Axons modulate myelin protein messenger RNA levels during central nervous system myelination in vivo.

Expression of myelin protein genes by myelinating Schwann cells in vivo is dependent on axonal influences. This report investigated the effect of axons on myelin protein mRNA levels in the central nervous system (CNS). In situ hybridization studies of rat spinal cord sections localized mRNAs encoding proteolipid protein (PLP) and myelin basic protein (MBP) 20 and 40 days after unilateral rhizotomy. Compared with control tissue, hybridization intensity was reduced in transected tissue, but there was little change in the number of oligodendrocytes labeled. Cellular RNA was extracted from transected and age-matched control optic nerves 5, 10, 20, and 40 days after surgery, and levels of the following mRNAs were determined by slot blot procedures: PLP, MBP, myelin-associated glycoprotein (MAG), and 2',3' cyclic nucleotide 3'-phosphodiesterase (CNP). In transected nerves, PLP and MBP mRNA levels were approximately 85%, 45%, and 25% of control values at 5, 20 and 40 days posttransection, respectively. Axonal transection had a lesser effect on CNP and MAG mRNA levels, which declined to approximately 60% of control levels at 40 days. Immunocytochemical studies indicated that the number of oligodendrocytes was not decreased 40 days after optic nerve transection. These data demonstrate that axons modulate myelin protein mRNA levels in oligodendrocytes. In contrast to Schwann cells, however, oligodendrocytes continue to express significant levels of myelin protein mRNA in vivo following loss of axonal contact.     

Proton MR spectroscopy to assess axonal damage in multiple sclerosis and other white matter disorders

Proton MR spectroscopy allows in vivo measurement of N-acetylaspartate in white matter, providing a biochemical index of axonal integrity. Several recent studies of patients with multiple sclerosis and other white matter disorders have shown both transient and sustained decreases in N-acetylaspartate in white matter lesions and in brain regions appearing normal on conventional MRI. These data have emphasised that a substantial amount of axonal damage or loss (presumably secondary to myelin pathology) is consistently present in most of these disorders. Recent post-mortem studies support these results. In contrast to changes seen with conventional MR imaging, decreases in N-acetylaspartate have shown a close correlation with changes in neurological status. This suggests that axonal damage may be more relevant than demyelination for determining chronic functional impairments in primary white matter diseases. Thus, serial measurement of brain N-acetylaspartate with proton MR spectroscopy can provide a reliable and clinically-relevant monitor of disease evolution. As pathological changes responsible for long-term morbidity are logically important targets for therapeutic agents, early treatment directed at axonal protection should be useful in these disorders.     

Slowly progressive axonal degeneration in a rat model of chronic, nonimmune-mediated demyelination

Axonal degeneration in the central nervous system (CNS) is associated with neurologic disability. In some diseases, it has been postulated that axonal degeneration may be caused by loss of trophic support normally provided by oligodendrocytes and myelin. To investigate this phenomenon, we studied axonal pathology in the taiep mutant rat, which develops nonimmune oligodendrocyte dysfunction and myelin loss. Using immunohistochemical analysis of several CNS regions, we show that accumulation of dephosphorylated neurofilaments occurs in taiep axons. These changes become more pronounced as myelin loss increases and characteristic spheroids, representing transected axons, become abundant. Amyloid precursor protein staining is increased in taiep white matter tracts, indicating abnormalities of axonal transport. These changes do not occur in wild type controls. Optic nerve counts demonstrate progressive axonal loss throughout the life of the rat; early axonal loss occurs in the context of dysmyelination; later axonal loss is likely to be related to chronic demyelination. The axonal pathology in the taiep rat provides evidence that CNS axonopathy may, in certain situations, be related to a loss of trophic support normally provided by cells of the oligodendrocyte lineage and/or myelin; this may occur in the absence of significant inflammation.     

Axonal and neuronal pathology in multiple sclerosis: What have we learnt from animal models

Axonal and neuronal injury and loss are of critical importance for permanent clinical disability in multiple sclerosis patients. Axonal injury occurs already early during the disease and accumulates with disease progression. It is not restricted to focal demyelinated lesions in the white matter, but also affects the normal appearing white matter and the grey matter. Experimental studies show that many different immunological mechanisms may lead to axonal and neuronal injury, including antigen-specific destruction by specific T-cells and auto-antibodies as well as injury induced by products of activated macrophages and microglia. They all appear to be relevant for multiple sclerosis pathogensis in different patients and at different stages of the disease. However, in MS lesions a major mechanism of axonal and neuronal damage appears to be related to the action of reactive oxygen and nitrogen species, which may induce neuronal injury through impairment of mitochondrial function and subsequent energy failure.     

Multiple neuroprotective mechanisms of minocycline in autoimmune CNS inflammation

Axonal destruction and neuronal loss occur early during multiple sclerosis, an autoimmune inflammatory CNS disease that frequently manifests with acute optic neuritis. Available therapies mainly target the inflammatory component of the disease but fail to prevent neurodegeneration. To investigate the effect of minocycline on the survival of retinal ganglion cells (RGCs), the neurons that form the axons of the optic nerve, we used a rat model of myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis. Optic neuritis in this model was diagnosed by recording visual evoked potentials and RGC function was monitored by measuring electroretinograms. Functional and histopathological data of RGCs and optic nerves revealed neuronal and axonal protection when minocycline treatment was started on the day of immunization. Furthermore, we demonstrate that minocycline-induced neuroprotection is related to a direct antagonism of multiple mechanisms leading to neuronal cell death such as the induction of anti-apoptotic intracellular signalling pathways and a decrease in glutamate excitotoxicity. From these observations, we conclude that minocycline exerts neuroprotective effects independent of its anti-inflammatory properties. This hypothesis was confirmed in a non-inflammatory disease model leading to degeneration of RGCs, the surgical transection of the optic nerve.     

Reduction of proliferating non-neuronal cells in dried nerve segments partly impairs nerve regeneration

To determine whether drying of nerve grafts can affect axonal outgrowth and proliferation of non-neuronal cells, nerve segments dried for 0-60 min were used as nerve grafts to bridge a gap in transected rat sciatic nerves. Axonal outgrowth was measured by pinch reflex test and confirmed by immunocytochemical staining of neurofilaments. The proliferation of non-neuronal cells was measured by incorporation of BrdU in the dried nerve segments. Drying of the nerve segment for 60 min reduced the length of axonal outgrowth to 66 and 76% 3 and 6 days, respectively, after the grafting procedure. At that time point the number of proliferating cells was reduced by 51%. It is concluded that the number of proliferating non-neuronal cells is reduced in dried nerve segments which only partly impairs axonal outgrowth. Factors other than Schwann cells are probably important for an optimal mileue for regeneration in nerve grafts.     

Axonal degeneration and progressive neurologic disability in multiple sclerosis

Accumulating data support axonal degeneration as the major determinant of irreversible neurological disability in patients with multiple sclerosis (MS). The extent of axonal injury correlates with the degree of inflammation in active MS lesions and occurs at early stages of disease, indicating that inflammatory demyelination is an important factor behind axon pathology at the relapsing-remitting stage of MS. Axonal loss from disease onset can remain clinically silent for many years, and permanent neurological disability develops when a threshold of axonal loss is reached and the CNS compensatory resources are exhausted. Lack of myelin-derived trophic support due to long term demyelination may cause continuous axonal degeneration in chronic inactive lesions at the secondary-progressive stage of MS. Axonal pathology is not limited to demyelinated lesions, but also extends into normal appearing white matter. The concept of MS as a neurodegenerative disorder has important clinical implications: First, proactive anti-inflammatory and immunomodulatory treatment should prevent or delay chronic disability since inflammation influences axonal injury. Second, the pathophysiological mechanisms underlying axonal degeneration in MS need to be clarified in order to develop novel neuroprotective therapeutics. Finally, surrogate markers of axonal pathology, such as N-acetyl aspartate, can be used to monitor axonal dysfunction, axonal loss and treatment efficiency in patients with MS.     

Dynamics of oligodendrocyte responses to anterograde axonal (Wallerian) and terminal degeneration in normal and TNF-transgenic mice

The inflammatory cytokine tumour necrosis factor (TNF) can both induce oligodendrocyte and myelin pathology and promote proliferation of oligodendrocyte progenitor cells and remyelination. We have compared the response of the oligodendrocyte lineage to anterograde axonal (Wallerian) and terminal degeneration and lesion-induced axonal sprouting in the hippocampal dentate gyrus in TNF-transgenic mice with the response in genetically normal mice. Transectioning of the entorhino-dentate perforant path axonal projection increased hippocampal TNF mRNA expression in both types of mice, but to significantly larger levels in the TNF-transgenics. At 5 days after axonal transection, numbers of oligodendrocytes and myelin basic protein (MBP) mRNA expression in the denervated dentate gyrus in TNF-transgenic mice had increased to the same extent as in nontransgenic littermates. At this time, transgenics showed a tendency towards a greater increase in the number of juxtaposed, potentially proliferating oligodendrocytes. Noteworthy, at day 5 we also observed upregulation of MBP mRNA expression in adjacent hippocampal subregions with lesion-induced axonal sprouting, which were devoid of axonal degeneration, raising the possibility that sprouting axons provide trophic stimuli to the oligodendrocyte lineage. Twenty-eight days after lesioning, oligodendrocyte numbers and MBP mRNA expression were reduced to near normal levels. However, oligodendrocyte densities in the TNF-transgenic mice were significantly lower than in nontransgenics. We conclude that the early response of the oligodendrocyte lineage to axonal lesioning and lesion-induced axonal sprouting appears unaffected by the supranormal TNF levels in the TNF-transgenic mice. TNF may, however, have long-term inhibitory effects on the oligodendrocyte response to axonal lesioning.     

Quantitative assessment of ischemic pathology in axons, oligodendrocytes, and neurons: attenuation of damage after transient ischemia.

Axons and oligodendrocytes are vulnerable to cerebral ischemia. The absence of quantitative methods for assessment of white matter pathology in ischemia has precluded in vivo evaluation of therapeutic interventions directed at axons and oligodendrocytes. The authors demonstrate here that the quantitative extent of white matter pathology was reduced by restoration of cerebral blood flow after 2 hours of middle cerebral artery occlusion. Focal ischemia was induced in anesthetized rats by intraluminal thread placement, either transiently (for 2 hours) or permanently. At 24 hours after induction of ischemia, axonal damage was determined by amyloid precursor protein (APP) immunohistochemistry, and the ischemic insult to oligodendrocytes was assessed by Tau-1 immunostaining in the same sections. In adjacent sections, ischemic damage to neuronal perikarya was defined histologically. The hemispheric extent of axonal damage was reduced by 70% in the transiently occluded animals from that in permanently occluded animals. The volumes of oligodendrocyte pathology and of neuronal perikaryal damage were reduced by 62% and 58%, respectively, in the transiently occluded animals. These results demonstrate that this methodologic approach for assessing ischemic damage in axons and oligodendrocytes can detect relative alterations in gray and white matter pathology with intervention strategies.     

Imaging axonal damage in multiple sclerosis by means of MR spectroscopy

Axonal damage in multiple sclerosis has become an important issue. This has been emphasized by recent in vivo proton magnetic resonance (MR) spectroscopy and in vitro pathology studies that have found axonal damage in both lesions and the surrounding normal-appearing white matter. In particular, proton MR spectroscopy, by monitoring levels of N-acetylaspartate (a putative marker of axonal integrity), has been particularly illuminating, as the extent of axonal injury associated with white matter inflammation and demyelination had not been well appreciated from classical pathology studies. Recent MR data demonstrate that cerebral axonal damage begins and contributes to disability from the earliest stages of the disease. This implies that the apparently primary role of axonal damage and loss in the pathogenesis of the disease should be given due importance, and argues for the early treatment of multiple sclerosis with agents directed not only against inflammation, but also towards axonal protection.     

Adhesion and proliferation are enhanced in vitro in Schwann cells from nerve undergoing Wallerian degeneration

Proliferation of Schwann cells during nerve degeneration or regeneration is well documented in vivo. We investigated whether the proliferative response of Schwann cells to injury is retained in vitro. Using 5-month-old male C57BL mice, Schwann cells were isolated from sciatic nerves under 3 experimental conditions: (1) uninjured, (2) after permanent nerve-transection, or (3) after nerve-crush, which permits axonal regeneration. Schwann cells rarely attached to polylysine-coated coverslips when isolated from uninjured or 1 day posttransection/crush nerves. The number of adherent cells increased when Schwann cells were isolated 3 days after nerve-transection or -crush. When cells were isolated from transected nerves, cell adhesion reached a peak 2 weeks after the injury and then declined. Maximal attachment of Schwann cells occurred when the cells were isolated 2-4 weeks after nerve-crush. The percentage of Schwann cells with spreading processes corresponded closely with the number of thymidine-labeled cells at 1 day in vitro. The in vitro capacity of cells to spread and incorporate thymidine reached maximal levels at 5 days posttransection/crush. Capacity of cells to spread and incorporate thymidine subsequently decreased with time following transection. However, a biphasic elevation in cell spreading and thymidine incorporation was observed in Schwann cells isolated from crushed nerves. Maximal growth of Schwann cells in vitro occurred at 1-2 weeks posttransection and at 1-4 weeks postcrush. Adhesion and spreading of Schwann cells were promoted by coating coverslips with laminin or fibronectin. Preincubation of Schwann cells with soluble laminin or fibronectin prevented the initial cell attachment induced by the corresponding protein. Our results suggest that Schwann cells from injured nerves possess binding sites for laminin and fibronectin, which are, in part, responsible for the enhanced adhesion of Schwann cells in vitro. This study provides a new method for preparation of Schwann cells from peripheral nerves of adult mice.     

Axonal loss and gray matter pathology as a direct result of autoimmunity to neurofilaments

Axonal damage is considered the major cause of irreversible disability in multiple sclerosis (MS). Which mechanisms underlie the damage and whether this is secondary to myelin damage remains to be clarified. Recently, we have demonstrated that autoimmunity to the axonal/neuronal cytoskeletal protein neurofilament light (NF-L) induces axonal damage and neurological disease including spasticity — a common feature of MS. To examine the relationship between axonal damage and demyelination we have characterized the detailed neuropathology of NF-L-induced disease in Biozzi mice compared to classical experimental autoimmune encephalomyelitis (EAE) induced with myelin oligodendrocyte glycoprotein (MOG).In NF-L-induced neurological disease the lesions were predominantly located in the dorsal column displaying extensive axonal degeneration, but were also abundant in the gray matter. In contrast, lesions in MOG-EAE were restricted to the lateral and ventral columns and displayed less axonal damage and little gray matter involvement. The differential lesion location was confirmed by quantitation of leukocyte subsets. In both diseases myelin damage was a common feature although the numerous empty myelin sheaths in NF-L-disease indicative of axonal damage suggest that myelin damage was a secondary event.In summary, autoimmunity to NF-L induces a distinct lesion topology, axonal damage and gray matter lesions supporting the notion that axonal loss and gray matter pathology can be the direct consequence of a primary autoimmune attack against axonal antigens such as NF-L rather than merely a secondary event to myelin damage.     

White-matter astrocytes, axonal energy metabolism, and axonal degeneration in multiple sclerosis

In patients with multiple sclerosis (MS), a diffuse axonal degeneration occurring throughout the white matter of the central nervous system causes progressive neurologic disability. The underlying mechanism is unclear. This review describes a number of pathways by which dysfunctional astrocytes in MS might lead to axonal degeneration. White-matter astrocytes in MS show a reduced metabolism of adenosine triphosphate-generating phosphocreatine, which may impair the astrocytic sodium potassium pump and lead to a reduced sodium-dependent glutamate uptake. Astrocytes in MS white matter appear to be deficient in β(2) adrenergic receptors, which are involved in stimulating glycogenolysis and suppressing inducible nitric oxide synthase (NOS2). Glutamate toxicity, reduced astrocytic glycogenolysis leading to reduced lactate and glutamine production, and enhanced nitric oxide (NO) levels may all impair axonal mitochondrial metabolism, leading to axonal degeneration. In addition, glutamate-mediated oligodendrocyte damage and impaired myelination caused by a decreased production of N-acetylaspartate by axonal mitochondria might also contribute to axonal loss. White-matter astrocytes may be considered as a potential target for neuroprotective MS therapies.     

Regeneration of axons after nerve transection repair is enhanced by degradation of chondroitin sulfate proteoglycan

Our past work indicates that growth-inhibiting chondroitin sulfate proteoglycan (CSPG) is abundant in the peripheral nerve sheaths and interstitium. In this study we tested if degradation of CSPG by chondroitinase enhances axonal regeneration through the site of injury after (a) nerve crush and (b) nerve transection and coaptation. Adult rats received the same injury bilaterally to the sciatic nerves and then chondroitinase ABC was injected near the injury site on one side, and the contralateral nerve was injected with vehicle alone. Nerves were examined 2 days after injury in the nerve crush model and 4 days after injury in the nerve transection model. Chondroitinase-dependent neoepitope immunolabeling showed that CSPG was thoroughly degraded around the injury site in the chondroitinase-treated nerves. Axonal regeneration through the injury site and into the distal nerve was assessed by GAP-43 immunolabeling. Axonal regeneration after crush injury was similar in chondroitinase-treated and control nerves. In contrast, axonal regrowth through the coaptation of transected nerves was markedly accelerated and the ingress of axons into the distal segment was increased severalfold in nerves injected with chondroitinase. On the basis of these results we concluded that growth inhibition by CSPG contributes critically to the poor regenerative growth of axons in nerve transection repair. In addition, degradation of CSPG by injection of chondroitinase ABC at the site of nerve repair increased the ingress of axonal sprouts into basal laminae of the distal nerve segment, presumably by enabling more latitude in growth at the interface of coapted nerve. This suggests that chondroitinase application may be used clinically to improve the outcome of primary peripheral nerve repair.     

Molecular mechanisms of axonal damage in inflammatory central nervous system diseases

PURPOSE OF REVIEW: Axonal dysfunction and damage is an early pathological sign of autoimmune central nervous system disease, viral and bacterial infections, and brain trauma. Axonal injury has attracted considerable interest during the past few years because the degree of axonal damage appears to determine long-term clinical outcome. RECENT FINDINGS: Advanced magnetic resonance spectroscopic imaging techniques have suggested that axonal loss and dysfunction is responsible for the persistent neurological deficits that occur in patients with multiple sclerosis. Histopathological methods have shown that axonal damage is defined primarily by dysfunction of axonal transport, and finally by complete transection and degeneration of axons. Recent studies have demonstrated that the extent of axonal damage in the primary demyelinating lesion of multiple sclerosis patients is associated with the number of activated microglia/macrophages and cytotoxic CD8+ T lymphocytes. In addition, diffuse axonal dysfunction independent of demyelination develops in normal appearing white matter, possibly due to indirect effects of inflammation. SUMMARY: The fact that axonal damage in response to overt inflammatory reactions may occur gradually, leaving a window for therapeutical intervention, has important clinical implications. Determination of the exact molecular mechanism might help in finding new therapies for inflammatory axonal damage.     

Neurological disability correlates with spinal cord axonal loss and reduced N-acetyl aspartate in chronic multiple sclerosis patients

Axonal degeneration has been proposed as a cause of irreversible neurological disability in multiple sclerosis (MS) patients. The purpose of this study was to quantify axonal loss in spinal cord lesions from 5 paralyzed (Expanded Disability Status Scale score > or =7.5) MS patients and to determine if axonal number or volume correlated with levels of the neuronal marker N-acetyl aspartate (NAA). Axonal loss in MS lesions ranged from 45 to 84% and averaged 68%. NAA levels were significantly reduced (>50%) in cross sections of spinal cords containing MS lesions. Reduced NAA correlated with reduced axonal numbers within lesion areas. In addition, NAA levels per axonal volume were significantly reduced in demyelinated axons (42%) and in myelinated axons in normal-appearing white matter (30%). The data support axonal loss as a major cause of irreversible neurological disability in paralyzed MS patients and indicate that reduced NAA as measured by magnetic resonance spectroscopy can reflect axonal loss and reduced NAA levels in demyelinated and myelinated axons.     

White matter damage following systemic injection of the mitochondrial inhibitor 3-nitropropionic acid in rat

Oxidative stress has been implicated as a pathogenic mediator of neuronal perikarya cell death. Axons and oligodendrocytes, components of white matter, could also be vulnerable to oxidative damage. An experimental model of oxidative stress was induced by systemic injection of 3-nitropropionic acid (3-NPA). Animals received an i.p. injection of 10, 15, 20 or 30 mg/kg 3-NPA or vehicle and were killed 24 h later. 3-NPA produced a concentration-dependent increase in axonal pathology within the striatum reflected by the amount of beta-APP and SNAP-25 accumulation. Axonal damage was anatomically coincident with the neuronal lesion. There was no neuronal or axonal damage in the subcortical white matter or cerebral cortex in any of the animals treated with 3-NPA. Manganese superoxide dismutase (Mn-SOD) immunoreactivity was present in the vehicle and all 3-NPA treated groups. The amount of Mn-SOD cellular staining was concentration-dependently increased within the striatum supporting a role for oxidative stress in the mechanism of 3-NPA neurotoxicity. Oligodendrocyte-like cells within the subcortical white matter were immunopositive for calpain-mediated spectrin breakdown products and increased in a concentration-dependent manner. Therefore in this experimental model, mitochondrial inhibition may lead to the initiation of oxidative stress and calpain activation, which could mediate cytoskeletal breakdown in axons and oligodendrocytes suggesting an interaction between at least two pathogenic mechanisms.     

Notch signaling: Key role in intrauterine infection/inflammation,embryonic development,and white matter damage?

The mechanisms or pathophysiologies that lead to cerebral white matter damage during development are complex and not fully understood. It is postulated that exposure of the preterm brain to inflammatory cytokines during intrauterine infection/inflammation contributes to brain white matter damage, and this damage may affect the function and differentiation of progenitor oligodendrocyte cells under physiological conditions. The Notch pathway, an important signaling pathway controlling various cells' differentiation, functions in the timing of oligodendrocyte differentiation, and Notch signaling may contribute to white matter damage and may mediate neurogenesis in a pathophysiological phase. Recent studies have led to recognition of the role of the Notch pathway in neurogenesis in cerebral ischemic damage and in myelination and axonal damage of neurodegenerative diseases. Moreover, Notch plays a critical role in steering an immune response toward inflammation by regulating expression of various cytokines and proinflammatory cytokines resulting in the activation of Notch signaling. Thus, the Notch signaling pathway likely plays a key role in intrauterine infection/inflammation, brain development, and white matter damage, and future research directed toward understanding its role will be important. Insofar as Notch signaling could have an important effect on neurogenesis, mobilization of progenitor cells is one strategy for compensating for the neuronal losses seen in white matter damage after intrauterine infection/inflammation. © 2009 Wiley‐Liss, Inc.     

Axonal degeneration stimulates the formation of NG2+ cells and oligodendrocytes in the mouse

Proliferation of the adult NG2-expressing oligodendrocyte precursor cells has traditionally been viewed as a remyelination response ensuing from destruction of myelin and oligodendrocytes, and not to the axonal pathology that is also a characteristic of demyelinating disease. To better understand the response of the NG2+ cells to the different components of demyelinating pathology, we investigated the response of adult NG2+ cells to axonal degeneration in the absence of primary myelin or oligodendrocyte pathology. Axonal degeneration was induced in the hippocampal dentate gyrus of adult mice by transection of the entorhino-dentate perforant path projection. The acutely induced degeneration of axons and terminals resulted in a prompt response of NG2+ cells, consisting of morphological transformation, cellular proliferation, and upregulation of NG2 expression days 2-3 after surgery. This was followed by a reduction of cellular NG2 expression to subnormal levels from day 5 to 7 and reappearance of normal appearing NG2+ cells from day 10. Mice that had received repeated injections of bromodeoxyuridine from 24 to 72 h after surgery contained significant numbers of bromodeoxyuridine-incorporating oligodendrocytes in the areas with axonal degeneration at day 7. The results suggest that axonal degeneration induces a unique sequence of changes of NG2+ cells and that a subpopulation of the newly generated NG2+ cells differentiate into oligodendrocytes.     

Lineage tracing reveals dynamic changes in oligodendrocyte precursor cells following cuprizone‐induced demyelination

The regeneration of oligodendrocytes is a crucial step in recovery from demyelination, as surviving oligodendrocytes exhibit limited structural plasticity and rarely form additional myelin sheaths. New oligodendrocytes arise through the differentiation of platelet‐derived growth factor receptor α (PDGFRα) expressing oligodendrocyte progenitor cells (OPCs) that are widely distributed throughout the CNS. Although there has been detailed investigation of the behavior of these progenitors in white matter, recent studies suggest that disease burden in multiple sclerosis (MS) is more strongly correlated with gray matter atrophy. The timing and efficiency of remyelination in gray matter is distinct from white matter, but the dynamics of OPCs that contribute to these differences have not been defined. Here, we used in vivo genetic fate tracing to determine the behavior of OPCs in gray and white matter regions in response to cuprizone‐induced demyelination. Our studies indicate that the temporal dynamics of OPC differentiation varies significantly between white and gray matter. While OPCs rapidly repopulate the corpus callosum and mature into CC1 expressing mature oligodendrocytes, OPC differentiation in the cingulate cortex and hippocampus occurs much more slowly, resulting in a delay in remyelination relative to the corpus callosum. The protracted maturation of OPCs in gray matter may contribute to greater axonal pathology and disease burden in MS.