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

Uncompacted myelin lamellae in dysglobulinemic neuropathy

Uncompacted lamellae, located preferentially in inner layers of myelin sheath, were observed in biopsied sural nerves of 3 cases of dysglobulinemic neuropathy, in which the main pathological findings of myelinated fibers were those of segmental demyelination and remyelination, and axonal degeneration with concurrent marked decrease of myelinated fiber density. The presence of uncompacted myelin lamellae is well explained by the irregular distribution of adaxonal, incisural and paranodal cytoplasm of the Schwann cell.

The periaxonal space in an experimental model of neuropathy: the mutant Syrian hamster with hindleg paralysis

The periaxonal space of peripheral myelinated axons was studied in the mutant Syrian hamster with hindleg paralysis, an experimental model of neuropathy. Despite pronounced alterations of the axon and the surrounding sheath, sometimes leading to demyelination, the periaxonal space showed remarkable resistance to change in most instances. When the space was widened as the result of the infiltration of extracellular fluid, the axon was found at the periphery of the enlarged inner perimeter of the sheath. Even under these extreme conditions the axon maintained close to normal distance from the inner collar of cytoplasm. The significance of these findings with regard to both the normal anatomical relationship within the sheath and to the mechanisms of demyelination are discussed.

Evidence for a "dying-back" gliopathy in demyelinating disease

Recurrent demyelination was produced in mice by Cuprizone administration. During the second course of Cuprizone, the animals showed greater resistance to the toxin and demyelination occurred slowly and was complete only after prolonged periods. The earliest changes in oligodendrocytes occurred in the most distal processes, the inner cytoplasmic tongues, which showed degenerative changes 3 to 4 weeks before degeneration of the oligodendrocyte cell bodies or demyelination occurred. The results show for the first time that in demyelinating disease, a "dying-back" process similar to that described in axonal disease can affect the oligodendrocyte.

Interlamellar tight junctions of central myelin. I. Developmental mechanisms during myelogenesis

The process of myelination in the central nervous system (CNS) of the rat (optic nerve) was studied with the freeze-fracturing technique and ultrathin sectioning to obtain information on the developmental mechanisms of interlamellar tight junctions. Using a tilting cartridge for analysis of thin sections, it could be demonstrated that during the initial phase of wrapping a tight junction formation develops between the joining tips of the oligodendrocytic process. In tannic acid-stained samples these junctions appear as typical quintuple-layered membrane fusions, while in potassium permanganate-stained material membrane thickenings between the apposing glial tips are prevalent. The latter configuration represents the characteristic feature of the so-called radial component of central myelin. Using the freeze-fracturing technique, a biphasic mode of the myelinic tight junction assembly was detected. It is suggested that tight junctions represent a prerequisite of the myelination process.

The precise geometry of large internodes

The interrelations, in each given internode, between internodal length, axon caliber, axoplasmic volume and several parameters of sheath thickness were studied in a set of 37 large internodes from the sciatic nerve of the German shepherd dog, using multiple electron-microscopic measurements of isolated fibers. Internodal length varied in a nonlinear relation with axon caliber. Nonlinear relations were also found for the number of lamellae in the sheath as related to either axon caliber or to internodal length, particularly for large internodes: the deviations observed for axon caliber (thick fibers had fewer myelin lamellae than would correspond to axon caliber) were the opposite of for internodal length (thick fibers had more lamellae than would correspond to internodal length). A near linear relationship was found if the volume of myelin per internode was related to both the length and the circumference of the axon segment ensheated by it, i.e., axolemmal surface area. The deviations in the proportions of large internodes probably indicate the existence of upper limits of internodal growth conditioned by the enormous metabolic demands placed on the individual Schwann cell by the growth of internodes beyond 1.5--2 mm length or 100 thousand micrometer3 of myelin, respectively. The demands on myelin metabolism from unabated internodal growth and the resultant logistic problems of the Schwann cells are demonstrated from measurement of observed internodes and calculations of extrapolated growth.

Remodelling during remyelination in the peripheral nervous system

Morphologic analysis of longitudinal sections of lumbar anterior root tissue from rabbits at different stages of chronic experimental allergic encephalomyelitis has revealed new information on Schwann cell behaviour during early remyelination. Some short remyelinated internodes were displaced laterally towards nodes as other internodes elongated in a stepise fashion to occupy the vacated axonal segments. This suggests that a Schwann cell possesses the ability to remodel its myelin sheath after the initial establishment of the internode. Subaxolemmal densification was found to be formed wherever pairs of Schwann cells overlapped and gap junctions were observed between young Schwann cells.

The incubation period of multiple sclerosis

Movement of myelin through the spiral laminar arrangement of the myelin sheaths of neurones might act as a biological clock in determining the prolonged incubation period of multiple sclerosis. A defect incorporated peripherally in the myelin sheath as a result of neonatal virus infection might be revealed years later when the defective myelin reaches the axolemma and provokes an immunological reaction. This process could, directly or indirectly, be prevented by the effects of solar radiation, thus accounting for the geographical distribution of the disease.

Polyaxonal myelination in developing dystrophic and normal mouse nerves

Myelin-forming Schwann cells in the peripheral nervous system characteristically surround and myelinate only single axons. Polyaxonal myelination is an anomaly of this one-to-one relationship whereby one normal-appearing Schwann cell myelinates multiple axons. We examined the ventral roots and the proximal sciatic and posterior tibial nerves of developing normal mice and of dy2J/dy2J dystrophic mice with proximal failure of myelination. Polyaxonal myelination was a rare feature in normal nerves. Examples of polyaxonal myelination were observed six times more often in dystrophic than in normal mice and were most abundant in proximal sciatic nerves. Polyaxonal myelination could result from either an axonal or a Schwann-cell abnormality, or it may be the nonspecific response of uncommitted Schwann cells to an early failure of myelination.

Atypical axon-Schwann cell relationships in the common peroneal nerve of the dystrophic mouse: an ultrastructural study

Several atypical features of myelination of the peripheral nervous system are reported in common peroneal nerve of dystrophic mice (129 Re J dy/dy): (i) central nervous system-like contact between myelin sheaths of adjacent nerve fibres; (ii) nodes and internodes of myelinated fibres enwrapped with cytoplasmic processes of Schwann cells from adjacent nerve fibres; (iii) Schwann cells of adjacent nerve fibres co-operating in formation of a single myelin sheath; and (iv) a single Schwann cell myelinating two separate axons. In view of the presence of similar features of myelination in the central nervous system, where the myelin producing cells lack basement membrane, we suggest that in the dystrophic peripheral nerves the development of these features can be attributed to the partial deficiency of the Schwann cell basement membrane. Two types of widened nodes of Ranvier are also identified: (i) nodes with paranodal damage; and (ii) nodes without paranodal damage. In addition, abnormal features of myelination are described which are likely to represent altered Schwann cell/axon relationships during demyelination and remyelination and/or decreased myelinating ability of Schwann cells. We interpret these findings as indicating a metabolic disorder of Schwann cells. They provide an experimental model for the investigation of factors involved in the origin and maintenance of the structural organization of peripheral nerve.

Remyelination in multiple sclerosis

Chronic plaques in central nervous system tissue fixed by in situ perfusion for electron microscopy were examined for evidence of remyelination in 2 patients with multiple sclerosis (MS). Fibers with abnormal central myelin sheaths of several types were found at the margins of most of the plaques studied. The most common of these were: (1) the presence of bare stretches of axon between contiguous internodes, (2) the presence of thin paranodes, (3) internodes which changed markedly in thickness along their length due to premature termination of superficial or deep myelin lamellae that ended as hypertrophic lateral loops, and (4) abnormally thin internodes which were of uniform thickness along their length, which were shorter than normal, and which terminated in the form of normal nodal complexes. The finding of internodes of the last type at the edges of many plaques indicates that remyelination by oligodendrocytes can occur in the adult human CNS and that it is common in some cases of MS, although limited in its extent.

Characteristics of interhemispheric impulse conduction between prelunate gyri of the rhesus monkey

Cells of origin of the corpus callosum (callosal efferent neurons) in prelunate gyrus (area OA) of the rhesus monkey were studied using electrophysiological techniques. Monkeys were chronically prepared and callosal efferent neurons were identified by their antidromic activation following electrical stimulation of the contralateral prelunate gyrus and/or the splenium of the corpus callosum. Interhemispheric antidromic latencies ranged from 2.6--18.0 ms (median = 7.0 ms) while the conduction velocity along the length of the axon ranged from 2.8 to 22.5 M/s (median = 7.4 M/s) while the conduction velocity along the length of the axon ranged from 2.8 to 22.5 M/s (median = 7.4 M/s. Following the relative refractory period of a single prior impulse, all but one of 61 callosal efferent neurons studied showed a supernormal period of increased axonal conduction velocity and excitability. Following several prior impulses, the supernormal period was followed by a subnormal period of decreased axonal conduction velocity and excitability, which, depending on the number of prior impulses, lasted from several hundred ms to nearly 2 min.

Quantitative studies on the maturation of central and peripheral parts of individual ventral motoneuron axons. I. Myelin sheath and axon calibre

The entire lengths of central and peripheral myelin sheaths on the same ventral motoneuron axons of young rats were studied during the rapid early phase of myelination which occurs during the first 2 weeks after birth. Sheath thickness varied along central internodes at all ages, though this variation was less in the more mature sheaths. Usually there was a single level of maximum thickness, most often near the middle of the internode, but frequently close to one end. Some sheaths varied little in thickness along their lengths; a few were bimodal. The number of turns decreased on either side of the maximum as the sheath was traced towards the bounding nodes. Comparison of serially adjacent sheaths along the same axon showed no relationship between their mean thickness or patterns of longitudinal thickness variation. Patterns of thickness variation in peripheral sheaths were very similar to those found centrally. At a given age, however, peripheral sheaths were less variable along more than twice that of central internodes belonging to the same fibre. At first, the unrolled glial unit was oval in outline while the unrolled Schwann cell was trapezoidal. Subsequently, both eventually became trapezoidal. The calibre of central and peripheral stretches of the same ventral motoneuron axons were also compared with one another in young rats during the first 2 weeks after birth. At each age, the great majority of axons were thicker peripherally than centrally. The mean circumference of the peripheral segment exceeded that of the central by a factor which averaged 1.3 over the period studied. Circumference varied from one level to another along both central and perpheral stretches of the same axons. The degree of this variation differed considerably from one axon to another. In many central fibres, and in all peripheral stretches of fibres, calibre variation followed no particular pattern. However, the calibre of some central axon segments gradually increased as the fibre was traced distally towards the cord surface. A proportion of axons was dilated just deep to the cord surface. These dilatations were frequently myelinated. No association was found between patterns of longitudinal variation in axon calibre and in sheath thickness along the same fibre.

An experimental analysis of interlamellar tight junctions in amphibian and mammalian C.N.S. myelin

The distribution of interlamellar tight junctions was examined in myelin sheaths of Xenopus tadpole optic nerve and rabbit epiretinal tissue fixed with aldehydes, postfixed with osmium ferrocyanide and embedded in a water-soluble medium, Durcupan. Intramyelinic zonulae occludentes were clearly formed by fusion of adjacent intraperiod lines which corresponded to the external leaflets of oligodendrocytes. These occurred in register with other tight junctions present within successive lamellae and appeared as a series of radial lines extending either partially or totally across the thickness of the myelin sheath. This distribution of zonulae occludentes corresponded with that of tight junctional particle strands observed in freeze-fracture replicas. Analysis of intramyelinic vacuolation induced by hexachlorophene (HCP) intoxication indicated that lamellar splitting was frequently limited by the tight junctions. The intramyelinic zonulae occludentes also restricted the diffusion of colloidal lanthanum which had penetrated the myelin intraperiod gap following in vivo perineural injection. The results of this study provide evidence favouring a correspondence between interlamellar tight junctions and the 'radial component' of myelin described earlier by other investigators. Furthermore, observations of swollen myelin sheaths, resulting from HCP intoxication, suggest that these junctions may play a major role in maintaining myelin sheath integrity and limiting the extent of breakdown during certain pathological conditions.

Autoimmune and virus-induced demyelinating diseases. A review

Patterns of demyelination are described in several autoimmune and virus-induced demyelinating diseases of the peripheral and central nervous system. Myelin can be destroyed by injuries that affect either the myelin-supporting cells and/or the myelin lamellae. After destruction of the supporting cells, the related disintegrating sheaths are stripped off axons by invading phagocytes. Virus-induced cytolysis can occur with or without participation of immune responses, as demonstrated in subacute sclerosing panencephalitis and progressive mutlifocal leukoencephalopathy, respectively. Autoimmune demyelination is characterized by disintegration of myelin sheaths in periventular, mononuclear cell infiltrates. Myelin lamellae rather than the myelin-supporting cells are the target of the allergic reaction. The lamellae are lysed in focal areas when in contact with presumably sensitized mononuclear cells. The damaged sheaths are then removed in a nonspecific manner by invading macrophages that strip the myelin remnant off the axons. This sequence of changes is best revealed in experimental and human autoimmune demyelination of peripheral nerves, ie, allergic neuritis and idiopathic polyneutris (the Guillain-Barré syndrome). Autoimmune demyelination triggered by virus infection is described in Marek's disease and postinfectious Theiler's virus myelitis. Changes in canine distemper are discussed with reference to both autoimmune and virus-induced demyelination. The observations are compared with lesions in multiple sclerosis, the most common human demyelinating disease of unknown etiology.

Quaking mouse: an ultrastructural study of the peripheral nerves

Ultrastructural study of the peripheral nervous system of quaking mice has revealed several unusual features in the pattern of myelination in addition to hypomyelination. These are 1) the presence of 'atypical' Schmidt-Lanterman incisures, 2) irregularities of the nodal and internodal termination of Schwann cell cytoplasm and 3) the presence of non-myelinated segments without associated Schwann cell processes, but covered by basal lamina. In view of the observation of similar features during the development of the P.N.S. and also during P.N.S. remyelination, we suggest that these features are the results of modified myelination due to a defect in the control mechanisms necessary for normal myelination.

Status spongiosus of rat central nervous system induced by actinomycin D

The effect on central myelin of Actinomycin D, an RNA--and, secondarily, a protein-synthesis inhibitor, has been studied by light and electron microscopy. The intracranial injection of this drug produced an extensive status spongiosus of the white matter in the cerebrum, cerebellum, brain stem and optic nerve within 48 h. The status spongiosus was due to vacuole formation within the myelin sheath and to enlargement of the extracellular space. Three types of vacuoles were observed: (a) the most common varieties formed between the inner tongue and the remainder of the myelin sheath; (b) a second variety formed by enlargement of the periaxonal space with separation of the axon from its myelin sheath, and (c) a less common type of vacuolization was due to splitting of the myelin lamellae at the interperiod line to form large intramyelinic vacuoles. Myelinic vacuoles were preceded by nuclear and cytoplasmic changes in oligodendrocytes, which included nucleolar segregation, disaggregation, and diminution in number of ribosomes. These changes were similar to those previously reported in a variety of cells exposed to Actinomycin D. It is suggested that myelin vacuoles result secondarily from the Actinomycin D inhibitory effect on oligodendroglial RNA--and protein-synthesis, rather than from a direct effect of this drug on the myelin sheath.

[The Schmidt-Lantermann incisures of the myelin sheath of Mauthner axons: site of longitudinal myelin growth]

The myelin sheath of the Mauthner axons in the spinal cord of young and adult fish belonging to the family Cyprinidae was examined by phase and electron microscopy. The sheath thickened considerably with age, the number of lamellae increasing from 200-230 in young fish (length 2--4 cm) to 250-300 in adult animals (length 20-25 cm). During this growth, the myelin sheath remained fairly compact and of optimal thickness for impulse propagation, the axon/fiber thickness ratio being 0.67-0.72 in young and 0.76-0.78 in adult fish. As already observed by previous investigators, the Mauthner axons lacked nodes of Ranvier. However, Schmidt-Lantermann's clefts (SLC) were present at irregular intervals. Within the SLC, parallel arranged osmiophilic bands of variable length and with a thickness of approx. 42-44 nm were observed to form a transitional zone between the cytoplasmic areas of the oligodendrocyte and the typical myelin structure. Between such adjacent 'D-bands', which thinned out to build up the major dense lines, an electron translucent area measuring approx. 25--26 nm in width contained a thin 'I-band' (8--10 nm) which was continuous with the intraperiod line. It is speculated that, in the Mauthner axon, the SLCs may be sites where freshly synthesized myelin is added to the lamellae already present, thus permitting their longitudinal growth.

The growth and myelination of central and peripheral segments of ventral motoneurone axons. A quantitative ultrastructural study

This study compares the growth and myelination of those parts of cervical ventral motoneurone axons in the spinal cord (the intramedullary segments) and in the ventral roots of fetal and young rats (up to 21 days postnatal). The same fibre bundles are examined centrally and peripherally. Myelination begins centrally and peripherally at about birth. However, the peripheral segments of some fibres may begin to become myelinated before the central. Over the first 3 weeks after birth the minimum circumference of peripheral segments of myelinated axons remains relatively constant at 3 mum but that of central segments falls from 2.5 mum to just over 1 mum. Axons within the same fibre bundles tend to be thinner and less heavily myelinated centrally than peripherally. With ageing, axon circumference becomes more strongly correlated with sheath thickness. The thickness of the sheath surrounding an axon of a given circumference does not differ statistically from one age to another or between central and peripheral segments. Studies of myelin sheath growth rate show that in the early stages glial and Schwann cells vary independently of one another in the rates at which they add new turns to sheaths around central and peripheral segments of axons in the same bundles.

Membrane particles on fracture faces of frozen myelin

Freeze-fracture electron microscopy reveals constant and widespread presence of membrane particles on the fracture faces of frozen myelin. In unfixed myelin frozen shortly after dissection the distribution of the particles is uniform. In glutaraldehyde-fixed and/or glycerol-impregnated myelin the particles frequently occupy a network interspersed with circumscribed particle-free areas of variable dimension. In these membranes the network of particles is propagated radially across many lamellae. Particle-rich areas are closely apposed and contrast with frequent delamination of adjacent particle-free regions. We propose that, as in other plasma membranes, the particles of myelin represent protein-containing structures intercalated across the hydrophobic matrix of a membrane with bilayer organization. Our results indicate that these structures contain sites which mutually interact at the surface of apposed membranes and may be important in maintaining the organizational integrity of myelin.

Pathology of the early lesion in multiple sclerosis

Ideas concerning the nature of multiple sclerosis continue to be strongly influenced by the unusual morphology of the disease. A review of classic histology studies, however, reveals that there is less agreement than might be supposed concerning several important histiological features of the early lesion. Electron microscopy of brain biopsies, of immersion fixed autopsy tissue and of autopsy tissue fixed by early in situ brain perfusion suggests that the mechanism of demyelination in multiple sclerosis may be an unusual one that involves a progressive reduction in the number of myelin lamellae around nerve fibers in the vicinity of cells of uncertain origin that contain filamentous and multilamellated cytoplasmic inclusions unlike the usual pleomorphic inclusions seen in myelin phagocytes. Lymphocytes are not directly involved in this process but are observed to contact the inclusion material following its delivery to the Virchow-Robin spaces. The putative neurogenic or viral antigen in multiple sclerosis may be contained in this material. The occurrence of filamentous nuclei in early lesions fixed by immersion is an unrelated phenomenon, which may be an autolytic or drug induced artifact although this has not yet been established.

The node of Ranvier in experimental allergic neuritis: an electron microscope study

Pathological breakdown of the node of Ranvier in experimental allergic neuritis (EAN) in the rabbit was examined by electron microscopy. The study was concerned principally with the mechanism of disruption of paranodal myelin. Two types of change in myelin structure were observed, both apparently related to the presence of cytoplasmic processes of macrophages: (1) a vesicular disorganization of myelin lamellae and (2) a separation of myelin lamellae. Both methods of disorganization were used by macrophages as they apparently lifted off the myelin terminal loops of the paranode from the underlying axon and also penetrated between myelin loops and adjoining myelin lamellae. In some pathologically damaged nodes of Ranvier no macrophages were observed. The findings raise important immunological and electrophysiological questions concerning the involvement of the macrophage in the autoimmune response and the effect of the loss of paranodal myelin on nerve conduction.

A quantitative study of anterior root fibres during early myelination. II. Longitudinal variation in sheath thickness and axon circumference

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Membrane structure: some general principles

The arrangement of lipids and some proteins in the erythrocyte membrane has been discussed. The conclusions from this are listed here as a set of general guidelines for the structure of membranes of higher organisms: some of these rules may be wrong. But at this stage it seems useful to sharpen our thoughts in this way and thereby focus attention on various specific points. 1) The basis of a membrane is a lipid bilayer with (i) choline phospholipids and glycolipids in the external half and (ii) amino (and possibly some choline) phospholipids in the cytoplasmic half. There is effectively no lipid exchange across the bilayer (unless enzymatically catalyzed) (68). 2) Some proteins extend across the bilayer. Where this is so, they will in general have carbohydrate on their surface remote from the cytoplasm. This carbohydrate may prevent the protein diffusing out of the membrane into the cytoplasm; it acts as a lock on the protein. 3) Just as lipids do not flip-flop, proteins do not rotate across the membrane. Lateral motion or rotation of lipids and proteins in the plane of the bilayer may be expected. 4) Most membrane protein is associated with the inner, cytoplasmic, urface of the membrane. Proteins are not usually associated exclusively with the outer half of the lipid bilayer. 5) Membrane proteins are a special class of cytoplasmic proteins, not of secreted proteins.

Transitory traumatic paraplegia: electron microscopy of early alterations in myelinated nerve fibers

✓ The white matter of the monkey spinal cord was examined by electron microscopy during the first 4 hours following a contusion sufficient to produce a transitory paraplegia. At 5 min after injury the myelinated nerve fibers resembled those of the control animals. By 15 and 30 min after contusion, selected fibers were noted to have moderately enlarged periaxonal spaces. Attenuated myelin sheaths, splaying of the myelin lamellae, and a marked increase in the periaxonal spaces were present in affected nerve fibers at 1 hr following trauma. By 4 hrs after contusion approximately one-fourth of the fibers showed breakage of the myelin sheaths and consequent denuding of axons or marked attenuation of the myelin sheaths, greatly enlarged periaxonal spaces, and degeneration of the associated axons.

An electron microscopic study of central and peripheral nodes of Ranvier

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