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

Adult-onset depletion of sulfatide leads to axonal degeneration with relative myelin sparing

3-O-sulfogalactosylceramide (sulfatide) constitutes a class of sphingolipids that comprise about 4% of myelin lipids in the central nervous system. Previously, our group characterized a mouse with sulfatide's synthesizing enzyme, cerebroside sulfotransferase (CST), constitutively disrupted. Using these mice, we demonstrated that sulfatide is required for establishment and maintenance of myelin, axoglial junctions, and axonal domains and that sulfatide depletion results in structural pathologies commonly observed in Multiple Sclerosis (MS). Interestingly, sulfatide is reduced in regions of normal appearing white matter (NAWM) of MS patients. Sulfatide reduction in NAWM suggests depletion occurs early in disease development and consistent with functioning as a driving force of disease progression. To closely model MS, an adult-onset disease, our lab generated a "floxed" CST mouse and mated it against the PLP-creERT mouse, resulting in a double transgenic mouse that provides temporal and cell-type specific ablation of the Cst gene (Gal3st1). Using this mouse, we demonstrate adult-onset sulfatide depletion has limited effects on myelin structure but results in the loss of axonal integrity including deterioration of domain organization accompanied by axonal degeneration. Moreover, structurally preserved myelinated axons progressively lose the ability to function as myelinated axons, indicated by the loss of the N1 peak. Together, our findings indicate that sulfatide depletion, which occurs in the early stages of MS progression, is sufficient to drive the loss of axonal function independent of demyelination and that axonal pathology, which is responsible for the irreversible loss of neuronal function that is prevalent in MS, may occur earlier than previously recognized.

Ultrastructural anatomy of nodes of Ranvier in the peripheral nervous system as revealed by STED microscopy

We used stimulated emission depletion (STED) superresolution microscopy to analyze the nanoscale organization of 12 glial and axonal proteins at the nodes of Ranvier of teased sciatic nerve fibers. Cytoskeletal proteins of the axon (betaIV spectrin, ankyrin G) exhibit a high degree of one-dimensional longitudinal order at nodal gaps. In contrast, axonal and glial nodal adhesion molecules [neurofascin-186, neuron glial-related cell adhesion molecule (NrCAM)] can arrange in a more complex, 2D hexagonal-like lattice but still feature a ∼190-nm periodicity. Such a lattice-like organization is also found for glial actin. Sodium and potassium channels exhibit a one-dimensional periodicity, with the Na_v channels appearing to have a lower degree of organization. At paranodes, both axonal proteins (betaII spectrin, Caspr) and glial proteins (neurofascin-155, ankyrin B) form periodic quasi-one-dimensional arrangements, with a high degree of interdependence between the position of the axonal and the glial proteins. The results indicate the presence of mechanisms that finely align the cytoskeleton of the axon with the one of the Schwann cells, both at paranodal junctions (with myelin loops) and at nodal gaps (with microvilli). Taken together, our observations reveal the importance of the lateral organization of proteins at the nodes of Ranvier and pave the way for deeper investigations of the molecular ultrastructural mechanisms involved in action potential propagation, the formation of the nodes, axon-glia interactions, and demyelination diseases.

Making the connection – shared molecular machinery and evolutionary links underlie the formation and plasticity of occluding junctions and synapses

The pleated septate junction (pSJ), an ancient structure for cell–cell contact in invertebrate epithelia, has protein components that are found in three more-recent junctional structures, the neuronal synapse, the paranodal region of the myelinated axon and the vertebrate epithelial tight junction. These more-recent structures appear to have evolved through alterations of the ancestral septate junction. During its formation in the developing animal, the pSJ exhibits plasticity, although the final structure is extremely robust. Similar to the immature pSJ, the synapse and tight junctions both exhibit plasticity, and we consider evidence that this plasticity comes at least in part from the interaction of members of the immunoglobulin cell adhesion molecule superfamily with highly regulated membrane-associated guanylate kinases. This plasticity regulation probably arose in order to modulate the ancestral pSJ and is maintained in the derived structures; we suggest that it would be beneficial when studying plasticity of one of these structures to consider the literature on the others. Finally, looking beyond the junctions, we highlight parallels between epithelial and synaptic membranes, which both show a polarized distribution of many of the same proteins – evidence that determinants of apicobasal polarity in epithelia also participate in patterning of the synapse.

Organization and maintenance of molecular domains in myelinated axons

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

Structural bases for central nervous system malfunction in the quaking mouse: dysmyelination in a potential model of schizophrenia

The dysmyelinating mouse mutant quaking (qk) is thought to be a model of schizophrenia based on diminution of CNS myelin (Andreone et al., 2007) and downregulation of the Qk gene (Haroutunian et al., 2006) in the brains of schizophrenic patients. The purpose of this study was to identify specific structural defects in the qk mouse CNS that could compromise physiologic function and that in humans might account for some of the cognitive defects characteristic of schizophrenia. Ultrastructural analysis of qk mouse CNS myelinated fibers shows abnormalities in nodal, internodal, and paranodal regions, including marked variation in myelin thickness among neighboring fibers, spotty disruption of paranodal junctions, abnormal distribution of nodal and paranodal ion channel complexes, generalized thinning and incompactness of myelin, and on many axonal profiles complete absence of myelin. These structural defects are likely to cause abnormalities in conduction velocity, synchrony of activation, temporal ordering of signals, and other physiological parameters. We conclude that the structural abnormalities described are likely to be responsible for significant functional impairment both in the qk mouse CNS and in the human CNS with comparable myelin pathology.

Paranodal reorganization results in the depletion of transverse bands in the aged central nervous system

Paranodal axo-glial junctional complexes anchor the myelin sheath to the axon and breakdown of these complexes presumably facilitates demyelination. Myelin deterioration is also prominent in the aging central nervous system (CNS); however, the stability of the paranodal complexes in the aged CNS has not been examined. Here, we show that transverse bands, prominent components of paranodal junctions, are significantly reduced in the aged CNS; however, the number of paired clusters of both myelin and axonal paranodal proteins is not altered. Ultrastructural analyses also reveal that thicker myelin sheaths display a "piling" of paranodal loops, the cytoplasm-containing sacs that demarcate the paranode. Loops involved in piling are observed throughout the paranode and are not limited to loops positioned in either the nodal- or juxtanodal-most regions. Here, we propose that as myelination continues, previously anchored loops lose their transverse bands and recede away from the axolemma. Newly juxtaposed loops then lose their transverse bands, move laterally to fill in the gap left by the receded loops and finally reform their transverse bands. This paranodal reorganization results in conservation of paranodal length, which may be important in maintaining ion channel spacing and axonal function. Furthermore, we propose that transverse band reformation is less efficient in the aged CNS, resulting in the significant reduction of these junctional components. Although demyelination was not observed, we propose that loss of transverse bands facilitates myelin degeneration and may predispose the aged CNS to a poorer prognosis following a secondary insult.

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.

Disposition of axonal caspr with respect to glial cell membranes: Implications for the process of myelination

Neurofascin-155 (NF155) and caspr are transmembrane proteins found at discrete locations early during development of the nervous system. NF155 is present in the oligodendrocyte cell body and processes, whereas caspr is on the axonal surface. In mature nerves, these proteins are clustered at paranodes, flanking the node of Ranvier. To understand how NF155 and caspr become localized to the paranodal regions of myelinated nerves, we have studied their distribution over time in myelinating cultures. Our observations indicate that these two proteins are recruited to the cell surface at the contact zone between axons and oligodendrocytes, where they trans-interact. This association explains the early pattern of caspr distribution, a helical coil that winds around the axon, resembling the turns of the myelin sheath. Caspr, an axonal membrane protein, therefore seems to move in register with the overlying myelinating cell via its interactions with myelin proteins. We suggest that NF155 is the glial cell membrane protein responsible for caspr distribution. The pair act as interacting partners on either side of the axoglial contact area. Most likely, there are other proteins on the axonal surface whose distribution is equally influenced by interaction with the nascent myelin sheath. The fact that caspr follows the movement of the spiraling membrane has a direct affect on the interpretation of the way in which myelin is formed.

Electrophysiological abnormalities precede apparent histological demyelination in the central nervous system of mice overexpressing proteolipid protein

Myelin proteolipid protein (plp), a major myelin protein in the CNS, has been proposed to function in myelin assembly. Transgenic mice overexpressing the plp gene by introduction of two extra wild-type (Wt) mouse plp genes (plp(tg/-)) exhibit normal myelination and ion channel clustering at the age of 2 months. However, at the age of 5 months, demyelination becomes observable, accompanied by a reduction in the number of K+ channel clusters at Ranvier's node and a progressive increase in motor deficit. To clarify how these age-dependent changes are related to nerve conduction in the CNS, we analyzed the conduction velocity (CV) and relative refractory period (RRP) of identified spinal ascending or descending tracts, such as the dorsal column pathway, the vestibulospinal and reticulospinal tracts, and the pyramidal tract, in plp(tg/-) mice 2, 5, and 8 months of age. We found that CVs decreased as age increased. Importantly, CVs were significantly reduced and prolonged RRPs were observed in 2-month-old (2M) plp(tg/-) mice that had no apparent demyelination. Immunohistological examination revealed that densities of Na+ and K+ channel clusters decreased as plp(tg/-) and Wt mice aged. However, a clear correlation was not observed between CVs and mean channel cluster densities or between mean channel cluster densities and progress of demyelination. Performance in the rotarod test was normal in 2M plp(tg/-) mice but deteriorated in mice older than age 5 months. These results suggest that electrophysiological analysis can detect the abnormalities of the plp(tg/-) mice earlier than histological or behavioral measures.

Polarized domains of myelinated axons

The entire length of myelinated axons is organized into a series of polarized domains that center around nodes of Ranvier. These domains, which are crucial for normal saltatory conduction, consist of distinct multiprotein complexes of cell adhesion molecules, ion channels, and scaffolding molecules; they also differ in their diameter, organelle content, and rates of axonal transport. Juxtacrine signals from myelinating glia direct their sequential assembly. The composition, mechanisms of assembly, and function of these molecular domains will be reviewed. I also discuss similarities of this domain organization to that of polarized epithelia and present emerging evidence that disorders of domain organization and function contribute to the axonopathies of myelin and other neurologic disorders.

Dependence of nodal sodium channel clustering on paranodal axoglial contact in the developing CNS

Na(+) channel clustering at nodes of Ranvier in the developing rat optic nerve was analyzed to determine mechanisms of localization, including the possible requirement for glial contact in vivo. Immunofluorescence labeling for myelin-associated glycoprotein and for the protein Caspr, a component of axoglial junctions, indicated that oligodendrocytes were present, and paranodal structures formed, as early as postnatal day 7 (P7). However, the first Na(+) channel clusters were not seen until P9. Most of these were broad, and all were excluded from paranodal regions of axoglial contact. The number of detected Na(+) channel clusters increased rapidly from P12 to P22. During this same period, conduction velocity increased sharply, and Na(+) channel clusters became much more focal. To test further whether oligodendrocyte contact directly influences Na(+) channel distributions, nodes of Ranvier in the hypomyelinating mouse Shiverer were examined. This mutant has oligodendrocyte-ensheathed axons but lacks compact myelin and normal axoglial junctions. During development Na(+) channel clusters in Shiverer mice were reduced in numbers and were in aberrant locations. The subcellular location of Caspr was disrupted, and nerve conduction properties remained immature. These results indicate that in vivo, Na(+) channel clustering at nodes depends not only on the presence of oligodendrocytes but also on specific axoglial contact at paranodal junctions. In rats, ankyrin-3/G, a cytoskeletal protein implicated in Na(+) channel clustering, was detected before Na(+) channel immunoreactivity but extended into paranodes in non-nodal distributions. In Shiverer, ankyrin-3/G labeling was abnormal, suggesting that its localization also depends on axoglial contact.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells

Rat sciatic nerve fibres were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase EM has been used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. At the peak of demyelination, 1 week post-injection, only isolated sites, suggestive of the original nodes, were labelled. As Schwann cells adhered and extended processes along the axons, regions of axonal Na+ channel immunoreactivity were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labelled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighbouring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel label was found primarily just outside the region of close contact between axon and Schwann cell. This suggests that Schwann cell adherence acts in part to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact.

Renal vascular reactivity to ATP in hyper- and hypothyroid rats

The effects of adenosine triphosphate (ATP) on the renal vasculature of isolated kidneys from control, hyper- and hypothyroid rats were characterized. ATP responsiveness was evaluated in basal tone and in raised tone (phenylephrine 10(-6) M) preparations. These responses were compared with those obtained with barium chloride or sodium nitroprusside (SNP), used respectively as nonreceptor agonist for vasoconstriction or vasodilation. In preparations at basal tone, ATP produced dose-related vasoconstriction, which was increased in hyperthyroid kidneys, and was severely attenuated in kidneys from hypothyroid rats. In raised tone preparations from control rats ATP produced a dual response: vasoconstriction at low doses, which declined with increasing doses to give way to vasodilator responses; biphasic responses were found in some kidneys. Hyperthyroid kidneys showed increased pressor responses and a vasodilator response similar to those seen in kidneys from control rats. However, in hypothyroid kidneys the vasodilator response was abolished. The responses to barium chloride and to SNP were significantly increased and decreased in hyper- and hypothyroid kidneys, respectively; vasoconstrictor responses to SNP were also found in hypothyroid kidneys. Hence the abnormal responses to ATP observed in both thyroid dysfunctions may be partially explained by unspecific alterations in the contractile machinery of the renal vasculature in these kidneys. However, ATP responsiveness (vasoconstriction at low tone and vasodilation at raised tone) was more severly affected in hypothyroid kidneys, suggesting that purinergic (P2X and P2Y) receptor activity may be decreased in these organs.

Axonal coding of action potentials in demyelinated nerve fibers

Conduction in individual axons of Xenopus has been measured optically in response to short trains of stimuli, following demyelination of the sciatic nerve. In many cases the initial action potential in a burst is absent. Failure may also occur later in the train, resulting in a profound alteration of signal coding by the axon. Integration leading to delayed transmission occurred at the heminode forming the proximal border of the demyelinated zone, as well as at new nodes of Ranvier forming in remyelinating axons. This process appeared to involve a depolarizing afterpotential and seemed to be analogous to the threshold changes involved in superexcitability. Axonal coding was very sensitive to small changes in the stimulus pattern. Neither 1 mM tetraethylammonium ion, which blocks nodal and Ca2+ activated K+ channels, nor 1 mM 4-aminopyridine, which blocks fast internodal K+ channels, prevented loss of the initial spike in a burst. Similarly, neither block of Ca2+ channels by Cd2+ nor lowering of Cl- had a notable effect. Ouabain, on the other hand, had small but possibly significant effects on responses to repetitive stimuli. A computational model was used to test mechanisms involving passive cable properties. Lowering the myelin resistance and the nodal leakage conductance, in accord with recent evidence from intracellular recordings, reproduced many of the results and was accurate with respect to stimulus frequency, temperature and sensitivity to average potential. The coding of action potentials observed here may have clinical consequences in demyelinating diseases such as multiple sclerosis.

Freeze-fracture studies on unmyelinated axolemma of rat cervical sympathetic trunk: correlation with saxitoxin binding

The density and diameter distributions of intramembranous particles (IMPs) within unmyelinated axolemma from rat cervical sympathetic trunk were examined with freeze-fracture electron microscopy. The axolemma displays a highly asymmetrical partitioning of IMPs with ca. 1200 IMPs microns-2 on P-faces and ca. 100 IMPs microns-2 on E-faces. Particle sizes (diameters) are unimodally distributed on both fracture faces, with a range from 2.4 nm to 15.6 nm. Approximately 16% of the particles on P-faces and 28% of particles on E-faces are of a large (greater than 9.6 nm) diameter. On both fracture faces, the IMPs appear to be randomly distributed; no aggregations of particles were observed. The results indicate that there are ca. 230 large IMPs microns-2 of unmyelinated axolemma from rat cervical sympathetic trunk. The density of these IMPs is similar to the density of saxitoxin binding sites on unmyelinated axolemma from rat cervical sympathetic trunk (Pellegrino et al. 1984 (Brain Res. 305, 357-360)), which suggests that many of the large diameter particles may be the morphological correlate of voltage-sensitive Na+ channels.

Development of axonal-oligodendroglial relationships and junctions during myelination of the optic nerve

The early stages of myelination were examined in optic nerves of rats aged 12-15 days. The initial association between oligodendroglial processes and bare axons involves no junctional specialization, as the axoglial extracellular space remains unaltered. Following ensheathment by a collar of glial cytoplasm, at least one full rotation of mesaxon was evident before compact myelin formed. Furthermore, myelin was generally evident before a second rotation was completed. In longitudinal sections, an axoglial junction was always observed beginning on the first paranodal loop, continuing through to the last (or outermost) loop. Thus, the formation of myelin and elaboration of a junctional complex in the paranodal region follow a promyelination phase and appear to be synchronous (and possibly related) events. Although the paranodal plasmalemma and axolemma are in close apposition, there is a material in the extracellular space that precipitates phosphotungstic acid, a characteristic that appears to be featured in a number of different types of cell junctions.

Activity-dependent calcium transients in central nervous system myelinated axons revealed by the calcium indicator Fura-2

Optical measurements from rat optic nerve, loaded with the new Ca2+ indicator Fura-2, provide the first evidence for the presence of activity-dependent fast intracellular [Ca2+] transients in mammalian central nervous system (CNS) myelinated axons. The results suggest that voltage-dependent Ca2+ channels are present in some of the myelinated axons. Optical measurements from axons stained with anterogradely transported voltage-sensitive dye suggest the presence of Ca2+-dependent potassium conductances in these axons. This report also demonstrates that Fura-2 can readily detect changes in [Ca2+] inside cells as a result of electrical activity, and establishes its suitability for measurements of intracellular Ca2+ transients in the millisecond time domain.

Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord

The macromolecular structure of axonal membrane from dorsal funiculi of control and irradiated spinal cord of 45-day-old rats was examined with freeze-fracture electron microscopy. In control spinal cords, virtually all myelination is mediated by oligodendrocytes, and the internodal axonal membrane of these fibres displays highly asymmetrical partitioning of intramembranous particles (IMPs). The internodal P-face particle density is approximately 2350IMPs per micron 2, whereas the E-face IMP density is approximately 150 per micron 2. In control dorsal spinal roots, myelination is mediated by Schwann cells, and the ultrastructure of the internodal axolemma of the myelinated fibres is similar to that displayed by myelinated fibres of dorsal funiculi. On the internodal P-face of Schwann cell-myelinated fibres the IMP density is approximately 2350 per micron 2, whereas on the E-face the density is approximately 175 per micron 2. Irradiation of the lumbosacral spinal cord at 3 days of age results in a glial cell-deficient region within the spinal cord such that myelination in irradiated dorsal funiculi is delayed and subsequent myelination is mediated by both oligodendrocytes and Schwann cells. By 45 days of age, dorsal funiculi of irradiated spinal cords are well populated with fibres myelinated by oligodendrocytes and Schwann cells. However, fibres myelinated by oligodendrocytes display very thin myelin sheaths whereas Schwann cell-myelinated fibres exhibit myelin sheaths with normal thicknesses. Internodal membrane of fibres myelinated by Schwann cells and oligodendrocytes exhibit similar macromolecular structure, with approximately 2400 IMPs per micron 2 on P-faces and approximately 150 IMPs per micron 2 on E-faces. Occasional large (greater than 1.5 micron diameter) axons without glial-Schwann cell ensheathment are observed. These axons display a high density of P-face particles (approximately 2000 per micron 2) and a moderate density (approximately 350 per micron 2) of E-face IMPs on their fracture faces. These results demonstrate that CNS fibers exhibit similar axonal membrane ultrastructure irrespective of whether they are myelinated by Schwann cells or oligodendrocytes, or whether myelination is delayed. Moreover, when myelination does not occur, the axolemmal E-face IMP density, which may be related to the density of voltage-sensitive sodium channels, is not reduced.

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.

Glial cells of an insect ganglion

The rapid development of the study of insect neurobiology, which is currently occurring principally because individual neurons can be re-identified and because their activities can be recorded in situ and related to behavior, is generating a demand for more knowledge concerning insect glial cells and their functional relationships with neurons. This study examines the ultrastructure of glial cells in locust metathoracic ganglia in relation to general locale within the ganglion and also to specific identified neurons and neuron types. Seven major types of glial cell form are recognized, with subdivisions, requiring a new scheme for classification. Glial invaginations into neurons are of four different kinds: regular, chunky, filigree, and ridge (found only at axon hillocks). They also range from only intrusive to fully reciprocal. In addition, some neurons make projections of various lengths into surrounding glia and between neighboring neuron somata, and some glia make long, branched projections into other glial cells. The differences show that insect glial cells develop highly specific functional specializations; they may not be interchangeable. The complexity and intimacy of relationships of glia with neurons suggest that some glial cells may have roles other than that of nursemaids, possibly in modulation of behavior-determining neural activity, and in learning and other adaptive acts.

Ultrastructure of the squid axon membrane as revealed by freeze-fracture electron microscopy

The structure of the axolemma of the squid giant axon was studied by freeze-fracture electron microscopy. Three types of preparations were examined: intact axons, axons with their Schwann cell sheaths stripped off prior to freezing, and axons with their Schwann cell sheaths chemically detached but not mechanically removed. Because of a problem of cross-fracturing, the first two types of preparations revealed very few membrane faces of the axolemma. This cross-fracturing problem, however, was eliminated when we used a complementary replication method to fracture the third type of preparation. We found that the E-face of the axon membrane was smooth relative to the P-face, which showed many prominent intramembrane particles (IMP). The diameters of the typical IMP range from 6 to 15 nm. The P-face of the adjacent Schwann cells also showed many large IMP. The sizes and heights of the Schwann-cell IMP, however, appear to be more homogeneous than the P-face axolemma.

Membrane ultrastructure of developing axons in glial cell deficient rat spinal cord

In order to investigate axolemmal development in a glial cell deficient environment, normal and irradiated dorsal funiculus in rat lumbosacral spinal cord was examined by freeze-fracture electron microscopy. At 3 days of age, normal fibres are all unmyelinated and of small (less than 0.5 micron) diameter. The unmyelinated axons have a moderate density (approximately 850 microns-2) of intramembranous particles (IMPs) on P-fracture faces and a low IMP density (approximately 300 microns-2) on E-faces. IMPs are homogeneously distributed along both fracture faces. By 19 days of age, the normal dorsal funiculus is well populated with myelinated axons and glial cells, as well as a sizable population of unmyelinated fibres. Nearly all of the myelinated fibres have a large (greater than 1.0 micron) diameter; whereas, most unmyelinated axons are of small (less than 0.5 micron) calibre. The axolemma of unmyelinated axons is relatively undifferentiated, with an asymmetrical distribution of IMPs (P-face: approximately 1100 microns-2; E-face: approximately 450 microns-2). Myelinated fibres show nodal and paranodal regions with P-face and E-face ultrastructure similar to previous descriptions. Internodal axolemma appears relatively homogeneous, with P-faces being highly particulate (approximately 2100 microns-2) and a low IMP density (approximately 200 microns-2) on E-faces. Following irradiation of the lumbosacral spinal cord at 3 days of age, there is a severe reduction in the number of glial cells and myelinated fibres in this region when the tissue is examined at 19 days of age. Despite the deficiency of glial cells in this tissue, axonal and axolemmal development continue. Numerous large (greater than 1.0 micron) diameter axons are present in this irradiated tissue. Large diameter axons show a high (approximately 2000 microns-2) density of IMPs on P-faces; E-face IMP density remains at approximately 440 micron-2. Small calibre axons also have an asymmetrical distribution of particles (P-face: approximately 1100 microns-2; E-face: 280 microns-2). The axolemmal E-faces of some glial cell deprived fibres exhibit regions with greater than normal (approximately 750 microns-2) density of IMPs. These results demonstrate that some aspects of axonal and axolemmal development continue in a glial cell deficient environment, and it is suggested that axolemmal ultrastructure is, at least in part, independent of glial cell association.

Nodes of Ranvier and Schmidt-Lanterman incisures: an in vivo lanthanum tracer study

The permeability of the tight junctional system of myelin, at the juxtanodal myelin terminal loops and Schmidt-Lanterman incisures, was investigated using the ionic tracer lanthanum (a) in vivo followed by fixation, (b) concurrently with fixation, (c) following fixation. Employing the same methods the juxtanodal membrane complex formed between myelin loops and axolemma was also tested. The results of this study demonstrate that the periaxonal space (between axon and Schwann cell) is apparently accessible to lanthanum via the myelin loop-axolemmal junction, irrespective of the mode of exposure of myelinated fibres to the tracer. Similarly, the tight junctions between adjacent myelin terminal loops apparently do not prevent lanthanum penetration either in living or in fixed nerves. By contrast the tracer obtained access to the extracellular space within incisures only in vivo. The results are interpreted in terms of the permeability of nodes and incisures in vivo to physiologically important ions and related to current concepts of the electrophysiology of the myelinated nerve fibre.

Structure of the proteolipid protein extracted from bovine central nervous system myelin with nondenaturing detergents

As a basis for attempts to define the structures of the proteins within myelin, methods have been developed for their extraction and isolation in solutions of non-denaturing detergents. With use of solutions of deoxycholate or Triton X-100, up to 90% of the protein has been extracted from bovine CNS myelin, along with most of the phospholipid. The proteolipid protein has been purified in deoxycholate solutions by chromatography on a blue dye-ligand column, which retained all of the basic protein and 2',3'-cyclic nucleotide-3'-phosphodiesterase, and then on Sephacryl S300, which separated proteolipid protein from phospholipid and high-molecular-weight proteins. The proteolipid protein was isolated from Triton X-100 extracts of myelin by adsorption onto phosphocellulose resin, with subsequent elution by 0.5 M sodium chloride. Gel permeation chromatography was used as the final purification step. Sedimentation equilibrium experiments gave a monomer molecular weight of 134,000 +/- 8000 in deoxycholate and 145,000 +/- 17,000 in Triton X-100 solutions. On the basis of an apparent subunit molecular weight of 23,500 it was deduced that the native protein is probably hexameric. Above 0.2 gL-1 in Triton X-100 solutions and 0.5 gL-1 in deoxycholate solutions the protein aggregated. In deoxycholate solutions the protein adopts the highly helical conformation expected for an intrinsic membrane protein.

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.

'Shaking pups': a disorder of central myelination in the spaniel dog. IV. Freeze-fracture electron microscopic studies of axons, oligodendrocytes and astrocytes in the spinal cord white matter

Large axons identified in freeze-fracture replicas of the spinal cord white matter of 'shaking pups' were encircled by spiral processes at the paranodes that appeared to arise from oligodendrocytes. In the axolemma adjacent to some of these processes there were paranode-like intramembranous specializations and node-like accumulations of E-face particles; in other instances, no special contacts formed between axons and the spiral processes. The outer surfaces of the spiral processes, which often apposed astrocytes, frequently contained gap junctions. Although many of the abnormalities identified in this dysmyelinating mutant by freeze-fracture electron microscopy are probably secondary to a more fundamental defect of myelin formation, the prominence of the spiral processes suggests that the encircling of axons by oligodendrocytes may be an independent state of ensheathment and not a passive effect of myelin formation.

Axolemmal differentiation in myelinated fibers of rat peripheral nerves

In developing rat peripheral fibers, nodal specialization appears early, prior to myelin compaction, and is first detected as a junction between the axon and the overhanging Schwann cell process characterized by a uniformly wide (approximately 18 nm) intercellular gap containing a patchy dense substance and a cytoplasmic undercoating subjacent to the axolemma. The gap width is rather consistent but the axolemmal undercoating is more variable and lower in density than that found at more mature nodes of Ranvier, and it is also highly variable in length, ranging from 0.5 to 3 micron. The outermost Schwann cell layer is usually prominent with a large volume of cytoplasm and many organelles. In freeze-fracture replicas, modal specializations are characterized by accumulations of large (approximately 10 nm) particles in the axolemma, especially the E face, but immature nodes generally have a lower particle concentration than mature nodes. No node-like particle aggregates have been found in axons not intimately associated with Schwann cells. Mature paranodal axon-Schwann cell junctions are usually formed first by the loops closest to the node and are characterized by a 2-3 nm gap between the apposed membranes, periodic intercellular densities (transverse bands) in the gap and cisternae flattened against the junctional Schwann cell membrane. The loops further removed from the node display a wider gap containing irregularly spaced or diffuse intercellular densities, or none. Mature junctions appear relatively late in the rat, and it is not unusual to find developing nodes with several Schwann cell loops present that do not indent the axolemma significantly and are not associated with the paracrystalline pattern characteristic of the mature junctional axolemma. In such instances, the nodal particle aggregates do not have sharply circumscribed boundaries. The majority of the developing nodes are asymmetric with one paranodal segment more mature than the other.

Electron microscopic serial section analysis of nodes of Ranvier in lumbar spinal roots of the cat: a morphometric study of nodal compartments in fibres of different sizes

Serially sectioned nodes of Ranvier from nerve fibres 2-20 micron in diameter of feline ventral and dorsal spinal roots were examined electron microscopically, reconstructed to scale and analysed morphometrically. The assumed 'fresh-state' value of several structural variables, considered to be of functional significance, were calculated by the use of compensation factors. The compensated data were plotted against fibre and axon diameters. It was calculated that the membranous area of the 'fresh-state' nodal axon segment increased more or less exponentially from less than 5 micron2 to 30 micron2 with increasing fibre diameter (D). Most variables associated with the nodal gap and the Schwann cell initially increased rapidly with D and then levelled out or even decreased in fibres with a D value greater than 8-12 micron. The area open for communication between the nodal axolemma and the endoneurial space was 30-100 times smaller than the membrane area of the nodal axolemma. The volume of the extracellular space in the nodal gap, outside the nodal axolemma, increased linearly from less than 0.1 micron3 to about 0.6 micron3 with increasing fibre size. The Schwann cell membrane area facing the nodal gap outnumbered the membrane area of the nodal axon by 10-15 times in nerve fibres with a D value between 5 and 15 microns. Some functional implications of the 'fresh-state' nodal model are discussed.

Electron microscopic serial section analysis of nodes of Ranvier in lumbosacral spinal roots of the cat: ultrastructural organization of nodal compartments in fibres of different sizes

The general ultrastructural organization of nodes of Ranvier in peripheral nerve fibres from 2 to 20 microns in diameter (D) was investigated in the adult cat using serially sectioned ventral and dorsal spinal roots. The study was performed in order to collect and systematize information considered necessary for a morphometric analysis of the node of Ranvier. In all cases a node of Ranvier could be divided into a central nodal axon segment and a surrounding nodal Schwann cell compartment. The latter included a nodal gap matrix substance, more or less overlapping nodal Schwann cell collars and, as a rule, also a Schwann cell brush-border emanating from the nodal Schwann cell collars and occupying the nodal gap. The relative size and the organization level of the nodal Schwann cell compartment increased with increasing fibre size up to a fibre diameter of 8-10 microns. At this fibre size the nodal gap was of a fairly even height (1 micron) all around the nodal axon and contained a thick brush-border of densely packed, more or less radially arranged Schwann cell microvilli. In very small fibres (D less than 3 microns) the nodal gap was low (less than 0.1 microns) and contained no or few microvilli. In fibres greater than 10 microns in diameter the relative size and the degree of structural order of the nodal Schwann cell compartment decreased with increasing fibre size. Drastic sectorial variations in nodal gap height and local thinning-out of the brush-border became prominent features in the largest fibres. The possible in vivo organization of the nodal Schwann cell compartment is discussed. Preliminary calculations indicate that the extracellular space directly surrounding the nodal axon might be quite small and that the area open for free communication between this extracellular space and the endoneurial space might be very much restricted, measuring as little as 2% of the area of the nodal axolemma. Algorithms for calculating various nodal structural parameters are discussed.

Myelinating glia of earthworm giant axons: thermally induced intramembranous changes

The median and lateral giant axons in the ventral nerve cord of the earthworm Lumbricus terrestris are ensheathed by extensive spiral glial cell wrappings which resemble vertebrate myelin. The other, smaller, axons are encompassed by attenuated glial processes, as is typical of invertebrates. The fine structural details of the glial cells have been studied in thin sections and in replicas produced by freeze-fracturing where the intramembranous particle (IMP) populations within the lipid bilayer are visible. These consist of both low-profile IMPs as well as prominent ones 6-8 nm in diameter, scattered at random over the lipid interface in the myelinating glia. The larger IMPs on both P and E faces number about 80/mum2 at 16 degrees C in contrast to the IMP density of 400/mum2 in the other glial membranes. After acclimation to 5, 16 and 26 degrees C, the loose myelin glial membranes show variations in the density of their larger IMP population; in animals acclimated over 3 or more weeks to 5 degrees C, the number of these IMPs is significantly (P less than 0.001) less per unit area than in animals acclimated to 16 or 26 degrees C. The size of the particles at 5 degrees C is significantly (P less than 0.001) smaller than those at 16 or 26 degrees C. When animals are subjected to a sudden differential in ambient temperature, from 26 or 16 to 5 degrees C, or from 5 to 26 degrees C, and their giant axons with encompassing glia are fixed and frozen 30 min after this temperature change, the IMP population of the glial membranes remaining does not appear to alter. The differences in the IMP population of the myelinating glial membranes at different temperatures may reflect the extent to which they insulate and/or influence the velocity of impulse propagation.

Demyelination: a failure of cell communication?

A hypothesis is proposed that demyelination in both the CNS and PNS involves a failure of cell communication between the axon and oligodendrocyte/Schwann cell, as a primary event. The site of communication is assumed to be the paranodal myelin loop-axolemma membrane complex. It is postulated that "cross-talk" between the two cell types can be interrupted, and hence demyelination initiated, by pathophysiological changes in either the axon or myelinating cell. Experimental evidence in support of the hypothesis is cited in so far as it exists.

Rat optic nerve: freeze-fracture studies during development of myelinated axons

This freeze-fracture study examines the development of myelinated fibers in the rat optic nerve. Axolemma of optic nerve fibers were studied before, during, and after myelination. At birth, the optic nerve is composed entirely of non-myelinated (premyelinated) axons, while in the adult, virtually all fibers acquire compact myelin. Myelination begins at 6-8 days postparturition and proceeds rapidly, such that by 28 days of age approximately 85% of the axons are myelinated. The axolemma of premyelinated fibers from 2-day-old animals exhibits an asymmetrical partitioning of intramembranous particles (IMPs) between E- and P-fracture faces; the E-face had approximately 125 particles/micron2 and the P-face approximately 550 particles/micron2. Particle densities for premyelinated axolemma from 8, 12, 14, 16, and 28-day-old nerves were similar to those observed at 2 days. Beginning at 8-12 days postnatal, definitive association between oligodendroglial processes and axons (termed 'ensheathed' fibers) was observed. At the time of glial ensheathment, there was a 50-100% increase in the number of P-face particles; in contrast, the E-face did not display an overall increase in particle density. In certain regions, however, localized aggregations of E-face particles were observed. IMPs on P-faces of ensheathed axons had a greater mean particle size and higher percentage of 'large' (greater than 9.6 nm) particles than did IMPs on the corresponding fracture face of premyelinated fibers. Myelinated axons from 14-16 day optic nerves displayed several differences from adult myelinated fibers. The P-face of the internodal axolemma had approximately 45% fewer particles than that of adult internodal membrane, and the percentage of large IMPs on the P-face of the younger internodal membrane was approximately 50% of the value for adult internodal axolemma. E-faces of internodal axolemma from 14-16-day-old and adult animals had equivalent IMP densities and size distributions. The nodal region of myelinated axons from 14-16-day-old rats had fewer large particles on both E- and P-faces than did adult fibers, though particle densities on both fracture faces were similar for the two age groups. These studies demonstrate a clear reorganization of axon membrane structure concomitant with axo-glial ensheathment, followed by continued gradual axolemmal changes as myelination progresses.

Differentiation of the nodal and internodal axolemma in the optic nerves of neonatal rats

Axon plasma membranes (axolemma) were studied by freeze-fracture electron microscopy at stages prior to and during myelination in the optic nerves of neonatal rats. In unensheathed axons, intramembranous particles associated with the internal (P) and external (E) leaflets of the axolemma increased in number before reaching a plateau (approximately 600/micron2 in both leaflets) at about 9 days postnatally. In newly myelinated fibres, by contrast, the distribution of particles was asymmetrical; fewer particles (approximately 200/micron2) were found on the E-face and greater numbers (approximately 1400/micron2) were present on the P-face, distributions similar to those observed in mature myelinated fibres. Node-like aggregations of particles were not found in unensheathed pre-myelinated axons nor were they present in axons presumed to be ensheathed by glial cytoplasm but not yet myelinated, although nodal specializations could be easily identified in fibres with only a few turns of compact myelin. These observations show first that there is a redistribution of particles in the P- and E-faces of the internodal axolemma coincident with the onset of myelination and secondly, that nodal specializations (represented by the increased densities of E-face particles) appear after ensheathment but before the formation of compact myelin in fibres of the rat optic nerve.

Repetitive propagation of action potentials destabilizes the structure of the myelin sheath. A dynamic x-ray diffraction study

Time courses of myelin lattice swelling in toad sciatic nerves preexposed to different treatments were determined by x-ray diffraction using a one-dimensional position-sensitive detector. In the nerves supramaximally stimulated for 1 h at 200 Hz, the subsequent process of myelin swelling occurred 45.0 +/- 7.3 min (n = 24) sooner than in resting controls. Sciatic nerves incubated for 1 h in a Ringer's solution deprived of divalent cations (Ca++ and Mg++) exhibited a kinetics of swelling similar to that shown by the stimulated nerves, that is, 52.5 +/- 14.2 min (n = 6) sooner than controls preincubated for the same time in normal Ringer's solution (with divalent cations). The fact that both pretreatments supramaximal stimulation and removal of divalent cations from the perfusion solution produced a similar effect; namely, a decrease of the myelin lattice stability against swelling in distilled water, suggests that the repetitive propagation of action potentials could modify the ionic composition at either the intraperiod channel or the paranodal axoglial junction complexes.

Development of nodal and paranodal membrane specializations in amphibian peripheral nerves

Peripheral nerves from the hind legs of frog tadpoles were examined in order to ascertain the pattern of development of nodal and paranodal specializations in myelinated fibers. In thin sections the earliest detectable node-related specializations resemble "intermediate" junctions between axons and Schwann cell processes. These occur in individually ensheathed axons near the edges of the sheath segments and could represent early nodal or paranodal components or transient structures. The characteristic nodal "undercoating" is indistinct and highly variable in thickness in immature fibers and its density is lower in developing nodes than in adult nodes. Corresponding freeze-fracture replicas of developing axons demonstrate aggregates of nodal E face particles whose concentration is lower than that in the adult. Such aggregates usually occur immediately adjacent to Schwann cell indentations, even though early in development the latter may not exhibit the paracrystalline pattern seen in the adult paranodal axolemma. On rare occasions, node-like particle aggregates and presumptive nodal undercoatings have been observed without recognizable paranodal junctions or indentations nearby. However, neither specialization has been found in axons not individually ensheathed by Schwann cells. Paranodal Schwann cell loops are widely separated and irregularly arranged in the developing nodes, and the paranodal regions flanking a node usually mature asymmetrically. Differentiated paranodal junctions appear early in axons ensheathed by only a few loose Schwann cell lamellae. However, such junctions are not formed by all paranodal loops; they consistently appear first in the loops close to the node and only later in those further removed. No junctional specialization has been observed in either the axolemma or the Schwann cell membrane without the close association of the other.

Freeze-fracture ultrastructure of rat C.N.S. and P.N.S. nonmyelinated axolemma

The axolemma of nonmyelinated fibres from the corpus callosum and cerebellar cortex (C.N.S.) and the vagus nerve (P.N.S.) was investigated with freeze-fracture electron microscopy. The major observations of this study are as follows: (1) there is a highly asymmetrical distribution of intramembranous particles between the E- and P-fracture faces in both C.N.S. and P.N.S. fibres; (2) the total number of particles on the P-faces of all axonal types studied is considerably greater than that on the E-face; (3) the number of particles on the E-faces of C.N.S. axons is greater than that on the E-faces of P.N.S. axons; and (4) the percentage of large (greater than 9.6 nm) particles is greater on the E-face than on the P-face regardless of the axon studied. The results are compared with previous freeze-fracture investigations on the nodal and intermodal membranes of myelinated fibres.

Myelination-dependent axonal membrane specializations demonstrated in insufficiently myelinated nerves of the dystrophic mouse

"Dystrophic' mice of the 129/ReJ-Dy strain have a genetic defect affecting Schwann cell proliferation. Spinal nerve roots of these animals contain myelinated and unmyelinated axons in addition to groups of large "amyelinated' axons. In affected regions of the spinal roots, myelinated axons are missing their myelin sheaths. Where the myelination terminates or begins, half-nodes are created. Freeze-fracture analysis of these half-nodes shows that only the myelinated side contains rows of dimeric particles in the axonal P-face of the paranode. The P-face on the amyelinated side of a half-node, and the remainder of the amyelinated axon. contains a dense even distribution of particles, many of which are the size of dimeric-particle subunits, but only a few of which are arranged into short rows. As the long circumferential rows are not found on the unmyelinated side of the myelinated side of the half-node we conclude that the paranodal rows of dimeric particles are dependent upon myelination for their organization.

Shaking pups: a disorder of central myelination in the spaniel dog. II. Ultrastructural observations on the white matter of the cervical spinal cord

The ultrastructure of the cervical cord is described in a new canine mutant with severe hypomyelination of the C.N.S. Axons were either non-myelinated or surrounded by a myelin sheath that was markedly reduced in both its thickness and length of internode. Myelinated and non-myelinated zones were present on a single axon. There was no paucity of oligodendrocytes but many of those present contained empty or granular vacuoles within the cytoplasm. Features suggesting immaturity of myelination were commonly found at paranodes and along the internode. Abnormal inter-relationships of oligodendrocytes and astrocytes were present at many paranodes. These observations suggest an intrinsic defect of oligodendrocyte metabolism such that they are incapable of normal extension of their plasma membranes, while the cytoplasmic vacuoles may represent breakdown of defective lipids.

The node of Ranvier in early Wallerian degeneration: a freeze-fracture study

Using the freeze-fracture technique, the sciatic nerve of the rat and rabbit was examined distally at 24 h after crush, with particular reference to the node of Ranvier and paranode. The paranodes, in the majority of myelinated fibres, showed a loss of the cytoplasmic circumferential bands and longitudinal columns and their associated membrane pores which characterise the normal Schwann cell surface. Axonal changes consisting of accumulations of axoplasmic organelles occurred at both the node and paranode. At the nodes large intramembraneous particles in the axolemma (E face) appeared unchanged. Nodal Schwann cell microvilli and paranodal myelin terminal loops were generally unaffected. The findings are discussed in terms of the decrease in amplitude of the action potential which occurs in early Wallerian degeneration.

Nodal and paranodal membrane structure in complementary freeze-fracture replicas of amphibian peripheral nerves

Complementary freeze-fracture replicas of frog peripheral nerves have revealed new details of membrane structures at the node of Ranvier and paranodal axon-Schwann cell junction. At the node both E and P fracture faces of the axolemma have high particle concentrations (approximately 1350/sq. micron and 1600/sq. micron respectively) and these particles do not overlap when tracings from the respective fracture faces are superimposed. A high proportion of the E face particles are large (> 9.5 nm) and cast long shadows while the proportion of large particles in the P face is much lower. In the paranodal region the diagonal pattern of parallel rows in the junctional axolemma always has the same orientation within a given fracture face. In the E face, the parallel rows form a positive (+ 30 degrees) angle to the groove below and in the P face, a negative (-30 degrees) angle to the ridge above. This implies that the diagonal pattern derives from asymmetric subunits that are able to associate along only one axis and are unable to 'flip over' with respect to the junctional membranes.

Binding of horseradish peroxidase-alpha-bungarotoxin to axonal membranes at the node of Ranvier

The binding of horseradish peroxidase (HRP)-labeled alpha-bungarotoxin (alpha-BuTx) was investigated in rat sciatic nerve. Activity was found to be localized to the axolemma of myelinated nerve fibers at the nodes of Ranvier. Activity was also seen in other regions of the axolemma where the myelin sheath was separated from the axon by enzymatic treatment. Pretreatment of nerves with native alpha-BuTx or curare blocked the binding of HRP-alpha-BuTx to the axonal membranes. This study demonstrates binding of alpha-BuTx to axonal membranes although the nature and significance of the toxin receptor is uncertain.

The localization of sodium and calcium to schwann cell paranodal loops at nodes of Ranvier and of calcium to compact myelin

High-voltage electron microscopy (HVEM) has been used to determine the distribution of cationic precipitates in myelinated axons resulting from the application of two cytochemical techniques: a direct osmium pyroantimonate treatment for precipitating Na+, Ca2+ and Mg2+; and a 5 mM Ca2+ inclusion procedure (Oschman & Wall) for imparting electron density to Ca2+ binding sites. Electron probe wavelength spectroscopy was then used on semi-thick tissue sections to identify the species of ions present in the following regions: Schwann cell paranodal loops, axoplasm at the node, compact myelin and extracellular matrix. With these combined procedures we were able to localize elevated concentrations of both Na+ and Ca2+ to cytoplasmic compartments of the Schwann cell paranodal loops, as well as to detect the presence of Ca2+ at elevated levels in compact myelin. The involvement of the Schwann cell paranodal loops in providing a source and/or sink for Na+ involved in impulse conduction is suggested by these results, and the significance of such a role is discussed. A role for Ca2+ in the formation and stabilization of myelin lamellae is also suggested.

Rows of dimeric-particles within the axolemma and juxtaposed particles within glia, incorporated into a new model for the paranodal glial-axonal junction at the node of Ranvier

Using freeze-fracture techniques, we have analyzed the glial-axonal junction (GAJ) between Schwann cells and axons in the peripheral nervous system, and between oligodendrocytes and axons in the central nervous system of the rat. We have identified a new set of dimeric-particles arranged in circumferential rows within the protoplasmic fracture faces (P-faces) of the paranodal axolemma in the region of glial-axonal juxtaposition. These particles, 260 A in length, composed of two 115-A subunits, are observed in both aldehyde-fixed and nonfixed preparations. The rows of dimeric-particles within the axonal P-face are associated with complementary rows of pits within the external fracture face (E-face) of the paranodal axolemma. These axonal particles are positioned between rows of 160-A particles that occur in both fracture faces of the glial loops in the same region. We observed, in addition to these previously described 160-A particles, a new set of 75-A glial particles within the glial P-faces of the GAJ. These 75-A particles form rows that are centered between the rows of 160-A particles and are therefore superimposed over the rows of dimeric-particles within the paranodal axolemma. Our new findings are interpreted with respect to methods of specimen preparation as well as to a potential role for the paranodal organ in saltatory conduction. We conclude that this particle-rich junction between axon and glia could potentially provide an intricate mechanism for ion exchange between these two cell types.

Molecular specializations of the axon membrane at nodes of Ranvier are not dependent upon myelination

Nodes of Ranvier from normal and 'dystrophic' mice have been examined with quantitative freeze-fracture electron microscopy. Regions of nodal, paranodal and interparanodal axolemma of normal fibres are clearly distinguishable on the basis of particle size distributions in electron micrographs of freeze-fractured replicas. Protoplasmic fracture faces of normal nodes of Ranvier, contain approximately 40% 100 A particles and about 25% elongated particles 150 by 250 A. Paranodal and interparanodal membranes contain a more uniform distribution of smaller diameter particles. 'Dystrophic', mice of the 129/ReJ-Dy strain have a genetic defect of Schwann cell development and myelinogenesis. Axons of the sciatic and deep peroneal nerves in dystrophic mice, which appear to be normally myelinated, possess approximately the same distributions of particles as axons in normal mice. However, in affected regions of the ventral and dorsal roots, Schwann cell wrappings may be missing, creating heminodes of Ranvier where the myelination terminates or begins again. At such heminodes, there is a circular band of axonal membrane which bears particles of sizes and packing densities similar to that found at normal nodes. High voltage electron microscopic examination of 0.25--1 micron thick sections from these hemi-nodal regions reveals the presence of a filamentous layer beneath the particle-rich membrane. In addition, completely amyelinated regions of root axons contain particle patches having size-density distributions similar to that of both normal and hemi-nodal membranes. Thus, the nodal membrane displays a characteristic particle-size distribution profile. The occurrence of this particle profile does not appear to be dependent upon the presence or absence of Schwann cells. These observations suggest that the functions subserved by the numerous particles at the node of Ranvier are not dependent upon myelination for their local differentiation within the axonal membrane.