THE FINE STRUCTURAL ORGANIZATION OF NERVE FIBERS, SHEATHS, AND GLIAL CELLS IN THE PRAWN, PALAEMONETES VULGARIS

In view of reports that the nerve fibers of the sea prawn conduct impulses more rapidly than other invertebrate nerves and look like myelinated vertebrate nerves in the light microscope, prawn nerve fibers were studied with the electron microscope. Their sheaths are found to have a consistent and unique structure that is unlike vertebrate myelin in four respects: (1) The sheath is composed of 10 to 50 thin (200- to 1000-A) layers or laminae; each lamina is a cellular process that contains cytoplasm and wraps concentrically around the axon. The laminae do not connect to form a spiral; in fact, no cytoplasmic continuity has been demonstrated among them. (2) Nuclei of sheath cells occur only in the innermost lamina of the sheath; thus, they lie between the sheath and the axon rather than outside the sheath as in vertebrate myelinated fibers. (3) In regions in which the structural integrity of the sheath is most prominent, radially oriented stacks of desmosomes are formed between adjacent laminae. (4) An ~200-A extracellular gap occurs around the axon and between the innermost sheath laminae, but it is separated from surrounding extracellular spaces by gap closure between the outer sheath laminae, as the membranes of adjacent laminae adhere to form external compound membranes (ECM's). Sheaths are interrupted periodically to form nodes, analogous to vertebrate nodes of Ranvier, where a new type of glial cell called the "nodal cell" loosely enmeshes the axon and intermittently forms tight junctions (ECM's) with it. This nodal cell, in turn, forms tight junctions with other glial cells which ramify widely within the cord, suggesting the possibility of functional axon-glia interaction.     

THE FORMATION AND STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

The development and structure of myelin sheaths have been studied in the optic nerves of rats and of Xenopus laevis tadpoles. Both potassium permanganate- and osmium-fixed material was examined with the electron microscope. In the first stage of myelinogenesis the nerve fibre is surrounded by a cell process which envelops it and forms a mesaxon. The mesaxon then elongates into a loose spiral from which the cytoplasm is later excluded, so that compact myelin is formed. This process is similar to myelinogenesis in the peripheral nervous system, although in central fibres the cytoplasm on the outside of the myelin is confined in a tongue-like process to a fraction of the circumference, leaving the remainder of the sheath uncovered, so that contacts are possible between adjacent myelin sheaths. The structure of nodes in the central nervous system has been described and it is suggested that the oligodendrocytes may be the myelin-forming cells.     

ELECTRON MICROSCOPIC OBSERVATIONS OF RAT AND MOUSE CEREBELLUM IN TISSUE CULTURE

Closely ordered stages of myelin formation in cultures of newborn rat and mouse cerebellum, selected by direct light microscopy, were studied with the electron microscope. Electron micrographs of these cultures reveal the presence of neurons, axons, neuroglia, microglia, and ependymal cells. The appearance of the neuron is identical to that previously described in vivo. The neuroglial cell has long, branching processes, and its cytoplasm is characterized by packets of long, narrow fibrils. During myelin formation, a glial cell process surrounds the axon. This process may form an internal mesaxon and may spiral for several turns around the axon. Other glial cell processes may interdigitate with or overlay the innermost process to contribute to the multilamellated structure. The glial processes flatten and the cytoplasmic surfaces of the cell membrane come into contact to form the lamellae of the myelin sheath. These adhesions may be temporarily incomplete as evidenced by sequestered islands of glial cytoplasm among the myelin lamellae. Ultimately, a compact, apparently spiral, myelin sheath is formed. These findings are discussed in relation to in vivo central myelin formation.     

Serial Observations on Patterns of Growth, Myelin Formation, Maintenance and Degeneration in Cultures of New-Born Rat and Kitten Cerebellum

New-born rat and kitten cerebellum may be maintained for prolonged periods (over 5 months) in the Maximow assembly if explanted on to a coverslip previously coated with a thin gel of reconstituted rat tail collagen and fed a glucose-enriched "natural" medium. After a 2 week period of adjustment and early outgrowth, most cultures exhibit myelin formation. Axons located within the surrounding neuroglial sheet of the explant area myelinate. The sheaths are first evident as long, unsegmented, smooth, parallel, refractile lines. Simultaneously, neuronal nuclei tend to assume central positions and powdery granules of Nissl substance and lipoid materials begin to accumulate within the cytoplasm. During prolonged maintenance, axons may increase in width and the myelin may thicken. Some exhibit transient irregularities and swellings. Degeneration of some axons occurs manifested either by (a) progressive swellings and distortions of the myelin sheath and thinning of intervening portions of the axons which finally yield, leaving the swellings as myelin bodies, or by (b) small aneurysm-like distortions of myelin sheaths on thinning axons which become dull, irregular, and thread-like filaments beaded by the former herniations. The observations are compared with previous studies of in vitro and in vivo myelin formation with particular reference to neuronal-neuroglial relationships.     

The Structure of Myelin Sheaths in the Central Nervous System of Xenopus laevis (Daudin)

The structure of myelinated nerve fibres has been studied in the spinal cord and optic nerve of the tadpoles of Xenopus laevis. Potassium permanganate-fixed material was examined with the electron microscope. The myelin sheath itself is made up of spirally arranged lamellae in which the intraperiod and dense lines alternate. Inside the myelin sheath an inner cytoplasmic process surrounds the axon and where the external surfaces of its bounding membrane come together an internal mesaxon is formed. The intraperiod line begins within the mesaxon and the dense line usually begins in the same region by apposition of the cytoplasmic surfaces of the membrane. The width of each lamella is 140 A. The outer line in the sheath is the dense line and this terminates in a tongue where the cytoplasmic surfaces of the myelin-forming glial cell separate. Thus, central myelin in Xenopus tadpoles is arranged in the same way as peripheral myelin, the only difference being that in central fibres, cytoplasm on the outside of the sheath is confined to that present in the tongue. For this reason adjacent central sheaths come into apposition without any intervening material being present. When this occurs an intraperiod line is formed between them.     

Correction

The Schwann cell myelin sheath is a multilamellar structure with distinct structural domains in which different proteins are localized. Intracellular dye injection and video microscopy were used to show that functional gap junctions are present within the myelin sheath that allow small molecules to diffuse between the adaxonal and perinuclear Schwann cell cytoplasm. Gap junctions are localized to periodic interruptions in the compact myelin called Schmidt–Lanterman incisures and to paranodes; these regions contain at least one gap junction protein, connexin32 (Cx32). The radial diffusion of low molecular weight dyes across the myelin sheath was not interrupted in myelinating Schwann cells from cx32-null mice, indicating that other connexins participate in forming gap junctions in these cells. Owing to the unique geometry of myelinating Schwann cells, a gap junction-mediated radial pathway may be essential for rapid diffusion between the adaxonal and perinuclear cytoplasm, since this radial pathway is approximately one million times faster than the circumferential pathway.     

THE STRUCTURAL BASIS OF CILIARY BEND FORMATION : Radial Spoke Positional Changes Accompanying Microtubule Sliding

The sliding microtubule model of ciliary motility predicts that cumulative local displacement (Δl) of doublet microtubules relative to one another occurs only in bent regions of the axoneme. We have now tested this prediction by using the radial spokes which join the A subfiber of each doublet to the central sheath as markers of microtubule alignment to measure sliding displacements directly. Gill cilia from the mussel Elliptio complanatus have radial spokes lying in groups of three which repeat at 860 Å along the A subfiber. The spokes are aligned with the two rows of projections along each of the central microtubules that form the central sheath. The projections repeat at 143 Å and form a vernier with the radial spokes in the precise ratio of 6 projection repeats to 1 spoke group repeat. In straight regions of the axoneme, either proximal or distal to a bend, the relative position of spoke groups between any two doublets remains constant for the length of that region. However, in bent regions, the position of spoke groups changes systematically so that Δl (doublet 1 vs. 5) can be seen to accumulate at a maximum of 122 Å per successive 860-Å spoke repeat. Local contraction of microtubules is absent. In straight regions of the axoneme, the radial spokes lie in either of two basic configurations: (a) the parallel configuration where spokes 1–3 of each group are normal (90°) to subfiber A, and (b) the tilted spoke 3 configuration where spoke 3 forms an angle (θ) of 9–20°. Since considerable sliding of doublets relative to the central sheath (~650 Å) has usually occurred in these regions, the spokes must be considered, functionally, as detached from the sheath projections. In bent regions of the axoneme, two additional spoke configurations occur where all three spokes of each group are tilted to a maximum of ± 33° from normal. Since the spoke angles do not lie on radii through the center of bend curvature, and Δl accumulates in the bend, the spokes must be considered as attached to the sheath when bending occurs. The observed radial spoke configurations strongly imply that there is a precise cycle of spoke detachment-reattachment to the central sheath which we conclude forms the main part of the mechanism converting active interdoublet sliding into local bending.     

THE FINE STRUCTURE OF NERVE CELL BODIES AND THEIR MYELIN SHEATHS IN THE EIGHTH NERVE GANGLION OF THE GOLDFISH

The eighth cranial nerve ganglion consists of bipolar nerve cell bodies each occupying part of an internodal segment. The perikaryal sheaths range from a single layer of Schwann cell cytoplasm on the smallest cells to typical thick compact myelin on the largest. On most perikarya, the sheath displays an intermediate form, consisting of multiple layers of Schwann cell cytoplasm (loose myelin), or of loose and compact myelin continuous with each other. Internodes beyond the one containing the cell body bear only compact myelin. In loose myelin the thickness of each layer of Schwann cell cytoplasm is about 100 A. It may be much greater (~ 3000 A) particularly in the outermost layers of the sheath, or the cytoplasm may thin and even disappear with formation of a major dense line. The cytoplasmic layers are separated from each other by a light zone, 40 to 200 A wide, which in its broader portions may contain an intermediate line. Desmosomes sometimes occur between lamellae. In addition to the usual organelles, the perikaryal cytoplasm contains granular and membranous inclusions. Large cells covered by compact myelin have a consistently higher concentration of neurofilaments, and some of the largest cells, in addition, show a reduced concentration of ribosomes. The functional significance and possible origins of perikaryal myelin sheaths are discussed.     

A TIME-SEQUENCE AUTORADIOGRAPHIC STUDY OF THE IN VIVO INCORPORATION OF [1, 2-3H]CHOLESTEROL INTO PERIPHERAL NERVE MYELIN

A time-sequence study of the incorporation and distribution of cholesterol in peripheral nerve myelin was carried out by electron microscope autoradiography. [1,2-³H]Cholesterol was injected into 10-day old mice and the sciatic nerves were dissected out at 10, 20, 40, 60, 90, 120, and 180 min after the injection. 20 min after injection the higher densities of grains due to the presence of [³H]cholesterol were confined to the outer and inner edges of the myelin sheath. Practically no cholesterol was detected in the midzone of the myelin sheath. 1 ½ h after injection, cholesterol showed a wider distribution within the myelin sheath, the higher densities of grains occurring over the two peripheral myelin bands, each approximately 3,100 Å wide. Cholesterol was also present in the center of the myelin sheath but to a considerably lesser extent. 3 h after injection cholesterol appeared homogeneously distributed within the myelin sheath. Schwann cell and axon compartments were also labeled at each time interval studied beginning 20 min postinjection. These observations indicate that preformed cholesterol enters myelin first and almost simultaneously through the inner and outer edges of the sheath; only after 90 min does the density of labeled cholesterol in the central zone of myelin reach the same density as that in the outer and inner zones. These findings suggest that cholesterol used by the nerve fibers in the formation and maintenance of the myelin sheath enters the lamellae from the Schwann cell cytoplasm and from the axon. The possibility of a bidirectional movement of molecules, i.e. from the Schwann cell to the axon and from the axon to the Schwann cell through the myelin sheath, is noted. The results are discussed in the light of recent observations on the exchange, reutilization, and transaxonal movement of cholesterol.     

ULTRASTRUCTURAL STUDY OF REMYELINATION IN AN EXPERIMENTAL LESION IN ADULT CAT SPINAL CORD

This report presents ultrastructural observations on the cytological events that attend myelin formation occurring in the wake of demyelination in adult cat spinal cord. Lesions were induced in subpial cord by cerebrospinal fluid (c.s.f.) exchange (1, 2). Tissue from eleven cats at nine intervals from 19 to 460 days was fixed in situ by replacing c.s.f. with buffered OsO₄ and embedded in Araldite. After demyelination, axons are embraced by sheet-like glial processes. An occasional myelin sheath is first seen at 19 days; by 64 days, all axons are at least thinly myelinated. The cytoplasm of the myelin-forming cells, unlike that of either oligodendrocyte or fibrous astrocyte in normal cord, is dense with closely packed organelles and fine fibrils. Many of the myelinogenic cells become scarring astrocytes and at 460 days the lesion teems with their fibril-filled processes. Oligodendrocytes appear in the lesion after remyelination is under way. Phagocytes disappear gradually. A myelin sheath is formed by spiral wrapping of a sheet-like glial process around an axon. Where the first turn of the spiral is completed, a mesaxon is formed. As cytoplasm is lost from the process, the plasma membrane comes together along its outer and cytoplasmic surfaces to form compact myelin. Only a small amount of cytoplasm is retained; it is confined to the paramesaxonal region and, on the sheath exterior, to a longitudinal ridge which appears in profile as a small loop. This outer loop has the same rotational orientation as the inner mesaxon. These vestiges of spiral membrane wrapping are also found in normal adult and new-born cat cord. Nodes are present in all stages of remyelination and in normal adult cat and kitten cord. These observations suggest that myelin is reformed in the lesion in the same way it is first formed during normal development. The mechanism of myelin formation is basically similar to that proposed for peripheral nerve and amphibian and mammalian optic nerve; it does not agree with present views on the mechanism of myelinogenesis in mammalian brain and cord. This is the first demonstration of remyelination in adult mammalian central nervous tissue.     

THE FINE STRUCTURE OF ACOUSTIC GANGLIA IN THE RAT

Nerve cell bodies in the spiral and vestibular ganglia of the adult rat are surrounded by thin (about ten lamellae) myelin sheaths which differ in several respects from typical axonal myelin. In some instances lamellae surrounding perikarya appear as typical major dense lines, and in others as thin Schwann cell sheets in which cytoplasm persists. Discontinuities and irregularities appear in the structure of perikaryal myelin. Lamellae may terminate anywhere within the sheaths; they may bifurcate; they may reverse their direction; or they may merge with each other. The number of lamellae varies from one part of a sheath to another. In addition, the myelin of a single perikaryal sheath may receive contributions from more than one Schwann cell, which overlap and interleave with each other. The ganglion cells are of two types: those which are densely packed with the usual cytoplasmic organelles but have few neurofilaments (granular neurons), and those which exhibit large areas containing few organelles but have a high concentration of neurofilaments (filamented neurons). The latter cell type is ensheathed by myelin which is generally more compact that that surrounding the former. The formation and the physiologic significance of perikaryal myelin are discussed.     

FURTHER OBSERVATIONS ON THE STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

Direct evidence has been presented to confirm the existence of a spiral in the myelin sheaths of the central nervous system. An account of some of the variations in structure of central myelin sheaths has been given and it has been shown that the radial component of myelin sheaths has the form of a series of rod-like thickenings of the intraperiod line. These thickenings extend along the intraperiod line in a direction parallel to the length of the axon. The relative position of the internal mesaxon and external tongue of cytoplasm has been determined in a number of transverse sections of sheaths from the optic nerves of adult mice, adult rats, and young rats. In about 75 per cent of the mature sheaths examined, these two structures were found within the same quadrant of the sheath, so that the cytoplasm of the external tongue process tends to lie directly outside that associated with the internal mesaxon. The frequency with which the internal mesaxon and external tongue lie within the same quadrant of the sheath increases both with the age of the animal and with the number of lamellae present within a sheath. The possible significance of these findings is discussed.     

Myelin glycosphingolipids,galactosylceramide and sulfatide,participate in carbohydrate–carbohydrate interactions between apposed membranes and may form glycosynapses between oligodendrocyte and/or myelin membranes

Glycosphingolipids (GSLs) can interact with each other by homotypic or heterotypic trans carbohydrate–carbohydrate interactions across apposed membranes, resulting in cell–cell adhesion. This interaction can also provide an extracellular signal which is transmitted to the cytosolic side, thus forming a glycosynapse between two cells. The two major GSLs of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I³-sulfate (SGC), are an example of a pair of GSLs which can participate in these trans carbohydrate–carbohydrate interactions and trigger transmembrane signaling. These GSLs could interact across apposed oligodendrocyte membranes at high cell density or when a membranous process of a cell contacts itself as it wraps around the axon. GalC and SGC also face each other in the apposed extracellular surfaces of the multilayered myelin sheath. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.     

ELECTRON MICROSCOPE AND LOW-ANGLE X-RAY DIFFRACTION STUDIES OF THE NERVE MYELIN SHEATH

1. A close correlation has been obtained between high resolution electron microscopy and low-angle x-ray diffraction studies of the myelin sheath of frog and rat peripheral and central nerves. Extensive studies were performed by application of both techniques to the same specimens, prepared for examination by OsO₄ or KMnO₄ fixation, and embedding either in methacrylate or in gelatin employing a new procedure. Controlled physical and chemical modifications of the myelin sheath prior to fixation were also investigated. 2. A correspondence was established between the layer spacings observed in electron micrographs and the fundamental radial repeating unit indicated by the low-angle x-ray diffraction patterns. The variations in relative intensities of the low-angle x-ray reflections could be related to the radial density distributions seen in the electron micrographs. 3. An analysis of the preparation procedures revealed that OsO₄ fixation introduces a greater shrinkage of the layer spacings and more pronounced changes in the density distribution within the layers than KMnO₄ fixation. The effects of methacrylate and gelatin embedding are described, and their relative merits considered in relation to the preservation of myelin structure by OsO₄ fixation. 4. The experimental modifications introduced by freezing and thawing of fresh whole nerve are described, particularly the enhancement of the intermediate lines and the dissociation of the layer components in the myelin sheath. A characteristic collapsing of the radial period of the sheath is observed after subjecting fresh nerve trunks to prolonged and intense ultracentrifugation. 5. Controlled extraction of fresh nerve with acetone at 0°C., which preferentially removes cholesterol, produces characteristic, differentiated modifications of the myelin sheath structure. Electron microscopy reveals several types of modifications within a single preparation, including both expanded and collapsed layer systems, and internal rearrangements of the layer components. Alcohol extraction leads to a more extensive structural breakdown, but in certain areas collapsed layer systems can still be observed. The components of the lipide extracts could be identified by means of x-ray diffraction. These modifications emphasize the importance of cholesterol in the myelin structure, and disclose a resistance of the dense osmiophilic lines to lipide solvents. 6. The significance of these structures is discussed in relation to present concepts of the molecular organization of myelin. The available evidence is consistent with the suggestion that the primary site of osmium deposition is at the lipoprotein interfaces and that the light bands probably represent regions occupied by lipide chains. The electron microscope and x-ray diffraction data also indicate the possibility of a regular organization within the plane of the layers, probably involving units of 60 to 80 A. The myelin sheath is regarded as a favourable cell membrane model for detailed analysis by combined application of x-ray diffraction and electron microscopy.     

A series of desmosomal attachments in the schwann sheath of myelinated mammalian nerves

Summary This paper reports the occurrence of a band of desmosome-like complexes between schwannian loops near the nodes of Ranvier in the myelin sheath of the sciatic nerves of rats. The complexes are characterized by focal widening of the space between adjacent loops and a conspicuous density of the subjacent cytoplasm. These complexes are arranged back-to-back in a row thereby forming a band-like structure. In serial and step-section reconstructions it appears that one such band is present on either side of each nodal gap. Although the function of these complexes is unknown, it is suggested that they may serve to bond together adjacent loops of the Schwann sheath near the point where the myelin sheath is interrupted at the node of Ranvier.This investigation was supported in part by United States Public Health Service Research Grant NB 04330-01, -02 and research career program award NB-K3-16,731 from the National Institute of Neurological Diseases and Blindness and by Grant No. 358 from the National Multiple Sclerosis Society.Presented in part at the 3rd Annual Meeting of the American Society for Cell Biology, New York City, 1963 (Harkin 1963a).     

THE GEOMETRY OF PERIPHERAL MYELIN SHEATHS DURING THEIR FORMATION AND GROWTH IN RAT SCIATIC NERVES

In rat sciatic nerves, a small bundle of fibers was identified in which myelin sheaths were absent at birth, appeared within 3 days, and grew rapidly for 2 wk. During this interval, nerves were removed from littermates and were sectioned serially in the transverse plane. Alternating sets of thin and thick sections were used to prepare electron micrograph montages in which single myelinating axons could be identified and traced distally. During the formation of the first spiral turn, the mesaxon's length and configuration varied when it was studied at different levels in the same Schwann cell. The position of the mesaxon's termination shifted while its origin, at the Schwann cell surface, remained relatively constant. Along myelin internodes composed of two to six spiral turns, there were many variations in the number of lamellae and their contour. Near the mesaxon's origin, longitudinal strips of cytoplasm separated the myelin layers. Thicker sheaths were larger in circumference, more circular in transverse sections, and more uniform at different levels. Irregularities were confined to the paranodal region, and separation of lamellae by cytoplasm occurred at Schmidt-Lantermann clefts. Approximate dimensions of the bundle, its largest fibers, and their myelin sheaths were measured and calculated. The myelin membrane's transverse length and area increased exponentially with time; the growth rate increased rapidly during the formation of the first four to six spiral layers and remained relatively constant during the subsequent enlargement of the compact sheath.     

ELECTRON MICROSCOPE STUDIES OF THE FORMATION OF NODES OF RANVIER IN MOUSE SCIATIC NERVES

Observations with the electron microscope of longitudinal sections of the sciatic nerves of infant mice during the period of early myelin formation are described. These observations are interpreted in relation to previous studies of transverse sections, and a general picture of the formation of an internodal length of the myelin sheath in three dimensions is formulated. In general, an internodal length of myelin sheath is attained by the spiral wrapping of the infolded Schwann cell surface; the increase in length of the internode during maturation is at least partially explained by the increased length of axon covered by the overlapping of successive layers during the wrapping of the infolded Schwann cell surface; and the nodes of Ranvier refer to the structure complex at the junctions of adjacent non-syncytial Schwann cells. The fact that the mode of formation of myelin brings each of its layers into intimate contact with the axon surface at the nodes is emphasized because of the possible functional significance of this arrangement. The manner of origin of Schmidt-Lantermann clefts remains obscure. Certain isolated observations provide evidence for the possibility that occasional internodes of myelin may form from several small segments of myelin within a single Schwann cell.     

Tuning PAK Activity to Rescue Abnormal Myelin Permeability in HNPP

Schwann cells in the peripheral nervous systems extend their membranes to wrap axons concentrically and form the insulating sheath, called myelin. The spaces between layers of myelin are sealed by myelin junctions. This tight insulation enables rapid conduction of electric impulses (action potentials) through axons. Demyelination (stripping off the insulating sheath) has been widely regarded as one of the most important mechanisms altering the action potential propagation in many neurological diseases. However, the effective nerve conduction is also thought to require a proper myelin seal through myelin junctions such as tight junctions and adherens junctions. In the present study, we have demonstrated the disruption of myelin junctions in a mouse model (Pmp22+/-) of hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of Pmp22 gene. We observed a robust increase of F-actin in Pmp22+/- nerve regions where myelin junctions were disrupted, leading to increased myelin permeability. These abnormalities were present long before segmental demyelination at the late phase of Pmp22+/- mice. Moreover, the increase of F-actin levels correlated with an enhanced activity of p21-activated kinase (PAK1), a molecule known to regulate actin polymerization. Pharmacological inhibition of PAK normalized levels of F-actin, and completely prevented the progression of the myelin junction disruption and nerve conduction failure in Pmp22+/- mice. Our findings explain how abnormal myelin permeability is caused in HNPP, leading to impaired action potential propagation in the absence of demyelination. We call it “functional demyelination”, a novel mechanism upstream to the actual stripping of myelin that is relevant to many demyelinating diseases. This observation also provides a potential therapeutic approach for HNPP.     

Loss of ABCA8B decreases myelination by reducing oligodendrocyte precursor cells in mice

The myelin sheath, which is wrapped around axons, is a lipid-enriched structure produced by mature oligodendrocytes. Disruption of the myelin sheath is observed in several neurological diseases, such as multiple sclerosis. A crucial component of myelin is sphingomyelin, levels of which can be increased by ABCA8, a member of the ATP-binding cassette transporter family. ABCA8 is highly expressed in the cerebellum, specifically in oligodendroglia. However, whether ABCA8 plays a role in myelination and mechanisms that would underlie this role remain unknown. Here, we found that the absence of Abca8b, a mouse ortholog of ABCA8, led to decreased numbers of cerebellar oligodendrocyte precursor cells (OPCs) and mature oligodendrocytes in mice. We show that in oligodendrocytes, ABCA8 interacts with chondroitin sulfate proteoglycan 4 (CSPG4), a molecule essential for OPC proliferation, migration, and myelination. In the absence of Abca8b, localization of CSPG4 to the plasma membrane was decreased, contributing to reduced cerebellar CSPG4 expression. Cerebellar CSPG4+ OPCs were also diminished, leading to decreased mature myelinating oligodendrocyte numbers and cerebellar myelination levels in Abca8b^?/? mice. In addition, electron microscopy analyses showed that the number of nonmyelinated cerebellar axons was increased, whereas cerebellar myelin thickness (g-ratio), myelin sheath periodicity, and axonal diameter were all decreased, indicative of disordered myelin ultrastructure. In line with disrupted cerebellar myelination, Abca8b^?/? mice showed lower cerebellar conduction velocity and disturbed locomotion. In summary, ABCA8 modulates cerebellar myelination, in part through functional regulation of the ABCA8-interacting protein CSPG4. Our findings suggest that ABCA8 disruption may contribute to the pathophysiology of myelin disorders.     

Studies of Impedance in Cardiac Tissue using Sucrose Gap and Computer Techniques: II. Circuit Simulation of Passive Electrical Properties and Cell-to-Cell Transmission

The impedance measured in a strip of heart tissue from the moth Hyalophora cecropia is fitted by circuit models of several configurations. The circuits include: (a) a single R-C circuit (b) a double R-C circuit (c) terminated transmission lines, and (d) a pattern of cells with cell-to-cell transmission paths. The last of these is found to give the best fit. Calculation of the model impedances and optimization of element values are performed by a computer. The possibility that the mechanism of cell-to-cell transmission may be capacitative rather than conductive is explored using values of capacitance derived from the circuit models to calculate the effect of capacitative coupling alone on signal transmission. The calculations show that sufficient voltage can be transmitted from the excited cell to an adjacent cell to effect excitation.