Myelin membrane structure and composition correlated: a phylogenetic study

We have correlated myelin membrane structure with biochemical composition in the CNS and PNS of a phylogenetic series of animals, including elasmobranchs, teleosts, amphibians, and mammals. X-ray diffraction patterns were recorded from freshly dissected, unfixed tissue and used to determine the thicknesses of the liquid bilayer and the widths of the spaces between membranes at their cytoplasmic and extracellular appositions. The lipid and protein compositions of myelinated tissue from selected animals were determined by TLC and sodium dodecyl sulfate-polyacrylamide gel electrophoresis/immunoblotting, respectively. We found that (1) there were considerable differences in lipid (particularly glycolipid) composition, but no apparent phylogenetic trends; (2) the lipid composition did not seem to affect either the bilayer thickness, which was relatively constant, or the membrane separation; (3) the CNS of elasmobranch and teleost and the PNS of all four classes contained polypeptides that were recognized by antibodies against myelin P0 glycoprotein; (4) antibodies against proteolipid protein (PLP) were recognized only by amphibian and mammalian CNS; (5) wide extracellular spaces (ranging from 36 to 48 A) always correlated with the presence of P0-immunoreactive protein; (6) the narrowest extracellular spaces (approximately 31 A) were observed only in PLP-containing myelin; (7) the cytoplasmic space in PLP-containing myelin (approximately 31 A) averaged approximately 5 A less than that in P0-containing myelin; (8) even narrower cytoplasmic spaces (approximately 24 A) were measured when both P0 and 11-13-kilodalton basic protein were detected; (9) proteins immunoreactive to antibodies against myelin P2 basic protein were present in elasmobranch and teleost CNS and/or PNS, and in mammalian PNS, but not in amphibian tissues; and (10) among mammalian PNS myelins, the major difference in structure was a variation in membrane separation at the cytoplasmic apposition. These findings demonstrate which features of myelin structure have remained constant and which have become specifically altered as myelin composition changed during evolutionary development.

Computer-assisted morphometry of the peripheral nervous system. A diagnostic tool

A formal systematization of the set of procedures required in peripheral nerve morphometry has been implemented in a computer program, in an attempt to provide an easy-to-handle diagnostic tool in current neuropathological practice. The hardware is composed of an IBM-PC compatible computer and a graphic tablet. Programs are written in BASIC and run in computers compatible with the MS-DOS operating system. The flow of operative steps involved in the morphometric study is controlled by the software. Sequential measurements are also tested for congruence. Nerve fibre measurements are stored in pluri-dimensional matrices for subsequent statistical calculations, i.e. evaluation of size-frequency distribution of nerve fibres and correlation analysis between nerve fibre subcomponents. Stereological estimates of the parenchymal components are also derived. The increase in accuracy achieved using area instead of diameter measurements is stressed. Compared to "dedicated" machines such personal computer-assisted systems exhibit substantial advantages in terms of low cost and flexibility.

Microtubular packing varies along the course of motor and sensory axons: possible regulation of microtubules by environmental cues

In the toad Xenopus laevis, the microtubular density of 3-microns myelinated fibres was assessed in peripheral nerves, dorsal and ventral roots, and dorsal and ventral funiculi of the spinal cord. In the roots, the axonal microtubular density was 6 microtubules/microns 2 and twice as much at the other sampling sites. This indicates that the pattern of the microtubular packing may vary along the course of the axon. We propose that axonal microtubules are regulated by local cues.

Wallerian degeneration in the peripheral nervous system: participation of both Schwann cells and macrophages in myelin degradation

This study examined the role of Schwann cells and hematogenous macrophages in myelin degradation and Ia antigen expression during Wallerian degeneration of rodent sciatic nerve. To identify and distinguish between macrophages and Schwann cells we used, in addition to electron microscopy, immunocytochemical staining of teased nerve fibres and 1 microns thick cryosections. Before the appearance of adherent macrophages the myelin sheath fragmented into ovoids, small whorls of myelin debris appeared within Schwann cell cytoplasm and the Schwann cell displayed numerous lipid droplets. However, at least in large fibres most myelin degradation and removal was accomplished or assisted by macrophages, identified by their expression of the ED1 marker. These cells began entering the nerve from blood vessels by day 2, migrated to degenerating nerve fibres and adhered to nerve fibres in the regions of the ovoids. There they penetrated the Schwann cell basal lamina to occupy an intratubal position and phagocytose myelin. During Wallerian degeneration a subpopulation of ED1-positive monocytes/macrophages expressed Ia antigen; Schwann cells were Ia-negative. Ia expression by monocytes/macrophages appeared to be a transient event and was not seen in post-phagocytic macrophages, as indicated by the fact that ED1-positive phagocytes with large vacuoles were Ia-negative. Our data show that both Schwann cells and macrophages play important roles in degrading and removing myelin during Wallerian degeneration. The expression of Ia antigen during Wallerian degeneration indicates that Ia expression need not necessarily reflect specific immune events but in some instances can represent a nonspecific response to PNS damage.

Slow recovery of central axons in acrylamide neuropathy

To study the differential vulnerability of central versus peripheral axons, we observed serial changes in conduction over comparable segments of central and peripheral axons of the primary sensory neuron in 17 rats with acrylamide neuropathy using somatosensory evoked potentials. Central conduction abnormalities persisted even after peripheral conduction and clinical abnormalities had recovered. Morphometric studies showed prompt restoration of the largest-diameter fibers in the peripheral nerve after clinical recovery but persistent or even more severe loss of large- and medium-sized fibers in the cervical gracile tract. This finding suggests that recovery from central-peripheral distal axonopathy begins in the largest peripheral axons, perhaps even at the expense of central axons, and that clinical recovery can occur at a time when central conduction remains abnormal. The selective central axonopathy found in certain chronic degenerative disorders may be a consequence of this slow central recovery process associated with chronic or intermittent metabolic derangements.

Peripheral nerve regeneration through blind-ended semipermeable guidance channels: effect of the molecular weight cutoff

Synthetic nerve guidance channels are used to better understand the cellular and molecular events controlling peripheral nerve regeneration. In the present study, the contribution of wound-healing molecules to peripheral nerve regeneration was assessed by varying the molecular weight cutoff of the tubular membrane. Nerve regeneration through polysulfone tubular membranes with molecular weight (Mw) cutoffs of 10(5) and 10(6) Da was analyzed in a transected hamster sciatic nerve model. Cohorts of 6 animals received tubes of either type for 4 or 8 weeks with the distal end of the polymer tube capped. Other cohorts of 6 animals received tubes of either type for 4 weeks with the distal nerve stump secured within the guidance channel so as to create a 4 or 8 mm gap between both nerve stumps. Both types of channels contained regenerated tissue cables extending to the distal end of the guidance channel at both 4 and 8 weeks in the absence of a distal nerve stump. The cables regenerated in the 10(5) Da channels were composed of nerve fascicles surrounded by a loose epineurial sheath, whereas those regenerated in the 10(6) Da channels were composed mainly of granulation tissue. The numbers of myelinated and unmyelinated axons were significantly greater in the 10(5) Da than in the 10(6) Da channels at both 4 and 8 weeks. Both types of channel contained regenerated tissue cables with numerous nerve fascicles when the distal nerve stump was present with either gap length. However, when the gap distance was 8 mm, the 10(6) Da channels contained significantly fewer myelinated axons than the 10(5) Da channels. The present study reveals that the Mw cutoff of a semipermeable guidance channel strongly influences the outcome of peripheral nerve regeneration, possibly by controlling the exchange of molecules between the channel's lumen and the external wound-healing environment. These results suggest that the wound-healing environment secretes humoral factors that can either promote or inhibit the nerve-regeneration process.

Peripheral nerve repair across a gap studied by repeated observation in a new window implant chamber

We have developed a new peripheral nerve chamber system which allows high resolution observation of the cellular events involved in nerve regeneration. The growth into the chamber is confined to a two-dimensional sheet resembling tissue culture. An intact blood supply forms within the chamber and by 100 days the nerve bridging the chamber has a nearly normal perineurium surrounding unmyelinated and myelinated axons. Degenerating axons are very rarely seen. The early growth in the chamber has a tendency to spread widely and form a two-dimensional sheet. This results in morphologies similar to those found in tissue culture and facilitates observation of individual elements. The initially wide tissue growth gradually re-models to form a bridge with a constant width similar to the width of the peroneal nerve. Occasionally a 'side arm' containing myelinated axons was retained even though the majority of axons appeared to loop back and rejoin the main bridge. Prefilling the chamber with Matrigel did not produce a significant enhancement of growth rate over that found following prefilling with sterile saline but did result in a more normally organized structure in the long term. Proximal ingrowth occurred at a similar rate in the absence of the distal stump. The structure of the proximal stump in the absence of the distal stump was similar to the structure when both stumps were present, including the presence of myelinated axons near the proximal port by 20 days. However, at subsequent stages the absence of a distal stump led to withdrawal of the proximal growth.

Use of lead aspartate block staining in quantitative EM autoradiography of phospholipids: application to myelinating peripheral nerve

For quantitation of electron microscope (EM) autoradiographs, micrographs must contain clear images which are relatively free of heavy metal precipitates. Satisfactory contrast is usually obtained by staining individual ultra-thin sections with lead citrate. It was recently reported that sequential block staining of tissue with ferrocyanide-reduced osmium tetroxide and lead aspartate produced excellent contrast for EM autoradiography, with sections relatively free of lead precipitate. This protocol avoids the manipulation involved in staining individual ultra-thin sections. We have adapted this method to quantitative EM autoradiographic studies, primarily of phospholipid metabolism in peripheral nerve. We show that block staining with lead aspartate provides: (a) ultrastructural contrast of routinely high quality for myelinated peripheral nerve; (b) high (greater than 98%) retention of glycero-labeled lipid during dehydration and embedment; and (c) a distribution of de novo tritiated glycerol-labeled lipid in ultra-thin sections that is quantitatively identical to the distribution recorded for samples stained by the more laborious post-embedment method. During a 2-hr labeling period in vivo, tritiated glycerol is incorporated into phosphatidylcholine (44%), phosphatidylethanolamine (22%), other phospholipids (16%), and neutral lipids (15%). The analysis of grain distribution in developing sciatic nerve labeled for 2 hr with tritiated glycerol demonstrates that myelinating Schwann cells play the major role in synthesis of endoneurial lipids. Lipid synthesis in myelinated fibers is localized in perinuclear regions of Schwann cell cytoplasm. These regions lie external to compact myelin. Unmyelinated fibers and other endoneurial cells independently incorporate glycerol into lipids.

Homing behaviour of regenerating axons in the amphibian limb

Following peripheral nerve deviation in the limbs of urodele amphibians axons regrow distally toward their previous target muscles (Holder et al. 1984; Proc. Roy. Soc. Lond. B 222, 477–489). This study describes analysis of this axon regeneration over time following deviation of the forearm flexor nerve in Triturus cristatus and the extensor cranialis nerve in the axolotl. Using horseradish peroxidase (HRP) axonal tracing, electrophysiology and electron microscopy, we describe the sequence of events leading to reestablishment of functional innervation. HRP fills reveal axons leaving the deviated nerve via a number of possible routes and they invariably grow distally. Many axons take a path close to that of the original nerve but others fasciculate forming parallel paths. Electrophysiology and electron microscopy show that axons in the deviated region of the nerve degenerate extensively compared with cut, but undeviated, controls. The results are discussed in terms of the possible axon-growth-promoting mechanisms that result in directed growth.

Laser-assisted nerve repair in primates

Studies on the peripheral nerves in rats and other species have helped in the development of laser-assisted nerve anastomosis (LANA), but offer little in evaluating the efficacy of this technique in primates. The authors present a study of LANA in the peripheral nerves of rhesus monkeys. Twelve adult rhesus monkeys underwent bilateral resection of a portion of the peroneal nerve followed by placement of autogenous sural nerve interposition fascicular grafts. The grafts were completed with conventional microsurgical suture technique on one side and with LANA on the other. At 5, 8, 10, and 12 months, the grafted nerves were evaluated for continuity, nerve conduction, and histology (both light and electron microscopy). No significant difference in continuity, conduction velocity, nerve degeneration, nerve regeneration, axon fiber number, or axon fiber density was found in any animal between grafts performed by conventional microsuture and LANA grafts. There was no difference in distal or proximal myelinated fiber density between the LANA grafts and the conventional microsuture grafts. It was concluded that LANA is as effective as microsurgical suture nerve anastomosis in a primate model of nerve repair and grafting.

Local enhancement of major histocompatibility complex (MHC) class I and II expression and cell infiltration in experimental allergic encephalomyelitis around axotomized motor neurons

The effect of unilateral peripheral nerve lesions on the inflammatory response of experimental allergic encephalomyelitis (EAE) in rat central nervous system (CNS) was studied. Immunostaining for major histocompatibility complex (MHC) antigens and T-cell subsets demonstrated that MHC class I expression was markedly enhanced in as well as around axotomized motor neurons and that MHC class II expression was induced on several cells, probably microglial cells, in close proximity to the axotomized motor neurons. There was also a pronounced increase in interleukin 2 receptor-positive lymphocytes as well as T-cells and the T-cell subsets on the injured as compared to the non-injured contralateral side. These effects were present particularly in the initial phase of EAE and persisted for several weeks. The results suggest that neurons may communicate immunoregulatory signals to their microenvironment and that retrograde axonal signals from the distant periphery may alter the immune response locally within the CNS.

Localization of Fc gamma receptors and complement receptors CR1 on human peripheral nerve fibres by immunoelectron microscopy

The localization of receptors for the Fc part of IgG (Fc gamma R) and for the complement C3b/C4b components (CR1) on human peripheral nerve fibres was investigated by indirect immunoperoxidase staining of frozen nerve sections with monoclonal antibodies. Transmission electron microscopy revealed that Fc gamma R and CR1 are localized to the entire surface membrane and inner membrane (axolemma) of the Schwann cell. Myelin and axons were not stained. The presence of Fc gamma R and CR1 in human Schwann cells adds further evidence for the immunocompetence of these cells.

Spinal cord and peripheral nerve pathology in AIDS: the roles of cytomegalovirus and human immunodeficiency virus

We examined spinal cords, nerve roots, or peripheral nerves of 27 patients who died with acquired immunodeficiency syndrome (AIDS) for the presence of cytomegalovirus (CMV) and human immunodeficiency virus (HIV) by immunoperoxidase techniques in paraffin-embedded tissue. Vacuolar myelopathy was seen in 8 of 26 spinal cords (31%) and microglial nodules were seen in 13 (50%). All of the patients with lateral column vacuolar myelopathy showed severe brain pathology. HIV antigens had been detected in the brains of 15 (55%) of the 27 patients but were detected in only 3 (11%) of 26 spinal cords and were not localized to regions of vacuolar myelopathy. This suggests that the vacuolar myelopathy may be due to a remote or indirect effect of HIV or other infectious agent. CMV antigens were detected in none of the patients who showed vacuolar myelopathy but were detected in 2 of the 13 with microglial nodules. Focal nerve root or peripheral nerve inflammation was seen in 7 patients; 4 had CMV antigens and none had HIV antigens. CMV appears to be an important cause of inflammatory peripheral neuropathy in AIDS patients.

The spectrum of changes on 20 nerve biopsies in patients with HIV infection

Nerve and muscle biopsies were performed on 20 patients with HIV infection and peripheral neuropathy. Nine patients had distal symmetrical peripheral neuropathy (DSPN) (six ARC and three AIDS), six had inflammatory demyelinating polyneuropathy (IDP) (three ARC, one AIDS, and two otherwise asymptomatic patients), one had mononeuropathy multiplex (MM) (AIDS), 1 had mononeuropathy (ARC), one had meningoradiculitis (AIDS), and two had areflexia-associated lymphocytic meningitides (ARC), DSPN exhibited axonal degeneration in four of nine cases and was associated with segmental demyelination in five of nine cases. IDP exhibited segmental demyelination associated with axonal degeneration in four of six cases. Demyelination was more frequent in asymptomatic patients (2 of 2 cases) and in ARC (7 of 12 cases), whereas axonal degeneration was predominant in AIDS (6 of 6 cases). Mononuclear cell infiltration was seen in 1 of 2 asymptomatic patients and in 11 of 12 ARC patients but was exceptionally found in AIDS (1 of 6 cases). Involvement of the walls of small vessels, mostly venules ("subacute microvasculitis"), was found in 1 of 2 asymptomatic patients, in 8 of 12 ARC patients, and never in AIDS. The polyclonal mononuclear cell population was composed mainly of Leu 2 (T8) positive cells in seven cases of ARC. No virions were seen in electron microscopy. HIV was isolated in two cases from the CSF or the nerve biopsy.

Diabetic and hypoglycemic neuropathy--a comparison in the BB rat

Functional and structural neuropathy was examined in hyperglycemic (diabetic) BB rats maintained on small maintenance doses of insulin, hyperglycemic BB rats receiving no insulin, and BB rats in whom hypoglycemia was induced by the administration of excessive insulin doses. The data were compared with those of non-diabetic age- and sex-matched BB rats. Functional deficits and structural abnormalities were comparable in diabetic rats with and without insulin supplementation, suggesting that the generally necessary insulin dosing in this model does not per se account for the neuropathy. Hypoglycemic neuropathy was characterized by slowing of nerve conduction velocity, marked loss of anterior horn motoneurons and Wallerian degeneration, as well as loss of large myelinated fibers, suggesting a neuropathy involving predominantly motoneurons. Diabetic neuropathy was not associated with nerve cell loss but showed marked axonal atrophy involving predominantly sensory fibers. Thus, diabetic and hypoglycemic neuropathies are two distinguishable entities under strict experimental conditions, but may overlap in human diabetic subjects in whom tight insulin control is desirable.

Cytokeratin filaments and desmosomes in the epithelioid cells of the perineurial and arachnoidal sheaths of some vertebrate species

Using electron microscopy and immunohistochemistry with a large panel of antibodies to various cytoskeletal proteins we have noted that the single- or multi-layered sheaths of epithelioid cells ("neurothelia") surrounding peripheral nerves (perineurial cells) or structures of the central nervous system, including the optic nerve (arachnoid cells), show remarkable interspecies differences in their cytoskeletal complements. In two anuran amphibia examined (Xenopus laevis, Rana ridibunda), the cells of both forms of neurothelia, i.e., perineurial and arachnoid, are interconnected by true desmosomes and are rich intermediate-sized filaments (IFs) of the cytokeratin type. Among higher vertebrates, a similar situation is found in the bovine and chicken nervous systems, in which the arachnoid cells of the meninges contain desmosomes and IFs of both the cytokeratin (apparently with restricted epitope accessibilities in the chicken) and the vimentin type, whereas the perineurial cells of many nerves contain cytokeratin IFs, often together with vimentin, but no desmosomes. In contrast, in rat arachnoidal and perineurial cells significant reactions have been observed neither for cytokeratins nor for desmosomes. In the human nervous system, cytokeratins and desmosomes have also not been seen in the various perineuria studied whereas desmosomes are frequent in arachnoidal cell layers which are dominated by vimentin IFs and only in certain small regions of the brain contain some additional cytokeratins. The occurrence of cytokeratins in the tissues found positive by immunohistochemistry has been confirmed by gel electrophoresis of cytoskeletal proteins, followed by immunoblotting. Our results emphasize both similarities and differences between the neurothelia on the one hand and epithelia or endothelia on the other, justifying classification as a separate kind of tissue, i.e., neurothelium. The observations of interspecies differences lead to the challenging conclusion that neither desmosomes nor cytokeratins are essential for the basic functions of neurothelial sheaths nor does the specific type of IF protein expressed in these cells appear to matter in this respect. The results are also discussed in relation to the cytoskeletal characteristics of other epithelioid tissues and of human neurothelium-derived tumors.

Congenital hypomyelinating polyneuropathy in two golden retriever littermates

Serial peripheral nerve biopsies from two golden retriever littermates with chronic neurologic disease were taken for morphologic and morphometric evaluation. Teased nerve preparations were difficult to interpret due to the lightness of myelin staining. Light and electron microscopic findings were characterized by the following: reduced number of myelinated axons, presence of myelinated sheaths inappropriately thin for the caliber of the fiber, poor myelin compaction, increased numbers of Schwann cell nuclei, increased concentration of neurofilaments in myelinated axons, many Schwann cells with voluminous cytoplasm, and increased perineurial collagen. Onion bulb formation was not seen. In contrast to control data, a poor correlation was seen between numbers of myelin lamellae (ML) and axonal circumference (AC). The frequency distribution of ML ranged from 5 to 55 lamellae in affected animals (mean, 28 lamellae) compared to 20 to 140 lamellae in controls (mean, 66 lamellae). The ML/AC ratio was significantly reduced (P less than 0.001) in nerves of affected dogs. Morphometric results indicated that fibers of all calibers were hypomyelinated.

Degenerative and regenerative processes in the peripheral nerve after disconnection and reanastomosis using the Neodym Yag laser

The development of neuroma after peripheral nerve surgery is still an unsolved problem. Such lesions are not yet controllable by conventional methods. Reports of results achieved using various types of laser surgery appear contradictory. In this experimental study on rats, the Neodym Yag laser was found to be superior the conventional methods. After severance and reanastomosis of the sciatic nerve using this type of laser, no development of neuroma was observed in any of the animals during the first three months of observation.

Light- and electron-microscopic study of M. leprae-infected armadillo nerves

Lesions in peripheral nerves of armadillos experimentally infected with Mycobacterium leprae were studied by light- and electron-microscopy. Bacilli could be found clearly inside axons of unmyelinated nerve fibers. Heavily bacillated Schwann cells were seen embracing unmyelinated axons with interrupted cytoplasmic membranes. This indicated the initiation of rupture of those cells which were responsible for the liberation of bacilli into the axons. The nerve lesions were divided into three grades according to their severity: grade I showed lesions focalized in the perineurium; grade II lesions were scattered inside nerve tissue; and in grade III lesions the nerve tissues were diffusely affected. No regressive changes, such as fibrosis or scar formation, were seen in the nerve lesions. Bacillated macrophages were not as foamy as those of human lesions, indicating that these bacillated cells were younger or more easily disrupted with a higher turnover than the cells in human lesions. This would promote the spread of lesions in armadillos, and would explain the less foamy appearance of the cells. We found bacilli inside lymphatics surrounding the nerves, substantiating the opinion that lesions spread to peripheral nerves not only by a hematogenous route but also by the lymphatics.

Myelin phagocytosis by peritoneal macrophages in organ cultures of mouse peripheral nerve. A new model for studying myelin phagocytosis in vitro

Organ cultures of degenerating nerve fascicles were exposed to cultured macrophages obtained by peritoneal lavage. Invasion of the nerve fascicle by phagocytes was shown by prelabeling with carbon and with electron microscopy. There was massive active phagocytosis of degenerating myelin sheaths. The invading phagocytic cell population was identified as Fc receptor-positive, Mac-1-positive macrophages by immunocytochemistry. The Schwann cell population persisted without significant myelin phagocytosis. The vitality of the Schwann cell population was shown by subsequent reimplantation of the organ cultures into host animals. The reimplants had retained their ability to remyelinate regenerating axon sprouts. These observations were made in cultures exposed to cytostatic agents. If cytostatic agents were omitted, there was proliferation of endogenous phagocytes in the nerve fascicles without added peritoneal cells. These endogenous phagocytes were identified as proliferating resident monocytes and were positive for the Fc receptor and Mac-1 markers. This model allows studies on how monocytes recognize and digest degenerating myelin apart from surviving Schwann cells.

There is no simple adequate sampling scheme for estimating the myelinated fibre size distribution in human peripheral nerve: a statistical ultrastructural study

Morphometric studies of peripheral nerves (PN) usually involve some sampling of the myelinated fibres (MF). In order to scrutinize the statistical properties of the sampling processes in common use and the reliability of the resulting estimates, a quantitative analysis of human superficial peroneal nerves from 8 different normal subjects was undertaken at the ultrastructural level, both in terms of MF spatial distribution and of their size distribution. This study used sampling rates involving more than 10% of the whole myelinated fibre population observed in each nerve fascicle. However, in nearly all the fascicles evaluated, the sampling fluctuations are so high that neither the number of axons nor their diameter distribution can be assessed with enough accuracy. A systematic study of the myelinated fibres shows that the spatial distribution of their size is not uniform. This marked heterogeneity in the MF size distribution imposes measurement of large enough samples (500 or 600 MFs usually represent about one-half or two-thirds of the whole MF population) in a way to secure a reliable enough estimate of the density and size distributions. However, the practical usefulness of sampling schemes requiring more than one-half of the whole MF population in a nerve fascicle, is questionable.

Ultrastructural changes of the peripheral nerve induced by vibration: an experimental study

To investigate the effects of vibration on the peripheral nerves, rabbits were exposed to vibration of 60 cycles/s frequency with 0.35 mm amplitude (acceleration: 51 m/s2) for two hours daily. After 150, 250, 450, and 600 hours vibration, thin sections of the saphenous and median nerves were examined under the electron microscope. Vibration was found to induce the following changes: (1) disruption of the myelin sheath and constriction of the axon, (2) accumulation of vacuoles in the nodal gap and paranodal region, (3) disorganisation of the paranodal end loops and detachment of the paranodal end loops from the axolemma, (4) dilatation of the Schmidt-Lanterman incisures (SLI) and increased density of SLI, and (5) disappearance of neurotubules and neurofilaments in axons. The diameters of myelin sheaths disrupted by vibration varied from 2 to 12 microns. The extent of the myelin disruption is proportional to the vibration dose.

Neuropathy in IgM lambda paraproteinemia. Immunoreactivity to neural proteins and chondroitin sulfate

In axonal neuropathies associated with IgM paraproteinemia, reports of antigen specificity of the M-protein are few. A patient with IgM paraproteinemia presented with progressive mononeuritis multiplex. IgM was found deposited in striking amounts in endoneurium and shown to bind specifically to neural proteins and chondroitin sulfates. Direct immune mechanisms, as well as the physical effects of IgM deposition, likely contributed to the development of the neuropathy.

Nerve fibre regeneration across the PNS-CNS interface at the root-spinal cord junction

Root-spinal cord regeneration was investigated in immature and adult rats. The elongation in the dorsal root of regrowing dorsal root axons, rerouted ventral root nerve fibres (cholinergic neurons) or hypogastric nerve fibres (catecholaminergic neurons) is impeded as they meet the astrocyte dominated CNS tissue of the root. The establishment of synaptoid nerve terminals as the regrowing axons encounter astrocytes indicates a mechanism for growth inhibition other than a physical impediment in the CNS environment. The glial cells of the CNS segment in the root are influenced by the type of regenerating nerve fibres in terms of maintenance, multiplication and phenotypic expression. After a dorsal root lesion in the neonatal rat several root axons may reinnervate the spinal cord. In these rats, the normal establishment of a CNS root segment has been disrupted and the PNS-CNS border is situated central to the root-spinal cord junction. Implantation of cut dorsal roots into the spinal cord of adult rats results in the extension of processes from intrinsic spinal cord neurons out into the root. After implantation of avulsed ventral roots into the ventro-lateral aspect of the cord, axonal regrowth and functional restitution of alpha-motoneurons could be demonstrated by intracellular recordings and injections with horseradish peroxidase. These results show that regeneration can occur across a PNS-CNS interface that has been established secondary to a trauma in the mature animal and in the immature animal before the astrocyte-rich CNS root segment has been developed.

Peripheral nerve regeneration through optic nerve grafts

Grafts of optic nerve were placed end-to-end with the proximal stumps of severed common peroneal nerves in inbred mice. It was found that fraying the proximal end of adult optic nerve grafts to disrupt the glia limitans increased their chances of being penetrated by regenerating peripheral nerve fibres. Suturing grafts to the proximal stump also enhanced their penetration by axons. The maximum distance to which the axons grew through the CNS tissue remained about 1.5 mm from 2-12 weeks after grafting. Schwann cells were seldom identified in the grafts. Varicose and degenerating nerve fibres were often seen within the grafts. Some varicose profiles were shown to be the terminal parts of axons within the grafts. Axons containing clusters of organelles resembling synaptic vesicles became more abundant in the longer-term grafts. Immunohistochemical studies performed on sutured grafts using a polyclonal antiserum to neurofilaments confirmed the impressions given by the electron microscopical observations. Grafts of neonatal optic nerve lacked myelin debris but were not usually penetrated by regenerating peripheral axons within a 6-week period. Sixty minutes after the intravenous injection of horseradish peroxidase, reaction product could be detected in the extracellular spaces around blood vessels in all types of living optic nerve graft. This indicates that blood-borne macromolecules could penetrate the grafts. However, the profiles of axons which were found within living optic nerve grafts had no obvious relationship to blood vessels and were usually surrounded by astrocytic processes. These results suggest that living astrocytes, rather than the absence of serum-derived trophic factors or the presence of CNS myelin, constitute the major barrier to the extension of axons and the migration of Schwann cells into CNS tissue.