Background Multiple sclerosis is an inflammatory disease ofthe central nervous system that destroys myelin, oligodendrocytes,and axons. Since most of the lesions of multiple sclerosis arenot remyelinated, enhancement of remyelination is a possibletherapeutic strategy that could perhaps be achieved with thetransplantation of oligodendrocyte-producing cells into thelesions. We investigated the frequency distribution and configurationof oligodendrocytes in chronic lesions of multiple sclerosisto determine whether these factors limit remyelination.
Methods Forty-eight chronic lesions obtained at autopsy from10 patients with multiple sclerosis were examined immunocytochemicallyfor oligodendrocytes and oligodendrocyte progenitor cells. Usingconfocal microscopy, we examined the three-dimensional relationsbetween axons and the processes of premyelinating oligodendrocytes.
Results Thirty-four of the 48 chronic lesions of multiple sclerosiscontained oligodendrocytes with multiple extended processesthat associated with demyelinated axons but failed to myelinatethem. These axons were dystrophic and contained multiple swellings.In some regions, the densities of premyelinating oligodendrocytes(25 per square millimeter of tissue) were similar to those inthe developing rodent brain (23 per square millimeter). In thepatients with disease of long duration (more than 20 years),there were fewer lesions with premyelinating oligodendrocytes(P<0.001).
Conclusions Premyelinating oligodendrocytes are present in chroniclesions of multiple sclerosis, so remyelination is not limitedby an absence of oligodendrocyte progenitors or their failureto generate oligodendrocytes. Our findings suggest that in thechronic lesions of multiple sclerosis, the axons are not receptivefor remyelination. Understanding the cellular interactions betweenpremyelinating oligodendrocytes, axons, and the microenvironmentof lesions of multiple sclerosis may lead to effective strategiesfor enhancing remyelination.
Multiple sclerosis is an inflammatory disease of the centralnervous system that destroys myelin, the insulation that surroundsaxons. Oligodendrocytes (the cells that produce myelin) andnerve fibers are also destroyed.1,2,3,4 Most patients with multiplesclerosis have an initial relapsing–remitting course for5 to 15 years that then takes a secondary progressive courseof irreversible neurologic disability.5 Relapses result frominflammation and demyelination, whereas restoration of nerveconduction and remission is accompanied by resolution of inflammation,redistribution of sodium channels on demyelinated axons, andremyelination.6,7
Demyelination is not always permanent in multiple sclerosis.Remyelination during early stages of the disease process hasbeen documented by histologic analysis of tissue specimens fromboth biopsy and postmortem examination.8,9,10,11 Most chroniclesions of multiple sclerosis, however, are not remyelinated.Remyelination requires generation of new oligodendrocytes.12Oligodendrocyte progenitor cells, identified by the expressionof the platelet-derived growth factor receptor and the sulfatedproteoglycan NG2, have been characterized in developing brain,13,14normal adult human brain,15,16,17 and chronic lesions of multiplesclerosis.16,18,19 Isolated progenitor cells can give rise tooligodendrocytes in vitro.20,21,22,23 During development ofthe rodent brain, oligodendrocyte progenitor cells differentiateinto premyelinating oligodendrocytes that radially extend multipleprocesses positive for myelin proteins that do not immediatelymyelinate axons.24 These premyelinating oligodendrocytes havea limited life span (approximately three days) and either myelinateaxons or die by programmed cell death.24,25 Premyelinating oligodendrocytesare not detected in white matter in the brains of normal adultrodents or humans.
Transplantation of oligodendrocyte-producing cells into lesionsof multiple sclerosis is being considered as a therapeutic strategyto enhance remyelination.26 It remains to be determined, however,whether oligodendrocyte progenitors or production or differentiationof oligodendrocytes limits remyelination in chronic lesionsof multiple sclerosis. Despite detection of cells with phenotypiccharacteristics of oligodendrocyte progenitor cells in chroniclesions of multiple sclerosis,16,18,19 the potential for thesecells to produce oligodendrocytes has not been demonstrated.This report describes oligodendrocytes in chronic lesions ofmultiple sclerosis that extend multiple processes that associatewith but fail to myelinate axons.
Methods
Tissue
The brains from 10 deceased patients with multiple sclerosiswere investigated. Six brains were obtained from patients whohad been followed at the Cleveland Clinic Foundation. Thesebrains were sliced (1 cm thick) and fixed in 4 percent paraformaldehyde.Lesions were removed, cryoprotected, and sectioned (30 µmthick) on a freezing–sliding microtome. Fresh-frozen brainslices from four other deceased patients with multiple sclerosiswere obtained from the Multiple Sclerosis Human NeurospecimenBank in Los Angeles and were simultaneously thawed and fixedin 4 percent paraformaldehyde and processed as described above.Clinical data for the patients are summarized in Table 1.
Table 1. Characteristics of Patients and Lesions Studied.
Patient 1 died from respiratory failure accompanying severebrain-stem inflammation in the setting of relapsing multiplesclerosis. All the other patients died of medical complicationsfrom severe, debilitating multiple sclerosis, as listed in Table 1.Two patients received disease-modifying therapy in the yearbefore death. Patient 1 was treated with cyclophosphamide andmethylprednisolone one month before death; Patient 7 was receivinginterferon beta treatment at the time of death. On the basisof the distribution of myelin protein and the staining for major-histocompatibility-complex(MHC) class II molecules, 52 lesions were identified and classifiedas active (4), chronic active (5), and chronic inactive (43),as described previously.4
Immunocytochemical Analysis
Free-floating sections (30 µm thick) were microwaved in10 mM citric acid buffer (pH 6.0) twice for 5 minutes, incubatedin 1 percent hydrogen peroxide and 10 percent Triton X-100 inphosphate-buffered saline for 30 minutes, and immunostainedby the avidin–biotin complex procedure with diaminobenzidine,as described previously.24 Sections were incubated in proteolipid-proteinantibodies for five days at 4°C. Sections stained for NG2were not microwaved; they were pretreated with 0.3 percent TritonX-100 and immunostained with the use of the tyramide signal-amplificationmethod (PerkinElmer Life Sciences, Boston), as previously described.16Sections for double labeling were pretreated as above and incubatedwith two primary antibodies for five days and with fluorescein-conjugatedand biotinylated secondary antibodies (Jackson ImmunoResearchLaboratories, West Grove, Pa.) for one hour.
Antibodies
Sections were immunostained with the following antibodies: ratanti–proteolipid protein (Agmed, Bedford, Mass.), mouseanti–myelin oligodendrocyte glycoprotein (a gift fromDr. Minnetta Gardinier, University of Iowa, Iowa City), mouseanti–human NG2 (clone 9.2.27, Pharmingen, San Diego, Calif.),mouse anti–human MHC class II (Dako, Glostrup, Denmark),mouse anti–nonphosphorylated neurofilament (SMI32, SternbergerMonoclonals, Baltimore), mouse anti–phosphorylated neurofilament(SMI31, Sternberger Monoclonals), and rabbit antineurofilament(Serotec, Raleigh, N.C.).
Confocal Microscopy
Sections were examined with a laser scanning confocal microscope(Leica Microsystems, Exton, Pa.). Antibody combinations includedproteolipid protein plus neurofilament and myelin oligodendrocyteglycoprotein plus neurofilament. Laser intensity was adjustedto eliminate "bleed-through." The images presented are stacksof 18 to 36 optical sections that were scanned synchronously.
Quantification of Premyelinating Oligodendrocytes
Premyelinating oligodendrocytes in lesions of multiple sclerosiswere identified as cells positive for proteolipid protein withmultiple processes that did not contact myelin internodes. Todetermine the size of the demyelinated area, sections stainedwith proteolipid protein antibodies were scanned (ScanMaker4, Microtek Lab, Redondo Beach, Calif.); the borders of thelesions were outlined on a digital image, and the demyelinatedarea was measured with software from the National Institutesof Health (NIH Image). The density of premyelinating oligodendrocyteswas determined in the total demyelinated area in 34 lesions,in 30 lesions enriched in premyelinating oligodendrocytes, andin developing rat brain (postnatal day 14). Areas of lesionsof multiple sclerosis containing more than six premyelinatingoligodendrocytes per 0.38 mm2 were calculated separately andclassified as enriched in premyelinating oligodendrocytes; thisdensity was chosen because it is similar to that found in areasof the developing rodent brain. The density of premyelinatingoligodendrocytes in developing rat brain was determined by counting15 regions of unmyelinated cerebral cortex. Densities of premyelinatingoligodendrocytes were compared by Student's t-test. The relationbetween the percentage of lesions with premyelinating oligodendrocytesand the duration of disease, the age of the patient at the timeof death, the type of disease, and the score on the ExpandedDisability Status Scale (EDSS; possible range, 0 to 10, witha higher score indicating a greater degree of disability)27was determined by the Pearson correlation coefficient.
Results
Premyelinating Oligodendrocytes in Chronic Lesions of Multiple Sclerosis
The clinical history and number of lesions studied from eachpatient are shown in Table 1. Forty-eight chronic lesions ofmultiple sclerosis from 10 deceased patients with multiple sclerosiswere examined for oligodendrocytes. Chronic lesions of multiplesclerosis were identified by the presence of demyelination anda low density of cells positive for MHC class II molecules (insetsin Figure 1A). Premyelinating oligodendrocytes were detectedin 34 lesions (71 percent). These premyelinating oligodendrocyteswere not evenly distributed within the lesions and tended tooccur in groups (Figure 1A). The general morphologic appearanceof the cells varied, depending on location. The perikarya ofthe premyelinating oligodendrocytes were often detected in theaxon-free subventricular zone (Figure 1B and Figure 1C). Thesecells asymmetrically extended multiple processes positive forproteolipid protein into the area of demyelinated axons. Mostpremyelinating oligodendrocytes were clustered throughout thelesions (inset in Figure 1D) and radially extended multipleprocesses positive for proteolipid protein that were orientedparallel to demyelinated axons (Figure 1D).
Figure 1. Premyelinating Oligodendrocytes in Chronic Lesions of Multiple Sclerosis.
Chronic demyelinated lesions of multiple sclerosis were identified by a lack of myelin (left inset in Panel A) and a low density of cells positive for MHC class II molecules within the lesion (right inset in Panel A). Premyelinating oligodendrocytes were often detected in clusters (Panel A) just beneath the subventricular zone (SVZ). Some perikarya of premyelinating oligodendrocytes were located within the axon-free subventricular zone (Panels B and C) and extended processes that were oriented parallel to the demyelinated axons. Clusters of premyelinating oligodendrocytes were detected within lesions (inset in Panel D). These cells radially extended multiple processes that were oriented parallel to demyelinated axons (Panel D). All premyelinating oligodendrocytes were stained with proteolipid-protein antibodies. V denotes ventricle. The scale bar in Panel A represents 100 µm; the scale bars in Panels B, C, and D represent 30 µm.
In the 34 chronic lesions with premyelinating oligodendrocytes,the average density of premyelinating oligodendrocytes was 2per square millimeter (Figure 2). However, in 30 lesion areasenriched in premyelinating oligodendrocytes, the density ofoligodendrocytes (25 per square millimeter) was similar to thatfound in developing rat brain (23 per square millimeter) (Figure 2).Thirteen of the 30 areas were located close to the subventricularzone. In the 34 lesions analyzed, premyelinating oligodendrocyteswere enriched in approximately 7 percent of the total area ofthe lesions.
Figure 2. Scatter Plot of the Density of Premyelinating Oligodendrocytes in the Total Areas of Chronic Lesions, in Lesion Areas Enriched in Premyelinating Oligodendrocytes, and in Rat Cerebral Cortex (Postnatal Day 14).
The density of premyelinating oligodendrocytes was significantly enriched in approximately 7 percent of the total area of chronic lesions (P<0.001). These areas contained densities of premyelinating oligodendrocytes similar to those in developing rat brain.
Fourteen of the 48 chronic lesions studied did not contain detectablepremyelinating oligodendrocytes (Table 1). Eleven lesions withoutpremyelinating oligodendrocytes came from the three patientswith the longest duration of disease (23, 35, and 44 years),whereas only 1 of 14 lesions from Patient 6 was negative forpremyelinating oligodendrocytes. The percentage of lesions withpremyelinating oligodendrocytes varied inversely with the durationof disease (Pearson correlation coefficient, –0.90; 95percent confidence interval, –0.62 to –0.98; P<0.001).The age of the patient at the time of death, the type of disease,and the EDSS score did not correlate with the percentage oflesions with premyelinating oligodendrocytes (P>0.05).
Oligodendrocyte Progenitor Cells in Chronic Lesions of Multiple Sclerosis
Cells expressing the sulfated proteoglycan NG2 may be oligodendrocyteprogenitor cells in lesions of multiple sclerosis.16 The distributionof NG2-positive cells was investigated in sections cut adjacentto sections stained with proteolipid protein antibodies. Subventricularareas of the lesions with premyelinating oligodendrocytes containedstellate NG2 cells (Figure 3A) that had an appearance similarto that of NG2 cells in brains from deceased patients withoutneurologic disease. As in these brains, many of the NG2-positivecells projected processes to blood vessels that were also NG2-positive.Other regions of the lesions of multiple sclerosis enrichedin premyelinating oligodendrocytes contained elongated (Figure 3A)or stellate (Figure 3B) NG2 cells.
Figure 3. Oligodendrocyte Progenitor Cells in Chronic Lesions of Multiple Sclerosis.
Stellate NG2-positive cells lined the ventricle (area above solid arrows in Panel A) of chronic lesions of multiple sclerosis. Deeper within the lesion, elongated (arrowheads in Panel A) or stellate (solid arrows in Panel B) NG2-positive cells were present. In lesions without premyelinating oligodendrocytes, stellate NG2-positive cells (solid arrows in Panel C) were detected at a lower density than lesions with premyelinating oligodendrocytes, and many had shorter and fewer processes. NG2 antibodies also stained blood vessels in the central nervous system (open arrows in Panels A, B, and C). V denotes ventricle; the scale bars represent 100 µm.
Stellate NG2 cells were detected in lesions without premyelinatingoligodendrocytes. The density of these cells, however, was lessthan that in nonlesion areas of the same sections. In addition,these NG2 cells had fewer, shorter, and thicker processes thanthose in nonlesion areas (Figure 3C). Elongated NG2 cells werenot detected in lesions without premyelinating oligodendrocytes.
Processes of Premyelinating Oligodendrocytes Associated with Axons
The orientation of processes of premyelinating oligodendrocytes(Figure 1) suggests that they physically associate with demyelinatedaxons. This possibility was investigated by determining thethree-dimensional relation between processes of premyelinatingoligodendrocytes and axons in confocal images of sections immunostainedwith proteolipid protein and neurofilament antibodies. Figure 4Ashows a premyelinating oligodendrocyte located in the axon-freesubventricular zone. This cell extended processes positive forproteolipid protein into the zone of demyelinated axons. Whenthese processes reached the demyelinated axons, many assumeda parallel orientation with individual axons (Figure 4A). Three-dimensionalanalysis of rotated images indicated that many processes longitudinallyspiraled around axons (Figure 4B). Radial ensheathment of axons,however, was rare.
Figure 4. Processes of Premyelinating Oligodendrocytes Associated with Axons.
Confocal micrographs of lesions of multiple sclerosis stained with proteolipid protein antibodies (red in Panels A, B, and D), myelin oligodendrocyte glycoprotein antibodies (red in Panel C), and neurofilament antibodies (green in Panels A, B, C, and D) are shown. A premyelinating oligodendrocyte (red in Panel A) in the subventricular zone (SVZ) extended processes into the region of demyelinated axons (green in Panel A) in a chronic lesion of multiple sclerosis. Many of these processes (arrows in Panel A) spiraled around axons, as shown at higher magnification (Panel B). Some premyelinating oligodendrocytes in chronic lesions of multiple sclerosis were also positive for myelin oligodendrocyte glycoprotein and extended processes that associated with axons (arrows in Panel C). Premyelinating oligodendrocytes within the lesion (red in Panel D) radially extended processes that also contacted and longitudinally associated (arrows in Panel D) with axons (green in Panel D). Dying premyelinating oligodendrocytes had fragmented processes that were positive for proteolipid protein and condensed perinuclear cytoplasm (inset in Panel D). The scale bars represent 20 µm.
During normal development of the rodent brain, myelin oligodendrocyteglycoprotein is considered a marker of mature or myelinatingoligodendrocytes28 and has not been detected in premyelinatingoligodendrocytes (unpublished data). Many premyelinating oligodendrocytesin chronic lesions of multiple sclerosis, however, were stainedby myelin oligodendrocyte glycoprotein antibodies (Figure 4C).Perikarya of premyelinating oligodendrocytes located withinlesions radially extended processes that also associated withindividual axons (Figure 4D). Some processes extended considerabledistances before associating with axons, and many axons closeto oligodendrocyte-cell bodies were not ensheathed by oligodendrocyteprocesses. Dying premyelinating oligodendrocytes, which werecharacterized by fragmented processes, condensed perinuclearcytoplasm, and nuclei stained for proteolipid protein (insetin Figure 4D), were also detected in chronic lesions of multiplesclerosis.
Oligodendrocytes in Remyelinating Lesions
Previous studies identified abundant oligodendrocytes in somesubacute lesions of multiple sclerosis.11,29 These oligodendrocytesextended a few short processes, but they did not myelinate axons.Four acute lesions with abundant macrophages that were positivefor MHC class II molecules contained regions with abundant,small, round oligodendrocytes with few or no processes (datanot shown). Premyelinating oligodendrocytes with multiple radiallyoriented processes were not detected in these acute lesions.Remyelination was evident, however, at the edge of many chroniclesions. The relation between remyelinating oligodendrocytesand axons was investigated by confocal microscopy in these partiallyremyelinated lesions or "shadow plaques."
Remyelinating oligodendrocytes had relatively large perikaryapositive for proteolipid protein and extended processes to shortmyelin internodes (Figure 5A). The number of processes was oftenmatched to the number of internodes, particularly in remyelinatingoligodendrocytes that formed longer internodes (Figure 5B).In areas of shadow plaque with more remyelination and longerinternodes, oligodendrocyte perikarya were smaller and lessintensely stained by proteolipid-protein antibodies (Figure 5C)than in premyelinating oligodendrocytes (Figure 1B, Figure 1C,Figure 1D, and Figure 4D) or in early remyelinating oligodendrocytes(Figure 5A and Figure 5B). Oligodendrocyte processes extendingto myelin internodes were not always detected within shadowplaques. Premyelinating oligodendrocytes were not detected inshadow plaques of the chronic lesions of multiple sclerosisthat we analyzed.
Figure 5. Remyelinating Oligodendrocytes in Chronic Lesions of Multiple Sclerosis.
Remyelinating oligodendrocytes at the edge of shadow plaques (red areas in Panel A) extended processes to very short myelin internodes. Many unmyelinated axons (green areas in Panel A) appeared dystrophic. As shown in Panel B, in demyelinated regions of brain that contained straight, healthy-appearing axons (green), oligodendrocytes (red) extended processes (arrowheads) to myelin internodes. Within shadow plaques, many but not all axons were ensheathed by myelin internodes (red in Panel C). Occasional perikarya of oligodendrocytes were positive for proteolipid protein (arrowhead in Panel C), but oligodendrocyte processes that were positive for proteolipid protein were rarely detected. Axons within shadow plaques were straight and of a consistent diameter (green in Panel C). The scale bars represent 20 µm.
The morphologic appearance of axons in chronic lesions containingpremyelinating oligodendrocytes and remyelinating oligodendrocytesdiffered. Chronic lesions with premyelinating oligodendrocytescontained axons (Figure 4A, Figure 4B, and Figure 4C) with multipleswellings, marked variations in diameter, and a more tortuouscourse. In contrast, remyelinated axons (Figure 5B and Figure 5C)were straight, with relatively consistent diameters. Theedge of shadow plaques often contained a combination of dystrophicand healthy-appearing axons (Figure 5A). In general, the majorityof remyelinated internodes surrounded the healthy-appearingaxons.
Discussion
In the central nervous system of patients with multiple sclerosis,remyelination may be able to restore rapid nerve conductionand protect demyelinated axons from degeneration. To developstrategies for successful remyelination, one needs to identifywhy remyelination fails. We detected premyelinating oligodendrocytesin 34 of 48 chronic lesions of multiple sclerosis, which establishesthat some other factors limit remyelination of such lesions.The premyelinating oligodendrocytes physically associated withaxons and were abundant in patients with clinical disease of1 to 15 years' duration. This provides an extended window ofopportunity to deliver remyelinating therapeutic agents thattarget premyelinating oligodendrocytes, the microenvironmentof chronic lesions of multiple sclerosis, or both.
The differentiation of oligodendrocytes in chronic lesions ofmultiple sclerosis appears to recapitulate the premyelinatingstage identified during the development of the rodent brain.24The positive correlation between the distribution of NG2 cellsand premyelinating oligodendrocytes suggests that NG2 cellsare a source of these premyelinating oligodendrocytes. It remainsto be determined whether premyelinating oligodendrocytes aregenerated from a stem or progenitor cell that repopulates thelesion or from the NG2 cells in or near the lesion. In eitherevent, the detection of premyelinating oligodendrocytes unequivocallyestablishes the presence of oligodendrocyte progenitor cellsin chronic lesions of multiple sclerosis. All patients examinedhad lesions with premyelinating oligodendrocytes (Table 1).It remains to be determined, however, whether proposed variationsin the pathogenesis of multiple sclerosis30 will affect thenumber of premyelinating oligodendrocytes in chronic lesions.Fourteen chronic lesions were negative for premyelinating oligodendrocytes.Eleven of these lesions were from patients with disease of morethan 20 years' duration, suggesting that chronic lesions eventuallylose the ability to maintain or produce new oligodendrocytes.
We confirmed previous reports11,31,32,33 describing abundant,small, round oligodendrocytes that were positive for myelinoligodendrocyte glycoprotein in acute lesions of multiple sclerosis.Although premyelinating oligodendrocytes were not detected immunocytochemicallyin previous studies, cells positive for proteolipid proteinmessenger RNA were detected by in situ hybridization in tissuesections of chronic lesions of multiple sclerosis11,31,32 andare likely to represent, at least in part, the premyelinatingoligodendrocytes identified in the present report. Optimal immunocytochemicaldetection of premyelinating oligodendrocytes in developing brain24or lesions of multiple sclerosis (Figure 1, Figure 4, and Figure 5)requires microwave pretreatment and five days of incubationwith primary antibody. In addition, resolution of elaborateprocesses, the morphologic hallmark of premyelinating oligodendrocytes,is best achieved in sections of 30 µm or thicker. As inother studies, we have not detected premyelinating oligodendrocyteswith more conventional staining procedures.
The remyelinating potential of premyelinating oligodendrocytesin chronic lesions of multiple sclerosis will depend on thelife span of the cells. Although it is impossible to determinethe life spans of cells in the human brain, the detection ofapoptotic premyelinating oligodendrocytes in chronic lesionsof multiple sclerosis (inset in Figure 4D) indicates that theirlife spans are limited. If the life span is two to three days,as reported for the developing rodent central nervous system,25,34the total number of oligodendrocytes produced in chronic lesionsthat are years or decades old should be more than enough toremyelinate the lesions. Rodent premyelinating oligodendrocytesthat are produced in the absence of axons of the optic nerveextend fewer and shorter processes than those in normal nerves35or in chronic lesions of multiple sclerosis (Figure 1 and Figure 4).Axons in the chronic lesions of multiple sclerosis thereforeappear capable of supporting differentiation of an oligodendrocyteto a premyelinating phenotype, to association with axons, andto expression of myelin oligodendrocyte glycoprotein, but notto myelination.
Little is known about the molecular mechanisms responsible forthe initiation of myelination or whether the inhibition of myelinationin lesions of multiple sclerosis is due to dysregulated growthfactors, the altered molecular composition of axons, or thepresence of an inhibitory signal. The correlation between dystrophicaxons with swellings and lesions with premyelinating oligodendrocytesbut no remyelination supports the hypothesis that axonal pathologicprocesses limit the remyelination of chronic lesions of multiplesclerosis. This hypothesis is also supported by data in animals36,37,38and the morphologic integrity of remyelinated axons in shadowplaques described here and in previous studies.37 At present,there is no evidence of genetic defects specific for oligodendrocytesthat would limit remyelination in patients with multiple sclerosis.
With regard to possible remyelination therapies, an importantquestion is whether transplantation of the appropriate cellinto lesions of multiple sclerosis would promote remyelination.Our data indicate that the environment of many chronic inactivelesions of multiple sclerosis supports the production of newpremyelinating oligodendrocytes from endogenous cell populationsand suggest that it would support such production from transplantedprogenitor cells. However, it appears that the environment withinchronic lesions will not provide the appropriate signals forremyelination. We speculate that failure of remyelination bypremyelinating oligodendrocytes in chronic lesions of multiplesclerosis is due to an abnormal molecular composition of chronicallydemyelinated axons or an imbalance of growth factors that regulatemyelination. The challenge is therefore to understand the interactionsbetween premyelinating cells, axons, and the microenvironmentof the lesions better. If lesions of multiple sclerosis canbe modified to promote the myelination of axons by processesof premyelinating oligodendrocytes, cell transplantation intochronic lesions could prove beneficial.
Supported by grants (PO1 NS38667 and RO1 NS35058) from the NationalInstitutes of Health.
We are indebted to the Cleveland Clinic Foundation MultipleSclerosis Tissue Donation Program and the Multiple SclerosisHuman Neurospecimen Bank for providing tissue specimens; toDrs. Richard Ransohoff, Jeff Cohen, and Robert Miller for helpfulcomments; to Dr. Grahame Kidd for confocal-image analysis; toRenata Klinkosz for technical help; and to Victoria Pickettfor editorial assistance.
Source Information
From the Department of Neurosciences, Lerner Research Institute (A.C., B.D.T.), and the Mellen Center for Multiple Sclerosis (R.R.), Cleveland Clinic Foundation, Cleveland; and the Department of Neurology, West Los Angeles Veterans Affairs Medical Center, Los Angeles (W.W.T.).
Address reprint requests to Dr. Trapp at the Department of Neurosciences, NC30, Lerner Research Institute, Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195, or at trappb{at}ccf.org.
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and the sulfated proteoglycan NG2, have been characterized in developing brain,13,14 normal adult human brain,15,16,17 and chronic lesions of multiple sclerosis.16,18,19 Isolated progenitor cells can give rise to oligodendrocytes in vitro.20,21,22,23 During development of the rodent brain, oligodendrocyte progenitor cells differentiate into premyelinating oligodendrocytes that radially extend multiple processes positive for myelin proteins that do not immediately myelinate axons.24 These premyelinating oligodendrocytes have a limited life span (approximately three days) and either myelinate axons or die by programmed cell death.24,25 Premyelinating oligodendrocytes are not detected in white matter in the brains of normal adult rodents or humans. 
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