Axonal regeneration, but not myelination, is partially dependent on local cholesterol reutilization in regenerating nerve

A recycling pathway in peripheral nerve permits cholesterol from degenerating myelin to be salvaged by macrophages and resupplied to myelinating Schwann cells by locally produced lipoproteins. A similar reutilization of cholesterol by regenerating axons has been proposed but not demonstrated. Neurites in culture, however, do take up cholesterol and cholesterol-containing lipoproteins, where these molecules are found to promote neurite extension. To test the requirement for cholesterol reutilization in axon regeneration and myelination, we examined 2 models of blocked intracellular cholesterol transport: 1) bone marrow transplants from Niemann-Pick C mice into wild-type recipient mice, and 2) imipramine treatment. Following nerve crush in these models, we found that unusually large, debris-filled macrophages appeared and persisted for many weeks. A morphometric analysis of regenerating nerves revealed that myelination proceeded at a normal rate (normal g-ratios), but that axon growth was retarded (decreased fiber numbers and diameters) in these animals. Cholesterol synthesis was elevated in these nerves, indicating that Schwann cells compensated for the decreased exogenous supply of cholesterol by up-regulating de novo synthesis to support myelination. These data indicate that Schwann cells are not dependent on cholesterol reutilization to support myelination, but that optimal axonal regeneration is dependent on a local supply of cholesterol.     

Peripheral nerve regeneration and cholesterol reutilization are normal in the low-density lipoprotein receptor knockout mouse

Following peripheral nerve injury, cholesterol from degenerating myelin is retained locally within macrophages and subsequently reutilized by Schwann cells for synthesis of new myelin during nerve regeneration. Substantial evidence indicates this conservation and reutilization of cholesterol is accomplished via lipoprotein-mediated intercellular transport, although the identities of the lipoproteins and their receptors are unresolved. Because Schwann cells in regenerating nerve are reported to express the low-density lipoprotein (LDL) receptor (LDLR), we used the LDLR knockout mouse to examine the potential role of this receptor in cholesterol reutilization. Sciatic nerves were crushed in knockout and wild-type mice and examined 3 days to 10 weeks later. Morphometric analyses and measures of mRNA levels for myelin protein P(0), indicate that axon regeneration and myelination proceed normally in the LDLR knockout mouse. We therefore measured hydroxy-methylglutaryl-coenzyme A (HMG-CoA) reductase activity and mRNA levels to determine whether Schwann cells compensated for the absence of the LDLR by upregulating cholesterol synthesis. Unexpectedly, these measures remained at the same downregulated levels found in regenerating nerves of wild-type animals. The apparently normal nerve regeneration, coupled with the lack of any compensatory upregulation of cholesterol synthesis in the LDLR knockout mice, indicates that other lipoprotein receptors must be primarily involved in cholesterol uptake by Schwann cells.     

Local administration of vasoactive intestinal peptide after nerve transection accelerates early myelination and growth of regenerating axons

Our goal was to determine whether local injections of vasoactive intestinal peptide (VIP) promote early stages of regeneration after nerve transection. Sciatic nerves were transected bilaterally in 2 groups of 10 adult mice. In the first group, 15 microg (20 microL) of VIP were injected twice daily into the gap between transected ends of the right sciatic nerve for 7 days (4 mice) or 14 days (6 mice). The same number of mice in the second group received placebo injections (20 microL of 0.9% sterile saline) in the same site, twice daily, for the same periods. After 7 days, axon sizes, relationships with Schwann cells and degree of myelination were compared in electron micrographs of transversely sectioned distal ends of proximal stumps. Fourteen days after transection, light and electron microscopy were used to compare and measure axons and myelin sheaths in the transection gap, 2-mm distal to the ends of proximal stumps. Distal ends of VIP-treated proximal stumps contained larger axons 7 days after transection. More axons were in 1:1 relationships with Schwann cells and some of them were surrounded by thin myelin sheaths. In placebo-treated proximal stumps, axons were smaller, few were in 1:1 relationships with Schwann cells and no myelin sheaths were observed. In VIP-treated transection gaps, measurements 14 days after transection showed that larger axons were more numerous and their myelin sheaths were thicker. Our results suggest that in this nerve transection model, local administration of VIP promotes and accelerates early myelination and growth of regenerating axons.     

Myelinated fiber regeneration after crush injury is retarded in sciatic nerves of aging mice

To compare nerve regeneration in young adult and aging mice, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two weeks later, both groups of mice were perfused with an aldehyde solution, and, after additional fixation, the sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. The same level was sectioned in unoperated contralateral nerves; these nerves served as controls. Electron micrographs and the Bioquant Image Analysis System IV were used to measure areas of posterior tibial fascicles and count the number of myelinated axons, the number of unmyelinated axons, and their frequency in Schwann cell units. In aging mice, the total number of regenerating myelinated axons was significantly reduced, but totals of regenerating unmyelinated axons in aging and young adults did not differ significantly. In aging mice, the frequency of Schwann cells that contained a single unmyelinated axon was greater, suggesting that before myelination began, Schwann cell ensheathment of axons also was slowed. After axotomy by a crush injury, the area of the posterior tibial fascicle was less than that in young adults and the distal disintegration of myelin sheath remnants also appeared to be retarded. The results indicate that responses of neurons, axons, and Schwann cells could be important in slowing the regeneration of myelinated fibers found in sciatic nerves from aging mice.     

Quantitative ultrastructural studies of the axon Schwann cell abnormality in spinal nerve roots from dystrophic mice.

Lumbo-sacral spinal roots and the nerve to the medial gastrocnemius muscle (NMG) from normal and dystrophic mice were examined by quantitative ultrastructural techniques. It was demonstrated that, although many axons at this level were totally devoid of Schwann cells, totatl axonal numbers were approximately normal. Some axons in these roots were surrounded by Schwann cells but their myelin sheaths were abnormally thin. In addition, cells resembling oligodendrocytes were observed in the dorsal roots of the dystrophic mice. In contrast, Schwann cells and myelin sheaths were normal in the more peripherally situated NMG and the regenerative response of these nerves to crush injury was not significantly different from that of control nerves. Thus, the main abnormality of neural morphology in dystrophic mice is a localized absence of Schwann cells. Such a deficiency does not appear to have influenced axonal growth or the capacity of these axons to regenerate when crushed distally.     

Influence of laminin-2 on Schwann cell-axon interactions

The dy/dy mouse suffers from a form of muscular dystrophy caused by a substantial reduction in laminin alpha2-chain protein, a major component of both muscle and Schwann cell basal laminae. This article examines the effect of laminin alpha2 deficiency on Schwann cell-axon interactions both in vivo at varying intervals after nerve crush, and in vitro, in cocultures of neurons and Schwann cells. The morphological spectrum of aberrant Schwann cell-axon associations seen in uncrushed dy/dy sciatic nerves was recapitulated during regeneration: myelination of regenerating axons was delayed compared with the process in unaffected mice and the relatively few myelin sheaths which were formed in dy/dy distal nerve stumps were often uncompacted. In vitro, Schwann cells dissociated from adult dy/dy sciatic nerves predictably failed to express detectable laminin alpha2-chain and displayed an unusual multipolar morphology. Branching of neurites, in terms both of numbers of terminal branches and of complexity of branching, from dorsal root ganglia neurons grown on dy/dy Schwann cells, was significantly less extensive than that seen when neurons were cocultured with Schwann cells from unaffected littermates, but this effect was reversed by exogenous laminin-2. Our results lend strong support to the view that laminin-2 is essential for establishing and/or maintaining Schwann cell-axon interactions, in normal and in regenerating nerves.     

In Vivo Expression of the Arf6 Guanine-Nucleotide Exchange Factor Cytohesin-1 in Mice Exhibits Enhanced Myelin Thickness in Nerves

The myelin sheath consists of a unique multiple layer structure that acts as an insulator between neuronal axons to enhance the propagation of the action potential. In neuropathies such as demyelinating or dismyelinating diseases, chronic demyelination and defective remyelination occur repeatedly, leading to more severe neuropathy. As yet, little is known about the possibility of drug target-specific medicine for such diseases. In the developing peripheral nervous system (PNS), myelin sheaths form as Schwann cells wrap individual axons. It is thought that the development of a drug promoting myelination by Schwann cells would provide effective therapy against peripheral nerve disorders: to test such treatment, genetically modified mice overexpressing the drug target molecules are needed. We previously identified an Arf6 activator, the guanine-nucleotide exchange factor cytohesin-1, as the signaling molecule controlling myelination of peripheral axons by Schwann cells; yet, the important issue of whether cytohesin-1 itself promotes myelin thickness in vivo has remained unclear. Herein, we show that, in mouse PNS nerves, Schwann cell-specific expression of wild-type cytohesin-1 exhibits enhanced myelin thickness. Downstream activation of Arf6 is also seen in these transgenic mice, revealing the involvement of the cytohesin-1 and Arf6 signaling unit in promoting myelination. These results suggest that cytohesin-1 may be a candidate for the basis of a therapy for peripheral neuropathies through its enhancement of myelin thickness.     

Targeted disruption of the FGF-2 gene affects the response to peripheral nerve injury

Basic fibroblast growth factor (FGF-2) is involved in the development, maintenance, and survival of the nervous system. To study the physiological role of endogenous FGF-2 during peripheral nerve regeneration, we analyzed sciatic nerves of FGF-2-deleted mice by using morphometric, morphological, and immunocytochemical methods. Quantification of number and size of myelinated axons in intact sciatic nerves revealed no difference between wild-type and FGF-2 knock-out (ko) animals. One week after nerve crush, FGF-2 ko mice showed about five times more regenerated myelinated axons with increased myelin and axon diameter in comparison to wild-types close to the injury site. In addition, quantitative distribution of macrophages and collapsed myelin profiles suggested faster Wallerian degeneration in FGF-2-deleted mice close to the lesion site. Our results suggest that endogenous FGF-2 is crucially involved in the early phase of peripheral nerve regeneration possibly by regulation of Schwann cell differentiation.     

Chronic nerve compression induces local demyelination and remyelination in a rat model of carpal tunnel syndrome

In a previous study, we demonstrated that chronic compression of rat sciatic nerve, a model of compressive neuropathies, triggered dramatic Schwann cell proliferation and concurrent apoptosis. Importantly, this Schwann cell response occurred before there are signs of overt axonal pathology, raising the question of whether there are alterations in axonal myelination in the areas of the nerve in which Schwann cell apoptosis and proliferation occur. Here, we use nerve teasing techniques and unbiased stereology to assess myelination in nerves after 1 and 8 months of compression. Evaluations of myelin thickness and axonal diameter (AD) using design-based, unbiased stereology revealed alterations in myelin structure that indicate remyelination, specifically a dramatic decrease in the average internodal length (IL) and an increase in the proportion of axons with thin myelin sheaths. The mean IL was reduced after 1 month of chronic nerve injury with no further decrease in IL at 8 months. There was limited change in average axonal diameter at both 1 and 8 months. Measures of myelin thickness revealed not only a greater than 6-fold increase in the number of axons with very thin (<5 microm thickness) myelin sheaths, but also a proportional decrease in the number of axons with the thick myelin sheaths characteristic of normal nerve. These results confirm that an early consequence of chronic nerve compression (CNC) is local demyelination and remyelination, which may be the primary cause of alterations in nerve function during the early period post-compression.     

The aberrant retino-retinal projection during optic nerve regeneration in the frog. III. Effects of crushing both nerves

Previous reports from this laboratory have shown that a substantial number of optic axons are misrouted after optic nerve regeneration in the adult frog, Rana pipiens. Regenerating axons from a crushed optic nerve are distributed throughout the opposite nerve. In this study, we report the effect of crushing both optic nerves (double crush) on the pattern and degree of axonal misrouting. In 28 frogs both optic nerves were crushed at the same time (simultaneous double crush) and animals survived for varying periods before the right eye was injected with ³H-proline and the brain processed for autoradiography 24 hours later. In every frog with postoperative survivals longer than 2 weeks, labeled axons from the right eye were found in the left optic nerve. However, when compared to the amount of label seen in frogs in which only the right optic nerve was crushed (single crush) there was substantially less label within the left nerve of frogs after crushing both nerves. Label was also found only at the edge of the left nerve in material from double crush frogs, unlike that found after single crush. In four of six frogs where the left nerve was crushed 1 week after the right nerve (delayed double crush), the proximal end of the left nerve was completely filled with label, but more distally, label was found only along the edge of this nerve. Although fewer optic axons were labeled in the opposite optic nerve of double crush frogs, label did extend to the optic disc of that eye. However, label was not apparent in the ganglion cell fiber layer of the opposite eye. Instead, it was confined to the edge of the optic disc in a space apparently associated with papilledema resulting from crushing the optic nerve of that eye. In six frogs the retina of the left eye was removed at the same time the right optic nerve was crushed. Labeled axons of the right eye filled the left optic nerve to the retina-less shell of the left eye. Thus, these data show that the amount and distribution of axonal misrouting into the opposite optic nerve during optic nerve regeneration is affected by intact or regenerating optic axons from the other eye.     

SCHWANN CELL RESPONSES DURING REGENERATION AFTER ONE OR MORE CRUSH INJURIES TO MYELINATED NERVE FIBRES

The responses of Schwann cells during regeneration of myelinated nerve fibres were studied ultrastructurally in the distal segment of mouse phrenic nerve after a single or repeated localized crush injury. Chronological observations on nerves after a single crush confirmed the occurrence of myelination of only single regenerating axons among many that appeared in individual Büngner bands. The redundant axon sprouts often showed the structural features of degeneration and decreased in number with time. During the process, supernumerary Schwann cells not related to myelin formation were produced. They commonly failed not only to make a one-to-one relationship with an axon, but they also failed to acquire a new basal lamina of their own. With time, they showed shrinkage of their cytoplasm and became arranged circumferentially around the myelinating axon with unipolar or bipolar cytoplasmic processes. Electron microscopic, quantitative assessment of the nuclear population of Schwann cells following repeated crushes up to four times, clearly indicated a progressive and predominant increase in the number of the supernumerary Schwann cells with the number of crushes. Also, they were found to form separate concentric cytoplasmic lamellae around the myelinating axons, developing structures resembling onion-bulbs. It was concluded that essentially the same regenerating process as that observed after a single crush was repeated following re-crush, thereby resulting in the successive accumulation of supernumerary Schwann cells around a myelinating axon.     

Delayed nerve regeneration in streptozotocin diabetic rats

Nerve regeneration across a 10-mm gap was delayed in streptozotocin diabetic rats 3 and 4 weeks after transecting the sciatic nerve. Opposite ends of each cut nerve were introduced into a silicone tube, leaving a 10-mm gap. Electron microscopy was used to evaluate the progress of regeneration in sections at 2-mm intervals across the 10-mm gap. After 3 weeks, control axons had bridged the 10-mm gap, and myelin sheaths extended for 6-8 mm. By contrast, axons and their myelin sheaths were seen no further than 2 mm from the proximal stump in diabetic animals. By 4 weeks, axons had bridged the gap in diabetics; however, they appeared immature and showed dystrophic changes. The findings suggest that although regeneration does occur in diabetic nerves, it is significantly delayed and qualitatively impaired.     

Peripheral nerve pathology in Niemann-Pick type C mouse

The sciatic nerve of the mouse mutant with Niemann-Pick type C disease (NPC mouse) was investigated using light and electron microscopy, and teased-fiber preparations. As early as postnatal day 20, when clinical symptoms were not yet apparent, focal paranodal swellings with an accumulation of small myelin figures in the Schwann cell cytoplasm were noted. These paranodal changes were more pronounced in the distal segment and became progressively conspicuous with increasing age. The morphometric analysis revealed a hypomyelination of large myelinated fibers in the NPC nerves at 70 days, whereas an essentially similar histogram pattern was noted in both control and NPC nerves at 20 days, suggesting progressively defective utilization of cholesterol in the NPC nerves with age. Intraxonal accumulation of dense bodies was noted in older mice, but no segmental demyelination or Wallerian type of axonal degeneration was observed at any age. The changes noted in the paranodal regions in the NPC mouse closely resemble those found in rats treated with an inhibitor of cholesterol biosynthesis, as well as those seen in remodeling fibers during an early stage of peripheral nerve development. Thus, the morphological changes seen in the sciatic nerve of the NPC mouse may be an expression of perturbation in myelin maintenance as a result of defective cholesterol metabolism.Preliminary findings were presented at the Annual Meeting of the American Association of Neuropathologists, Baltimore, Maryland, June 1991     

Migration and myelination by adult glial cells: reconstructive analysis of tissue culture experiments

Adult glia are capable of at least limited myelination of CNS axons. However, it is difficult to quantitate their myelination or migratory capacities and to examine contributions of the CNS environment or exogenous factors that might promote or inhibit this process in situ. We have therefore developed a mouse tissue culture system in which optic nerve glia (in the form of appropriately handled optic nerve) are added to chemically demyelinated cerebellar axons. Optic nerve up to postnatal day 411 (P411) contains cells that can migrate out of the nerve into the cerebellar explant and form myelin around its axons. The success rate for myelin formation in these cultures is 57% for immature (P7-11) glia and 55% for adult (P50-411) glia. Computer-generated reconstructions of cultures containing immature (P8) and adult (P89 and P139) nerves demonstrate that in all 3 cases the glia may migrate more than 0.6 mm before myelinating axons, assuming the shortest possible track. Both the age limit for myelination and distance limit for migration, if any, remain to be determined for these adult glia. In successful cultures, myelin always directly abuts the optic nerve surface, whether or not it also extends further, suggesting that migrating glia may depend upon contact guidance by myelin-receptive axons. We conclude that this culture system is a useful model of adult CNS myelin regeneration, in which one can examine the influence of potential trophic or toxic factors on specific aspects of myelinating glial cell behavior.     

Effects of advancing age on peripheral nerve regeneration.

Following axotomy, the regrowth of peripheral axons takes longer in older individuals than in young ones. The present study compares the crush-induced process of degeneration and regeneration in the buccal branch of the facial motor nerve in groups of rats aged 3 months and 15 months. Observations are based on qualitative and quantitative analyses of the nerve 20 mm from the site of injury in rats 1, 2, 4, 16, 21, 28, and 56 days after crush. The buccal branch is purely motor and contains a unimodal population of about 1,600 axons commonly in a single fascicle. During the first 28 days post crush (dpc) in the 3-month animals, the progression of myelin and axon degeneration, myelin clearance, regrowth of axon sprouts, and axon maturation are relatively synchronized and uniform. In the older rats, the degeneration of myelin and axons, myelin clearance, and the appearance of axon sprouts at the site of sample are all delayed. In the younger animals, axon sprouts increase in numbers from their first appearance at 4 dpc through the 2 weeks examined following the restoration of whisking behavior. The numbers of regenerating older axons increase at a rate comparable to that in the younger animals through the time that bilaterally symmetrical whisking behavior is evident, but afterwards the number of axon sprouts decreases. At 2 months after crush the young animals have 30% more fibers in the buccal branch than control nerves, while the older animals have fewer than control numbers. In the 3-month regenerated nerve, 2 months post crush, 30% of the regenerated fibers are of very small caliber, less than 3 microns2 in cross sectional area, and typically these small axons have unusually thick myelin sheaths; the older nerves do not have such a skewed distribution of axon areas. The older regenerated axons at 2 months post crush have an unusually high density of microtubules compared to the younger regenerated ones (and controls), and the ratio of neurofilaments to microtubules is very low. The conclusions are that motor neurons in older animals regenerate damaged axons after a delay not apparent in the young; the strong regenerative response apparent initially in animals of both age groups is not maintained in the older animals; and the relationship between the numerical density of cytoskeletal elements and the axon cross-sectional area deviates from normal in the regenerated axons of the older animals.     

Diffusion tensor imaging to assess axonal regeneration in peripheral nerves

Development of outcome measures to assess ongoing nerve regeneration in the living animal that can be translated to human can provide extremely useful tools for monitoring the effects of therapeutic interventions to promote nerve regeneration. Diffusion tensor imaging (DTI), a magnetic resonance based technique, provides image contrast for nerve tracts and can be applied serially on the same subject with potential to monitor nerve fiber content. In this study, we examined the use of ex vivo high-resolution DTI for imaging intact and regenerating peripheral nerves in mice and correlated the MRI findings with electrophysiology and histology. DTI was done on sciatic nerves with crush, without crush, and after complete transection in different mouse strains. DTI measures, including fractional anisotropy (FA), parallel diffusivity, and perpendicular diffusivity were acquired and compared in segments of uninjured and crushed/transected nerves and correlated with morphometry. A comparison of axon regeneration after sciatic nerve crush showed a comparable pattern of regeneration in different mice strains. FA values were significantly lower in completely denervated nerve segments compared to uninjured sciatic nerve and this signal was restored toward normal in regenerating nerve segments (crushed nerves). Histology data indicate that the FA values and the parallel diffusivity showed a positive correlation with the total number of regenerating axons. These studies suggest that DTI is a sensitive measure of axon regeneration in mouse models and provide basis for further development of imaging technology for application to living animals and humans.     

Myelin formation by Schwann cells in the absence of beta4 integrin.

The interaction of the Schwann cell with its basal lamina has been hypothesized to be an important prerequisite for the formation of a myelin sheath in the peripheral nervous system. One possible player in this interaction is beta4 integrin; it is up-regulated during myelin formation and, in association with alpha6 integrin, can interact with particular components of the Schwann cell basal lamina. In order to characterize the functional roles of beta4 integrin during myelination, we investigated myelination in the absence of beta4 integrin, i.e., in peripheral nerve tissue from beta4 integrin-deficient mice. Because the mutants die within several hours after birth, we cultured dorsal root ganglia from neonatal mutants under conditions that promote myelination, quantified the myelin segments by immunofluorescence, and investigated the ultrastructure of the cultured myelin sheaths. In another approach, we quantified the few myelin sheaths that are detectable in femoral nerves of newborn animals. Based on both approaches, we conclude that myelination by Schwann cells can occur in the absence of beta4 integrin demonstrating that this Schwann cell component is dispensable for myelin formation in peripheral nerves.     

Myelinated fiber regeneration after sciatic nerve crush: morphometric observations in young adult and aging mice and the effects of macrophage suppression and conditioning lesions.

To study myelinated nerve fiber regeneration during aging, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two, 4, and 8 weeks later, both groups of mice were perfused. The sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. Two weeks after axotomy, fascicles of aging mice contained significantly fewer regenerated myelinated fibers than those of young adults. After 4 weeks, the difference in the number of myelinated fibers was less. However, measurements of myelinated fibers in fascicles of aging mice showed that areas of Schwann cell cytoplasm and myelin were significantly reduced at all intervals. In contrast, although axon diameters in aging mice were somewhat less 2 weeks after crushing, the difference decreased with time, suggesting that in nerves of aging mice, regenerative responses of Schwann cells were more affected than those of axons. Other experiments in young mice showed that myelinated fiber regeneration could be retarded by suppressing macrophage responses and was not significantly changed by conditioning lesions before crush injury.     

Morphometric effects of vincristine on nerve regeneration in the rat

Administration of vincristine (200, 100 or 50 micrograms/kg/week) for 6 months during regeneration of the sciatic nerve after crush injury caused a dose-dependent reduction in nerve fibre size and failure of removal of myelin debris. Successfully regenerating neurites showed an unusual amount of shape distortion. The ratio of myelin sheath thickness to axon circumference was reduced, but the ratio of myelin sheath thickness to axon area was normal. Microtubule concentration was diminished in axons, but neurofilament density was unaffected. Unmyelinated axons were reduced in number but their axon diameter distribution was not affected. Fibres on the non-crushed side appeared normal. The toxicity of vincristine to regenerating nerves is probably related to increased blood-nerve permeability occurring both at the site of crush and along the degenerating nerve.     

The acute response of Schwann cells to taxol after nerve crush

Summary The effect of taxol, an antimitotic drug which stabilizes microtubules and promotes their assembly, was studied with regard to Schwann cells over a 4-week period following a crush injury to rat sciatic nerve. A single intraneural injection of taxol in dimethyl sulfoxide (DMSO) was given immediately after the crush into the site of injury in one sciatic nerve and was compared with the other side which was crushed but injected with DMSO only. Sampled sites were taken proximal and distal to the lesion, as well as from the lesion itself, and studied by light and electron microscopy. The Schwann cell response was most marked during the degenerative phase immediately following the crush. At this time, there was a decrease of all cytoplasmic structures except microtubules and smooth endoplasmic reticulum. At the site of the crush lesion in taxol-treated nerves, Schwann cells possessed accumulations of myelin debris and lipid droplets. Mitotic Schwann cells were also engorged with myelin breakdown products. Multinucleated Schwann cells, believed to be the result of abnormal mitotic activity, were also apparent and were filled with large numbers of cytoplasmic microtubules. The latter were sometimes regularly arranged around phagocytosed or intracytoplasmic debris. Some recovery from the crush injury was noted with time, although the number of Schwann cells was much lower than would have been anticipated in the absence of taxol, in that long stretches of naked axon bundles were common and microtubule-related abnormalities persisted up to 4 weeks. Myelination of regenerating axonal sprouts was delayed and might have been related to axons being swollen due to the build-up of microtubules. However, some myelination was noted sporadically along a few axons in taxol-treated nerves after 4 weeks. The present results suggest that the rapid Schwann cell reaction after nerve crush was impeded by the adverse effect of taxol upon mitosis and cell migration and that Schwann cells play an active role in the degradation of myelin phagocytosis of debris during Wallerian degeneration.Supported by the Finnish Cultural Foundation and USPHS grants NS 08952 and NS 11920