Specialized paranodal and interparanodal glial-axonal junctions in the peripheral and central nervous system: a freeze-etching study

Cation binding at the node of Ranvier: I. Localization of binding sites during development

Cations are known to bind to the node of Ranvier and the paranodal regions of myelinated fibers. The integrity of these specialized structures is essential for normal conduction. Sites of cation binding can be microscopically identified by the electrondense histochemical reaction product formed by the precipitate of copper sulfate/potassium ferrocyanide. This technique was used to study the distribution of cation binding during normal development of myelinating fibers. Sciatic nerves of C57B1 mice, at 1, 3, 5, 6, 7, 8, 9, 13, 16, 18, 24 and 30 days of age, were prepared for electron microscopy following fixation in phosphate-buffered 2.5% glutaraldehyde and 1% osmic acid, microdissection and incubation in phosphate-buffered 0.1 M cupric sulfate followed by 0.1 M potassium ferrocyanide. Localization of reaction product was studied by light and electron microscopy. By light microscopy, no reaction product was observed prior to 9 days of age. At 13 days, a few nodes and paranodes exhibited reaction product. This increased in frequency and intensity up to 30 days when almost all nodes or paranodes exhibited reaction product. Ultrastructurally, diffuse reaction product was first observed at 3 days of age in the axoplasm of the node, in the paranodal extracellular space of the terminal loops, in the Schwann cell proper and in the terminal loops of Schwann cell cytoplasm. When myelinated axons fulfilled the criteria for mature nodes, reaction product was no longer observed in the Schwann cell cytoplasm, while the intensity of reaction product in the nodal axoplasm and paranodal extracellular space of the terminal loops increased. Reaction product in the latter site appeared to be interrupted by the transverse bands. These results suggest that cation binding accompanies nodal maturity and that the Schwann cell may play a role in production or storage of the cation binding substance during myelinogenesis and development.

Spatial heterogeneity of the axolemma of non-myelinated fibers in the optic disc of the adult rat. Freeze-fracture observations

This freeze-fracture study examined the structure of the axolemma of non-myelinated fibers in the optic disc region of the adult rat. Spatially heterogeneous patterns of intramembranous particles (IMPs) were observed on the E-fracture faces; particle distribution patterns ranged from clusters of 4-7 IMPs to linear arrays of particles 1-3 IMPs wide that apparently encircle the axon. These bands of particles displayed a periodicity of approximately 0.16 micrometers. The present findings demonstrate that, in specialized regions, the axolemma of non-myelinated fibers not in the region of synapses can exhibit distinct spatial heterogeneity.

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.

The pattern of recovery of axons in the nervous system of rats following 2,5-hexanediol intoxication: a question of rheology?

Rats have been dosed with 2,5-hexanediol for 48 days and then allowed to recover. The changes in the accumulations of neurofilamentous masses in various pathways in CNS and PNS have been followed by light microscopy over the subsequent 9 weeks. It was found that many CNS pathways allow the argyrophilic masses to pass to their terminals from whence they subsequently disappear, usually over 5-6 weeks. Little or no axonal degeneration is seen where this happens. The same occurs in many peripheral nerves, particularly cranial nerves. However, in many tracts in the spinal cord and in many axons in the longer peripheral nerves, filamentous masses remain and becomes associated with axon degeneration, and, in tracts, gliosis. The importance of paranodal constrictions at nodes of Ranvier which tend to be greater in larger diameter axons is emphasized as a likely mechanism for the axon degeneration which largely took place during the recovery period.

Freeze-fracture characterization of isolated myelin and axolemma membrane fractions

The macromolecular organization of membranes isolated from the rabbit optic nerve and tract was analyzed using the freeze-fracture technique. A myelin fraction and two axolemma-enriched fractions were prepared from a preparation of myelinated axons isolated by flotation in a buffered salt-sucrose medium. In the myelinated axon preparation, axolemma and myelin membranes were easily identified. Larger areas of the axon membrane and myelin membrane totally lacked intramembranous particles. The particles remaining on the myelin membrane formed patches of evenly distributed elongated and globular particles. In contrast, the particles remaining on the axolemma were globular in shape and tightly clustered. Particle clustering and particle-free areas were not characteristic of either the axolemma or myelin membrane of whole nerves fixed in situ and processed for freeze-fracture. The isolated myelin membrane fraction contained a large number of vesicles completely lacking intramembranous particles. Of the remaining membrane vesicles, profiles with dispersed elongated and globular particles predominated. A small percentage of vesicles displayed intramembranous particles of the same size, shape and clustering pattern as that seen on the axolemma of the myelinated axon preparation. The two axolemma fractions were enriched in membrane containing tightly clustered globular particles. Particle-free vesicles as well as some myelin membrane vesicles were also seen in the axolemma fractions.

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.

Peripheral and central conduction times in hereditary pressure-sensitive neuropathy

Seven members of a family with histologically proven hereditary pressure-sensitive neuropathy (HPSN) agreed to be examine clinically and electrophysiologically. A sural nerve biopsy specimen taken from the propositus who suffered from a partial brachial plexus palsy showed typical 'sausage-like' myelin sheath thickenings reflecting a failure of axon-adjusted myelination. Reduced motor and sensory conduction velocities involving several nerves were found in the four family members with clinical signs of HPSN. In addition, central conduction times in the auditory and somatosensory pathways were determined measuring the interwave latency I-V in brainstem auditory-evoked potentials and the interpeak latency N14-N20 in median nerve sensory-evoked potentials. Central conduction times in both afferent systems were within normal limits. The absolute delay of peak N14 and N20 in median and P40 in tibial nerve-evoked potentials was probably due to an impaired conduction in the peripheral branch of the bipolar ganglion cell. Whether the central axon branch in the dorsal columns was also involved could not be decided.

Associated particle aggregates in juxtaparanodal axolemma and adaxonal Schwann cell membrane of rat peripheral nerve

Freeze-fracture observations have been made on unfixed cryoprotected, and glutaraldehyde-perfused and cryoprotected rat sciatic nerve. In the juxtaparanodal region of the internode, numerous particle clusters were observed on the axolemmal E face and rings of particles of uniform size on the P face of the adaxonal Schwann cell membrane. Both of these particle aggregates were concentrated in the internodal region immediately adjacent to the paranode (juxtaparanodal). The findings provide evidence for a close association between the two particle formations, suggesting a unitary structure forming links between the axolemma and Schwann cell membrane. Figures are given for the density distribution of these particles at the juxtaparanodal region. They were rarely observed on membrane fracture faces of the general internodal regions. It is possible that these particle formations may represent potassium channels or that they could provide channels for other metabolic communication between the Schwann cell and the axon.

Ultrastructural observations on nodes of Ranvier from isolated single frog peripheral nerve fibres

A preparative procedure is described by which well-preserved nodes of Ranvier from isolated frog peripheral nerve fibres may be obtained. The following steps are crucial: careful and gentle dissection and isolation of a single nerve fibre, mounting in a chamber limiting lateral movements of the fibre during the fluid exchange, simultaneous glutaraldehyde-OsO4 fixation and embedding in the same chamber used for fixation. Serial sectioning of individual nodes from both motor and sensory fibres made it possible to reconstitute three-dimensional models of several nodes and to study their morphology extensively. In addition to well-known ultrastructural features of the nodal and paranodal architecture, evaginations of a nodal membrane containing mitochondria and outpouchings of the paranodal axoplasm containing vesicles are described.

Myelin proteins, glycoproteins, and myelin-related enzymes in experimental demyelination of the rabbit optic nerve: sequence of events

Wallerian degeneration of the rabbit optic nerve was investigated by the technique of retinal ablation which precludes edema, hemorrhage, or macrophage infiltration. After 8 days of degeneration, marked degradation of axons and some myelin abnormalities appeared in the optic nerve, optic chiasma, and optic tract. Myelin lesions were maximal 32 days after retinal destruction. The amount of material stained with a myelin dye decreased drastically between 32 and 90 days after the operation. Biochemical parameters gave the following sequence of events. The concentration of the major periodic acid--Schiff staining glycoproteins was decreased after 2 days, and 6 days later the presence of cholesterol esters was detected in the optic tissue. After 16 days of Wallerian degeneration, the specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase not associated with myelin decreased, indicating a possible de-differentiation of oligodendrocytes. Degradation of myelin basic protein became significant at 32 days and the amount of myelin isolated decreased later. The loss of myelin basic protein coincided with a reduction of myelin periodicity as measured in purified fractions by electron microscopy. These results show that secondary myelin destruction in the absence of edema, hemorrhage, or macrophages is a very slow process, and in this situation myelin undergoes a selective and sequential loss of its constituents.

Axoglial junctions in the mouse mutant Shiverer

Analysis of Shiverer central nervous tissue by the freeze-fracture method shows that axoglial junctions of the type found normally in the paranodal region occur commonly despite the gross reduction in myelin. On a substructural level these junctions appear identical to those that form between paranodal oligodendroglial processes and axolemma. On a grosser level, however, they are bizarre in shape, arrangement and distribution. Isolated glial processes, or small sheaves of them, course among axons and form such junctions in an irregular patchy manner, usually without apparent relationship to paranodal regions. These aberrant junctions may be oriented transversely, obliquely or longitudinally with respect to the axonal axis. Axolemmal E face particle accumulations, which characterize normal nodes of Ranvier, are usually not found in the membrane adjacent to the aberrant junctional patches. Thus, axoglial junctional specializations of the paranodal type can form in this mutant in the absence of the myelin proteins that are deficient in Shiverer, and such junctions may appear in areas not related to other paranodal or nodal structures. The relevance of these findings to differentiation of the axolemma and to the neurological defects in this mutation is discussed.

The distribution of orthogonal arrays and their relationship to intercellular junctions in neuroglia of the freeze-fractured hypothalamo-neurohypophysial system

Using freeze-fracture techniques, we have investigated membrane specializations of the glia associated with the hypothalamo-neurohypophysial system of the rat. In the paraventricular (PVN) and supraoptic (SON) nuclei, astrocytes in areas of high neuronal density (i.e., magnocellular regions) display orthogonal arrays of 6--7 nm particles solely near gap junctions, while astrocytes in areas of lower neuronal density (i.e., parvocellular regions), contain additional arrays in membranes not displaying gap junctions. Arrays are especially numerous on astrocytic perivascular end-feet in both nuclei and in the laminations of the pial-glial limitans ventral to the SON. Ependymal cells near the PVN show arrays both on their lateral surfaces (displaying gap junctions) and on their apical surfaces (facing the CSF). Tight junctions are not noted on astrocytes or ependymal cells, but are noted on both the somas and myelin lamellae of oligodendroglia. Both of these latter membranes occasionally contain gap junctions as well; however, orthogonal arrays are never noted on oligodendroglia. The plasma membranes of pituicytes in the neurohypophysis display gap junctions, complex junctions, and tight junctions. Orthogonal arrays are noted near the first two of these, but not near the last. Arrays in the neutral lobe appear most dense on membranes adjacent to subpial or perivascular spaces. Pituicyte membranes containing orthogonal arrays appear infrequently near the neural stalk, increasing towards the distal end of the neural lobe. The distribution of orthogonal arrays in this system, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

A permeability change of myelin membrane vesicles towards cations is induced by MgATP but not by phosphorylation of myelin basic proteins

The existence of an endogenous protein kinase activity and protein phosphatase activity in myelin membrane from mammalian brain has now been well established. We found that under all conditions tested the myelin basic protein is almost the only substrate of the endogenous protein kinase in myelin of bovine brain. The protein kinase activity is stimulated by Ca2+ in the micromolar range. Optimal activity is reached at a free Ca2+ concentration of about 2 microM. Myelin membrane vesicles were prepared and then shown to be sealed by a light-scattering technique. After preloading with 45Ca2+, 86Rb+, or 22Na+, the self-diffusion (passive outflux) of these ions from myelin membrane vesicles was measured. Ionophores induced a rapid, concentration-dependent outflux of 80--90% of the cations, indicating that only a small fraction of the trapped ions was membrane bound. There was no difference in the diffusion rates of the three cations whether phosphorylated (about 1 mol phosphate per myelin basic protein) or non-phosphorylated vesicles were tested. In contrast, a small but significant decrease in permeability for Rb+ and Na+ was measured, when the vesicles were pretreated with ATP and Mg2+.

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.

Central myelin in the mouse mutant shiverer

The spinal cord, optic nerves, and cerebellum of the mouse mutant Shiverer were examined by electron microscopy of thin sections. Although central nervous system myelin is grossly deficient in amount, none of its basic structural elements are missing. Regions of compact myelin can be found composed of several layers of alternating major dense lines and intermediate lines repeating with normal periodicity. The "radial component" consisting of periodic thickenings of the intermediate line aligned through several lamellae was also identified. Axoglial junctions characteristic of the type found in paranodal regions are present in greater than normal numbers but occur in aberrant locations. Myelin sheaths have marked reduced numbers of lamellae, which often contain cytoplasm, terminate in cytoplasmic "loops" within and around myelin sheaths, and do not completely encircle axons. In addition, membranous debris appears within neuronal and glial profiles, suggesting some degree of myelin breakdown. Thus, the protein lacks in this mutant appear not to be associated with discrete deficiencies of specific structural components but rather with a variety of quantitative changes and irregularity of form.

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.

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.

Aberrant axon-Schwann cell junctions in dystrophic mouse nerves

'Amyelinated' axons in the spinal roots of dystrophic mouse nerves lack typical nodal and paranodal membrane specializations. However, at the periphery of the amyelinated bundles some of the naked axons form aberrant junctions with Schwann cells belonging to neighbouring myelinated axons. These junctions are characterized by a narrow intercellular cleft containing regularly-spaced densities that closely resemble the 'transverse bands' found at paranodal axoglial junctions with respect to both configuration and spacing. In addition, the Schwann cells sometimes extend fingerlike projections towards amyelinated axons in regions where the axolemma has a dense cytoplasmic undercoating. Such regions resemble nodes of Ranvier, where Schwann cell processes interlace over the axolemma. Freeze-fracture replicas show no typical nodal or paranodal membrane specializations in the amyelinated fibres where they are apposed to each other. However, isolated paracrystalline patches of membrane occur corresponding to the aberrant junctions between amyelinated axons and Schwann cells at the periphery of the bundles. The observations show that structural differentiation of the axolemma occurs only where axons are in intimate contact with myelinating cells and does not develop independently in the amyelinated regions. Sodium channels, which are normally concentrated in the specialized nodal membrane, are, therefore, probably distributed uniformly along the amyelinated axon segments that show no sign of such regional differentiation. In addition, it is shown that Schwann cells are capable of forming specialized junctions with more than one axon at the same time.

Membrane specializations and cytoplasmic channels of Schwann cells in mammalian peripheral nerve as seen in freeze-fracture replicas

Mammalian Schwann cells in rat, rabbit and human fetal nerves were studied using several cryoprotective agents for electron microscopic study of freeze-fracture replicas. The findings in fixed and unfixed tissue reveal surface plasmalemma caveolar specializations and the outer layer membrane junctional complexes found in non-mammalian species. The plasmalemma also reveals a complex arrangement of contours outlining cytoplasmic channel networks distinct from the long-recognized Schmidt-Lanterman incisures and paranodal cytoplasmic loops. A specialized interconnected channel system in the outer "loose" myelin layer displays relatively uniform dimensions comparable in diameter to nodal microvilli, paranodal loops and some incisures. An adaxonal tubular channel system constituting the "axon-Schwann network" is found in the internodal region in addition to other variants of the adaxonal Schwann plasmalemma. The several forms of sequestration of Schwann cell cytoplasm presumably underlie the specialized needs of cytoplasmic continuity in a dynamic functional entity in which large domains of cytoplasm have been displaced by the formation of compact myelin.

Percussive injury to peripheral nerve in rats

Weights were dropped on rat sciatic nerves. Teased fibers and light and electron micrographs of nerves removed between 10 minutes and 2 weeks later were examined. Axonal alterations were seen 10 minutes after injury, and subsequently interruption of axonal continuity with preservation of the basal lamina was apparent. Dissolution of myelin began within 30 minutes and progressed. At 14 days, a segment of some large fibers was devoid of myelin and, by 2 weeks, remyelination had commenced. Demyelination of significant number of fibers was always accompanied by Wallerian degeneration of other fibers of the same nerve. Percussive injury of nerves caused a mixed lesion in which the early and late pathological features were clearly distinguishable from those following crush or compression by a cuff. Any explanation of the transient interruption of function that has been described following such an injury is at present speculative.

Node-like areas of intramembraneous particles in the unensheathed axons of dystrophic mice

The unensheathed axons in the spinal roots of adult dystrophic mice were examined by freeze-fracture electron microscopy. In most areas of these abnormal fibers the distribution of intramembraneous particles was similar to that of the internodal segments of normal axons with many more particles on the PF (internal) leaflets of these axonal surface membranes than on their EF (external) leaflets. However, patches of axonal membranes were also observed in which the distribution of intramembraneous particles resembled that seen in nodes of Ranvier.

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.

Physiologically induced changes in intramembranous particle frequency in the axons of an osmoconforming bivalve

Freeze-fractured axonal membrane surfaces from the connectives of Mytilus edulis show an increment in particle frequency of 52% (fixed tissues) or 68% (unfixed tissues) after long-term adaptation to low salinity. Particle size distribution was unaffected by osmotic adaptation, but was significantly different in fixed and unfixed material. The possibility that these structural changes reflect the known increase in sodium pump frequency in this osmoconforming tissue is considered.

Glial membrane specializations in extraparanodal regions

Previous freeze-fracture studies of central myelinated nerve fibres have demonstrated a distinctive junction in the paranodal region formed between the terminal loops of the glial cell and the axolemma. This unique junction is characterized by the presence of diagonally oriented rows of particles in the P face and to a lesser extent in the E face of the glial cell and an equivalent pattern in the axolemma. In both, the rows are spaced at 250--300 A intervals. Although this junction was originally thought to be peculiar to the paranodal region, examples of the same pattern have now been seen in extraparanodal regions in the central nervous system where they appear as circumscribed patches of membrane exhibiting a pattern identical to that in the paranodal glial loops. All examples found were in the immediate vicinity of myelinated nerve fibres and in one case the membrane containing the specialized patch was identified as a lamella of a myelin sheath. These observations constitute evidence that this distinctive membrane specialization is not limited to the paranodal axoglial junction but can also be found in glial membrane specialization is not limited to the paranodal axoglial junction but can also be found in glial membranes not in immediate contact with the specialized membrane of the paranodal axolemma.

The oligodendrocytic junctional complex

The junctional complex of oligodendrocytes was studied by means of different electron microscopical techniques. This complex is composed of the following junctional membrane formations: 1) tight junctional domains in the oligodendrocytic membrane near the some of the cells, 2) fasciae occludentes or focal tight junctions on the outer oligodendrocytic loop of myelin and on the outermost myelin membrane, 3) gap junctions of considerable size variations, either on membranes near the soma or on peripheral oligodendrocytic processes, and 4) non-paranodal transverse bands. The different types of oligodendrocytic junctions are discussed in terms of their functional implications.

Preparation of membrane vesicles from isolated myelin: studies on functional and structural properties

Myelin membranes purified from bovine brain are shown to form membrane vesicles when incubated in hypotonic buffer. Following restoration of isotonicity a resealing of the membrane occurs as judged by a significant decrease in 22Na+ permeability. Electron spin resonance measurements using stearic acid spin label I indicate a small decrease in membrane fluidity with increasing ionic strength between 50 and 80 mM NaCl. Iodination of myelin membrane vesicles by lactoperoxidase shows a four-fold increase in the amount of iodine incorporation into the myeline basic protein from 0--150 mM NaCl, while the iodination of the proteolipid protein remains essentially unaffected by the change in ionic strength. This dependence of the iodination of the myelin basic protein on the ionic strength can be explained by the electrostatic interactions of this protein with membrane lipids. In view of striking analogies with studies on model membranes correlating protein binding with membrane permeability changes, we suggest a similar structure-function relationship for the myelin basic protein.

Intramembranous particles at the nodes of Ranvier of the cat spinal cord: a morphometric study

Size and distribution of intramembranous particles at nodes of Ranvier of the cat spinal cord were investigated by the freeze-etching technique and compared with those at the internodal axon. The particles are larger (up to 20 nm) at the nodal than at the internodal segment (up to 13 nm), and these large particles are more densely packed in the nodal (400 per sq micrometer) than in the internodal E (external) face (4 per sq micrometer). The nodal E face reveals a much denser overall population (1200--1300 per sq micrometer) of particles than the internodal E face (100--200 per sq micrometer), while at the P (protoplasmic) face the particle density is similar in nodal and internodal segments (1200--1600 per sq micrometer). It is suggested that the large nodal particles may be related to the mechanism of nerve excitation.

Particle arrays in earthworm postjunctional membranes

Analysis of freeze-fractured earthworm body wall muscle reveals distinctive trough-shaped concavities in the protoplasmic leaflet of the muscle cell membrane which contain diagonally oriented rows of particles sometimes in highly ordered arrays. The troughs correspond to the concave postjunctional patches of sarcolemma seen previously in thin sections of myoneural junctions identified as cholinergic, and the intramembranous particles within the troughs correspond in concentration and arrangement to granular elements present in the outer dense lamina of the postjunctional membrane which were interpreted as acetylcholine receptors. The freeze-fracture data provide a more accurate picture of the arrangement of these putative receptors within the plane of the membrane, and indicate also that they extend into the membrane at least as far as its hydrophobic layer.

Isolation of non-myelin plasma membranes unique to white matter

A procedure is described for isolating two membrane fractions from rabbit spinal-cord white matter enriched with 5'-nucleotidase, a nonspecific plasma membrane marker, 2',3'-cyclic nucleotide phosphohydrolase, an oligodendroglial plasma membrane marker, and acetylcholinesterase, an axonal plasma membrane marker. While the two membrane fractions exhibited similar enrichments with respect to cyclic nucleotide phosphohydrolase, enrichments of 5'-nucleotidase and acetylcholinesterase were significantly greater in the heavier membrane fraction. Selected enzyme markers for cyto- and mitochondrial membranes were not detected. Moreover, gray matter did not yield homologous membrane fractions in the gradient when subjected to the identical procedure, indicating that the two membrane fractions were unique to white matter. While electronmicroscopic examination revealed that both membrane fractions were comtaminated with myelin, the heavier fraction was least contaminated and exhibited a fair degree of homogeneity with respect to single membrane vesicular profiles. It was concluded that both membrane fractions were enriched with oligodendroglial and axonal plasma membranes, with the heavier fraction containing significantly more axolemma.

Electron microscopy of cutaneous nerves and receptors

Recent ultrastructural observations on the connective tissue sheaths of nerves, Schwann cell-axonal relations, and nerve terminals and receptors are reviewed. It seems likely that endoneurial collagen is formed by perineurial cells during development and postnatally. New observations on "collagen pockets" are presented. Attention is drawn to freeze-fracture studies of peripheral nerve, particularly in relation to junctional complexes associated with compact myelin, and further application of the technique is considered. Current views on Merkel cells, encapsulated endings, and free nerve terminals are discussed.

Characterization of two subcellular fractions isolated from myelinated axons

Myelin and a heavy membrane fraction (1.0/1.2 fraction) were isolated from rabbit white matter by a slight modification of the procedure for bovine CNS. The specific activities of acetylcholinesterase and Na+, K+-ATPase were higher in the 1.0/1.2 fraction than in myelin. In contrast, the cerebroside content and 2'3'-cyclic nucleotide 3'-phosphohydrolase activity in the 1.0/1.2 fraction were 4.5 and 3.4 times lower than in myelin. Total lipids accounted for only 30% of the 1.0/1.2 fracton's dry weight; for myelin, they represented 70%. Polacrylamide gel electrophoresis showed the presence of many high molecular weight proteins and glycoproteins in the 1.0/1.2 fraction but myelin components were practically missing. Cytochrome c oxidase and NADPH-cytochrome c reductase activities suggested about 15% contamination in the 1.0/1.2 fraction but less than 5% for myelin. In electron micrographs of the 1.0/1.2 fraction, there were many membraneous profiles that varied in size, some mitochondrial fragments, and only a few lamellar whorls of compact myelin. The results suggest that the 1.0/1.2 fraction is different from other myelin-related fractions and is probably enriched in axolemma.

Freeze-fracture properties of central myelin in the bullfrog

The freeze-fractured membrane of the central myelin sheath has three classes of particulate components: (i) Particles inherent to the compact myelin lamellae. These are distributed at random and cleave predominantly with the P (protoplasmic) face. (ii) Particles which comprise the intramyelinic tight junctions. These are arranged in strands and are located at the inner and outer mesaxon, the paranodal loops, the cytoplasmic incisures, and occasionally within the compact regions of the myelin sheath. (iii) Particles localized exclusively at the portion of the paranodal loop membrane involved in the septate-like junction with the axolemma. These are regularly spaced and are organized in parallel rows. In the central myelin sheaths of bullfrogs fixed by perfusion with aldehydes and cryoprotected in 30% glycerol, the randomly distributed particles differ in size and shape from those of the axolemma. They possess a reasonably well defined bimodal distribution with respect to particle shape--most can be described either as globules or as ellipsoids. The globular particles range in diameter from 60 to 150 A. The ellipsoidal particles are 100-200 A long and 15-50 A wide. The total number of particles per square micron on the P face is approximately 1500. About half of these are of the globular type and half of the ellipsoidal type. In poorly fixed specimens, loss of interlamellar adhesion and loss of randomly distributed particles seem to coincide. Evidence is presented against the hypothesis that the tight junctions between compact myelin lamellae represent the radial component of the myelin. The possible relation between the types of particulate components seen in freeze-fracture and the classes of protein isolated from central myelin fractions is briefly discussed.

Intramembranous particle distribution at the node of Ranvier and adjacent axolemma in myelinated axons of the frog brain

The plasma membrane of myelinated axons in the frog brain has been examined by the freeze-fracture technique. The cytoplasmic leaflet of the axolemma contains numerous randomly distributed particles in nodal and internodal regions but relatively fewer particles in the axoglial junctional portion of the paranodal region. Particle distribution is even less uniform in the outer leaflet of the axolemma, which contains a low concentration of particles in the internodal region and a relatively high concentration at the node of Ranvier (approximately 1200 particles mum-2). The nodal particles tend to be larger than most intramembranous particles, approaching 200 A diameter. The paranodal region of the leaflet is virtually devoid of such particles except in the narrow helical 'groove' which faces extracellular clefts between terminating glial processes. In places this pathway widens to form 'lakes' up to approximately 0.3 mum2 area which contain large numbers of large particles resembling those at the node. The concentration of particles at the node is in the same range as the concentration of sodium channels estimated to be in this region and it is suggested on the basis of their location and concentration that these particles represent ionophores. The distribution of particles in the paranodal region suggests that the large intramembranous particles do not have free access to the axoglial junctional portion of the membrane and therefore the movement of such particles along the paranodal region of the membrane may occur primarily in the membrane of the 'groove' spiraling through this portion of the axolemma. Such a restriction in surface area for particle movements on either side of the node of Ranvier could result in trapping of particles at the node and thus contribute to their concentration in the nodal axolemma.

Impulse conduction in the myelinated giant fibers of the earthworm. Structure and function of the dorsal nodes in the median giant fiber

The dorsal openings in the myelin sheath of the median giant fiber (MGF) of the earthworm (Lumbricus terrestris L.) have been studied with electronmicroscopical and electrophysiological methods. The fine structure of the dorsal nodes (DN) resembles in many aspects the Ranvier nodes in vertebrate and crustacean nerve fibers. The nodal membrane directly faces the extracellular collagenous capsule of the ventral cord and displays a conspicuous electrondense undercoat. The myelin sheath of the paranode shows a characteristic differentiation into large desmosomal contracts. Recordings of the transmembrane and longitudinal surface currents along the dorsal side of the MGF during spike propagation support the view that an active inward current is restricted there to the DN. The inward current density in the DN reaches outstandingly high values similar to those measured in vertebrate nodes of Ranvier. The nodal activity can be blocked by application of tetrodotoxin and local anaesthetics. Local electrical stimulation of only one DN may suffice to elicit propagated actions potentials up and down the MGF. It is concluded that the dorsal nodes of the median giant fiber of the earthworm are highly specialized excitable structures mediating saltatory impulse conduction in these fibers.