Journal o f Neurocytology 8, 6 5 5 - 6 7 2 (1979)

Aberrant axon-Schwann cell junctions in dystrophic mouse nerves J. ROSENBLUTH Departments of Physiology and Rehabilitation Medicine, New York University School of Medicine, New York, New York 10016, U.S.A.

Received 4 J a n u a r y 1979; revised 8 March 1979; accepted 23 March 1979

Summary '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.

Introduction The m o u s e m u t a n t ' d y s t r o p h i c ' has been shown to have a defect o f m y e l i n a t i o n m o s t conspicuous in spinal nerve roots where bundles o f large caliber ' a m y e l i n a t e d ' axons travel in close c o n t a c t with one a n o t h e r separated o n l y by n a r r o w intercellular clefts (Bradley and Jenkison, 1973; Stirling, 1975). More distally the d y s t r o p h i c axons acquire myelin sheaths, however, and cross-sections through peripheral nerves appear relatively normal, although defects of m y e l i n a t i o n have been reported there as well (Okada et al., 1976). The central nervous system appears to be normal.

0300-4864/79/050657-18 $3.80 9 1979 Chapman and Hall Ltd.

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Recent freeze-fracture studies of myelinated axons from normal animals have shown them to be regionally differentiated and distinctly different in this respect from unmyelinated fibres. In the paranodal axolemma a unique paracrystalline pattern has been demonstrated (Livingston et al., 1973; Dermietzel, 1974; Schnapp and Mugnaini, 1975; Rosenbluth, 1976) corresponding to the distinctive axoglial junction visible in thin sections (Bargmann and Lindner, 1964; Andres, 1965; Hirano and Dembitzer, 1969). Study of the nodal axolemma (Rosenbluth, 1976) demonstrated large numbers of E face particles (~1200//1m 2) which, it was proposed, may represent the sodium channels shown by several other methods (Nonner et al., 1975; Conti etal., 1976; Ritchie and Rogart, 1977) to be concentrated in this region. Comparable concentrations of nodal E face particles occur in nerve fibres from a variety of animals (Kristol et al., 1977, 1978). Whether regional differentiation of myelinated axons in the nodal and paranodal regions occurs as a consequence of myelin formation around them, or whether these axons are programmed to differentiate independently even in the absence of myelin is, however, uncertain. Dystrophic nerves offer an opportunity to answer this question since amyelinated regions of axons that are known to be myelinated both more proximally and more distally can be studied in order to determine whether typical paranodal and nodal membrane specializations develop along the segments lacking myelin sheaths. Examination of dystrophic roots in fact shows that the axolemma does not differentiate normally in the amyelinated regions. However, certain aberrant membrane specializations do occur between amyelinated axons and Schwann cells that not only bear on the biology of the a x o n - S c h w a n n cell interaction in the mutant, but also clarify the structure of the normal a x o n - S c h w a n n cell junction. Indeed, some of the structural details of the aberrant junctions have now been seen at normal paranodal junctions as well (Rosenbluth, 1979b). Preliminary reports of this study have been presented previously (Rosenbluth, 197B, 1979a).

Materials and methods Dystrophic mice (C57BL/6J-dy), obtained from the Jackson Laboratory, Bar Harbor, Maine, were maintained in the laboratory and sacrificed at either 2 or 9 months of age. Animals were anaesthetized with chloral hydrate and fixed by perfusion through the heart with a solution of 3% glutaraldehyde, made up from Ladd 70% glutaraldehyde, and 2% paraformaldehyde (freshly made up) in 0.1 M phosphate buffer at pH 7.4. Tissues were dissected into small fragments, rinsed overnight in Ringer's solution, postfixed in 2% osmium tetroxide in 0.1 M phosphate buffer (pH 7.4) for 2--3 h, and then rinsed, dehydrated and embedded in Araldite according to standard procedures. Thin sections were stained by potassium permanganate followed by uranyl acetate and examined in a Philips EM 300 electron microscope. Some tissues were stored in fixative for examination by freeze-fracturing. When needed these were cut into fragments of suitable size, rinsed, and glycerinated in a graded series of glycerol-Ringer's solutions ranging from 10 to 30%. Tissue fragments were then frozen in liquid Freon 22 cooled by liquid nitrogen, mounted in a complementary replica holder and fractured in a Balzer's freeze-fracture apparatus. After platinum and carbon coating, tissue was removed by soaking in Clorox. A methanol soak was sometimes used in order to remove myelin debris. Replicas were mounted on carbon-coated grids for examination.

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Results Examination of cross-sections through lumbar roots shows the characteristic appearance of bxmdles of naked axons adjacent to myelinated fibres (Figs. 1 and 2). Two classes of fibres can be distinguished in the amyelinated bundles: (1) those surr o u n d e d entirely by other axons; (2) those situated peripherally that adjoin other axons along one part of their circumference but, along another, face either connective tissue or Schwann cells, some of which belong to neighbouring myelinated axons. The apposed membranes of the adjacent axons of the first group parallel each other very closely b u t usually form no specialized junctions with each other. On occasion, however, a specialized region resembling an 'adherens' junction can be f o u n d between two adjacent fibres (Fig. 15). These axons have the normal c o m p l e m e n t of

Fig. 1. Photomicrograph of dystrophic root showing characteristic dilated blood vessels and amyelinated bundles surrounded by apparently normal myelinated fibres, x 500. Fig. 2. Amyelinated bundle from dystrophic root consisting of axons directly apposed to each other without intervening Schw~tnn cells. The bundle as a whole is enveloped by a Schwann cell layer which separates it from connective tissue containing large amounts of collagen (C), normally myelinated fibres, and isolated amyelinated axons (A). x 21 000.

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organelles, b u t because t h e y are in direct c o n t a c t w i t h each o t h e r w i t h o u t intervening collagen or o t h e r c o n n e c t i v e tissue e l e m e n t s t h e y resemble a x o n s in the central nervous system. Fibres o f the s e c o n d g r o u p , w h i c h o c c u r at the p e r i p h e r y o f the a m y e l i n a t e d bundles, a p p e a r n o d i f f e r e n t f r o m those l o c a t e d deep in the bundles. H o w e v e r , w h e r e the larger a x o n s ( m i n i m u m d i a m e t e r ~ 0 . 5 / a m ) adjoin S c h w a n n cells belonging t o n e i g h b o u r i n g m y e l i n sheaths t w o t y p e s o f specialized c o m p l e x e s f o r m Fig. 3. Amyelinated fibres adjacent to myelinated fibres. A thin apparently immature myelin sheath is shown at the upper right and a degenerating sheath at the lower left. Myelinated fibre at the upper left appears normal. An elongated junction (between arrows) is formed between the outermost Schwann cell of this sheath and a large amyelinated axon. Periodic densities in the junctional cleft are barely visible at this magnification. A cisterna of ER in the Sehwann cell cytoplasm follows the junctional membrane closely. The basement membrane of the Schwann cell is discontinuous, x 60 000. Fig. 4. Detail of another such junction at high magnification. The plasma membrane of the Schwann cell (SC) is clearly trilaminar in the junctional region but only the inner dense lamina of the axonal membrane is clear. Membranes are separated by a cleft of 20-30 A containing transverse bands at intervals of ~280 A. A thin lamina (L) within the axoplasm parallels the axolemma and is interconnected with it periodically, x 237 000. Fig. 5. Detail of junction between Sehwann cell (SC) and amyelinated axon (A). In the junctional region at the left two triangular cytoplasmic densities extend between the Schwann cell plasma membrane and apposed cisterna of ER. These triangular densities are situated over the light regions between transverse bands. Equivalent but fainter densities are present between the ER cisterna and the Schwann cell membrane of the junction to the right of centre, x 220 000. Fig. 6. Junction between Schwann cell and am)?elinated axon (A) showing distinct transverse bands in one region (right) but not another (left). The extent of the junctions is relatively short in this nine-month-old animal, x 100 000. Fig. 7. Two short junctional regions with Schwann cell cisternae (ER) associated. A mitochondrion adjoins the cisterna at the right, x 123 000. Fig. 8. Longitudinal section showing interface between myelinated axon (top) and amyelinated bundle. Arrows indicate four regions where specialized junctions containing transverse bands occur. The one at the extreme right is enlarged in Fig. 9. x 6000. Fig. 9. Detail of Fig. 8 showing one example of transverse bands in a longitudinally cut junction. An ER cisterna is present in the Schwann cell. x 165 000. Fig. 10. Junction in which the orientation of the transverse bands changes with respect to the section plane. The junction has periodic densities in one region (arrows) but elsewhere resembles a gap junction, x 188 000. Fig. 11. Axon at the periphery of an amyelinated bundle. A Schwann cell process surrounds the axon and forms a single lamella of compact myelin (M). The leading edge of the process forms a junction with the axon (arrow). x 68 000. Fig. 12. Detail of Fig. 11 showing transverse bands between the junctional membranes, x 153 000. Fig. 13. Cytoplasmic process extending from perinuclear Schwann cell cymi~lasm toward an amyelinated axon and forming a junction with it. The basement membrane of the Schwann cell is interrupted. Note multiple omega-shaped pits along the Sehwann cell surface, x~!01 000. Fig. 14. In this instance two Schwann cell processes form junctions with two different axons. One of these is connected to the Schwann cell shown at the right by a narrow neck. x 43 000. Fig. 15. Amyelinated axons showing a specialized region resembling an 'adherens' junction between two of them (arrow). x 63 000.

Fig. 16. A Schwann cell process extends towards a neighbouring amyelinated axon and indents it in one region as well as spreading along its surface. Transverse bands are n o t visible along most of the length of the junction, which in some places resembles a gap junction (*), b u t can be seen clearly in one region (arrows). A thin dense lamina (L) is prominent in the axoplasm subjacent to the junction, x 77 000. Fig. 17. A Schwann cell process extends towards a neighbouring amyelinated axon and indents it. The junctional membranes are cut tangentially, x 79 000.

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involving the axolemma and Schwann cell plasma membrane. The first type illustrated in Figs. 3 and 4 consists of a junction in which the apposed membranes follow each other closely and are separated by a cleft only ~ 2 0 - 3 0 A wide. The membranes m a y be approximately straight (Figs. 3 - 5 , 8 and 9) or the Schwann cell membrane m a y indent the axon (Figs. 16 and 17). The m o s t characteristic feature of these junctions consists of periodic densities in the interspace between the two membranes at intervals of "~ 280 A (Figs. 4 - 1 2 ) . These densities are each 130 A wide and closely resemble the 'transverse bands' that occur in the paranodal region (Bargmann and Lindner, 1964; Andres, 1965; Hirano and Dembitzer, 1969). Typically at these junctions the plasma membrane of the Schwann cell is clearly defined as a trilaminar structure ~ 7 5 A wide; the axonal m e m b r a n e in contrast has a distinct inner dense lamina b u t often the outer dense lamina shows up only faintly (Fig. 4). In addition to these features, a flattened sac of endoplasmic reticulum is usually apposed to the junctional membrane within the Schwann cell cytoplasm (Figs. 3, 5 - 7 , 9 and 10), and sometimes periodic densities extend between the sac and the junctional plasma m e m b r a n e (Fig. 5). On the axonal side there appears to be a delicate layer of dense material subjacent to the membrane sometimes with periodic projections extending to the axolemma (Figs. 4, 9 and 16). The length of the junction in sections varies from ~ 0 . 0 8 tam (Fig. 5) to ~ 1.8 t~m, (Fig. 16) and the periodic densities within the junctional cleft have been seen both in transverse sections through the nerve fibres (Figs. 3 - 7 ) and in longitudinal sections (Figs. 8 and 9). Frequently, the cleft between the junctional membranes exhibits a uniform rather than a periodic density, however (Figs. 5, 6 a n d l 0 ) , suggesting that the densities are elongated in one axis and overlap when n o t sectioned across that axis. In a few instances, longitudinal sections have shown this type of junction f o r m e d at intervals between a Schwann cell and the same adjacent axon over an extended length ( ~ 1 5 / l m in Fig. 8). In addition, in transverse sections, Schwann cells may be seen forming such junctions with two or more different axons. Fig. 18. Transverse section showing an amyelinated axon, part of whose circumference faces a myelinated nerve fibre. This part of the axolemma has a distinct cytoplasmic undercoating. Fingerlike Schwann cell processes are apposed to its external surface and are separated from it by a thin interrupted basement membrane. The rest of the circumference of this axon faces other amyelinated axons and exhibits no membrane specialization, x 89 000. Fig. 19. The outermost Schwann cell layer of a myelin sheath gives rise to narrow finger-like projections which extend toward a neighbouring amyelinated axon. That portion of the axolemma facing these processes has a distinct cytoplasmic density associated with it (between arrows) but the remainder of the circumference facing other amyelinated axons does not exhibit this differentiation, x 46 000. Fig. 20. Longitudinal section showing fingerqike Schwann cell processes apposed to an axolemma which bulges outwards and has a cytoplasmic density underlying it (between arrows). Paranodal structures are not visible on either side of this region, x 28 000. Fig. 21. Trigeminal ganglion cell. The neuron appears fully differentiated but is bare of satellite cells in this region. The extracellular space contains cell debris, x 18 000. Fig. 22. Myelin sheath surrounding a group of axons, x 34 000.

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The Schwann cell surface in the region of these junctions is distinctive in two respects. Normal Schwann cells are completely invested by a continuous basement membrane that bridges across the extemal mesaxon and separates the space within the sheath from the connective tissue space. In the dystrophic roots, however (Figs. 3, 13, 14, 16 and 17), the basement membrane is incomplete or interrupted (Madrid et al., 1975) and the surrounding connective tissue occurs only in patches rather than filling up the extracellular space entirely. The junctions described above occur at such interruptions in the basement membrane, which apparently permit closer contact between the Schwann cells and adjacent axons than is ever seen in the normal peripheral nervous system. The second distinctive feature of the Schwann cell consists of cytoplasmic protrusions from the outermost layer. These projections , which are also not surrounded by a basement membrane, extend towards the adjacent naked axons and form junctions of the type described above (Figs. 13, 14, 16 and 17). Often these Schwann cell processes invaginate axons deeply and form junctions along the entire contact surface area (Fig. 16). Although Schwann cells show numerous cytoplasmic vesicles along their surfaces and many omega-shaped invaginations (Figs. 13 and 17) as well as occasional coated pits, no such profiles have been found within a junctional region. The second specialized complex formed at the interface between Schwann cells and amyelinated axons is illustrated in Figs. 18-20. It consists of multiple fingerlike processes of Schwann cell cytoplasm apposed to a naked axon. In this case, however, the projections remain separated from the axon by a larger cleft, and, in addition, a thin basement membrane is sometimes visible in this cleft between the processes and the axolemma (Fig. 18). Moreover, the axolemma itself in such regions has a distinct dense layer associated with its cytoplasmic surface (Figs. 19 and 20) but sometimes separated from it by a lucent region (Fig. 18). This constellation resembles the node of Ranvier of a peripheral myelinated nerve fibre where the axon is covered by fingerqike processes of the Schwann cells adjoining the node, and the axon has a characteristic cytoplasmic density underlying the plasma membrane (Peters, 1968). In the dystrophic roots, however, this configuration extends only part way around the axonal circumference. That portion of the axolemma facing the interior of the amyelinated bundle and apposed to other axons remains unspecialized; only the membrane facing the Schwann cell exhibits the differentiated features. Several other aspects of the nerve roots are also distinctive. Schwann cell processes sometimes extend in among the amyelinated fibres (Fig. 11) partially separating them from each other, and on occasion these processes enwrap individual axons and form junctions in which periodic densities are visible (Fig. 12). Other instances have been found of what appear to be immature myelinated axons which have relatively few lamellae and rather abundant Schwann cell cytoplasm. In addition, there appear to be some degenerating myelin sheaths in these roots marked by frank disruption of the lamellar system (Fig. 3). Nodes of Ranvier are sometimes flanked by a normal paranodal region on one side of the node and a degenerating or immature Schwann cell forming the paranodal region on the opposite side. Markedly

Fig. 23. Freeze-fracture replica of nerve root. The upper two fibres probably represent myelinated axons separated by a wide cleft. The remaining fibres are amyelinated axons, closely packed together, which exhibit randomly scattered intramembranous particles in both P and E fracture faces with no sign of clustering or paracrystailine regions, x 36 000. Fig. 24. Paranodal axotemma from a myelinated fibre in a dystrophic root. The paracrystalline pattern is characteristic of the E fracture face of this region and appears normal, x 24 000.

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widened nodes (Bradley etal., 1977), 'heminodes', and myelin formation around multiple axons (Fig. 22) have also been seen. Cranial nerves are known to be affected in this mutant (Biscoe et al., 1974), and within the trigeminal nerve, ganglion cells were encountered with extensive defects in their satellite cell sheaths and basement membranes (Fig. 21). Specimens fixed at two months and nine months of age are comparable qualitatively but the frequency and size of the specialized junctions between axons and Schwann cells appear to be different. In the younger specimens, these junctions are encountered more frequently and are more extensive with lengths of junctional membrane greater than 0.5 #m not uncommon. The Schwann cells are also more prone to invaginate the axons as in Figs. 16 and 17. In the older specimens, in contrast, these junctions are found less frequently, are smaller in size, often consisting of only a few transverse bands, and tend not to deform the axonal contour as much (Figs. 6 and 7). These changes with age suggest that such junctions once formed do not necessarily persist unchanged as the animal matures. Freeze-fracture replicas of dystrophic nerve roots (Figs. 23--27) reveal both myelinated and amyelinated nerve fibres. The amyelinated fibres (Fig. 23) were examined extensively for evidence of structuraUy-differentiated membranes comparable to those occurring in nodal and paranodal regions (Fig. 24). In well-fixed specimens only randomly distributed intramembranous particles were found, and typical paranodal membrane specializations were never seen. However, where amyelinated axons adjoin Schwann cell membranes, identified by the characteristic pits in the P face of the membrane (Fig. 25) or projections in the E face, occasional patches of aberrant paracrystalline membrane were seen. In Fig. 26, for example, the axolemma exhibits a region of paracrystalline striations which, however, have no consistent orientation and are clearly not part of any paranodal structure. They appear to radiate from a central 'hole' which probably represents the fractured neck of a Schwann cell process inx~aginating an axon (cf. Fig. 16). This region of paracrystalline axonal membrane presumably corresponds to one of the aberrant junctions that occur between the amyelinated axons and Schwann cells. In a few instances it has also been possible to identify paracrystaUine patterns in extraparanodal P faces that presumably represent the complementary specialization (Fig. 27). Fig. 25. Freeze-fracture replica showing the P face of a Schwann cell membrane containing a cluster of pits (P). The E face of the immediately adjacent membrane contains paracrystalline structures shown at higher magnification in Fig. 26. x 11 000. Fig. 26. Detail of Fig. 25. The paracrystalline pattern changes in orientation in several regions surrounding what probably represents the fractured neck of a Schwann cell process (of. Fig. 16). x 66 000. Fig. 27. P fracture face showing diagonal array of particles resemblingthat found in terminal glial loops in the paranodal region. In this case, however, the specialized membrane occurs as an isolated patch probably corresponding to the kind of extraparanodal membrane specialization shown in Fig. 26. x 73 000.

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Discussion

The principal findings reported here are that the amyelinated segments of axons in dystrophic mouse roots do not exhibit the typical nodal and paranodal membrane specializations characteristic of myelinated axons, but where they are directly apposed to Schwann cells belonging to neighbouring myelinated axons, isolated atypical nodal and paranodal membrane specializations do occur. These findings imply that axons are not able to form the normal differentiated membrane structures independently but only in response to immediate contact with appropriate Schwann or glial cells. Histochemical data also indicating that amyelinated dystrophic axons do not differentiate normally have been published recently by Waxman et al. (1978). Evidence that intimate contact with myelinating cells is required for the normal structural differentiation of the axolemma has been derived from studies of other mutants as well. It was reported previously (Rosenbluth, 1977, 1979a) that Jimpy mice, which lack central nervous system myelin almost completely but have apparently normal peripheral myelin, exhibit normal axolemmal differentiation peripherally but not centrally. In central nervous system fibres such as those of the dorsal columns, which are extensions of peripheral nerve fibres, the absence of these membrane specializations is not likely to be the result of an intrinsic axonal defect since the peripheral segments are normal. Presumably, it is the paucity and immaturity of the oligodendroglial cells and their failure to develop myelin around axons centrally that underlies the failure of differentiation of the axolemma within the Jimpy central nervous sytem. The Quaking mutant, which exhibits a variety of irregularities in its myelin has corresponding abnormalities in the form of the axolemmal membrane specializations (Rosenbluth, 1979a). The fact that the amye|tnated axons in dystrophic nerves form aberrant and incomplete membrane specializations where they are apposed to adjacent Schwann cells not only emphasizes the importance of the axon-Schwann cell interaction to axolemmal differentiation but indicates also that the axon need not be completely enveloped to be influenced in this way. As in the case of normal myelin, however, only axons of a certain minimum diameter are capable of this type of differentiation. It is also implicit from the observations that Schwann cells are fully able to form such specialized junctions with more than one axon at the same time, that is, one within the myelin sheath and one or more outside. Oligodendrocytes have long been regarded as capable of myelinating many axons simultaneously, while Schwann cells form myelin around only one at a time. However, it appears that where a Schwann cell that has already formed a myelin sheath becomes intimately apposed to other axons, which are amyelinated, it can form at least some of the structures characteristic of myelinated fibres. The Schwann cell is thus capable of interacting with more than a single axon at the same time. Similarly, it has been shown that a Schwann cell is capable of forming multiple paranodal type junctions at short intervals along the length of the same amyelinated axon. This too differs from what happens in normal

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myelinated nerve fibres where an extensive axoglial junction forms at both ends of an internode, markedly narrowing the gap between the axolemma and Schwann or glial cell membranes, but in the intervening region the respective membranes are normally separated by a 100 A wide gap. What prevents extensive junction formation in this intervening region, or, conversely, what normally induces junction formation at the ends of the internode remains unknown. Studies of pathological or experimentally altered tissues are sometimes useful in clarifying their normal counterparts. In this instance some of the structures observed in the aberrant junctions appear to be common to normal paranodal junctions, but have been overlooked in that location. Specifically, membranous cisternae within the Schwann cell loops and thin axoplasmic laminae are associated with the junctional membranes at normal paranodal axon-Schwann cell junctions as well (Rosenbluth, 1979b). The interconnections between the cisternal profiles and Schwann cell membrane are reminiscent of those occurring in muscle cells at triads and dyads between terminal cisternae of the sarcoplasmic reticulum and the sarcolemma (Rosenbluth, 1969; Franzini-Armstrong, 1970) and suggest the possibility of comparable functions. It is of interest also that the aberrant junctions that form in the dystrophic animal appear to be related to the defects in the basement membrane surrounding the Schwann cell and in the connective tissue of the amyelinated portion of the nerve. Bunge and Bunge (1978) have recently shown that in cultured nervous tissue amyelinated axons also can be demonstrated in regions where the growing nerve fascicles have failed to establish a proper contact with the underlying collagen substrate. The functional significance of the paranodal axoglial junction remains uncertain. Among the functions that have been proposed are: (1) Mechanical attachment between the respective membranes. The aberrant junctions formed by Schwann cell 'projections' in the dystrophic animals could thus represent regions in which the axons were at one time apposed to neighbouring Schwann cells, became attached to them, and during later movements of the cells became separated again, pulling along Schwann cell processes that remained attached to the original junctional sites. (2) Intercellular communication. The similarity between the junctions shown here and gap junctions with respect to proximity of the membranes and the presence of intercellular bridges suggests possible similarities in function. Communication between axons and Schwann cells or oligodendrocytes is well known as exemplified by the disintegration of distal myelin segments after axotomy even though those segments have not themselves been directly injured. The possibility of low-resistance ionic coupling at these junctions has also been proposed (Livingston et al., 1973) and could underlie the recently described 'ephaptic' communication between amyelinated and myelinated axons in dystrophic roots (Rasminisky, 1978). (3) Isolation of the nodal from the internodal periaxonal space. A barrier to ionic movements longitudinally along the outer surface of the axolemma would presumably increase the efficiency of saltatory conduction. However, tracer studies indicate that the barrier created by the paranodal junction may not be a complete one (Feder et al., 1969;

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Hirano and Dembitzer, 1969) and indeed measurements of nodal capacitance (Tasaki, 1955) also suggest that the effective nodal surface area is larger than it appears to be anatomically. (4) Restriction of intramembranous particle movements. The axon-Schwann cell junction may have the effect of 'structuring' the axolemma in the paranodal junctional region in such a way as to impede the movement of intramembranous sodium channel particles within the plane of the axolemma thus 'trapping' them at the node of Ranvier (Rosenbluth, 1976). The absence of these paranodal structural specializations girdling the amyelinated regions of dystrophic axons could accordingly result in a lack of concentration of sodium channels at periodic intervals, and as a result the amyelinated segments may be equivalent to unmyelinated fibres, which presumably exhibit continuous conduction. Rasminisky et al. (1978) have in fact directly demonstrated continuous conduction in dystrophic nerve roots, as have Bostock and Sears (1976) in demyelinated axons, suggesting the additional possibility that demyelination of an axon may result in dedifferentiation of its axolemma. Although it has not yet been possible to establish the function of the a x o n Schwann cell junctions in normal myelinated fibres, the occurrence of such junctions in aberrant locations may permit experimentation not previously feasible bearing on their significance. Acknowledgements This work was supported by grant NS-07495 from the National Institutes of Health and by a grant from the Muscular Dystrophy Association. References ANDRES, K. H. (1965) tJber die Feinstruktur besonderer Einrichtungen in markhaltigen Nervenfasern des Kleinhirn der R~ttte. Zeitscbriftfi~r Zellforscbung und mikroskopiscbe Anatomie 65, 701-12. BARGMANN, W. and LINDNER, E. (1964) Uber den Feinbau des Nebennierenmarkes des Igels (Erinaceus europaeus, L.). Zeitschrift fiir Zellforscbung und mikroskopiscbe Anatomic 64, 868-912. BISCOE, T. J., CADDY, K., PALLOT, D. J., PHERSON, U. M. and STIRLING, C. A. (1974)The

neurological lesion in the dystrophic mouse. Brain Researcb 76, 534--6. BOSTOCK, H. and SEARS, T.A. (1976) Continuous conduction in demyelinated mammalian nerve fibers. Nature 263,786-7. BRADLEY, W.G. and JENKISON, M. (1973) Abnormalities of peripheral nerves in murine muscular dystrophy. Journal of the Neurological Sciences 18, 227-47. BRADLEY, W. G., JAROS. E.. and ]ENKISON, M. (1977) Nodes of Ranvier in nerves of mice with muscular dystrophy. Journal of Neuropatbology and Experimental Neurology 36, 797-806. BUNGE, R. P. and BUNGE, M. B. (1978) Evidence that contact with connective tissue matrix is required for normal interaction between Schwann cells and nerve fibers. Journal of Cell Biology 78, 943-50. CONTI, F., HILLE, B., NEUMCKE, B., NONNER, W. and STAMPFLI, R. (1976) Conductance of

the sodium channels in myelinated nerve fibers with moderate sodium inactivation. Journal of Physiology 262, 729-42.

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Aberrant axon-Schwann cell junctions in dystrophic mouse nerves.

Journal o f Neurocytology 8, 6 5 5 - 6 7 2 (1979) Aberrant axon-Schwann cell junctions in dystrophic mouse nerves J. ROSENBLUTH Departments of Physio...
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