, which were characterized by detached paranodal loops. Notably, we observed doubly myelinated axons enveloped by an additional concentric myelin sheath around the original sheath. Our data show that axonal GlcCer-based GSLs are essential for axonal homeostasis and correct myelin sheath formation.

Optically resolving individual microtubules in live axons

Microtubules are essential cytoskeletal tracks for cargo transportation in axons and also serve as the primary structural scaffold of neurons. Structural assembly, stability, and dynamics of axonal microtubules are of great interest for understanding neuronal functions and pathologies. However, microtubules are so densely packed in axons that their separations are well below the diffraction limit of light, which precludes using optical microscopy for live-cell studies. Here, we present a single-molecule imaging method capable of resolving individual microtubules in live axons. In our method, unlabeled microtubules are revealed by following individual axonal cargos that travel along them. We resolved more than six microtubules in a 1 microm diameter axon by real-time tracking of endosomes containing quantum dots. Our live-cell study also provided direct evidence that endosomes switch between microtubules while traveling along axons, which has been proposed to be the primary means for axonal cargos to effectively navigate through the crowded axoplasmic environment.

[Fine structure of neuronal and glial processes in neuropathology, a personal historical note]

Neurons and glia are characterized by their well formed processes and by cell-to-cell relationships. Neurons show cylindrical processes, which form synaptic junctions. On the other hand, the peripheral parts of the glial cells are sheet-like in nature. Thus, the oligodendroglial cells form shovel-shaped myelin sheets around axons. The astrocytes also form delicate sheet-like processes, which separate the central nervous system from the mesodermal tissue and surround neuronal soma, dendrites and synapses. Fine structural studies in neuropathological material provide many interesting new findings on neuronal and glial processes. This communication highlights my exciting experience studying neuropathology for over 50 years.

Glutaraldehyde fixation of central nervous tissue: an electron microscopic evaluation in the isolated rabbit retina

The isolated rabbit retina was studied electron microscopically after fixation with a 3% solution of glutaraldehyde in a 0.05 M Sørensen's phosphate buffer. In radial sections, the inner segments, nuclei, and synapses of the photoreceptor cells seemed similar in size to those from retinas that had been fixed in an isotonic solution containing 1 % crystalline osmium tetroxide in the incubating medium used for the isolation procedure. However, when the number of comparable structures was greatly increased by viewing them in tangential sections, the cellular shrinkage and mitochondrial swelling produced by this widely used, hypertonic, glutaraldehyde fixative were obvious.

[Neuropathological perspective on single slide]

During over 50 years of my career in Neuropathology at Montefiore Medical Center in New York, I have come across certain interesting neuropathological findings. In this communication, some photographs showing macroscopic, microscopic and electron microscopic significant findings are selected to illustrate the usefulness, not only for the diagnosis but also for the understanding of the nervous system. The 11 topics presented in this paper are: (1) alteration of dura mater associated with advanced aging; (2) orderly arrangement of tumor cells in leptomeningeal carcinomatosis; (3) horizontal section of brain with border zone infarct; (4) neurofibrillary tangle formation in the nucleus basalis Meynert ipsilateral to a massive cerebral infarct; (5) extracellular spread of hematogenous edema fluid in the white matter: (6) unrolled myelin sheath: (7) unattached presynaptic terminals in cerebellar neuroblastoma: (8) unattached post synaptic terminals in agranular cerebellar degeneration: (9) neurofibrillary tangles and Lewy bodes in a single neuron: (10) Cu/Zu superoxide dismutase positive Lewy body-like hyaline inclusions in anterior horn cells in familial motor neuron disease: (11) Hirano body. Analysis of these findings are presented for an educational purpose.

Fine structure of neuronal and glial processes in neuropathology

The cells of the nervous system are characterized by their well-formed cell processes and by cell-to-cell relationships that they form. The neuron reveals essentially cylindrical processes, which form synaptic junctions. On the other hand, the peripheral parts of the glial cells are mainly sheet-like in nature. Thus, the oligodendroglial cell elaborates many sheet-like processes, each of which forms a segment of the myelin sheath. Unique cell junction, transverse bands are present at the interface of oligodendroglial processes and the axon. Finally, the astrocytes also form elaborate sheet-like processes, which separate most of the CNS from the mesodermal tissue as well as surrounding certain neuronal surfaces, including synapses. Punctate adhesions, gap junctions and other adhesive devices are present between astrocytic processes. Defects or anomalies in the neuronal and glial cell processes characterize numerous pathological conditions.

The role of electron microscopy in neuropathology

The electron microscope has been an essential instrument for elucidating the morphology of cells and tissue. Fine structural investigation of nervous tissue has disclosed numerous new findings, which were once invisible. In this communication I present my personal perspective on the role of electron microscopy in neuropathology through the presentation of selected electron microscopic projects engaged in our institution. These include brain edema, structural analysis of the myelin in the central nervous system, endodermal cysts in the central nervous system, aberrant synaptic development, toxoplasmosis in AIDS, and Hirano bodies.

Connexin29 and connexin32 at oligodendrocyte and astrocyte gap junctions and in myelin of the mouse central nervous system

The cellular localization, relation to other glial connexins (Cx30, Cx32, and Cx43), and developmental expression of Cx29 were investigated in the mouse central nervous system (CNS) with an anti-Cx29 antibody. Cx29 was enriched in subcellular fractions of myelin, and immunofluorescence for Cx29 was localized to oligodendrocytes and myelinated fibers throughout the brain and spinal cord. Oligodendrocyte somata displayed minute Cx29-immunopositive puncta around their periphery and intracellularly. In developing brain, Cx29 levels increased during the first few postnatal weeks and were highest in the adult brain. Immunofluorescence labeling for Cx29 in oligodendrocyte somata was intense at young ages and was dramatically shifted in localization primarily to myelinated fibers in mature CNS. Labeling for Cx32 also was localized to oligodendrocyte somata and myelin and absent in Cx32 knockout mice. Cx29 and Cx32 were minimally colocalized on oligodendrocytes somata and partly colocalized along myelinated fibers. At gap junctions on oligodendrocyte somata, Cx43/Cx32 and Cx30/Cx32 were strongly associated, but there was minimal association of Cx29 and Cx43. Cx32 was very sparsely associated with astrocytic connexins along myelinated fibers. With Cx26, Cx30, and Cx43 expressed in astrocytes and Cx29, Cx32, and Cx47 expressed in oligodendrocytes, the number of connexins localized to gap junctions of glial cells is increased to six. The results suggested that Cx29 in mature CNS contributes minimally to gap junctional intercellular communication in oligodendrocyte cell bodies but rather is targeted to myelin, where it, with Cx32, may contribute to connexin-mediated communication between adjacent layers of uncompacted myelin.

The paranodal axo-glial junction in the central nervous system studied with thin sections and freeze-fracture

The lateral belts of the myelin sheath wind helically around the paranodal region of the axon. The lateral belt coil leaves an imprint on the axon and thus confers a conspicuous, indented configuration to the freeze-fracture faces of the axolemma. The contact area between the axolemma and the lateral belt membrane is the site of an extensive and unusual cell junction (axo-glial junction). In thin sections the junctional membranes are undulated, the peaks in one membrane mirroring the peaks in the other. The transverse bands (intercellular septa) are in register with the undulations. The intercellular space measures about 30 A. In freeze-fracture replicas, the undulations are evident as alternating ridges and grooves which run strictly parallel and are oriented at an angle with respect to the helical path of the lateral belt. Both junctional membranes contain parallel rows of intramembrane particles which coincide with the ridges and grooves and, therefore, with the intercellular septa. The center-to-center distance between septa or, equivalently, between adjacent rows of particles measures approximately 250 A. Although the axo-glial junction possesses structurally symmetrical features, there exist important differences between the two junctional membranes. The intramembrane particles of the glial and the axonal membrane differ in cleaving properties. Furthermore, in some of the fibres the E face of the junctional axolemma displays a crystalline array which is not present in the fracture faces of the glial membrane. The axo-glial junction is limited to the paranodal region, although the inner belt of the myelin sheath may form occasional junctional spots with the internodal region proper of the axolemma. The classification and the presumptive functions of the paranodal axo-glial junction are briefly discussed.

Organizing principles of the axoglial apparatus

On axonal surfaces that flank the node of Ranvier and in overlying glial paranodal loops, proteins are arranged within circumscribed microdomains that defy explanation by conventional biosynthetic mechanisms. We postulate that the constraint of proteins to these loci is accomplished in part by discriminative membrane-embedded molecular sieves and diffusion barriers, which serve to organize and redistribute proteins after delivery by vesicular transport to neural cell plasma membranes. One sieve likely comprises a moveable, macromolecular scaffold of axonal and glial cell-derived transmembrane adhesion molecules and their associated cytoplasmic binding partners, located at the ends of each elongating myelin internode; this sieve contributes to restricting the sodium channel complexes to the node. We also anticipate the existence of a passive paranodal diffusion barrier at the myelin/noncompact membrane border, which prohibits protein diffusion out of contiguous paranodal membranes.

Myelination and axonal regeneration in the central nervous system of mice deficient in the myelin-associated glycoprotein

The myelin-associated glycoprotein, a member of the immunoglobulin superfamily, has been implicated in the formation and maintenance of myelin sheaths. In addition, recent studies have demonstrated that myelin-associated glycoprotein is inhibitory for neurite elongation in vitro and it has therefore been suggested that myelin-associated glycoprotein prevents axonal regeneration in lesioned nervous tissue. The generation of mice deficient in the expression of myelin-associated glycoprotein by targeted disruption of the mag gene via homologous recombination in embryonic stem cells has allowed the study of the functional role of this molecule in vivo. This review summarizes experiments aimed at answering the following questions: (i) is myelin-associated glycoprotein involved in the formation and maintenance of myelin in the CNS? and (ii) does myelin-associated glycoprotein restrict axonal regeneration in the adult mammalian CNS? Analysis of optic nerves from mutant mice revealed a delay in myelination when compared to optic nerves of wild-type animals, a lack of a periaxonal cytoplasmic collar from most myelin sheaths, and the presence of some doubly and multiply myelinated axons. Axonal regeneration in the CNS of adult myelin-associated glycoprotein deficient mice was not improved when compared to wild-type animals. These observations indicate that myelin-associated glycoprotein is functionally involved in the recognition of axons by oligodendrocytes and in the morphological maturation of myelin sheaths. However, results do not support a role of myelin-associated glycoprotein as a potent inhibitor of axonal regeneration in the adult mammalian CNS.

Axon-glial relationships in the anterior medullary velum of the adult rat

The anterior medullary velum is a thin sheet of CNS tissue which roofs the rostral part of the IVth ventricle and contains fascicles of myelinated fibres which, in part, arise from the nucleus of the IVth cranial nerve. This study used histochemical, immunohistochemical, and intracellular dye-injection techniques to describe cellular interrelationships in the velum in whole-mounts and in sections. Rip antibody-stained whole mounts provided a unique description of both oligodendrocyte units (defined as an oligodendrocyte and the complement of myelinated internodal segments it forms), and consecutive myelin sheaths along the same axon. A broad range of unit morphologies was categorised into four arbitrary groups, according to classical criteria, which comprised small cells supporting the short, thin myelin sheaths of 15-30 small diameter axons (Type I), through intermediate types (II & III), to the largest cells forming the long, thick myelin sheaths of 1-3 large diameter axons. Rip antibody and ferric ion-ferrocyanide staining, together with intracellular dye injection, revealed oligodendrocyte process branching patterns and their mode of engagement of myelin sheaths, nodes of Ranvier, and the spatial disposition of the outer cytoplasmic rims of myelin sheaths. The latter formed a conspicuous spiral ridge on the exterior surface of myelin sheaths which connected with the paranodal loops at each heminode. Large bundles of axons decussated through the velum, the bulk of which were IVth nerve fibres which constituted the IVth nerve rootlet. The PNS/CNS transitional zone of the IVth nerve was located 0.25-0.50 mm along the root, where astrocytic end-feet defined an abrupt margin, convex towards the periphery, where the heminodes of central and peripheral myelin were apposed, and where the basal lamina tubes of the Schwann cell units were discontinued. The basal processes of ependymal cells lining the ventricular wall of the velum, passed between axon bundles before abutting on the basal lamina of the pia. Many of these processes branched and ran along the axonal bundles. A monolayer of microglia occupied a subependymal stratum in which the non-overlapping dendritic territories of each cell formed a regular mosaic throughout the velum without any obvious interaction with either axons or other glial cells. Astrocytes were also uniformly distributed; their fine processes made up a dense lattice amongst axons, often running parallel and within the fibre bundles; stouter ones had terminal end-feet which undercoated the basal lamina of both the glia limitans externa and the blood vessels in the velum.

Biochemical subtypes of oligodendrocyte in the anterior medullary velum of the rat as revealed by the monoclonal antibody Rip

Oligodendrocytes were studied in the anterior medullary velum (AMV) of the rat using the monoclonal antibody Rip, an oligodendrocyte marker of unknown function. Confocal microscopic imaging of double immunofluorescent labelling with antibodies to Rip and carbonic anhydrase II (CAII) revealed two biochemically and morphologically distinct populations of oligodendrocyte which were either Rip+CAII+ or Rip+CAII-. Double immunofluorescent labelling with Rip and myelin basic protein (MBP) or glial fibrillary acidic protein (GFAP) provided direct evidence that Rip-labelled cells were phenotypically oligodendrocytes and confirmed that Rip did not recognise astrocytes. Oligodendrocytes which were Rip+CAII+ supported numerous myelin sheaths for small diameter axons, whilst Rip+CAII- oligodendrocytes supported fewer myelin sheaths for large diameter axons. Morphologically, Rip+CAII+ oligodendrocytes corresponded to types I or II of classical nomenclature, whilst Rip+CAII- oligodendrocytes corresponded to types III and IV. The results demonstrated a biochemical difference between oligodendrocytes which myelinated small and large diameter fibres.

Imaging myelinated nerve fibres by confocal fluorescence microscopy: individual fibres in whole nerve trunks traced through multiple consecutive internodes

Current methods of morphological analysis do not permit detailed imaging of individual myelinated fibres over substantial lengths without disruption of neighbouring, potentially significant, cellular and extracellular relationships. We report a new method which overcomes this limitation by combining aldehyde-induced fluorescence with confocal microscopy. Myelin fluorescence was intense relative to that from other tissue components, enabling individual myelinated nerve fibres to be traced for distances of many millimeters in whole PNS nerve trunks. Image obtained with a Bio-Rad MRC-600 confocal laser scanning microscope clearly displayed features of PNS and CNS myelinated fibres including nodes of Ranvier; fibre diameter; sheath thickness and contour; branch points at nodes; as well as (in the PNS) Schmidt-Lanterman incisures and the position of Schwann cell nuclei. Direct comparisons using the same specimens (whole nerve trunks; also teased fibres) showed confocal imaging to be markedly superior to conventional fluorescence microscopy in terms of contrast, apparent resolution and resistance to photobleaching. Development of the fluorophore was examined systemically in sciatic nerves of young adult rats. In separate experiments, animals were perfused systemically using (1) 5% glutaraldehyde; (2) Karnovsky's solution; (3) 4% paraformaldehyde; buffered with either 0.1 M sodium phosphate or sodium cacodylate (pH 7.4). The concentration of glutaraldehyde in the fixative solution was the principal determinant of fluorescence intensity. Confocal imaging was achieved immediately following perfusion with 5% glutaraldehyde or Karnovsky's. Fluorescence intensity increased markedly during overnight storage in these fixatives and continued to increase during subsequent storage in buffer alone. The fluorophore was stable and resistant to fading during storage (15 months at least), enabling data collection over extended periods. To demonstrate application of the method in neuropathology, individual fibres in transected sciatic nerve trunks were traced through multiple successive internodes: Classical features of Wallerian degeneration (axonal swelling and debris; ovoid formation and incisure changes; variation among fibres in the extent of degeneration) were displayed. The method is compatible with subsequent ultrastructural examination and will complement existing methods of investigation of myelinated fibre anatomy and pathology, particularly where preservation of 3-dimensional relationships or elucidation of spatial gradients are required.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.

Mice deficient for the myelin-associated glycoprotein show subtle abnormalities in myelin

Using homologous recombination in embryonic stem cells, we have generated mice with a null mutation in the gene encoding the myelin-associated glycoprotein (MAG), a recognition molecule implicated in myelin formation. MAG-deficient mice appeared normal in motor coordination and spatial learning tasks. Normal myelin structure and nerve conduction in the PNS, with N-CAM overexpression at sites normally expressing MAG, suggested compensatory mechanisms. In the CNS, the onset of myelination was delayed, and subtle morphological abnormalities were detected in that the content of oligodendrocyte cytoplasm at the inner aspect of most myelin sheaths was reduced and that some axons were surrounded by two or more myelin sheaths. These observations suggest that MAG participates in the formation of the periaxonal cytoplasmic collar of oligodendrocytes and in the recognition between oligodendrocyte processes and axons.

Morphology of astrocytes and oligodendrocytes during development in the intact rat optic nerve

The detailed three-dimensional morphology of macroglial cells was determined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were analyzed by intracellular injection of horseradish peroxidase or Lucifer Yellow to provide a new perspective of glial differentiation in situ. Retrograde analysis of changes in glial morphology allowed us to identify developmental timetables for three morphological subclasses of astrocytes and oligodendrocytes, and to estimate their time of emergence from undifferentiated glial progenitors. Glial progenitors were recognised throughout postnatal development and persisted in 35-day-old nerves, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either transverse, random, or longitudinal process orientation. Several lines of evidence led us to believe that the majority of astrocytes in the rat optic nerve were morphological variations of a single cell type. Young oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During early development these cells extended a large number of fine processes, which then bifurcated and extended along axons. Later, as myelination proceeded, oligodendrocytes exhibited fewer processes which grew symmetrically and uniformly along the axons, resulting in a highly stereotypic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligodendrocytes.

Distribution of proteolipid protein and myelin basic protein in cultured mouse oligodendrocytes: primary vs. secondary cultures

The distribution of proteolipid protein (PLP) and myelin basic protein (MBP) was examined in differentiating oligodendrocytes of primary and secondary mouse brain cell cultures by single- and double-label indirect immunofluorescence. In primary cultures, MBP and PLP were differentially located in oligodendrocytes. MBP became concentrated as fine punctate dots lining the edges of processes and as coarse grains in flattened sheet-like structures. PLP was distributed diffusely throughout cell bodies and processes but was limited to the perimeter of sheets and some processes within sheets. To compare the detailed distribution of PLP and MBP in the absence of underlying cells, a simple method for the growth of isolated oligodendrocytes in secondary cultures was developed. Cells were maintained in primary culture for 39-41 days, harvested by scraping, enriched for oligodendrocytes, and plated at low cell density. After 1 week, isolated oligodendrocytes had developed long processes and large flattened membranous sheets. MBP and PLP were differentially localized in these cell structures. The sheets contained fine-grained patches of MBP, which were surrounded by networks of MBP- processes. In contrast, PLP was initially seen throughout the cell bodies and processes. In older cultures, PLP became strikingly concentrated in curvilinear membranous profiles. The observations show that PLP and MBP are differentially located in cultured mouse oligodendrocytes. Furthermore, the precise distribution of these myelin-specific antigens is dependent on culture conditions.

Relations between axons and oligodendroglial cells during initial myelination. II. The individual axon

Axo-glial relations in the ventral funiculus of the spinal cord (SC) and in the corpus callosum (CC) of the cat were examined by electron microscopy during initial myelination. In addition to random transverse and longitudinal sections from several stages, two series of sections were studied. As a first step in myelination the axons become ensheathed by one to three uncompacted glial lamellae (E-sheaths). E-sheaths present a length range from less than 5 microns to 149 microns (SC) or to 93 microns (CC). E-sheaths are more frequent along SC-axons than CC-axons, and the mean E-sheath is 3.3-fold longer in the former compared to the latter. In both areas naked axon portions occur between successive E-sheaths, but these gaps are insufficient to allow elongation of all short E-sheaths into long ones. Sheaths composed of mixed compacted (M-sheaths) and uncompacted segments have a length range of 66-212 microns in the SC and 66-171 microns in the CC. In relation to the undifferentiated terminations of E-sheaths or mixed E/M-sheaths, undercoated axolemmal domains are always lacking. Fully compacted sheaths were not found in the series from the SC. In the CC, 141-212 microns long compact sheaths were found, with tight axoglial junctions at their terminations. Axolemmal domains with a 'nodal' undercoating occur in relation to some of these terminations. In both areas, individual developing axons present a chaotic mixture of naked, ensheathed and myelinated portions; bulges with clusters of vesiculotubular profiles are frequent along naked and ensheathed axonal portions, particularly in the SC. The axon diameter is clearly larger in myelinated than in naked portions of the same axon. On the basis of these results, we propose that the early glial sheaths of developing CNS axons actively elongate and undergo extensive remodelling before compaction. The maximal length of uncompacted E-sheaths, and the sheath length at which axoglial junctions and nodes of Ranvier form, are markedly different in the two areas.

Intercellular septate-like junction of neoplastic cells in myxopapillary ependymoma of the filum terminale

Septate junction, a common intercellular feature of invertebrate epithelium, is absent in most vertebrate tissues. In an ultrastructural study of three cases of myxopapillary ependymoma of the filum terminale, structures similar to septate junction were observed in two cases. They were circumferential bands around the finger-like processes of neoplastic cells in which slightly widened 30-40-nm intercellular spaces (as compared to 15-20 nm in non-junctional region) were transversed at regular intervals by parallel septa resulting in characteristic ladder-like appearance. The electron-dense septa, 30-40 nm in cross-length and 20-30 nm in width, were arranged in a periodicity of 40-50 nm. The septa connected to the outer leaflets of the apposing cytoplasmic membranes which appear undisrupted. Most junctions were short; the longest one contained 15 septa in a length of 1.4 microns. Hemiseptate-like junctions with septa of the same measurements were noted between the processes and the investing basement membrane. Junctional complexes such as zonula adherens and gap junction were present in the vicinity. They may represent a specific intercellular feature of myxopapillary ependymoma, and function as cellular adhesions and a mechanical support of the neoplastic cells.

Inhibition of protein synthesis during CNS myelination produces focal accumulations of membrane vesicles in oligodendrocytes

Optic nerves of Xenopus tadpoles were exposed to cycloheximide to identify changes that occur during CNS myelin membrane formation when protein synthesis is inhibited. Groups of stage 51-56 tadpoles were immersed in either 10 or 20 micrograms ml-1 cycloheximide, and at specified times between 12 and 18 h after initial immersion tadpoles were killed and their optic nerves prepared for ultrastructural analysis. As early as 12 h there were alterations in oligodendrocytes from treated animals compared with control animals. The number of polyribosomes in the perikarya and cell processes was greatly reduced and the rough endoplasmic reticulum was disorganized. Mitochondria and microtubules were normal in appearance. Many oligodendroglial tongue processes at the inner margin of the myelin sheath were enlarged, occasionally indented the axon and were filled with vesicular profiles. Vesicles were noted in other cytoplasmic regions of oligodendrocytes and focal changes in the lamellar structure of myelin were found in paranodal regions. The internodal portions of the myelin sheath, axons and astrocytes appeared normal. Polyacrylamide gels of optic nerves showed that the incorporation of 35S-methionine into polypeptides had been almost completely inhibited by treatment with cycloheximide. These observations suggest that cycloheximide, by inhibiting synthesis of myelin proteins, alters the ability of oligodendrocytes to incorporate membrane components into CNS myelin sheaths.

Visualization of oligodendrocytes and astrocytes in the intact rat optic nerve by intracellular injection of lucifer yellow and horseradish peroxidase

The morphology of glial cells in the intact rat optic nerve, a central nervous system (CNS) white matter tract, was analysed by filling over 500 macroglial cells intracellularly with horseradish peroxidase (HRP) or Lucifer yellow (LY). Two main cell types were distinguished: fibrous astrocytes and cells presumed to be oligodendrocytes. Intracellularly stained astrocytes were highly complex, with 50-60 long branching processes which passed radially from the cell body and terminated in end-feet at the pial surface or on blood vessels; some processes ended freely in the nerve parenchyma. Astrocytes filled with LY were usually dye-coupled to other astrocytes after the first week of life. Filled oligodendrocytes had a unique appearance that unmistakably distinguished them from astrocytes and were occassionally dye-coupled to nearby oligodendrocytes. These cells had 20-30 longitudinally oriented processes 150-200 microns long, which passed exclusively along the long axis of the nerve parallel to axons; the longitudinal processes were connected to the cell body by thin branches 15-30 microns long. The longitudinal processes probably represent the tongue processes of the internodal myelin sheaths, and thus each oligodendrocyte appears to myelinate 20-30 axons with sheaths that are 150-200 microns in length.

[Myelination, demyelination and re-myelination in the central nervous system]

The myelin sheaths that surround axons in the CNS are made and maintained by oligodendrocytes. These glial cells can form variable numbers of myelin segments (internodules): from 1 to 200 so that when one oligodendrocyte is destroyed with preservation of the axon, many internodules can be lost, constituting a demyelinating process. As a consequence of the destruction of myelin and sheath cells a rapid and abundant cell response takes place. The response is made up by resident (microglia) and haematogenous phagocytes which phagocytose myelin and cellular debris leaving the axons demyelinated. Demyelinated axons may either stay demyelinated and clumped together or they may be separated by astrocytic processes, yet they can be remyelinated. The occurrence of remyelination depends upon the intensity and time of exposition to the demyelinating agent. Remyelination in the CNS with complete restoration of conduction may be made by oligodendrocytes or Schwann cells which invade the CNS when astrocytes are destroyed.

Nodal and paranodal structural changes in frog optic nerve during early Wallerian degeneration

Ultrastructural changes in the nodal and paranodal regions of myelinated nerve fibres of frog optic nerves were studied during early stages of Wallerian degeneration. The earliest changes seen include retraction of paranodal loops of myelin from the axolemma and disconnection of paranodal myelin loops from myelin lamellae. These paranodal changes are asymmetric around the node and may be more advanced on either the proximal or distal side. Axoplasmic changes, including segregation of microtubules from neurofilaments, disorientation of microtubules and accumulation of abnormal organelles at nodes, appear shortly. In some axons the 'undercoating' along the widened nodal surfaces becomes patchy, and blebs appear in the nodal axolemma. In freeze-fracture replicas a mixture of particle clusters and particle-free areas appears in both E- and P-faces of the nodal axolemma. Blebs remain particle free. Initially, E-face particles remain segregated to the node and are present only at much lower concentrations in the demyelinated paranodal axolemma, suggesting that they are not freely mobile at this stage. Nodal E-face particles begin to decrease on day 5 associated with an increase in particles at the adjacent demyelinated paranode, and by day 11 the particle distribution is uniformly low over the entire extent of the nodal and demyelinated paranodal axolemma. If nodal E-face particles represent sodium channels, as has been proposed, the sequence of changes in Wallerian degeneration would be compatible with a gradual redistribution of nodal sodium channels into the demyelinated paranode.

Ca2+- and K+-dependent communication between central nervous system myelinated axons and oligodendrocytes revealed by voltage-sensitive dyes

The interactions between myelinated axons and surrounding glia cells, in rat optic nerve, were investigated by optical recording with voltage-sensitive dyes. Electrical stimulation of the nerve evoked an optical signal revealing two clearly distinct components: a fast propagating component, corresponding to the compound action potential, and a prominent slow component. Several lines of evidence suggest that part of the slow component originated from depolarization of the oligodendrocytes by potassium accumulation in the paranodal or internodal region. In addition, the experiments suggest that in this preparation axons also have voltage-dependent Ca2+ channels, and a Ca2+-dependent K+ conductance involved in the depolarization of oligodendrocytes. Thus, axons and oligodendrocytes communicate in an intimate, ionically-mediated fashion, and oligodendrocytes may play an important functional role beyond that of providing the myelin sheath.

Biochemical abnormalities in spinal cord myelin and CNS homogenates in heterozygotes affected by the shiverer mutation

Myelin was purified from the spinal cords of normal mice and mice heterozygous for the shiverer mutation, and measurements were made of the major myelin proteins and lipids and the specific activities of three myelin-associated enzymes. The myelin purified from the spinal cords of the heterozygotes (shi/+) was deficient by 30-40% in yield and had an apparently unique composition. In particular, when compared with normal mouse spinal cord myelin, there were more high-molecular-weight protein, less myelin basic protein, a higher protein-to-lipid ratio, and higher specific activities of 2',3'-cyclic nucleotide-3'-phosphohydrolase (EC 3.1.4.37) and carbonic anhydrase (EC 4.2.1.1) in the myelin purified from the shi/+ animals. These abnormalities were reflected in the composition of shi/+ whole spinal cord, where the protein-to-lipid ratio was intermediate between the respective values for +/+ and shi/shi spinal cords. Whole brains from shi/+ mice showed deficiencies in galactocerebroside and galactocerebroside sulfate and an increase in total phospholipid, and the lipid composition in the brains of the shi/shi mice was similar to that reported for another dysmyelinating mutant, quaking. The findings provide the first values for the lipids in normal mouse spinal cord myelin and show that heterozygotes are affected by the shiverer mutation. The observations imply that there can be considerable deviation from the normal CNS myelin content and composition without apparent qualitative morphological abnormalities or loss of function and that the amount of myelin basic protein available during myelination may influence the incorporation of other constituents into the myelin membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropathologic study on chronic neurotoxicity of 5-fluorouracil and its masked compounds in dogs

5-Fluorouracil (FU) and its masked compounds tegafur (FT) and carmofur (HCFU) were administered orally to Beagle dogs daily for 6 months, and their chronic neurotoxic effects were examined morphologically. In ten dogs that survived the 6-month treatment large vacuoles produced by splitting of the intraperiod line of myelin were observed in the fornix in the wall of the third ventricle. In severely affected dogs large vacuoles developed in the medial preoptic area, medial portion of the internal capsule, the area around the subthalamic nucleus and the mammillo-thalamic tract. Axons of myelinated fibers affected by vacuolation were generally well maintained, and destruction of myelin was not detected. Though proliferation of glia cells or abnormality of oligodendroglia was not detected, a lipid deposit covered by a single layer membrane was observed in the cell bodies and processes of astrocytes. No abnormality was detected by electron microscopy in the cerebrum, inferior colliculus, cerebellum, or pons. Of eight dogs that died during the treatment period, large vacuoles were observed in the fornix in the wall of the third ventricle of four dogs treated for more than 1 month, and large vacuoles were present in the inferior colliculus in two dogs of the FT group in the above four dogs. In the HCFU group, the interruption of treatment for 6 months resulted in alleviation or disappearance of the vacuolar lesions. The above findings suggest that the neurotoxicity of FU and its masked compounds FT and HCFU in long-term treatment produces changes morphologically identical with one another in respect to the site of their manifestation and nature of lesion, that their common degraded product alpha-fluoro-beta-alanine (FBAL) plays a crucial role in their neurotoxic actions, and that vacuolar lesions, to which myelin was more vulnerable than neurons, can develop where the toxic substance readily deposits and accumulates.

Junctional complexes in the inner cyst tissue of the cysticercoid of Hymenolepis diminuta (Cestoda)

The inner cyst tissue development is anteriad and centripetal. The cells produce lamellar extensions which assume parallel alignment. The first contact points (approximately 4 days post-infection) establish heptalaminar (gap) junctions. Lamellar attenuation results in a decreased intercellular space, and at 5-6 days pentalaminar junctions (with fused outer plasmalemma leaflets to give an electron-dense, approximately 3 nm wide O-O line) occur. This is the first maturation (M1) stage. The O-O lines are permeable to lanthanum, and evidence of their possible transformation from heptalaminar junctions is presented. Continued lamellar attenuation, associated with scolex retraction and subsequent growth, results in cytoplasmic occlusion and contact between the inner leaflets of the same lamella. The resultant electron-dense I-I line is approximately 3 nm wide; the O-O line is now less electron-dense and thinner (approximately 2 nm). This final maturation (M2) stage, resembling vertebrate myelin, occurs over limited areas; closely adjacent regions either remaining at the M1 stage, or not displaying junctional complexes. Since in vivo and in vitro excystment can occur before the M2 stage, the inner cyst tissue is not considered to be protective against the definitive host. That the tissue may function in limiting nutrient flow, thus regulating the size of the presumptive adult, is discussed.

Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A study has been made of the structural changes of nodal and paranodal regions of the nodes of Ranvier of peripheral nerves of rats in which marked accumulations of neurofilaments have occurred within axons under the influence of 2,5-hexanediol over 10 weeks. The neurofilamentous masses caused distension of the axon at two points of apparent weakness as they attempted to slide through the axonal constriction at the nodes. Principally, a spiral axonal protrusion pushed into the zone of unattached myelin loops in the proximal paranodal spinous bracelet of Nageotte. This led to a conical widening of the paranodal constriction and considerable attenuation of the overlying myelin. No degeneration of the myelin occurred however. Alternatively, or additionally, a protrusion occurred of the axon at the nodal region which increased the nodal gap width and occasionally compressed and displaced the adjacent distal paranodal constriction which could have led to some obstruction of axoplasmic flow. Swelling of distal paranodal regions occurred later and was usually associated with proximal swelling. It was also accompanied by evidence suggesting transnodal passage of filamentous material. Sometimes, however, striking nodal constriction occurred in association with symmetrical paranodal swelling. These observations suggest that the spiral glial-axonal relationships at nodes of Ranvier are capable of marked deformation that might allow the intra-axonal neurofilamentous masses to move distally. These findings are discussed in relation to the structural features of the paranodal constrictions.

The ultrastructure of isolated rat oligodendroglial cell cultures

Cultured rat oligodendrocytes were studied by transmission electron microscopy. Cells were identified by an immunocytochemical stain for galactocerebroside, a specific cell surface marker for oligodendroglial cells. Oligodendroglial cell perikarya contained numerous ribosomal rosettes, microtubules, prominent networks of cisternae of the Golgi apparatus and residual bodies. Glycogen and intermediate filaments were absent. Oligodendrocytes gave rise to numerous processes. Pentalaminar and heptalaminar profiles, consistent with tight and gap junctions, were seen between plasma membranes of processes and between perikarya and processes. The cell surface of processes showed numerous gross ruffles which stained for galactocerebroside. Similar membranous profiles appeared in the vicinity of oligodendroglial processes and suggested to us that the plasma membrane of certain of its components may be released into the medium. We concluded that cultured rat oligodendrocytes maintain many similarities with oligodendrocytes in situ and, therefore, are valid models for morphologic, physiologic and biochemical studies.

Interactions of dicyclohexylcarbodiimide with myelin proteolipid

Dicyclohexylcarbodiimide (DCCD) is known to bind preferentially to a proteolipid subunit of proton-translocating systems and thereby to inhibit proton transport. In the present study we show that, in an aqueous medium, DCCD binds to the bovine white matter proteolipid apoprotein, the major protein of central nervous system myelin. The binding is dependent on time, temperature, and concentration and is not inhibited by the hydrophilic carbodiimide 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. By contrast, when the incubation is carried out in chloroform/methanol no labeling by DCCD is demonstrable. In isolated rat myelin, DCCD binds specifically to the proteolipid and not to the myelin basic proteins. Liposomes reconstituted with the myelin proteolipid apoprotein transport protons, as assayed by quenching of the fluorescence of 9-aminoacridine. Preincubation of proteolipid-containing liposomes with DCCD results in an inhibition of transport. These studies have important implications for a possible ionophoric function of the myelin proteolipid and for the occurrence of transport processes within myelin.