Ultrastructural cytochemical localization of 5'-nucleotidase activity in axon-myelin-Schwann cell complex

5'-Nucleotidase activity has been localized at the ultrastructural level in the axon-myelin-Schwann cell complex. Sciatic nerves of rabbits of pre- and postnatal development were used. Positive reaction was found on the plasma membrane, basal lamina, cytoplasm, and finger-like processes of the Schwann cells; on the intraperiod lines of the compact myelin, on the surface of myelin sheath, in the split myelin lamellae in the paranodal regions and Schmidt-Lanterman clefts, in segments of outermost and innermost lamellae, split off from the interparanodal myelin, in the mesaxons (outer and inner), in the loose myelin lamellae in the earlier stages of myelinization; on the axolemma (especially in the nodal and paranodal segments), in the periaxonal space, axoplasm. The alterations of 5'-nucleotidase distribution were associated with the developing myelin sheath.     

Abnormal Schwann cell/axon interactions in the Trembler-J mouse

The Trembler-J ( Tr ^J ) mouse has a point mutation in the gene coding for peripheral myelin protein 22 (PMP22). Disturbances in PMP22 are associated with abnormal myelination in a range of inherited peripheral neuropathies both in mice and humans. PMP22 is produced mainly by Schwann cells in the peripheral nervous system where it is localised to compact myelin. The function of PMP22 is unclear but its low abundance (∼5% of total myelin protein) means that it is unlikely to play a structural role. Its inclusion in a recently discovered family of proteins suggests a function in cell proliferation/differentiation and possibly in adhesion. Nerves from Tr ^J and the allelic Trembler ( Tr ) mouse are characterised by abnormally thin myelin for the size of the axon and an increased number of Schwann cells. We report ultrastructural evidence of abnormal Schwann cell-axon interactions. Schwann cell nuclei have been found adjacent to the nodes of Ranvier whereas in normal animals they are located near the centre of the internodes. In some fibres the terminal myelin loops faced outwards into the extracellular space instead of turning inwards and terminating on the axon. In severely affected nerves many axons were only partially surrounded by Schwann cell cytoplasm. All these features suggest a failure of Schwann cell–axon recognition or interaction. In addition to abnormalities related to abnormal myelination there was significant axonal loss in the dorsal roots.     

Schwann Cell Properties: II. The Identity of Phagocytes in the Degenerating Nerve

The present study was undertaken to identify the cells from which phagocytes originate following traumatic injury to the sciatic nerves in rats. The morphologic evolution of the phagocytes was correlated daily with changes in axon, myelin, Schwann cell and neurilemmal tube at light and electron microscopic levels. Autoradiography with tritiated thymidine was employed to label the proliferative cells immediately before and following the injury. The result indicated that degeneration of Schwann cells occurred with the onset of myelin breakdown and that the degenerated products of myelin, axon and Schwann cells were removed by macrophages. While most of the macrophages were originally blood monocytes, some were derived from vascular pericytes. They penetrated the neurilemmal tubes on the third postoperative day and began engulfing first the Schwann cells and then the myelin and axons. Having filled their cytoplasm with debris, some macrophages moved out of the neurilemmal tube while others remained temporarily inside the envelopes formed by the persisting neurilemmal tubes—thus the macrophages can inherit a basal lamina from degenerated Schwann cells.     

An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves

Summary The quantitative relationships between the crossectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.     

P1 deficiency in shiverer myelin is expressed by Schwann cells in shiverer ↔ normal mouse chimaera nerves

The myelin basic protein (P1) deficiency in shiverer myelin is expressed in shiverer ? normal mouse chimaera nerves. Chimaera nerves examined with immunocytochemical techniques have revealed populations of both densely labeled and unreacting myelinated Schwann cells. Single axons can innervate both Schwann cell types, demonstrating that the expression of P1 in Schwann cell myelin is unrelated to the shiverer or normal genotype of the neuron. The coexistence of both Schwann cell types in single nerves indicates that multiple progenitor Schwann cells are allocated to developing nerves and the mosaic patterns expressed further suggest that such cells tend to proliferate relatively small coherent clones of Schwann cells.     

On the development of bone marrow innervation in new-born rats as studied with silver impregnation and electron microscopy,

This study of innervation of the bone marrow in new-born rats demonstrates that major signs of differentiation occur in the nerves at the end of the second week after birth. Myelinated nerve fibers begin to acquire their myelin sheath at this time. The Schwann cells show abundant ergastoplasmic reticulum. Some of these cells separate individual axons and wrap them up with the double membranes that form the myelin sheath. From then on, the nerves of the marrow contain both myelinated and nonmyelinated fibers. Fibroblasts also differentiate during this time, producing collagen fibers around the nerves. Some fibroblasts are interconnected by desmosome-like structures. Fibroblasts and collagen fibers form the connective tissue sheaths of the nerve (perineurium and endoneurium). Upon completion of the myelin sheath by the Schwann cells and the connective tissue sheath by the fibroblasts, nerves of the marrow acquire the morphological characteristics of the peripheral nerves of the adult animal. The fine structure of the axons in contact with the muscle fibers of the arterial wall correspond to Type 2-a of Watari. These nerve fibers are considered to be of sympathetic type. The time of maturation of nerves in the bone marrow coincides with the beginning of responsiveness to stimulatory and inhibitory conditions demonstrated in this organ by other authors.     

Double myelination of axons in the sympathetic nervous system of the mouse. I. Ultrastructural features and distribution

Summary This study has examined the structural features and distribution of doubly myelinated axons in normal adult and aged mice. Investigation focused on the superior cervical ganglion (SCG) and paravertebral sympathetic ganglia, which were extensively serial-sectioned for light and electron microscopy. In the SCG, the principal features of doubly myelinated regions were that an apparently normal myelinated axon was enclosed for part of its length by an additional (outer) myelinating Schwann cell. The separate nature of the inner and outer Schwann cells was emphasized by the consistent presence of individual nuclei in each, and by the presence of endoneurial space, often containing collagen fibrils, between the inner and outer cells. In some cases more than a single outer Schwann cell was present, arranged serially along the inner myelinated fibre. While double myelination forms through a mechanism involving displacement of an original myelinating Schwann cell by an interposed Schwann cell (see companion paper), we here provide evidence that in some instances the outer Schwann cell fails to retain any direct axonal contact, either with the axon centrally enclosed within the configuration or with any neighbouring axon. In contrast to the rat, delicate cytoplasmic processes often extended from the lateral extremes of outer Schwann cells. However, again no evidence for axonal contact was found, and similar processes also extended from the paranodal region of some singly myelinated non-displaced Schwann cells. Without exception the outer myelin sheath remained structurally intact, and characteristically underwent a series of conformational changes (progressive infolding of the paranodes and new areas of myelin compaction) which infer a continuing capacity of the outer Schwann cell to translocate myelin-specific components in a co-ordinated manner. A basal lamina was always present on the abaxonal plasma membrane of the outer cell, but not on the adaxonal surface except in areas involved in infolding, thus retaining the polarity which existed at the time of displacement from the axon. At single cross-sectional levels through the SCG, up to approximately 4% of myelinated axons were involved in double myelination. Double myelination was not detected in the sciatic nerve or in the paravertebral ganglia, thus indicating a predilection for the SCG as a site of development of these configurations. Though not challenging the role of the axon in initiating the formation of myelin, these data indicate that in this tissue myelin maintenance does not require direct contact between axonal and Schwann cell plasma membranes.     

Development of early postnatal peripheral nerve abnormalities in Trembler-J and PMP22 transgenic mice

Mutations in the gene for peripheral myelin protein 22 ( PMP22 ) are associated with peripheral neuropathy in mice and humans. Although PMP22 is strongly expressed in peripheral nerves and is localised largely to the myelin sheath, a dual role has been suggested as 2 differentially expressed promoters have been found. In this study we compared the initial stages of postnatal development in transgenic mouse models which have, in addition to the murine pmp22 gene, 7 (C22) and 4 (C61) copies of the human PMP22 gene and in homozygous and heterozygous Trembler-J ( Tr ^J ) mice, which have a point mutation in the pmp22 gene. The number of axons that were singly ensheathed by Schwann cells was the same in all groups indicating that PMP22 does not function in the initial ensheathment and separation of axons. At both P4 and P12 all mutants had an increased proportion of fibres that were incompletely surrounded by Schwann cell cytoplasm indicating that this step is disrupted in PMP22 mutants. C22 and homozygous Tr ^J animals could be distinguished by differences in the Schwann cell morphology at the initiation of myelination. In homozygous Tr ^J animals the Schwann cell cytoplasm had failed to make a full turn around the axon whereas in the C22 strain most fibres had formed a mesaxon. It is concluded that PMP22 functions in the initiation of myelination and probably involves the ensheathment of the axon by the Schwann cell, and the extension of this cell along the axon. Abnormalities may result from a failure of differentiation but more probably from defective interactions between the axon and the Schwann cell.     

Formation of central and peripheral myelin sheaths in the rat: An electron microscopic study

Formation of the myelin sheath in peripheral nerve of the newborn rat is compared with its formation in postnatal cerebral white matter. The unmyelinated central axon is bare and myelination begins by the spiral wrapping of an oligodendrocytic process around the axon. The paired membranes of this process fuse on their inside surfaces, lengthen, and spiral around the axon to make a loose sheath of major dense lines. Compact myelin results after fusion of the outside surfaces to form the intraperiod line. Cytoplasm is sparse in developing central myelin, usually being restricted to inner and outer tongues. Unmyelinated peripheral axons are enclosed within a mesaxon formed by the invagination and fusion of the outside surfaces of the Schwann cell plasma membranes. Loose myelin is produced by lengthening and spiralling of the mesaxon (intraperiod line) around the axon. As Schwann cell cytoplasm is extruded from between the spirals, the major dense line forms and compact myelin results. Trapped cytoplasm, a characteristic of developing peripheral myelin, is found in the internodal compact myelin sheath as the inner and outer collars and the Schmidt-Lanterman clefts.     

Interactions between Schwann cells and CNS axons following a delay in the normal formation of central myelin

Summary Irradiation of the rat spinal cord during the first postnatal week results in a profound reduction of oligodendrocyte myelin formation in the dorsal funiculi (DF). Despite this absence of myelin, however, axons in the irradiated region in the DF increase in diameter and approximate the size distribution seen in the control spinal cord. By 25 days of age Schwann cells are present in the irradiated DF where they undergo cell division and myelinate the axons. During the early stages of this myelin formation, these intraspinal Schwann cells exhibit a relationship to axons that is somewhat different from that seen in the normal developing peripheral nervous system (PNS). For example, within a given region, intraspinal Schwann cells myelinate axons of large diameter prior to ensheathing bundles of small diameter axons. Additionally, during myelination a Schwann cell will surround a single axon with multiple processes which appear to compete for contact with the axolemma. On axons of larger diameter, the elaboration of these processes is so excessive that it is often difficult to trace them back to the parent Schwann cell. Later, when a single process establishes several spirals about an axon, additional processes are no longer elaborated, and the extra processes disappear as myelin formation advances to the stage of compact lamellae. Thereafter, the myelin sheath continues to form in a normal manner. Excess processes have been observed during myelinogenesis in the normal developing PNS, but their frequency in that environment is much less than in the irradiated cord. These observations support the hypothesis that the signal(s) to initiate myelin formation are expressed on the axolemmal surface and are controlled by the neuron. In addition, these observations suggest that the delay in myelination results in an affinity or tropism between axons and Schwann cells which exceeds the level existing at the normal time of myelin formation.     

Regeneration of axons in the optic nerve of the adult Browman-Wyse (BW) mutant rat

Summary We have studied the regeneration of axons in the optic nerves of the BW rat in which both oligodendrocytes and CNS myelin are absent from a variable length of the proximal (retinal) end of the nerve. In the optic nerves of some of these animals, Schwann cells are present. Axons failed to regenerate in the exclusively astrocytic environment of the unmyelinated segment of BW optic nerves but readily regrew in the presence of Schwann cells even across the junctional zone and into the myelin debris filled distal segment. In the latter animals, the essential condition for regeneration was that the lesion was sited in a region of the nerve in which Schwann cells were resident. Regenerating fibres appeared to be sequestered within Schwann cell tubes although fibres traversed the neuropil intervening between the ends of discontinuous bundles of Schwann cell tubes, in both the proximal unmyelinated and myelin debris laden distal segments of the BW optic nerve. Regenerating axons never grew beyond the distal point of termination of the tubes. These observations demonstrate that central myelin is not an absolute requirement for regenerative failure, and that important contributing factors might include inhibition of astrocytes and/or absence of trophic factors. Regeneration presumably occurs in the BW optic nerve because trophic molecules are provided by resident Schwann cells, even in the presence of central myelin, oligodendrocytes and astrocytes. All the above experimental BW animals also have Schwann cells in their retinae which myelinate retinal ganglion cell axons in the fibre layer. Control animals comprised normal Long Evans Hooded rats, BW rats in which both retina and optic nerve were normal, and BW rats with Schwann cells in the retina but with normal, i.e. CNS myelinated, optic nerves. Regeneration was not observed in any of the control groups, demonstrating that, although the presence of Schwann cells in the retina may enhance the survival of retinal ganglion cells after crush, concomitant regrowth of axons cut in the optic nerve does not take place.     

Experimental diabetic neuropathy. Morphometric studies on the rat N. suralis in short-term streptozotocin-induced diabetes

Morphometric studies of sural nerves were performed in diabetic rats 35 or 44 d, respectively, after the administration of 60 mg/kg b.w. streptozotocin. Morphometry of photographed semithin sections was performed after whole-body glutaraldehyde perfusion both with the semiautomatic MOP Videoplan and the MOP AM 02 (Kontron, Munich, F.R.G.). The following parameters were registered: Area of nerves and fibers, perimeter of fibers, diameter of axons, thickness of myelin sheaths, form factor. No decrease of the total nerve area or of the myelinated area were found. Parameters area of fibers, thickness of myelin sheath and form factor decreased in diabetic animals. Axon diameter, ratio axon diameter-myelin sheath thickness and perimeter of fibers increased in the diabetic nerves. It is suggested that primary Schwann cell lesion is responsible for the observed myelin reduction.     

De- and remyelination in spinal roots during normal perinatal development in the cat: a brief summary of structural observations and a conceptual hypothesis

We have studied the perinatal development of large myelinated axons (adult D > 10 microm) in cat ventral and dorsal lumbosacral spinal roots using autoradiography and electron microscopy (serial section analysis). These axons acquire their first myelin sheaths 2-3 weeks before birth and show nearly mature functional properties first at a diameter of 4-5 microm, i.e. 3-4 weeks after birth. The most conspicuous event during this development takes place around birth, when a transient primary myelin sheath degeneration strikes already well myelinated although short 'aberrant' Schwann cells. The aberrant Schwann cells become completely demyelinated, then measuring about 10 microm in length, and are subsequently eliminated from their parent axons. Morphometry indicates that on average 50% of the Schwann cells originally present along a prospective large spinal root axon suffer elimination. Here it should be noted that in cat lumbo-sacral spinal roots, the longitudinal growth of myelinated Schwann cells that belong to the group containing what will be the largest fibers is on average twice that of their parent axons. The elimination phenomenon is particularly striking in the dorsal roots close to the spinal cord where CNS tissue invades the root for several hundred micrometres. Our observations suggest that, once demyelinated and then eliminated, Schwann cells (i.e. aberrant Schwann cells) colonize neighbouring axons, future myelinated as well as future unmyelinated ones. In the former case the immigrant Schwann cells appear to start myelin production, possibly risking a second demyelination and elimination. We take our observations to indicate that Schwann cells in the cat, during normal development, may switch iteratively between a 'myelin-producing' and a 'non-myelin-producing' phenotype. From a functional point of view the transient presence along a myelinated axon of intercalated unmyelinated segments approximately 10 microm long, due to aberrant Schwann cells, would mean a slowing down of the action potential. The rapid disappearance of aberrant Schwann cells during the two first postnatal weeks could then explain the progressing normalization of the leg-length conduction time.     

Myelin synthesis in the peripheral nervous system

By imposing saltatory conduction on the nervous impulse, the principal role of the myelin sheath is to allow the faster propagation of action potentials along the axons which it surrounds. Peripheral nervous system (PNS) myelin is formed by the differentiation of the plasma membrane of Schwann cells. One of the biochemical characteristics that distinguishes myelin from other biological membranes is its high lipid-to-protein ratio. All the major lipid classes are represented in the myelin membrane, while several myelin-specific proteins have been identified. During development, the presence of axons is required for the initiation of myelination, but the nature of the axonal signal is still unknown. The only certainties are that this signal is synthesized by axons whose diameter is greater than 0.7 microm, and that the signal(s) include(s) a diffusible molecule. Morphological studies have provided us with information concerning the timing of myelination, the mechanism by which immature Schwann cells differentiate into a myelinating phenotype and lay down the myelin sheath around the axon, and the accumulation and the structure of the myelin membrane. The last 20 years have seen the identification and the cDNA and gene cloning of the major PNS myelin proteins, which signalled the beginning of the knock-out decade: transgenic null-mutant mice have been created for almost every protein gene. The study of these animals shows that the formation of myelin is considerably less sensitive to molecular alterations than the maintenance of myelin. During the same period, important data has been gathered concerning the synthesis and function of lipids in PNS myelin, although this field has received relatively little attention compared with that of their protein counterparts.     

Segregation of NGF receptor in sensory receptors, nerves and local cells of teeth and periodontium demonstrated by EM immunocytochemistry

Summary Nerve growth factor receptor immunoreactivity (NGFR-IR) in sensory nerves and somatosensory receptors of adult rat dental and periodontal tissue was analysed using a monoclonal antibody (192-IgG) and electron microscopy. In dental and periodontal nerves, the unmyelinated axons and their Schwann cells had occasional labelling of their cell membranes, and myelinated axons had none. Dental free nerve endings in predentin had varied NGFR-IR: 15% were unlabelled, 25% had some axonal membrane NGFR-IR, and 60% had intense membrane label and cytoplasmic staining. In periodontal ligament there were two types of NGFR-IR somatosensory receptors: Ruffini mechanoreceptors had extensive NGFR-IR on apposed membranes of the terminal Schwann cell and nerve endings, but no labelling of the neural fingers which extended out into the ligament tissue; and thin fibres had intense membrane NGFR-IR and cytoplasmic stain.Non-neuronal NGFR-IR had cell specific patterns: perineurial and endoneurial cells and Ruffini terminal Schwann cells had NGFR-IR on cell membranes and inside numerous pinocytotic vesicles; Schwann cells along unmyelinated axons had NGFR-IR cell membrane intensities which varied depending on the NGFR-IR intensity of the enclosed axons; odontoblasts were unlabelled except at sites of contact with the NGFR-IR pulpal or neural cells; pulp fibroblasts in the subodontoblast zone had intense NGFR-IR all along their cell membrane; and ligament fibroblasts were unlabelled.The diverse NGFR-IR patterns described here suggest that there are specific categories of cellular expression and localization which correlate with somatosensory receptor type, and that specific patterns also characterize various non-neuronal cells in dental and periodontal tissue. Only the endoneurial cells, perineurial cells, and Ruffini terminal Schwann cells had NGFR-IR endocytotic vesicles, suggesting NGF internalization by high-affinity receptors.     

Transient modulation of Schwann cell antigens after peripheral nerve transection and subsequent regeneration

Summary Schwann cells within the distal portion of a transected nerve undergo a series of poorly understood events in response to injury and loss of axonal contact. These events may influence the regeneration of PNS neurons. In this study we examined the alteration of antigens located in the basal lamina, plasma membrane and cytoplasm of Schwann cells within the distal nerve stump: (a) after a complete transection of the sciatic nerve, and (b) subsequent to reestablished contact between regenerating axons and dedifferentated Schwann cells separated from contact with neurons. Visualization of laminin and heparan sulphate proteoglycan molecules at various intervals after nerve transection always revealed intact basal lamina channels. In response to loss of axonal contact, vimentin expression by Schwann cells within the distal nerve stump increased, becoming a predominant intermediate filament protein of the cytoskeleton while glial fibrillary acid protein (GFAP) expression decreased. This reversal in the prominence of intermediate filament proteins was maintained until the onset of axonal reinnervation, at which point expression of GFAP increased and vimentin decreased. Expression of the Schwann cell plasma membrane associated protein, C4, closely mimicked GFAP expression during axon degeneration and subsequent reinnervation. In the normal uninjured nerve, tissue plasminogen activator (tPA) and S-100 were localized in the region near the Schwann cell-axon interface and the outer Schwann cell plasma membrane. In response to loss of axonal contact, the S-100 and tPA immunoreactivity associated with the Schwann cell-axon interface was lost while that localized around the outer Schwann cell plasma membrane remained unchanged. The results of this study demonstrate that Schwann cells modulate a portion of their antigenic repertoire in response to a loss of axonal contact and after contact with regenerating axons.     

去细胞异体神经的形态结构及移植后的组织相容性

目的:为面神经缺损寻找一种理想的异体神经移植物。方法:取Wistar大鼠胫神经,经Triton X-100和脱氧胆酸钠溶液进行化学去细胞处理。将处理后的神经行组织学染色和免疫组织化学染色;并行异体移植修复面神经缺损,观察其组织相容性。结果:去细胞神经为一中空的神经基质管,其中的细胞和髓鞘成分被有效清除,神经基底膜被保留;异体移植后无明显炎症反应,无排斥和吸收反应,能引导宿主轴突和Schwann细胞增殖。结论:去细胞异体神经移植物具有良好的仿生性和组织相容性,可能用于修复面神经缺损。     

Redistribution of Schwann cells at the developing PNS–CNS borderline. An ultrastructural and autoradiographic study on the S1 dorsal root of the cat

In order to test our hypothesis that myelin-forming Schwann cells early during development, after having been eliminated from their parent axons, colonize neighbouring unmyelinated axons, we studied the distribution of Schwann cells at the PNS–CNS border in the feline S1 dorsal spinal root during pre- and postnatal development using electron microscopy and autoradiography. Myelination of axons peripheral to the PNS–CNS border began about 1.5 weeks before birth. The adult distribution of one-third myelinated and two-thirds unmyelinated axons was noted 3 weeks after birth. Analysis based on to-scale reconstructions of axon and Schwann cell samples from the first 6 postnatal weeks gave the following results. 1) CNS tissue appeared in the proximal part of the root around birth and expanded peripherally during the first three postnatal weeks. (2) The number of Schwann cells associated with myelinated axons decreased. (3) The number of Schwann cells associated with unmyelinated axons increased. (4) The mitotic activity of the Schwann cells was low at birth and nil after the first postnatal weak. (5) Apoptotic cell units were virtually absent. (6) Aberrant Schwann cells, i.e. short and very short Schwann cells with distorted and degenerating myelin sheaths, were common. (7) The endoneurial space contained numerous Schwannoid cells i.e. solitary cells surrounded by a basal lamina. (8) Cytoplasmic contacts between unmyelinated axons and aberrant Schwann cells or Schwannoid cells were observed. We take these results to support our hypothesis.     

Laminin in traumatized peripheral nerve: Basement membrane changes during degeneration and regeneration

Summary The changes in Schwann cell basement membrane associated with degeneration and regeneration during 50 weeks after transection of rat sciatic nerve were studied immunohistochemically with antibodies to laminin. In half of the animals, regeneration was prevented by suturing the nerve stumps aside, whereas in the rest spontaneous regeneration was allowed. Axonal regeneration was monitored with anti-neurofilament protein antibodies.In control nerves, basement membranes surrounding Schwann cells were visualized as circular, laminin-positive structures within the endoneurium. By 8 weeks after transection, Schwann cells had formed columns which were laminin-positive throughout their cross-sectional area and indistinguishable from basement membrane zones in both non-regenerating and regenerating nerves. As axons repopulated the distal stump, the normal shape of Schwann cell basement membrane tubes was slowly restored in freely regenerating nerves. In non-regenerating nerves, however, a striking atrophy of Schwann cell columns was observed. Regenerating axons were only seen inside laminin-positive tubular structures in all phases after 8 weeks in regenerating nerves. On the other hand, restoration of normal shape in laminin-positive basement membrane zones was coincident with appearance of axons in the distal stump, but it did not take place in chronically degenerating nerves.The results show that chronic degeneration leads to an atrophy of Schwann cell columns and results in a decrease in laminin immunoreactivity associated with them.     

Close apposition among neighbouring axonal endings in a neuroma

Summary Axons in intact peripheral nerve trunks constitute independent afferent and efferent communication channels. However, when nerves are severed, several different forms of axon-axon cross-excitation develop in association with the injury site. In this study we have examined experimental sciatic nerve-end neuromas in rats with special interest in the compartmentalization of individual axons, and the barriers that separate close neighbours. At postinjury times at which functional coupling is known to occur, neuromas were found to contain many examples of axons in which adjacent membrane faces come into close contact without an intervening Schwann cell process. These occur in bundles containing from two to as many as 30 individual nerve fibres wrapped in a common Schwann cell sheath. The surface area of close apposition between axon pairs ranges up to several tens of m². Closely apposed axon profiles may be outgrowing branches of a single parent axon, but anterograde tracer data indicate that many belong to independent neurons. Closely apposed axons are separated from one another, and from associated Schwann cell processes, by a cleft about 130 Å wide. No synapses, gap junctions or tight junctions were observed. Extracellular tracer studies using La³⁺ and Ruthenium Red indicated that the cleft system is patent, permitting the free diffusion of small molecules between the space adjacent to the axolemma and the bulk extracellular compartment. Together, these data provide a structural basis for interfibre interactions based on local electrical current flow (ephaptic crosstalk), as well as coupling mediated by K⁺ ions and neurotransmitter molecules.