Schmidt-Lanterman Clefts Research Articles

Acute conduction block associated with experimental antiserum-mediated demyelination of peripheral nerve.

Intraneural injection of antisera from rabbits with high antigalactocerebroside antibody levels into rat sciatic nerve produced acute nerve conduction block. This was first apparent in some motor axons between 30 and 60 minutes after injection and progressed to completion within 2 to 4 hours. Concurrent morphological evidence of demyelination was present, but structural changes at the time of onset of block were mild and were restricted to the myelin and Schwann cell, particularly at the paranodal areas and Schmidt-Lanterman clefts. It is suggested that paranodal lesions could account for the observed conduction block.

The permeability of the extracellular spaces at the Schmidt-Lanterman clefts and paranodes in peripheral myelin sheaths.

Electron dense material was observed in the extracellular spaces between adjacent components of the Schmidt-Lanterman clefts and between lateral loops at the paranodes in some of the heavily myelinated fibers of the anterior roots and peripheral nerves of a patient with amyotrophic lateral sclerosis. Similar deposits were seen in tissue derived from patients who died with no neurological disease. These deposits were also present in the extracellular spaces of the endoneurium and beneath the outer cytoplasmic collar of the sheath, but did not penetrate the intraperiod line of the compact myelin or the periaxonal space and the lateral loops tended to remain in contact with the axon. The findings suggest that a pathway exists between the extracellular spaces outside the sheath to the innermost portion of the sheath but that the periaxonal space is resistant to the penetration of the extraneous material.

Remyelination in the chicken sciatic nerve

Remyelination in the chicken sciatic nerve occurring after the injection of diphtheria toxin was studied. The rates of fast axonal transport and conduction velocities were measured sixty days after the injection of the toxin. Fast axonal transport rates were found to have returned to normal in the remyelinated nerves, but conduction velocity was markedly reduced even though the birds appeared to walk normally. The remyelinated nerve fibres had on histological examination relatively thin myelin sheaths. Of greater interest was the number of Schmidt-Lanterman clefts observed in both the control and remyelinated nerves when viewed in the electron microscope.

Schwann cell and glial responses in beta, beta'-iminodipropionitrile intoxication. I. Schwann cell and oligodendrocyte ingrowths.

In rats continuously exposed to beta, beta'-iminodipropionitrile (DPN) for long periods, numerous Schwann cell and oligodendrocyte ingrowths extending into giant axonal swellings were found. These ingrowths, cytoplasmic processes of the Schwann cells and oligodendrocytes which deeply indented the axon, regularly arose from Schmidt-Lanterman clefts in the internodes. They dissected along an intra-axonal 'cleavage plane' which separated the giant axonal swellings into two compartments: an inner central channel of relatively normal axoplasm and an outer 'axoplasmic pad' comprised of circularly or spirally-arranged masses of neurofilaments. Multiple ingrowths often arose within a single internode, resulting in nearly complete segregation of the axoplasmic pad from the central channel over long distances. At the interface between the two axoplasmic compartments, internal myelin sheaths were occasionally found. The distinctive ingrowths found in this study appear to represent unusual responses of the Schwann cells and oligodendrocytes to the primary neurofilamentous axonal pathology produced by IDPN.

Immunocytochemical study of P0 glycoprotein, P1 and P2 basic proteins, and myelin-associated glycoprotein (MAG) in lesions of idiopathic polyneuritis.

To investigate the mechanism of myelin breakdown in idiopathic polyneuritis, paraffin and Epon sections of lesions were immunostained with antisera to four proteins in myelin sheaths. Three of these (P0, P2, and BP) are constituents of compact myelin whereas myelin-associated glycoprotein (MAG) is restricted to membranes near Schwann cell cytoplasm in periaxonal and paranodal regions and in Schmidt-Lanterman clefts. In early lesions, there were focal abnormalities in P2, P0, and BP immunostaining of paranodal and internodal myelin. No single protein was affected selectively and lesions occurred in fibres of all sizes, not just in larger fibres selectively stained by P2 antiserum. Early changes in MAG immunostaining occurred only in regions where myelin immunostaining also was abnormal. More severe, late changes in the distribution of P0, P2, and BP and MAG were consistent with the sequence of myelinated fibre alterations seen in segmental demyelination and Wallerian degeneration. In regenerating fibres, MAG antiserum stained periaxonal regions intensely; thin regenerating myelin sheaths were stained by P0, and BP antisera, but not by P2 antiserum. The results show that myelin sheath changes are identified more easily in immunostained sections than in conventional histological preparations. Our data also suggest that in idiopathic neuritis, myelin sheaths are the primary target and that the breakdown of myelin and its proteins is not secondary to Schwann cell damage.

Contribution of axonal transport to the renewal of myelin phospholipids in peripheral nerves. I. Quantitative radioautographic study

Kinetics of phospholipid constituents transferred from the axon to the myelin sheath were studied in the oculomotor nerve (OMN) and the ciliary ganglion (CG) of chicken. Axons of the OMN were loaded with transported phospholipids after an intracerebral injection of [2- 3H]glycerol of [ 3H]labeled choline. Quantitative electron microscope radioautography revealed that labeled lipids were transported in the axons mainly associated with the smooth endoplasmic reticulum. Simultaneously, the labeling of the myelin sheath was found in the Schmidt-Lanterman clefts and the inner myelin layers. The outer Schwann cell cytoplasm and the outer myelin layers contained some label with [methyl- 3H]choline, but virtually none with [2- 3H]glycerol. With time the radioactive lipids were redistributed throughout and along the whole myelin sheath. Since [2- 3H]glycerol incorporated into phospholipids is practically not reutilized, the occurrence of label in myelin results from a translocation of entire phospholipid molecules and from their preferential insertion into Schmidt-Lanterman clefts. In this way, the axon-myelin transfer of phospholipid contributes rapidly to the renewal of a limited pool of phospholipids in the inner myelin layers. When [methyl- 3H]choline was used as precursor of phospholipids, the rapid appearance of the label in the inner myelin layers was interpreted also as an axon-myelin transfer of labeled phospholipids. However, the additional labeling of the outer Schwann cell cytoplasm adjacent to Schmidt-Lanterman clefts and of the outer myelin layers reflects a local re-incorporation of the base released from the axon. By these two processes, the axon contributes to purvey the inner myelin layers with new phospholipids and the Schwann cells with new choline molecules.

Immunocytochemical localization of P0 protein in Golgi complex membranes and myelin of developing rat Schwann cells.

P0 protein, the dominant protein in peripheral nervous system myelin, was studied immunocytochemically in both developing and mature Schwann cells. Trigeminal and sciatic nerves from newborn, 7-d, and adult rats were processed for transmission electron microscopy. Alternating 1-micrometer-thick Epon sections were stained with paraphenylenediamine (PD) or with P0 antiserum according to the peroxidase-antiperoxidase method. To localize P0 in Schwann cell cytoplasm and myelin membranes, the distribution of immunostaining observed in 1-micrometer sections was mapped on electron micrographs of identical areas found in adjacent thin sections. The first P0 staining was observed around axons and/or in cytoplasm of Schwann cells that had established a 1:1 relationship with axons. In newborn nerves, staining of newly formed myelin sheaths was detected more readily with P0 antiserum than with PD. Myelin sheaths with as few as three lamellae could be identified with the light microscope. Very thin sheaths often stained less intensely and part of their circumference frequently was unstained. Schmidt-Lanterman clefts found in more mature sheaths also were unstained. As myelination progressed, intensely stained myelin rings became much more numerous and, in adult nerves, all sheaths were intensely and uniformly stained. Particulate P0 staining also was observed in juxtanuclear areas of Schwann cell cytoplasm. It was most prominent during development, then decreased, but still was detected in adult nerves. The cytoplasmic areas stained by P0 antiserum were rich in Golgi complex membranes.

Quantitative study of the non-circularity of myelinated peripheral nerve fibres in the cat.

1. One hind limb of each of four cats was either chronically de-efferentated, or chronically de-afferentated, and perfused with buffered glutaraldehyde fixative. Up to three different muscle nerves were dissected from each limb, post-fixed in osmium tetroxide and embedded in Epon. Ultrathin transverse sections were mounted on Formvar-coated single-hole specimen grids so that all the fibres in each nerve could be examined individually by electron microscopy.2. Non-circularity was expressed as the ratio (ø): [Formula: see text] The degree of non-circularity of all the afferent axons, or all the efferent axons, in each muscle nerve was determined. The proportion of fibres cut through the paranodal region, or through the Schwann cell nucleus, was as expected for group I afferent and for alpha and gamma efferent fibres, but hardly any typical paranodal sections of group II or III afferent fibres were encountered which suggests that their paranodal arrangement differs from that of other groups. In a quantitative comparison of noncircularity in different functional groups, fibres cut through paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were rejected.3. All the gamma efferent fibres in one nerve were studied in a series of sections cut at 25 mum intervals. The degree of non-circularity was found to be relatively constant along the internode of most fibres when the values at paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were ignored.4. The value of ø varied widely from 1.0 (circular) to 0.5 or less from fibre to fibre within every functional group. However, the mean value of ø was less for gamma axons (0.68) than for alpha axons (0.78), and less for group III axons (0.79) than for axons in groups I and II (both 0.84). When the results for all the nerves were aggregated, these differences were statistically very highly significant, as was the difference in ø between group I and alpha fibres. If values of ø < 0.5 were rejected, the difference between the mean ø for group III and group II was then of doubtful significance whereas that between alpha and gamma fibres was still very highly significant.5. The external perimeter (S) of a non-circular fibre differs from pi times the diameter of a circle just enclosing the fibre (D). It is shown that S = 0.95 pi D for group I and II fibres, S = 0.90 piD for alpha and group III fibres, and S = 0.85 piD for gamma fibres.6. The myelin period, or interperiod repeat distance, varied from 14.1 to 15.6 nm in different cats, implying radial shrinkage of the myelin sheath from 15 to 23%. The myelin period in a particular cat was the same for several nerves, and the same for fibres in different functional groups.7. The possibility that repetitive firing of axons during fixation contributed to the varying degree of non-circularity is considered but rejected as unlikely.8. It is deduced that about 10% radial shrinkage of the myelin sheath, but little or no osmotic shrinkage of the axon, occurred during fixation and rinsing. Further radial shrinkage of about 8% in all components of the fibre probably occurred as a result of subsequent histological processing. It is concluded that the non-circularity of all axons, and the greater non-circularity of small axons, is unlikely to have been due to histological processing.9. It is concluded that axons are non-circular in vivo. The hypothesis that non-circularity allows axons to accommodate swelling during repetitive activity is discussed. Suggestions are made as to why gamma axons may be more non-circular than alpha or group III axons in an anaesthetized cat immediately prior to fixation, and why alpha axons may be more non-circular than axons in groups I and II.

Peripheral nerve demyelination induced by intraneural injection of experimental allergic encephalomyelitis serum.

Intraneural injection of sera from rabbits with experimental allergic encephalomyelitis, induced by sensitization with bovine brain white matter in complete Freund's adjuvant, produced focal primary demyelinative lesions in rat sciatic nerves. Demyelinating activity was removed by prior incubation of antisera with central (CNS) and peripheral nervous system (PNS) myelin but not with liver or kidney, and was heat-labile and complement-dependent. Recipient animals developed a sensorimotor disturbance of their toes and ankles on the side injected with antiserum. Twenty minutes after antiserum injection, Schwann cells showed focal cytoplasmic outpouching and their external mesaxons opened. Between 1 and 8 hours after injection vacuolation, splitting and vesiculation of myelin became increasingly prominent at Schmidt-Lanterman clefts and paranodal regions, with concomitant degenerative changes in Schwann cell cytoplasm. Polymorphonuclear cell infiltration and endoneurial edema were apparent at this time. Substantial demyelination occurred before the appearance of phagocytic cells. Between 8 hours and 3 days many nerve fibers were surrounded and attacked by invading macrophages. Axons became demyelinated progressively over several internodes by macrophage phagocytosis. Early signs of remyelination were observed by 5 days. These findings suggest that antibodies directed against antigens common to both CNS and PNS myelin can produce in vivo peripheral nerve demyelination.

An untrastructural analysis of remyelination following segmental demyelination.

Electron microscopic observations were made of early remyelination after segmental demyelination in experimental allergic neuritis (EAN) and experimental diphtheritic neuropathy. In EAN 71% of axons which had been demyelinated were invaginated each within a single Schwann cell; remyelination associated with this cellular relationship was in accordance with the spiral myelin concept, 29% of demyelinated axons in EAN were initially enveloped each by several Schwann cell processes and the associated mechanism of remyelination was by “tunication” resulting in a transient irregular distribution of myelin lamellae around the axon circumference. In diphtheritic neuropathy regeneration more closely paralleled development in ontogeny. 88% of demyelinated axons were invaginated each within one Schwann cell, only 12% of axons were each enveloped by more than one Schwann cell process. Early remyelination by tunication was not observed in diphtheritic neuropathy. Additional loosely associated Schwann cell processes lying within the “old” Schwann cell basement membrane occurred frequently in both experimental conditions. Schmidt-Lanterman clefts, “redundant” myelin, and “desmosomes” were observed in the sheaths of incipient remyelination.

Movements in the myelin Schwann sheath of the vertebrate axon.

THE demonstration in our laboratory that radioactive amino-acid and uridine label the myelin lamellae of vertebrate peripheral nerve and penetrate into the axon1–3 raised the problem of the route and mechanism of transport of these materials through the layered sheath4. Two mechanisms seemed salient: direct transfer through the battery of stacked myelin membranes, governed by the forces involved in transport across membranes; and transport within the Schwann cytoplasm into the depths of the myelin via the Schmidt–Lanterman clefts, perhaps propelled by movements of the Schwann cell body. These speculations, particularly the second one, led us to examine the myelin sheaths of living nerve fibres with lapsed-time microcinematography. We report our findings on movements in the myelin sheath, with particular reference to the clefts.

A light and electron microscope study of long-term organized cultures of rat dorsal root ganglia.

Dorsal root ganglia from fetal rats were explanted on collagen-coated coverslips and carried in Maximow double-coverslip assemblies for periods up to 3 months. These cultured ganglia were studied in the living state, in stained whole mounts, and in sections after OsO(4) fixation and Epon embedment. From the central cluster of nerve cell bodies, neurites emerge to form a rich network of fascicles which often reach the edge of the carrying coverslip. The neurons resemble their in vivo counterparts in nuclear and cytoplasmic content and organization; e.g., they appear as "light" or "dark" cells, depending on the amount of cytoplasmic neurofilaments. Satellite cells form a complete investment around the neuronal soma and are themselves everywhere covered by basement membrane. The neuron-satellite cell boundary is complicated by spinelike processes arising from the neuronal soma. Neuron size, myelinated fiber diameter, and internode length in the cultures do not reach the larger of the values known for ganglion and peripheral nerve in situ (30). Unmyelinated and myelinated nerve fibers and associated Schwann cells and endoneurial and perineurial components are organized into typical fascicles. The relationship of the Schwann cell and its single myelinated fiber or numerous unmyelinated fibers and the properties of myelin, such as lamellar spacing, mesaxons, Schmidt-Lanterman clefts, nodes of Ranvier, and protuberances, mimic the in vivo pattern. It is concluded that cultivation of fetal rat dorsal root ganglia by this technique fosters maturation and long-term maintenance of all the elements that comprise this cellular community in vivo (except vascular components) and, furthermore, allows these various components to relate faithfully to one another to produce an organotypic model of sensory ganglion tissue.

The ultrastructure of Schmidt-Lanterman clefts and related shearing defects of the myelin sheath.

Schmidt-Lanterman clefts in frog sciatic nerves have been studied in thin sections by electron microscopy utilizing permanganate fixation and araldite embedding. It is shown that they are shearing defects in myelin in which the lamellae are separated widely at the major dense lines. Each lamella consisting of two apposed Schwann cell unit membranes approximately 75 A across traverses the cleft intact. The unit membranes composing each lamella sometimes are slightly ( approximately 50 to 100 A) separated in the clefts. The layers between the lamellae contain membranous structures which may be components of the endoplasmic reticulum. These layers are continuous with the outer layer of Schwann cytoplasm and the thin and inconstant cytoplasmic layer next to the axon (Mauthner's sheath). Each of these layers in perfect clefts constitutes a long helical pathway through the myelin from the axon. One of these is connected with Schwann cytoplasm and the other directly with the outside. A type of cross-sectional shearing defect, not hitherto recognized, is described and shown to be a kind of Schmidt-Lanterman cleft. Incomplete clefts are seen and interpreted as representing stages in a dynamic process whereby the myelin lamellae may be constantly separating and coming together again in life.