THE RELATIONSHIP BETWEEN SCHMIDT-LANTERMANN INCISURES AND MYELIN SEGMENTATION DURING WALLERIAN DEGENERATION

Toxic Peripheral Neuropathies: Agents and Mechanisms

Toxic peripheral neuropathies are an important form of acquired polyneuropathy produced by a variety of xenobiotics and different exposure scenarios. Delineating the mechanisms of neurotoxicants and determining the degenerative biological pathways triggered by peripheral neurotoxicants will facilitate the development of sensitive and specific biochemical-based methods for identifying neurotoxicants, designing therapeutic interventions, and developing structure-activity relationships for predicting potential neurotoxicants. This review presents an overview of the general concepts of toxic peripheral neuropathies with the goal of providing insight into why certain agents target the peripheral nervous system and produce their associated lesions. Experimental data and the main hypotheses for the mechanisms of selected agents that produce neuronopathies, axonopathies, or myelinopathies including covalent or noncovalent modifications, compromised energy or protein biosynthesis, and oxidative injury and disruption of ionic gradients across membranes are presented. The relevance of signaling between the main components of peripheral nerve, that is, glia, neuronal perikaryon, and axon, as a target for neurotoxicants and the contribution of active programmed degenerative pathways to the lesions observed in toxic peripheral neuropathies is also discussed.

Behind the pathology of macrophage-associated demyelination in inflammatory neuropathies: demyelinating Schwann cells

In inflammatory peripheral demyelinating disorders, demyelination represents segmental demyelination in which the myelin sheath of a myelinating Schwann cell (SC) is completely removed by macrophages or a partial myelin degeneration in the paranode occurring due to autoantibodies attacking the node/paranode. For the segmental demyelination from living myelin-forming SCs, macrophages infiltrate within the endoneurium and insinuate between myelin lamellae and the cytoplasm of SCs, and the myelin is then removed via phagocytosis. During the macrophage invasion into the SC cytoplasm from the node of Ranvier and internodal areas, the attacked SCs do not remain quiescent but transdifferentiate into inflammatory demyelinating SCs (iDSCs), which exhibit unique demyelination pathologies, such as myelin uncompaction from Schmidt-Lanterman incisures with myelin lamellae degeneration. The longitudinal extension of this self-myelin clearance process of iDSCs into the nodal region is associated with the degeneration of nodal microvilli and paranodal loops, which provides a potential locus for macrophage infiltration. In addition to the nodal intrusion, macrophages appear to be able to invade fenestrated internodal plasma membrane or the degenerated outer mesaxon of iDSC. These SC demyelination morphologies indicate that the SC reprogramming to iDSCs may be a prerequisite for macrophage-mediated inflammatory demyelination. In contrast, paranodal demyelination caused by autoantibodies to nodal/paranodal antigens does not result in iDSC-dependent macrophage infiltration and subsequent segmental demyelination. In the context of inflammatory demyelination, the novel perspective of iDSCs provides an important viewpoint to understand the pathophysiology of demyelinating peripheral neuropathies and establish diagnostic and therapeutic strategies.

Wallerian demyelination: chronicle of a cellular cataclysm

Wallerian demyelination is characteristic of peripheral nerve degeneration after traumatic injury. After axonal degeneration, the myelinated Schwann cell undergoes a stereotypical cellular program that results in the disintegration of the myelin sheath, a process termed demyelination. In this review, we chronologically describe this program starting from the late and visible features of myelin destruction and going backward to the initial molecular steps that trigger the nuclear reprogramming few hours after injury. Wallerian demyelination is a wonderful model for myelin degeneration occurring in the diverse forms of demyelinating peripheral neuropathies that plague human beings.

Autophagic myelin destruction by Schwann cells during Wallerian degeneration and segmental demyelination

As lysosomal hydrolysis has long been suggested to be responsible for myelin clearance after peripheral nerve injury, in this study, we investigated the possible role of autophagolysosome formation in myelin phagocytosis by Schwann cells and its final contribution to nerve regeneration. We found that the canonical formation of autophagolysosomes was induced in demyelinating Schwann cells after injury, and the inhibition of autophagy via Schwann cell-specific knockout of the atg7 gene or pharmacological intervention of lysosomal function caused a significant delay in myelin clearance. However, Schwann cell dedifferentiation, as demonstrated by extracellular signal-regulated kinase activation and c-Jun induction, and redifferentiation were not significantly affected, and thus the entire repair program progressed normally in atg7 knockout mice. Finally, autophagic Schwann cells were also found during segmental demyelination in a mouse model of inflammatory peripheral neuropathy. Together, our findings suggest that autophagy is the self-myelin destruction mechanism of Schwann cells, but mechanistically, it is a process distinct from Schwann cell plasticity for nerve repair.

Rac1 GTPase controls myelination and demyelination

After peripheral nerve injuries, Wallerian degeneration starts with a stereotypic fragmentation of myelin sheath into myelin ovoids, which occur near Schmidt-Lantermann incisures (SLI). This demyelination process requires a dramatic change in cytoskeletal structures in Schwann cells. We have recently shown that actin polymerization around SLI is an important step for cleavage of the myelin sheath. We described that Rac1 GTP ase regulates actin polymerization in SLI after injury. It has been previously reported that Rac-dependent cytoskeletal reorganization also plays an important role in myelination during the development of peripheral nerves. Thus, our findings suggest that Rac-dependent actin polymerization controls both myelination and demyelination in the peripheral nerves. We further discuss our new findings in relation to Schwann cell dedifferentiation and segmental demyelination.

ATP release through lysosomal exocytosis from peripheral nerves: the effect of lysosomal exocytosis on peripheral nerve degeneration and regeneration after nerve injury

Studies have shown that lysosomal activation increases in Schwann cells after nerve injury. Lysosomal activation is thought to promote the engulfment of myelin debris or fragments of injured axons in Schwann cells during Wallerian degeneration. However, a recent interpretation of lysosomal activation proposes a different view of the phenomenon. During Wallerian degeneration, lysosomes become secretory vesicles and are activated for lysosomal exocytosis. The lysosomal exocytosis triggers adenosine 5′-triphosphate (ATP) release from peripheral neurons and Schwann cells during Wallerian degeneration. Exocytosis is involved in demyelination and axonal degradation, which facilitate nerve regeneration following nerve degeneration. At this time, released ATP may affect the communication between cells in peripheral nerves. In this review, our description of the relationship between lysosomal exocytosis and Wallerian degeneration has implications for the understanding of peripheral nerve degenerative diseases and peripheral neuropathies, such as Charcot-Marie-Tooth disease or Guillain-Barré syndrome.

Adenosine 5'-triphosphate (ATP) inhibits schwann cell demyelination during Wallerian degeneration

Adenosine 5'-triphosphate (ATP) is implicated in intercellular communication as a neurotransmitter in the peripheral nervous system. In addition, ATP is known as lysosomal exocytosis activator. In this study, we investigated the role of extracellular ATP on demyelination during Wallerian degeneration (WD) using ex vivo and in vivo nerve degeneration models. We found that extracellular ATP inhibited myelin fragmentation and axonal degradation during WD. Furthermore, metformin and chlorpromazine, lysosomal exocytosis antagonists blocked the effect of ATP on the inhibition of demyelination. Thus, these findings indicate that ATP-induced-lysosomal exocytosis may be involved in demyelination during WD.

Actin polymerization is essential for myelin sheath fragmentation during Wallerian degeneration

The mechanisms that trigger Wallerian degeneration (WD) of peripheral nerves after injury are not well understood. During the early period of WD, fragmentation of myelin into ovoid structures occurs near the Schmidt-Lantermann incisures (SLI), a noncompact region of the myelin sheath containing autotypical adherens junction. In this study, we found that new filamentous actin polymerization occurs in the SLI of mouse sciatic nerves after injury and that its inhibition prevented not only the degradation of E-cadherin in the SLI but also myelin ovoid formation. However, the inhibition of actin polymerization could not block Schwann cell dedifferentiation. The activation of Rac GTPase was observed in the distal stump of the injured nerves, and a specific Rac inhibitor, a dominant-negative Rac, and Rac1-RNA interference blocked myelin ovoid formation. Together, these findings suggest that dynamic changes in actin in the SLI are essential for initiation of demyelination after peripheral nerve injury.

Demyelination secondary to chronic nerve compression injury alters Schmidt-Lanterman incisures

The role of Schmidt-Lanterman incisures (SLIs) within the myelin sheath remains the subject of much debate. Previous studies have shown a positive correlation between the number of SLIs per internode and internodal width for both normal and pathological myelin internodes. As chronic nerve compression (CNC) injury induces demyelination, we sought to evaluate if CNC injury altered the occurrence of SLIs using nerve-teasing techniques and light microscopy. Rigorous examination of the teased axons from nerves subjected to CNC injury for 1 month, 2 months or 8 months revealed that there is indeed a positive correlation between the number of SLIs per internode and the internodal width. However, unlike previous studies, the degree of positive correlation between these two parameters was greater in the internodes that had undergone remyelination in response to CNC injury as compared with the internodes from control nerves. These findings support the theory that SLIs are likely to assist in the metabolic processes of the myelin sheath, including growth and maintenance of the myelin sheath.

Connexin29 and connexin32 at oligodendrocyte and astrocyte gap junctions and in myelin of the mouse central nervous system

The cellular localization, relation to other glial connexins (Cx30, Cx32, and Cx43), and developmental expression of Cx29 were investigated in the mouse central nervous system (CNS) with an anti-Cx29 antibody. Cx29 was enriched in subcellular fractions of myelin, and immunofluorescence for Cx29 was localized to oligodendrocytes and myelinated fibers throughout the brain and spinal cord. Oligodendrocyte somata displayed minute Cx29-immunopositive puncta around their periphery and intracellularly. In developing brain, Cx29 levels increased during the first few postnatal weeks and were highest in the adult brain. Immunofluorescence labeling for Cx29 in oligodendrocyte somata was intense at young ages and was dramatically shifted in localization primarily to myelinated fibers in mature CNS. Labeling for Cx32 also was localized to oligodendrocyte somata and myelin and absent in Cx32 knockout mice. Cx29 and Cx32 were minimally colocalized on oligodendrocytes somata and partly colocalized along myelinated fibers. At gap junctions on oligodendrocyte somata, Cx43/Cx32 and Cx30/Cx32 were strongly associated, but there was minimal association of Cx29 and Cx43. Cx32 was very sparsely associated with astrocytic connexins along myelinated fibers. With Cx26, Cx30, and Cx43 expressed in astrocytes and Cx29, Cx32, and Cx47 expressed in oligodendrocytes, the number of connexins localized to gap junctions of glial cells is increased to six. The results suggested that Cx29 in mature CNS contributes minimally to gap junctional intercellular communication in oligodendrocyte cell bodies but rather is targeted to myelin, where it, with Cx32, may contribute to connexin-mediated communication between adjacent layers of uncompacted myelin.

Myelin basic protein gene dosage effects in the PNS

Myelin basic protein (MBP) plays an essential adhesive role in the formation of compact myelin in the central nervous system (CNS), but not in the peripheral nervous system (PNS). Morphologic data suggest that MBP controls the number of cytoplasmic channels or Schmidt-Lanterman incisures (SLI) present in PNS myelin. The levels of connexin-32 (Cx32) and myelin-associated glycoprotein (MAG), two components of the incisures, are inversely proportional to the levels of MBP in sciatic nerves of mice affected by the shiverer (shi) mutation, while protein zero (P0) and peripheral membrane protein 22 (PMP22), two structural components of compact myelin, remain constant. The levels of P0, PMP22, Cx32, and MAG mRNA do not vary in relationship to the levels of MBP. This indicates that MBP exerts its effect on Cx32 and MAG at a posttranscriptional level and suggests a new function for MBP in regulating gene expression in the PNS.

The number of Schmidt-Lanterman incisures is more than doubled in shiverer PNS myelin sheaths

In the PNS, myelin basic protein (MBP) appears not to be essential for myelination, for in shiverer (shi) and mld mutant mice peripheral nerves, where MBP is not or only poorly expressed, myelination occurs normally. Only a few morphological abnormalities, i.e. reduction in axon calibre and myelin sheath thickness, and aberrant Schwann cell-axon contacts, have been reported. Here, we document a consistent difference between shi and wild type (wt) myelinated sciatic nerve fibres. The number of Schmidt-Lanterman incisures seen in longitudinally and transversely-sectioned sciatic nerves, or in teased fibres stained for the presence of F-actin, is dramatically increased in homozygous shi mice. With both methods, a twofold increase in Schmidt-Lanterman incisure number is seen in 15-day-old mice, the earliest time examined. The increase is slightly greater in nerve fibres from 30- and 90-day-old mice. The overproduction of Schmidt-Lanterman incisures in shi occurs in spite of the fact that the mean diameter of myelinated fibres in shi sciatic nerves is smaller than in wt sciatic nerves. These results lead us to suggest that the increase in Schmidt-Lanterman incisure density in shi compensates for a defect in Schwann cell-axon communication.

Vimentin-positive rosary-bead-like formations in the rabbit superior colliculus during postnatal development

Strings of vimentin-positive rosary-bead-like formations in the rabbit superior colliculus were studied. The size and distance between beads varied from one string to another, and within a given string. The formations were similar in number and distribution in the two superior colliculi. Successions of beads were found mainly in the medial and deep layers, and represented not more than 1% of the radial glial fibers. The strings were in continuity at deep levels with segments of varicose fibers of radial glia, which in turn were continuous with fibers of normal appearance. These observations suggest a temporal sequence of evolution, starting with the normal radial glial fiber, which acquires swellings that in turn give rise to discrete beads; in subsequent stages, the beads become smaller and more widely separated. In the newborn rabbit, these formations are seen mainly in laterobasal regions of the superior colliculus, and spread throughout the colliculus by the end of week one of postnatal life, becoming more numerous in medial regions. We suggest that these formations represent a mechanism of removal of at least part of the radial glial fibers, and discuss the possible relation between these formations and the transformation of radial glia into astrocytes.

Fine structural evaluation of altered Schmidt-Lanterman incisures in human sural nerve biopsies

Fine structural alterations of Schmidt-Lanterman incisures (SLI) were investigated in a series of 242 unselected sural nerve biopsies that had been examined for diagnostic purposes. The series included cases with Friedreich's ataxia, HSAN I, HMSN I-III, HMSN VI, tomaculous neuropathy, metachromatic leukodystrophy, ceroidlipofuscinosis, dysproteinemic neuropathies, and myotonic dystrophy, in addition to several neuropathies less-specifically classified as either of a predominantly demyelinating, axonal, or neuronal type. The following classification of SLI alterations is proposed: (A) abnormal inclusions; (B) changes in shape and dimension; and (C) modes of disintegration. Abnormal inclusions comprised membranous whorls, uniform and pleomorphous lysosome-like bodies, and accumulation of granular substances at the site of the major dense line, or granular deposits at the site of the intraperiod line of the myelin sheath. Variations of incisural shape and dimension included folding, dilatation, and pocket formation (compartmentalization). Disintegration at incisures comprised a fine, vesicular and a gross, vacuolar type. Various combinations of these changes were observed. The most frequent change consisted of membranous whorls, detected in SLI of 89 biopsies. They were most prominent in chloroquine neuropathy where they occurred in SLI as well as in the adaxonal and abaxonal cytoplasm of Schwann cells. Compartmentalization of the myelin sheath at incisures associated with formation of myelin loops was a frequent feature in myotonic dystrophy. It is concluded, that changes of incisural ultrastructure are sensitive indicators of human neuropathies offering clues to the type of the underlying pathomechanism.

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.

Filipin-sterol complexes at Schmidt-Lanterman incisures

Employing the freeze-fracture technique, the distribution of filipin-sterol complexes was determined for membranes of peripheral nerve myelin. A heterogeneous distribution of complexes was observed with the greatest abundance on membranes associated with the cytoplasmic channels of Schmidt-Lanterman and longitudinal incisures. In addition there was an irregular network of well-labelled membrane bands in compact myelin. The results are related to a possible role for these channels and bands in the biochemical turnover of cholesterol in myelin.

The role of Schmidt-Lanterman incisures in Wallerian degeneration

The number of the Schmidt-Lanterman incisures and their intrasegmental distribution were studied at 36 h after transection of the rat sciatic nerve. Examination of teased, proximo-distally oriented, myelinated nerve fibers revealed no difference between the distal and the proximal stump. The results indicate that no proliferation of the incisures is required for the fiber fragmentation: numerous incisures are normally available in the midinternodal area where the degeneration begins.

Quantitation of the Schmidt-Lanterman incisures in juvenile, adult, remyelinated and regenerated fibres of the chicken sciatic nerve

Morphometric studies of peripheral nerves in various species have shown that the number of Schmidt-Lanterman incisures in an internode is proportional to the fibre diameter. In the rat sural nerve it appears that following remyelination, the relationship between the numbers of incisures per internode and the fibre diameter remains unaltered despite the fact that the remyelinated internodes are uniformly short. We quantitatively examined the insertion of incisures in adult, juvenile, remyelinated and regenerated fibres of the chicken sciatic nerve. Remyelinated fibres were examined 100 and 200 days following the intraneural injection of diphtheria toxin, and regenerated fibres 200 days after a nerve crush. Our results show that the incisures are more abundant in the chicken than in all other species previously reported, and for both juvenile and adult hens the number of incisures in an internode is proportional to the fibre diameter. Following both remyelination and regeneration the internodal lengths were shorter than control fibres and the distances between the incisures were reduced by approximately 20% for all fibre diameters. Significantly, the numbers of incisures in an internode were related to the length of the remyelinated or regenerated internode and not to the fibre diameter. This finding is in marked contrast to previous reports for rat peripheral nerve. Our findings are discussed in relation to the hypothesis that the Schmidt-Lanterman incisures are vital to the maintenance of the myelin sheath and/or the associated axon.

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

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

Pattern of myelin breakdown during sciatic nerve Wallerian degeneration: reversal of the order of assembly

Myelin sheaths of rapidly growing rats were sequentially labeled with the 3H and 14C isotopes of leucine as precursors of protein synthesis. The two injections were separated by time intervals ranging from 2 to 12 d. Wallerian degeneration was initiated by sciatic nerve neurotomy at 2 or 10 d after the second injection of radioactivity. After 5 d of degeneration, myelin was purified and the ratio of isotopes was determined in the delipidated protein. Regardless of the order in which the two isotopes were administered, the relative recovery of radioactivity resultant from the second injection was greatly reduced in degenerating nerves compared with sham-operated controls. Radioactivity incorporated from the first injection was also reduced, but to a lesser extent. Consequently, the isotope ratio corresponding to the first/second injection was greater in degenerating nerves than in controls, and the ratio increased in proportion to the time interval separating the two injections. The magnitude of the effect of degeneration was only slightly greater when degeneration was initiated 2 d after the second injection than when initiated 10 d after the last injection. Consequently, myelin disintegration rather than diminished incorporation of radioactivity accounts for the losses of radioactivity. Furthermore, the pattern of myelin degeneration preferentially involves the last myelin to be formed.

Schmidt-Lanterman Incisures. I. A quantitative teased fibre study of remyelinating peripheral nerve fibres

The quantitative relationship between the number of Schmidt-Lanterman incisures per internode and fibre diameter has been investigated previously in normal, developing, and regenerating fibres and from such data inferences have been made concerning the function of incisures. We have, therefore, examined this quantitative relationship in remyelinating fibres and assessed the implications for incisural function. Using the rat sural nerve, demyelination was induced by local microinjection of lysophosphatidyl choline. At 100 days and 200 days post-injection remyelinating fibres together with normal control fibres were examined quantitatively for the distribution of Schmidt-Lanterman incisures and nodes of Ranvier. Regression lines relating the number of incisures per internode to fibre diameter showed a positive correlation and were not significantly different between the three groups of fibres. The results are compatible with those for normal, developing and regenerating fibres and support the hypothesis of a homeostatic mechanism controlling the number of incisures in relation to fibre diameter and myelin volume. However, in remyelinating fibres, as in regenerating fibres, there is a greater incisural frequency as a consequence of the short internodes.

The role of Schmidt-Lanterman incisures in Wallerian degeneration. II. An electron microscopic study

The electron microscopy of changes at Schmidt-Lanterman incisures in Wallerian degeneration has been described only briefly previously. We have demonstrated that the changes up to 36 h after nerve crush are chiefly peri-incisural. At 12 h and 24 h 'incisural dilatation' consisted of an intraperiod line separation of peri-incisural myelin lamellae, which began among inner (adaxonal) lamellae extending later to outer (abaxonal) lamellae. The incisure itself showed little or no change. At 36 h, ovoid formation was apparent in most fibres. The sites of fibre cleavage to form ovoids occurred adjacent to incisures at the focal regions of myelin lamellae separation. Even within ovoids the incisures themselves remained intact at 36 h. The fine structural changes at incisures following nerve crush provide an understanding of the increased perceptibility of incisures by light microscopy during early Wallerian degeneration.

Endoneurial fluid pressure in wallerian degeneration

Endoneurial fluid pressure (EFP) was recorded by an active, servo-null pressure system after a glass micropipette was inserted into rat sciatic nerve undergoing wallerian degeneration. The lesions were produced by crushing the left sciatic nerve of the anesthetized animal at its point of entry into the thigh. Eighty-four animals were employed in this experiment, in which EFP was recorded from sham-operated rats and other controls as well as from rats with wallerian degeneration. The experiment was designed so that EFP could be recorded from 2 or more experimental animals at daily intervals starting at day 0 and concluding on day 28. Pressure progressively increased during the first week, reaching a peak elevation four to five times normal. The subsequent decline in EFP was more gradual, with values approaching normal during the third week after injury. Linear regression analysis showed the progressive increase in EFP to be statistically significant (p less than or equal to 0.01). To determine the time at which EFP was maximum, we used the Marquardt computer algorithm for lease-squares estimation of nonlinear variables. By this procedure the peak value for EFP occurred at six days. These biophysical observations were correlated with subsequent microscopic examination of 1 mu thick sections of Araldite-embedded sciatic nerve. Microscopy confirmed the presence of wallerian degeneration associated with edema, which was observed in every instance of elevated EFP.

Ultrastructural sequence of myelin degradation. II. Wallerian degeneration in the rat femoral nerve

Myelin degradation in Wallerian degeneration of rat femoral nerves has been studied with the electron microscope. In the initial stages, a decrease of myelin periodicity from 115 A to 88 A was noted, followed by the transformation of the myelin structure into uniformly layered lipid inclusions. 10--14 days after nerve section, most of the inclusions found represented unstructured lipid droplets or crystals. In the later stages of degeneration, numerous pleomorphic lamellar inclusions were found, some of them resembling the structure of pi and micron-granules. Lysosomal enzyme activity was found especially in pleomorphic inclusions during the late stages of myelin degradation. Normal myelin sheaths, as well as unstructured lipid droplets and crystals, were devoid of enzyme activity. The results are compared with alterations found in Wallerian degeneration of the central nervous system.

Lysomes in the rat sciatic nerve following crush

Peripheral nerves undergoing degeneration are favorable material for studying the types, origins, and functions of lysosomes. The following lysosomes are described: (a) Autophagic vacuoles in altered Schwann cells. Within these vacuoles the myelin and much of the axoplasm which it encloses in the normal nerve are degraded (Wallerian degeneration). The delimiting membranes of the vacuoles apparently form from myelin lamellae. Considered as possible sources of their acid phosphatase are Golgi vesicles (primary lysosomes), lysosomes of the dense body type, and the endoplasmic reticulum which lies close to the vacuoles. (b) Membranous bodies that accumulate focally in myelinated fibers in a zone extending 2 to 3 mm distal to the crush. These appear to arise from the endoplasmic reticulum in which demonstrable acid phosphatase activity increases markedly within 2 hours after the nerve is crushed. (c) Autophagic vacuoles in the axoplasm of fibers proximal to the crush. The breakdown of organelles within these vacuoles may have significance for the reorganization of the axoplasm preparatory to regeneration. (d) Phagocytic vacuoles of altered Schwann cells. As myelin degeneration begins, some axoplasm is exposed. This is apparently engulfed by the filopodia of the Schwann cells, and degraded within the phagocytic vacuoles thus formed. (e) Multivesicular bodies in the axoplasm of myelinated fibers. These are generally seen near the nodes of Ranvier.