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Alterations of oligodendrocytes and demyelination in the spinal cord of patients with mitochondrial encephalomyopathy
Journal of the Neurological Sciences, 1988, 86:19-29
19
Elsevier JNS 03015
Alterations of oligodendrocytes and demyelination in the spinal cord of patients with mitochondrial encephalomyopathy Shinji Ohara ~,*, Eisaku Ohama 1, Hitoshi Takahashi ~, Fusahiro Ikuta 1, Masatoyo Nishizawa 2, Keiko Tanaka 2 and Tadashi Miyatake 2 tDepartment of Pathology and 2Department of Neurology, Brain Research Institute, Niigata University, Niigata 951 (Japan)
(Received23 March, 1987) (Revised,received23 February, 1988) (Accepted 26 February, 1988)
SUMMARY The spinal cords of 2 autopsied patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) were examined. Histologically, the spinal cords showed a spongy state due to the presence of distended myelinated fibers with enlarged periaxonal spaces. Ultrastructurally, the affected fibers showed extensive microvacuolation of the inner myelin sheath with occasional vesicular changes. The presence of macrophages near the degenerated myelin was a frequent finding. The stripping of myelin lameUae by macrophage was observed, with frequent appearance of denuded axons. Furthermore, prominent morphological changes were observed in oligodendrocytes. These f'mdings indicate that demyelination, probably secondary to the degeneration ofoligodendrocytes, occurs in the spinal cord ofMELAS.
Key words: Oligodendrocyte; Degeneration; Demyelination; Spongy state; Mitochondrial encephalomyopathy
* Present address: The Third Department of Internal Medicine, Shinshu University, School of Medicine, Asahi 3-1-1,Matsumoto390, Japan. Correspondence to: Dr.E. Ohama, Department of Pathology, Brain Research Institute, Nligata University, 1 Asahimachi, Niigata 951, Japan.
0022-510X/88/$03.50 © 1988Elsevier SciencePublishers B.V.(BiomedicalDivision)
20 INTRODUCTION Since Shapira et al. (1977) first introduced the concept of mitochondrial encephalomyopathies, special attention has been paid to the involvement of the central nervous system (CNS) because of its predominance in the clinical manifestation. Several authors have proposed that some of these can be classified into distinctive clinical syndromes, such as Keams-Sayre syndrome, myoclonus epilepsy with raggedred fibers (MERRF) (Fukuhara et al. 1980) and mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) (Pavlakis et al. 1984). In this communication, we describe the occurrence of a demyelinating process associated with prominent morphological changes of oligodendrocytes in the spinal cords of 2 autopsied patients with MELAS.
MATERIALS AND METHODS Clinical, neuropathological and biochemical findings of 2 cases of MELAS in this study are described in detail elsewhere. Case 1 is a 16-year-old Japanese girl with a 10-year clinical history (Nishizawa et al. 1987; Ohama et al. 1987; Ohama and Ikuta 1987). Case 2 is a 14-year-old Japanese boy with a 12-year clinical history (Tanaka et al. 1986; Ohama et al. 1987; Ohama and Ikuta 1987). Both patients were diagnosed as having MELAS clinically and pathologically. In Case 1, the biochemical investigations of the postmortem heart muscle demonstrated a defect of complex I in the respiratory chain of the mitochondria (Nishizawa et al. 1987). Several segments of the cervical and upper thoracic spinal cords were removed from both cases within three hours postmortem and fixed in phosphate-buffered 3 ~/o glutaraldehyde/1 ~o paraformaldehyde for electron microscopy. Each segment of the spinal cord was cut into several blocks, each containing an anterior, lateral or posterior column. They were post-osmificated and embedded in Epon 812. Thick sections were stained with toluidine blue for light microscopy. Selected portions were thin-sectioned, stained with uranyl acetate and lead citrate, and examined in a Hitachi-11B electron microscope.
RESULTS
Light microscopicfindings In Case 1, the spinal cord showed a marked loss of myelinated fibers with gliosis in the gracile fasciculi. In addition, minute vacuoles were distributed especially in the lateral and posterior columns, forming a spongy state, without accompanying inflammatory cell infiltrations (Fig. 1). This change was observed throughout the spinal cord and lower brainstem and was most conspicuous at the lower cervical through upper thoracic segments. Neurons of the anterior horn and Clarke's column were well preserved.
21
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"!!:
.......
~......i~ i~~~ ...... ~?~
" ?
~ ~I~¸~~ ~
i!:i¸ii
Fig. 1. Lower cervical spinal cord of Case 1 showing a moderate spongy state in the lateral and posterior columns. The gracile fasciculus shows atrophy and myelin pallor. KIQver-Barrera.
The minute vacuoles corresponded to the distended myelin sheaths with abnormally enlarged periaxonal spaces on Epon sections (Fig. 2). Mononuclear cells were occasionally found in the distended fibers, often next to the displaced axon. In mild lesions, the affected fibers were intermingled with apparently healthy nerve fibers and gliosis was mild. In the subpial zones, the lesions were more advanced, showing a massive appearance of distended fibers and loss of myelin. In these areas, gliosis was more evident and demyelinated axons were often observed. In the spinal cord of Case 2, the same change could be found, although to a lesser degree than Case 1.
Ultra.structuralfindings Spinal cords of both cases were suitably preserved for electron microscopy. In both cases, a formation of multiple vacuoles with frequent vesiculation of the inner myelin sheaths was a conspicuous finding (Figs. 3 and 4). Some of the intramyelinic vacuoles contained cell bodies and/or processes which could be identified as those of macrophages by the presence o.fpseudopodia, lysosomal granules, and frequent myelin debris in their cytoplasm. These macrophages were often seen in the immediate vicinity of the intact axons (Figs. 3 and 4). On rare occasions, the stripping of myelin lamellae by an invading tongue of maerophages was also encountered (Fig. 5). Furthermore, many denuded axons were also observed in both eases (Figs. 6 and 7). In addition to the myelin sheaths, prominent morphological changes could be found in the oligodendrocytes. In Case 1, the inner loops of the oligodendrocytes occasionally showed an accumulation of fibrillated materials which were often organized
Fig. 2. Toluidine-blue section, 1 #m thick, showing distended myelinated fibers in the lateral column. Axons (arrows) are occasionally displaced by an enlarged periaxonal empty space. Case 1, x 860.
Fig. 3. Electron micrograph showing multivacuolar cysts and vesicular changes in the inner myelin sheath. A macrophage (M) with pseudopodial processes is seen next to the apparently intact axon. Case 1, × 16350.
Fig. 4. Electron micrograph showing multicystic and vesicular myelin disintegrations (arrows) from Case 2. The cytoplasms (*) within the periaxonal vacuoles are probably those of macrophages. × 14 300.
Fig. 5. Myelin debris within the cytoplasm of macrophages. A process of macrophage is seen to separate myelin lamella. Case 1, x 16500. Inset: a higher magnification of the area indicated by the arrow shows two extemai loops containing microtubules, x 41000.
Fig. 6. Electron micrograph showing a demyelinating axon ensheathed by the innermost myelin lamella and an adjacent macrophage. Case 2, × 14300.
Fig. 7. Electron micrograph showing a completely demyelinated axon surrounded by glial processes. A macrophage is seen in the vicinity. Case 1, x 17900.
25
Fig. 8. (a) Electronmicrographshowingfilamentousinclusionswithinthe innerloopof an oligodendrocyte. Case 1, x 27000. (b)Higher magnificationof the inner loop showinglattice-likearrangement, x 103650. (c) Highermagnificationof the area indicatedby (*).This inclusionconsistsof orderlypackedfilamentscut transversely, x 103650. in crystalloid or lattice-like structures (Fig. 8). An increase of microtubules was also observed in the enlarged internal and external tongues and free cell processes of the oligodendrocytes (Fig. 9). In both cases, oligodendrocytes often showed hypertrophic cytoplasms containing many ribosomes, rough endoplasmic reticulum, dense bodies, microtubules and mitochondria (Fig. 10). An enlarged and vacuolating oligodendrocyte was also observed (Fig. 11). Oligodendrocytes were often found to be thoroughly surrounded by astrocytic processes in the gliotic areas. No structurally abnormal mitochondria could be identified in the oligodendrocytes, astrocytes or endothelial cells of the intraparenchyrnal vessels although the number of mitochondria appeared to be increased in some of them.
26
Fig. 9. Electronmicrograph showing an oligodendrocytewith enlarged internal and external tongues which contain many microtubules. A punctate adhesion (arrow) is noted between the external tongues. An oligodendrocyte with a blunt cellular process is also seen on the left. Case 1, × 26000.
DISCUSSION The present cases appear to be of particular interest in that both spinal cords demonstrated a spongy state with common ultrastructural features of ongoing demyelinating processes. To our knowledge, the occurrence of demyelination in the central nervous system of MELAS has not been documented previously. The ultrastructural features of the demyelination in the present cases seem to be in accordance with those previously described in a variety of demyelinating conditions such as experimental allergic neuritis (Lampert 1965, 1967; Ralne et al. 1974), a viral infection (Dal Canto and Lipton 1975), intoxication (Blakemore 1973; Yajima and Suzuki 1979), trauma (Balentine 1978) and in hereditary metabolic disorders (Takahashi and Suzuki 1984). In these pathological conditions, in which degeneration of oligodendrocytes or focal damage to myelin takes place, the microvacuolar and vesicular changes in the myelin sheath as well as the stripping of myelin lamellae by invading macrophages have also been observed. A random distribution of demyelinating fibers as seen in our
Fig. 10. Enlarged oligodendrocyte showing a marked increase of free ribosomes, rough ER, dense bodies, microtubules and mitoehondria. Case 1, x 26500.
Fig. 1 I. Electron micrograph showing an enlarged oligodendrocyte containing many cytoplasmic vacuoles. Some of these vacuoles are apparently derived from mitochondria (arrows). Case 2, x 26900.
28 cases seemed to occur when degeneration of myelin-forming cells was the primary event (Lampert 1978). Alterations of oligodendrocytes as seen in our cases have also been observed in other demyelinating processes. The occurrence of filamentous accumulations in the inner loops of oligodendrocytes (Fig. 8) has been described in rats subjected to portocaval anastomosis (Cavanaugh et al. 1971) and in mice with cuprizone intoxication (Blakemore 1972, 1973; Ludwin and Johnson 1981). Cuprizone, a chelating agent which inhibits mitochondrial metabolism, has been shown to induce demyelination secondary to the degeneration of oligodendrocytes (Blakemore 1972). In the peripheral nerves of a mutant hamster with hindleg paralysis, Hirano observed identical fibrillary accumulations in the inner loops of Schwann ceils and suggested their possible role in producing demyelination by interfering with the intracellular transport of metabolites and certain organellae (Hirano 1978). The appearance of hypertrophic cytoplasms in oligodendrocytes, containing many ribosomes, dense bodies, rough ERs and mitochondria in addition to abundant microtubules (Fig. 10), may be considered a reaction of the oligodendrocyte to cellular injuries. An abundance of microtubules and cytoplasmic hypertrophy in oligodendrocytes has often been observed in other pathological conditions including intoxication (Cavanaugh 1971; Hodges and Walter 1980), a viral infection (Powel and Lampert 1975), and in hereditary metabolic disorders (Takahashi and Suzuki 1983). The vacuolating oligodendrocyte as seen in our cases has also been described in these conditions. It was suggested that the oligodendrocyte shows transient hypertrophy prior to degeneration (Powel and Lampert 1975). Thus, it is suggested that in our cases, the degeneration of oligodendrocytes was a primary event which resulted in the demyelination of the spinal white matter and thereby presented the spongy state noted on light microscopy. The occurrence of demyelination in the spinal cord may imply that this is a manifestation of a generalized involvement of the oligodendrocyte in the central nervous system. However, in light microscopy studies, sponginess was not a conspicuous feature in the white matter of the cerebrum and cerebellum of our cases. Unfortunately, materials taken from these tissues were inadequately preserved for electron microscopy to further elucidate this point.
ACKNOWLEDGEMENTS We thank Dr. K. Oyanagi for his support during the observations. We also express our appreciation to Mr. T. Ichikawa, Mr. K. Kobayashi, Mr. S. Egawa and Ms. S. Sekimoto for their expert technical assistance and to Ms. K. Murayama for typing the manuscript. This study was supported by a grant from the Ministry of Health and Welfare of Japan.
29 REFERENCES Balentine, J. D. (1978) Pathology of experimental spinal cord trauma. II. Ultrastructure of axons and myelin. Lab. Invest., 39: 254-266. Blakemore, W. F. (1972) Observation on oligodendrocyte degeneration, the resolution of status spongiosus and remyelination in cuprizone intoxication in mice. J. Neurocytol., 1: 413-426. Blakemore, W.F. (1973) Demyelination of the superior cerebellar peduncle in the mouse induced by cuprizone. J. Neurol. Sci., 20: 63-72. Cavanagh, J.B., W.F. Blakemore and Kyu Ma Ha (1971) Fihrillary accumulations in oligodendrogiial processes of ratssubjected to portocaval anastomosis. J. Neurol. ScL, 14: 143-152. Dal Canto, M.C. and H.L. Lipton (1975) Primary demyelination in Theiler's virus infection. An ultrastructural study, Lab. Invest., 33: 626-637. Fukuhara, N., S. Tokiguchi, K. Shirakawa and T. Tsubaki (1980) Myoclonus epilepsy associated with ragged-red fibers (mitochondrial abnormalities): disease entity or a syndrome? Light- and electronmicroscopic studies of two cases and review of literature. J. Neurol. ScL, 47:117-133. Hart, Z.H., C-H. Chang, E.V.D. Perrin, J.S. Neerunjun and R. Ayyar (1977) Familial poliodystrophy, mitochondrial myopathy, and lactate acidemia. Arch. Neurol., 34: 180-185. Hirano, A. (1978) A possible mechanism of demyelination in the Syrian hamster with hindieg paralysis. Lab. Invest., 38: 115-121. Hedges, G.R. and I. Watanahe (1980) Chemical injury of the spinal cord of the rabbit after intracisternal injection of gentamicin. An ultrastructural study. J. Neuropathol. Exp. Neurol., 39: 452-475. Kuriyama, M., H. Umezaki, Y. Fukuda, M. Osame, K. Koike, J. Tateishi and A. Igata (1984) Mitochondrial encephalomyopathy with lactate-pyruvate elevation and brain infarctions. Neurology, 34: 72-77. Lampert, P.W. (1965) Demyelination and remyelination in experimental allergic encephalomyelitis. Further electron microscopic observations. J. Neuropathol. Exp. Neurol., 24: 371-385. Lampert, P.W. (1967) Electron microscopic studies on ordinary and hyperacute experimental allergic encephalomyelitis. Acta Neuropathol. (Bed.), 9: 99-126. Lampert, P.W. (1978) Autoimmune and virus-induced demyelinating diseases. A review. Am. J. Pathol., 91 : 176-198. Ludwin, S. K. and E. S. Johnson (1981) Evidence for a "dying-back" gliopathy in demyelinating disease. Ann. Neurol., 9: 301-305. Mukoyama, M., H. Kazui, N. Sunohara, M. Yoshida, I. Nonaka and E. Satoyoshi (1986) Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke like episodes with acanthocytosis: a clinicopathological study of a unique case. J. Neurol., 233: 228-232. Nishizawa, M., K. Tanaka, K. Shinozawa, T. Kuwabara, T. Atsumi, T. Miyatake and E. Ohama (1987) A mitochondrial encephalomyopathy with cardiomyopathy. A case revealing a defect of complex I in the respiratory chain. J. Neurol. Sci. 78: 189-201. Ohama, E., S. Ohara, F. Ikuta, K. Tanaka and M. Nishizawa (1987) Mitochondrial angiopathy in the cerebral vessels of mitochondrial encephalomyopathy. Acta Neuropathol. (Berl.), 74: 226-233. Ohama, E. and F. Ikuta (1987) Involvement of cboroid plexus in mitochondrial encephalomyopathy (MELAS). Acta Neuropathol. (Berl.), 75: 1-7. Pavlakis, S. G., P. C. Phillips, S. Dimauro, D. C. De Vivo and L. P. Rowland (1984) Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes: a distinctive clinical syndrome. Ann. Neurol., 16: 481-488. Powel, H.C. and P.W. Lampert (1975) Oligodendrocytes and their myelin-plasma membrane connections in JHM mouse hepatitis virus encephalomyelitis. Lab. Invest., 33: 440-445. Shapira, Y., S.D. Cederbaum, P.A. CanciUa, D. Nielsen and B.M. Lippe (1975) Familial poliodystrophy, mitochondrial myopathy, and lactate acidemia. Neurology, 25: 614-621. Tanaka, K., M. Ueno, T. Atsumi, M. Fukagawa and T. Koike (1986) A case ofmitochondrial encephalomyopathy with nephrotic syndrome. Clin. NeuroL (Tokyo), 26:1190-1196. Takahashi, H., H. Igisu, K. Suzuki and K. Suzuki (1983) The twitcher mouse: an ultrastructural study on the oligodendrogiia. Acta Neuropathol. (Bed.), 59: 159-166. Takahashi, H. and K. Suzuki (1984) Demyelination in the spinal cord of routine gioboid cell leukodystrophy (the twitcher mouse). Acta Neuropathol. (Bed.), 62: 298-308. Yajima, K. and K. Suzuki (1979) Demyelination and remyelination in the rat central nervous system following ethidium bromide injection. Lab. Invest., 41: 385-392.
19
Elsevier JNS 03015
Alterations of oligodendrocytes and demyelination in the spinal cord of patients with mitochondrial encephalomyopathy Shinji Ohara ~,*, Eisaku Ohama 1, Hitoshi Takahashi ~, Fusahiro Ikuta 1, Masatoyo Nishizawa 2, Keiko Tanaka 2 and Tadashi Miyatake 2 tDepartment of Pathology and 2Department of Neurology, Brain Research Institute, Niigata University, Niigata 951 (Japan)
(Received23 March, 1987) (Revised,received23 February, 1988) (Accepted 26 February, 1988)
SUMMARY The spinal cords of 2 autopsied patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) were examined. Histologically, the spinal cords showed a spongy state due to the presence of distended myelinated fibers with enlarged periaxonal spaces. Ultrastructurally, the affected fibers showed extensive microvacuolation of the inner myelin sheath with occasional vesicular changes. The presence of macrophages near the degenerated myelin was a frequent finding. The stripping of myelin lameUae by macrophage was observed, with frequent appearance of denuded axons. Furthermore, prominent morphological changes were observed in oligodendrocytes. These f'mdings indicate that demyelination, probably secondary to the degeneration ofoligodendrocytes, occurs in the spinal cord ofMELAS.
Key words: Oligodendrocyte; Degeneration; Demyelination; Spongy state; Mitochondrial encephalomyopathy
* Present address: The Third Department of Internal Medicine, Shinshu University, School of Medicine, Asahi 3-1-1,Matsumoto390, Japan. Correspondence to: Dr.E. Ohama, Department of Pathology, Brain Research Institute, Nligata University, 1 Asahimachi, Niigata 951, Japan.
0022-510X/88/$03.50 © 1988Elsevier SciencePublishers B.V.(BiomedicalDivision)
20 INTRODUCTION Since Shapira et al. (1977) first introduced the concept of mitochondrial encephalomyopathies, special attention has been paid to the involvement of the central nervous system (CNS) because of its predominance in the clinical manifestation. Several authors have proposed that some of these can be classified into distinctive clinical syndromes, such as Keams-Sayre syndrome, myoclonus epilepsy with raggedred fibers (MERRF) (Fukuhara et al. 1980) and mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) (Pavlakis et al. 1984). In this communication, we describe the occurrence of a demyelinating process associated with prominent morphological changes of oligodendrocytes in the spinal cords of 2 autopsied patients with MELAS.
MATERIALS AND METHODS Clinical, neuropathological and biochemical findings of 2 cases of MELAS in this study are described in detail elsewhere. Case 1 is a 16-year-old Japanese girl with a 10-year clinical history (Nishizawa et al. 1987; Ohama et al. 1987; Ohama and Ikuta 1987). Case 2 is a 14-year-old Japanese boy with a 12-year clinical history (Tanaka et al. 1986; Ohama et al. 1987; Ohama and Ikuta 1987). Both patients were diagnosed as having MELAS clinically and pathologically. In Case 1, the biochemical investigations of the postmortem heart muscle demonstrated a defect of complex I in the respiratory chain of the mitochondria (Nishizawa et al. 1987). Several segments of the cervical and upper thoracic spinal cords were removed from both cases within three hours postmortem and fixed in phosphate-buffered 3 ~/o glutaraldehyde/1 ~o paraformaldehyde for electron microscopy. Each segment of the spinal cord was cut into several blocks, each containing an anterior, lateral or posterior column. They were post-osmificated and embedded in Epon 812. Thick sections were stained with toluidine blue for light microscopy. Selected portions were thin-sectioned, stained with uranyl acetate and lead citrate, and examined in a Hitachi-11B electron microscope.
RESULTS
Light microscopicfindings In Case 1, the spinal cord showed a marked loss of myelinated fibers with gliosis in the gracile fasciculi. In addition, minute vacuoles were distributed especially in the lateral and posterior columns, forming a spongy state, without accompanying inflammatory cell infiltrations (Fig. 1). This change was observed throughout the spinal cord and lower brainstem and was most conspicuous at the lower cervical through upper thoracic segments. Neurons of the anterior horn and Clarke's column were well preserved.
21
iiiii!iilii(~ ~ ii!!i!~iiiiii~!iiiiiiiii ~
"!!:
.......
~......i~ i~~~ ...... ~?~
" ?
~ ~I~¸~~ ~
i!:i¸ii
Fig. 1. Lower cervical spinal cord of Case 1 showing a moderate spongy state in the lateral and posterior columns. The gracile fasciculus shows atrophy and myelin pallor. KIQver-Barrera.
The minute vacuoles corresponded to the distended myelin sheaths with abnormally enlarged periaxonal spaces on Epon sections (Fig. 2). Mononuclear cells were occasionally found in the distended fibers, often next to the displaced axon. In mild lesions, the affected fibers were intermingled with apparently healthy nerve fibers and gliosis was mild. In the subpial zones, the lesions were more advanced, showing a massive appearance of distended fibers and loss of myelin. In these areas, gliosis was more evident and demyelinated axons were often observed. In the spinal cord of Case 2, the same change could be found, although to a lesser degree than Case 1.
Ultra.structuralfindings Spinal cords of both cases were suitably preserved for electron microscopy. In both cases, a formation of multiple vacuoles with frequent vesiculation of the inner myelin sheaths was a conspicuous finding (Figs. 3 and 4). Some of the intramyelinic vacuoles contained cell bodies and/or processes which could be identified as those of macrophages by the presence o.fpseudopodia, lysosomal granules, and frequent myelin debris in their cytoplasm. These macrophages were often seen in the immediate vicinity of the intact axons (Figs. 3 and 4). On rare occasions, the stripping of myelin lamellae by an invading tongue of maerophages was also encountered (Fig. 5). Furthermore, many denuded axons were also observed in both eases (Figs. 6 and 7). In addition to the myelin sheaths, prominent morphological changes could be found in the oligodendrocytes. In Case 1, the inner loops of the oligodendrocytes occasionally showed an accumulation of fibrillated materials which were often organized
Fig. 2. Toluidine-blue section, 1 #m thick, showing distended myelinated fibers in the lateral column. Axons (arrows) are occasionally displaced by an enlarged periaxonal empty space. Case 1, x 860.
Fig. 3. Electron micrograph showing multivacuolar cysts and vesicular changes in the inner myelin sheath. A macrophage (M) with pseudopodial processes is seen next to the apparently intact axon. Case 1, × 16350.
Fig. 4. Electron micrograph showing multicystic and vesicular myelin disintegrations (arrows) from Case 2. The cytoplasms (*) within the periaxonal vacuoles are probably those of macrophages. × 14 300.
Fig. 5. Myelin debris within the cytoplasm of macrophages. A process of macrophage is seen to separate myelin lamella. Case 1, x 16500. Inset: a higher magnification of the area indicated by the arrow shows two extemai loops containing microtubules, x 41000.
Fig. 6. Electron micrograph showing a demyelinating axon ensheathed by the innermost myelin lamella and an adjacent macrophage. Case 2, × 14300.
Fig. 7. Electron micrograph showing a completely demyelinated axon surrounded by glial processes. A macrophage is seen in the vicinity. Case 1, x 17900.
25
Fig. 8. (a) Electronmicrographshowingfilamentousinclusionswithinthe innerloopof an oligodendrocyte. Case 1, x 27000. (b)Higher magnificationof the inner loop showinglattice-likearrangement, x 103650. (c) Highermagnificationof the area indicatedby (*).This inclusionconsistsof orderlypackedfilamentscut transversely, x 103650. in crystalloid or lattice-like structures (Fig. 8). An increase of microtubules was also observed in the enlarged internal and external tongues and free cell processes of the oligodendrocytes (Fig. 9). In both cases, oligodendrocytes often showed hypertrophic cytoplasms containing many ribosomes, rough endoplasmic reticulum, dense bodies, microtubules and mitochondria (Fig. 10). An enlarged and vacuolating oligodendrocyte was also observed (Fig. 11). Oligodendrocytes were often found to be thoroughly surrounded by astrocytic processes in the gliotic areas. No structurally abnormal mitochondria could be identified in the oligodendrocytes, astrocytes or endothelial cells of the intraparenchyrnal vessels although the number of mitochondria appeared to be increased in some of them.
26
Fig. 9. Electronmicrograph showing an oligodendrocytewith enlarged internal and external tongues which contain many microtubules. A punctate adhesion (arrow) is noted between the external tongues. An oligodendrocyte with a blunt cellular process is also seen on the left. Case 1, × 26000.
DISCUSSION The present cases appear to be of particular interest in that both spinal cords demonstrated a spongy state with common ultrastructural features of ongoing demyelinating processes. To our knowledge, the occurrence of demyelination in the central nervous system of MELAS has not been documented previously. The ultrastructural features of the demyelination in the present cases seem to be in accordance with those previously described in a variety of demyelinating conditions such as experimental allergic neuritis (Lampert 1965, 1967; Ralne et al. 1974), a viral infection (Dal Canto and Lipton 1975), intoxication (Blakemore 1973; Yajima and Suzuki 1979), trauma (Balentine 1978) and in hereditary metabolic disorders (Takahashi and Suzuki 1984). In these pathological conditions, in which degeneration of oligodendrocytes or focal damage to myelin takes place, the microvacuolar and vesicular changes in the myelin sheath as well as the stripping of myelin lamellae by invading macrophages have also been observed. A random distribution of demyelinating fibers as seen in our
Fig. 10. Enlarged oligodendrocyte showing a marked increase of free ribosomes, rough ER, dense bodies, microtubules and mitoehondria. Case 1, x 26500.
Fig. 1 I. Electron micrograph showing an enlarged oligodendrocyte containing many cytoplasmic vacuoles. Some of these vacuoles are apparently derived from mitochondria (arrows). Case 2, x 26900.
28 cases seemed to occur when degeneration of myelin-forming cells was the primary event (Lampert 1978). Alterations of oligodendrocytes as seen in our cases have also been observed in other demyelinating processes. The occurrence of filamentous accumulations in the inner loops of oligodendrocytes (Fig. 8) has been described in rats subjected to portocaval anastomosis (Cavanaugh et al. 1971) and in mice with cuprizone intoxication (Blakemore 1972, 1973; Ludwin and Johnson 1981). Cuprizone, a chelating agent which inhibits mitochondrial metabolism, has been shown to induce demyelination secondary to the degeneration of oligodendrocytes (Blakemore 1972). In the peripheral nerves of a mutant hamster with hindleg paralysis, Hirano observed identical fibrillary accumulations in the inner loops of Schwann ceils and suggested their possible role in producing demyelination by interfering with the intracellular transport of metabolites and certain organellae (Hirano 1978). The appearance of hypertrophic cytoplasms in oligodendrocytes, containing many ribosomes, dense bodies, rough ERs and mitochondria in addition to abundant microtubules (Fig. 10), may be considered a reaction of the oligodendrocyte to cellular injuries. An abundance of microtubules and cytoplasmic hypertrophy in oligodendrocytes has often been observed in other pathological conditions including intoxication (Cavanaugh 1971; Hodges and Walter 1980), a viral infection (Powel and Lampert 1975), and in hereditary metabolic disorders (Takahashi and Suzuki 1983). The vacuolating oligodendrocyte as seen in our cases has also been described in these conditions. It was suggested that the oligodendrocyte shows transient hypertrophy prior to degeneration (Powel and Lampert 1975). Thus, it is suggested that in our cases, the degeneration of oligodendrocytes was a primary event which resulted in the demyelination of the spinal white matter and thereby presented the spongy state noted on light microscopy. The occurrence of demyelination in the spinal cord may imply that this is a manifestation of a generalized involvement of the oligodendrocyte in the central nervous system. However, in light microscopy studies, sponginess was not a conspicuous feature in the white matter of the cerebrum and cerebellum of our cases. Unfortunately, materials taken from these tissues were inadequately preserved for electron microscopy to further elucidate this point.
ACKNOWLEDGEMENTS We thank Dr. K. Oyanagi for his support during the observations. We also express our appreciation to Mr. T. Ichikawa, Mr. K. Kobayashi, Mr. S. Egawa and Ms. S. Sekimoto for their expert technical assistance and to Ms. K. Murayama for typing the manuscript. This study was supported by a grant from the Ministry of Health and Welfare of Japan.
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