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Neuroglial tissue in human eighth nerve specimens
Neuroglial Tissue in Human Eighth Nerve Specimens IUKKA YLIKOSKI, M.D.,* AND YRI~3 COLLAN, M.D.q
One hundred eighty nerve biopsy specimens from patients with eighth nerve tumors or other vertiginous diseases requiring vestibular neurectomy were studied. In many specimens light microscopy revealed pale areas among the myelinated nerve fibers in the neurolemmal portion of the eighth nerve, Electron microscopy showed that these were ectopic areas of gLial tissue consisting of bundles of numerous cytoplasmic processes of fibrous astrocytes. The abundance of filaments within each process and the occurrence of microtubuli suggest that these processes are reactive astrocytes and accordingly are present as a consequence of the reparative potential of the astroglial tissue after a neuronal lesion. The possible functional significance of the findings is discussed.
The human eighth nerve is approximately 17 mm. long and has a proximal glial segment and a distal nonglial segment, as described by Thomsen' in 1887. The proximal segment is approximately 10 mm. long and extends to the level of the internal acoustic orifice, where there is a transition zone joining the neurolemmal party In 1915 Henschen ~ described penetration of glial fingers into the distal segment of the eighth nerve. Tarlov a in his study of the cerebrospinal nerve roots concentrated on the junction of the central and peripheral nervous system and found not only glial processes but also islands of glial tissue in the distal segment. These islands were p r e s e n t - - o c c a s i o n ally as far as 2 ram. distally from the transition s i t e - in almost all cranial nerves. This phenomenon was particularly striking in the eighth nerve; in one instance, nests of glial cells were found as far distally as the lower pole of Scarpa's ganglion. The occurrence of glial tissue beyond the border zone of the eighth nerve has not been reported in monkeys, cats, or guinea pigs. 4' '~
This report describes the light and electron microscopic appearance of the glial tissue in surgical biopsy specimens of the human eighth nerve. The possible role of the glial tissue is discussed.
MATERIALS AND METHODS Eighth nerve specimens from 180 patients, half at the Department of Otolaryngology of the University of Helsinki and half at the Ear Research Institute in Los Angeles, were studied. Specimens were obtained during eighth nerve or vestibular nerve neurectomy from approximately 300 patients with Meniere's disease, approximately 20 patients with other vertiginous disease, and during surgical removal of acoustic tumors in 60 patients. After removal, the specimens were immediately fixed in cold buffered 3 per cent glutaraldehyde or Karnewsky solution for at least three hours and stored in buffer or sucrose until postfixation with I per cent osmium tetroxide,
Accepted for publication August 3, 1979. *Associate Professor, Department of Otolaryngology, University of Helsinki, Helsinkl, Finland. Visiting Professor, Ear Research Institute, Los Angeles, California. tAssociate Professor, Department of Pathology, University of Helsinki, Helsinki, Finland. American Journal of Otolaryngalogy--Velume 1, Number 2, February 1980
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d e h y d r a t i o n in a l c o h o l , a n d e m b e d d i n g in e p o x y resin. For l i g h t m i c r o s c o p y , 1 m i c r o n thick sections were stained with toluidine blue O or m e t h y l e n e b l u e . T h i n s e c t i o n s for electron m i c r o s c o p y w e r e s t a i n e d w i t h u r a n y l acetate a n d l e a d citrate. T h e v e s t i b u l a r n e r v e s f r o m 60 cases of M e n i e r e ' s d i s e a s e w e r e e x a m i n e d i n detail in serial s e c t i o n s of the e n t i r e b i o p s y s p e c i m e n a n d e l e c t r o n m i c r o s c o p i c s e c t i o n s at v a r i o u s levels.
R E S U LTS
Light Microscopy
142
Figure 2. Higher magnification of the same specimen as in Figure 1. Each pale area is composed of numerous rounded substructures. (Epon embedded sectiml. Toluidine blue 0 stain, x 830.}
T h e glial b u n d l e s w e r e e a s i l y d i s t i n g u i s h e d u n d e r light m i c r o s c o p y b e c a u s e t h e y a p p e a r e d clearly paler than the surrounding myelinated n e r v e fibers a n d g a n g l i o n cells (Fig. 1). T h e glial b u n d l e s w e r e c o m p o s e d of s m a l l e r r o u n d s u b u n i t s w i t h no or o n l y v e r y f e w m y e l i n a t e d n e r v e fibers a n d s o m e e n t r a p p e d r o u n d nuclei (Fig. 2). In m a n y b u n d l e s a d a r k e r , s l i g h t l y red s t a i n e d , s p h e r i c a l b o d y was o b s e r v e d (Fig. 3). It h a d an a v e r a g e d i a m e t e r of 10 /z a n d often s h o w e d a d e n s e r c e n t r a l a r e a . Glial b u n d l e s w e r e f o u n d in s o m e c o m p a r t m e n t s of the vest i b u l a r n e r v e in n i n e of 60 cases of M e n i e r e ' s disease. Pale bundles were sometimes present in t h e s a m e s e c t i o n as S c a r p a ' s g a n g l i o n cells (Fig. 4). T h e d i a m e t e r of p a l e areas, r a n g i n g f r o m 10 to 90 m i c r o n s in cross section, was g e n e r a l l y g r e a t e r t h a n t h a t of v e s t i b u l a r ganglion cells. I d e n t i c a l a r e a s w e r e also f o u n d in
nerve s p e c i m e n s f r o m p a t i e n t s w i t h diseases u n l i k e M e n i e r e ' s disease, i.e., in s o m e patients w i t h v e r t i g i n o u s s y m p t o m s r e s e m b l i n g vestibular n e u r o n i t i s and i n one p a t i e n t w i t h disabling t i n n i t u s after s u d d e n deafness. T h e pale n e r v e fiber free areas w e r e also o b s e r v e d in s p e c i m e n s of eighth n e r v e t u m o r s in w h i c h the gliotic c o n t e n t was c o n s i d e r a b l y greater than in other cases. The glial areas in t u m o r patients differed f r o m t h o s e in p a t i e n t s with M e n i e r e ' s disease b e c a u s e of their irregular shape. O c c a s i o n a l l y the section i n c l u d e d the g l i a l - S c h w a n n cell t r a n s i t i o n zone, a n d fingerlike p r o c e s s e s of glial tissue w e r e n o t e d (Fig. 5). In s o m e t u m o r cases glial islands were seen in the distal b u t not i n the central s t u m p .
Figure 1. Inferior vestibular nerve from a patient with Meniere's disease..Large areas of the nerve are occupied by light staining, rounded structures with a few entrapped myelinated nerw fibers end nuclei. (Epon embedded section. Toluidine blue 0 stain. • 330.)
Figure 3. Longitudinal section of tile inferior vestibular nerve from a patient with Meniere's disease. The cylindrical processes in the pale area appear to run parallel with myelinated nerve fibers. Note two spherical bodies withinthe pale area. (Epon embedded section. Toluidine blue 0 stain, x 580.]
NEUROGLIAL TISSUE IN HUMAN EIGHTH NERVE SPECIMENS
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Figure 4. Superior vestibular nerve at the level of Scarpa's ganglion from a patient with tinnitus after sudden deafness. Nots two round pale areas {C) in the immediate vicinity of the ganglion cells. (Epon embedded section. Toluidine blue 0 stain, x 400.]
Electron Microscopy The round pale bundles seen under the light microscope were confirmed by electron microscopy as being numerous, roughly parallel, generally cylindrical projections with diameters
Figure 5. Superior vestibular n e r v e from a patient with vertigo of unknown origin. The section is probably at the level of the transition area from glia to ueurolemma. From the glial " d o m e " finger-like projections of glial tissue (G} are seen to penetrate between myelinated nerve fibers. (Epon embedded section. Toluidine blue 0 stain. • 220.)
ranging from 0,6 to 3 microns {usually 1,3 to 1.6 microns). Each process contained tightly packed, parallel arrays of fibrils or filaments arranged in a longitudinal fashion (Fig. "6). The diameters of the filaments varied from 7 to 9 nm. In cross sections the filaments appeared to
_
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Figure 6. Electron micrograph of two pale art;as. These are seen to be composed of cylindrical processes o{ fibrous astrocytes. Each process contains numerous microfibrils, and in some there are numerous glycogen-like particles (arrow). Note ensheathing basal lamina and one nonmyelinated nerve fiber within each bundle (small arrows), i x 11,750.)
JUKKA YL1KOSKI AND YR}0 COLLAN
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Otolaryngology
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Figure 7, Higher magnification of the bundle shown in Figure 6, Microfibrils are seen to be hollow a n d have a diameter of approximately 7 to 9 nm. Some processes contain microtubules 20 to 30 nm. in diameter. Each process is separated from the other by a double membrene, (x 125,000.)
have a hollow light center surrounded by a dense wall composed of several subunits with diameters of about 2 nm. (Fig. 7). Many processes contained collections of small tubuli with diameters from 20 to 30 nm. in the periphery (Fig. 7). Some processes displayed a large number of small single or clustered dark particles with diameters ranging from 20 to 30 nm. (Fig. 6). Occasionally mitochondria and endoplasmic reticulum cisternae were also present. Each process was separated from others by a double membrane with a gap of about 40 nm. between the membranes. A few junctional complexes were seen between the processes. The ultrastructure of some nuclei seen by light microscopy resembled that of astrocytic nuclei. The whole bundle was ensheathed by a basal lamina. Some unmyelinated or mye]inated nerve fibers were closed in the bundle and surrounded by Schwann cell cytoplasm (Fig. 6). The basal lamina of the Schwann cell was directly connected with the basal lamina of the whole bundle. Collagen fibers were seen surrounding the bundle but not in it. The spherical bodies noted within many glial bundles were seen to be composed of randomly oriented fibers and small dark deposits that showed no substructures (Fig. 8). 144
DISCUSSION
Astrocytes, oligodendroglial cells, and microglial cells are classified as neuroglial cells of the central nervous system. Astrocytes are star shaped cells with many cytoplasmic processes. According to the amount of filamentous elements in their cytoplasm, they are divided into fibrous and protoplasmic astrocytes. From the perikaryon of fibrous astrocytes several broad processes arise, branching to form numerous smaller elongated cylindrical processes. These combine with similar processes of the other cells to form layers that arrange nerve fibers into bundles. Some of the processes have expansions applied to the surfaces of the blood vessels ("end-feet"), and others extend to the surfaces of the central nervous system to form the glial limiting membrane." Astrocytes characteristically appear by light microscopy to be the palest part of the region, and their processes appear paler than the tissue in which they occur. 7 However, the astrocytes and in particular their processes can be recognized only by electron microscopy, ~ The characteristic ultrastructural features of fibrous astroglial cells are the presence of bundles of fine fibrils or filaments 8 to 9 nm. in diameter,
NEUROGLIAL TISSUg IN HUMAN EIGHTH NERVE
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bundles of astrocytic processes studied may represent reactive proliferated fibrous astrocytes. The processes of reactive fibrous astrocytes have numerous densely packed filaments that may proliferate in pathological conditions. ~~ ~ The frequent occurrence of microtubules in the peripheral parts of particularly thin projections suggests a developing stage, for microtubules have been reported to occur mainly in immature astrocytes. ~" What induces astroglial proliferation in the eighth nerve? It has been shown in the central nervous system of the adult rat that the rate of astrocyte division increases with physiological stress, e.g., dehydration. ~4 In accordance with this theory and the hypothesis that glial cells divide throughout life, ~'~ astroglial islands in the eighth nerve could be a physiological man-
single or clustered dense particles with the dimensions and staining properties of glycogen, and collections of microtubules. ~' ~ These features were present in the bundles examined. The astrocytes are considered to have various functions: to support the neurons in the central nervous system, to act as components of the blood-brain barrier, and to isolate receptive surfaces. In addition they may be phagocytic and may possess tissue repairing capacity, s The reparative potential of the astrocytes is of particular interest. It has been known that neuroglial cells, especially astrocytes, can repair a defect in the central nervous system by proliferating to fill the spaces previously occupied by neurons and their processes."' ~0 The glial scar arises by an increase not in the number of astrocytes but in their size, ~ The
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Figure 8. Electron micrograph of a spherical body occasionally observed inside astroglial areas. The body appears to be composed of randomly oriented fibers and small dark deposits. A, astrocytic processes. (x 18,000.)
JUKKA YLIKOSKIAND YRJO COLLAN
14 5
American Journal
of Otolaryngology
ffestation of the aging process. It must be emphasized that Tarlov 3 found astroglial islands in the peripheral segment of the eighth nerve in only four of 18 individuals studied. On the other hand, islands of glial tissue in the eighth nerve m i g h t be reparative formations following various disorders, such as neuronal degeneration. An equivalent mechanism probably exists in the spinal roots of patients with Werdnig-Hoffmann disease (infantile spinal muscular atrophy), in w h i c h similar astrocytic proliferations were described by Chou and Fakadej 16 in 1971 and later b y other authors. Astroglial proliferation could even be a primary event and p a t h o g n o m o n i c for this disease. IT The general consensus, however, is that the degeneration of motor nerve fibers is the primary event and that astroglial invasion occurs secondarily, perhaps along the uni~yelinated nerve fibers.IS Since astrogliosis has been observed in different diseases, particularly in nerve specimens from patients with acoustic neuroma, it may be a nonspecific change. Its occurrence, however, has encouraged the formulation of hypotheses about causes of various diseases. Thus, the occurrence of astroglial scar tissue in the eighth nerve - - occasionally even in the immediate vicinity of the vestibular ganglion c e l l s - - i n Meniere's disease and the classic histopathological explanation that astroglial scars cause increased excitability of the central nervous system leading to focal epileptic seizures can be linked to explain the seizures in Meniere's disease as resulting from an increased neuronal excitability provoked b y astrogliotic loci. Similarly, it is k n o w n that S c h w a n n cell tumors almost always originate i n the non-neuronal elements of the distal or neurolemmal portion of the eighth nerve. It has been confirmed that there are often loci of astroglial tissue in the neurolemmal segment of the eighth nerve, and particularly in nerves r e m o v e d with Schwarm cell tumors. This suggests that astrocytic loci may also be the site of neoplasm of the eighth nerve.
Acknowledgments W e t h a n k W i l l i a m F. H o u s e , M.D., of the Ear Res e a r c h I n s t i t u t e i n Los A n g e l e s a n d T a u n o Palva, M.D., of the D e p a r t m e n t of O t o l a r y n g o l o g y , U n i v e r sity of H e l s i n k i , F i n l a n d , for p r o v i d i n g the m a t e r i a l for t h i s s t u d y . We a l s o t h a n k P r o f e s s o r I m r i c h F r i e d -
]46
m a n n , M.D., of N o r t h w i c k Park H o s p i t a l , London, E n g l a n d , for his v a l u a b l e r e v i e w of the m a n u s c r i p t .
References 1. Thomsen, R.: Uebar eigentiimliche aus ver~inderten Ganglienzellen hervorgegangene Gebilde in den St~-amen der Hirnnerven des Menschen. Virchows Arch. [Psthol. Anat.], 109:459-465, 1887, 2. Henschen, F.: Zur Histologie und Pathogenese der Kleinhimbrtickeuwinkeltumoren. Arch. Psychiat., 56:20--122, 1915. 3. Ta.rlov, M.: Structure of the nerve root, Arch. Neurol. Psychiat., 31:555-583, 1937. 4. Gacek, R. R., and Rasmussen, G. L: Fiber analysis of the statoacoustic nerve of guinea pig, cat and monkey. Anat. Rec., 139:455-463, 1961. 5. Sando, I., Black, F. O., and Hemenway, W. C.: Spatial distribution of vestibular nerve in the internal auditory canal. Ann. Otol. Rhinol. Laryngol., 81:305-314, 1972. 6. Peters, A., Palay, S. L., and Webster, H. F.: The Pine Structure of the Nervous System. Ed. 2. Philadelphia, W. B. Saunders Company, 1976. 7. Blackstad, T. W,: Cortical grey m a t t e r - - a correlation of light and electron microscopic data. In Hyden, H. (Editor]: The Neuron. Amsterdam, Elsevier, 1967, pp. 49-115. 8, Mugnaini, E., and Walberg, F.: Ultrastructure of neuroglia. Ergeb. Anat. Entwick. Gesch., 31:193-236, 1984. 9. Schultz, R. L., and Pease, D. C.: Cicatrix formation in rat cerebral cortex as revealed by electron microscopy. Am. J. Pathol., 35:1017-1041, 1959. 10. Maxwell, D. S., and Kruger, L.: The fine structure of astrocytes in the cerebral cortex and their response to focal injury produced by heavy ionizing particles. J. Cell Biol., 25:141-157, 1965. 11. Vaughn, J. E., Hinds, P. L., and Skoff, R. P.: Electron microscopic studies of Wallerian degeneration in rat optic nerves. I. The multipotential glia. J. Comp. Neurol., 140:175-206, 1970. 12. Luse, S. A.: Electron microscopic observations of the central nervous system. J. Biophys. Biechem. Cytol., 2:531-541, 1956. 13. Vaughan, D. W., and Peters, A.: Neuroglial cells in the cerebral cortex of rats from young adulthood to old age: an electron microscopic study. J. Neurocytol., 3:405--429, 1974. 14. Mm'ray, M.: Effects of dehydration on the rate of proliferation of hypothalamic glial cells. Exp. Neurol., 20:46{N-468, 1968. 15. Dalton, M. M., Heroines, O. R., and Leblond, C. P.: Correlation of glial proliferation with age in the mouse brain. J. Comp. Neurol., 134:397-400, 1968. 16. Chou, S. M., and Pakadej, A. V.: Ultrastructure of chromatolytic motor neurons and anterior spinal roots in a case of Werdnig-Hoffmann disease. J. Neuropathol. Exp. Neurol., 30:368-379, 1971. 17. Chou, S. M., and Nonaka, I.: Werdnig-Hoffmarm disease: proposal of a pathogenetic mechanism. Acta Neuropathol. (Berl.), 41:45--54, 1978. 18. Ghatak, N. R.: Spinal roots in Werdnig-Hoffmann disease. Acts Neuropathol. (Berl.), 41:1-7, 1978. The Ear Research Institute 256 South Lake Street Los Angeles, California 90057 (Dr. Ylikoskt)
NEUROGLIAL TISSUE IN H U M A N
EIGHTH NERVE SPECIMENS
One hundred eighty nerve biopsy specimens from patients with eighth nerve tumors or other vertiginous diseases requiring vestibular neurectomy were studied. In many specimens light microscopy revealed pale areas among the myelinated nerve fibers in the neurolemmal portion of the eighth nerve, Electron microscopy showed that these were ectopic areas of gLial tissue consisting of bundles of numerous cytoplasmic processes of fibrous astrocytes. The abundance of filaments within each process and the occurrence of microtubuli suggest that these processes are reactive astrocytes and accordingly are present as a consequence of the reparative potential of the astroglial tissue after a neuronal lesion. The possible functional significance of the findings is discussed.
The human eighth nerve is approximately 17 mm. long and has a proximal glial segment and a distal nonglial segment, as described by Thomsen' in 1887. The proximal segment is approximately 10 mm. long and extends to the level of the internal acoustic orifice, where there is a transition zone joining the neurolemmal party In 1915 Henschen ~ described penetration of glial fingers into the distal segment of the eighth nerve. Tarlov a in his study of the cerebrospinal nerve roots concentrated on the junction of the central and peripheral nervous system and found not only glial processes but also islands of glial tissue in the distal segment. These islands were p r e s e n t - - o c c a s i o n ally as far as 2 ram. distally from the transition s i t e - in almost all cranial nerves. This phenomenon was particularly striking in the eighth nerve; in one instance, nests of glial cells were found as far distally as the lower pole of Scarpa's ganglion. The occurrence of glial tissue beyond the border zone of the eighth nerve has not been reported in monkeys, cats, or guinea pigs. 4' '~
This report describes the light and electron microscopic appearance of the glial tissue in surgical biopsy specimens of the human eighth nerve. The possible role of the glial tissue is discussed.
MATERIALS AND METHODS Eighth nerve specimens from 180 patients, half at the Department of Otolaryngology of the University of Helsinki and half at the Ear Research Institute in Los Angeles, were studied. Specimens were obtained during eighth nerve or vestibular nerve neurectomy from approximately 300 patients with Meniere's disease, approximately 20 patients with other vertiginous disease, and during surgical removal of acoustic tumors in 60 patients. After removal, the specimens were immediately fixed in cold buffered 3 per cent glutaraldehyde or Karnewsky solution for at least three hours and stored in buffer or sucrose until postfixation with I per cent osmium tetroxide,
Accepted for publication August 3, 1979. *Associate Professor, Department of Otolaryngology, University of Helsinki, Helsinkl, Finland. Visiting Professor, Ear Research Institute, Los Angeles, California. tAssociate Professor, Department of Pathology, University of Helsinki, Helsinki, Finland. American Journal of Otolaryngalogy--Velume 1, Number 2, February 1980
141
American Journal of Otolaryngology
d e h y d r a t i o n in a l c o h o l , a n d e m b e d d i n g in e p o x y resin. For l i g h t m i c r o s c o p y , 1 m i c r o n thick sections were stained with toluidine blue O or m e t h y l e n e b l u e . T h i n s e c t i o n s for electron m i c r o s c o p y w e r e s t a i n e d w i t h u r a n y l acetate a n d l e a d citrate. T h e v e s t i b u l a r n e r v e s f r o m 60 cases of M e n i e r e ' s d i s e a s e w e r e e x a m i n e d i n detail in serial s e c t i o n s of the e n t i r e b i o p s y s p e c i m e n a n d e l e c t r o n m i c r o s c o p i c s e c t i o n s at v a r i o u s levels.
R E S U LTS
Light Microscopy
142
Figure 2. Higher magnification of the same specimen as in Figure 1. Each pale area is composed of numerous rounded substructures. (Epon embedded sectiml. Toluidine blue 0 stain, x 830.}
T h e glial b u n d l e s w e r e e a s i l y d i s t i n g u i s h e d u n d e r light m i c r o s c o p y b e c a u s e t h e y a p p e a r e d clearly paler than the surrounding myelinated n e r v e fibers a n d g a n g l i o n cells (Fig. 1). T h e glial b u n d l e s w e r e c o m p o s e d of s m a l l e r r o u n d s u b u n i t s w i t h no or o n l y v e r y f e w m y e l i n a t e d n e r v e fibers a n d s o m e e n t r a p p e d r o u n d nuclei (Fig. 2). In m a n y b u n d l e s a d a r k e r , s l i g h t l y red s t a i n e d , s p h e r i c a l b o d y was o b s e r v e d (Fig. 3). It h a d an a v e r a g e d i a m e t e r of 10 /z a n d often s h o w e d a d e n s e r c e n t r a l a r e a . Glial b u n d l e s w e r e f o u n d in s o m e c o m p a r t m e n t s of the vest i b u l a r n e r v e in n i n e of 60 cases of M e n i e r e ' s disease. Pale bundles were sometimes present in t h e s a m e s e c t i o n as S c a r p a ' s g a n g l i o n cells (Fig. 4). T h e d i a m e t e r of p a l e areas, r a n g i n g f r o m 10 to 90 m i c r o n s in cross section, was g e n e r a l l y g r e a t e r t h a n t h a t of v e s t i b u l a r ganglion cells. I d e n t i c a l a r e a s w e r e also f o u n d in
nerve s p e c i m e n s f r o m p a t i e n t s w i t h diseases u n l i k e M e n i e r e ' s disease, i.e., in s o m e patients w i t h v e r t i g i n o u s s y m p t o m s r e s e m b l i n g vestibular n e u r o n i t i s and i n one p a t i e n t w i t h disabling t i n n i t u s after s u d d e n deafness. T h e pale n e r v e fiber free areas w e r e also o b s e r v e d in s p e c i m e n s of eighth n e r v e t u m o r s in w h i c h the gliotic c o n t e n t was c o n s i d e r a b l y greater than in other cases. The glial areas in t u m o r patients differed f r o m t h o s e in p a t i e n t s with M e n i e r e ' s disease b e c a u s e of their irregular shape. O c c a s i o n a l l y the section i n c l u d e d the g l i a l - S c h w a n n cell t r a n s i t i o n zone, a n d fingerlike p r o c e s s e s of glial tissue w e r e n o t e d (Fig. 5). In s o m e t u m o r cases glial islands were seen in the distal b u t not i n the central s t u m p .
Figure 1. Inferior vestibular nerve from a patient with Meniere's disease..Large areas of the nerve are occupied by light staining, rounded structures with a few entrapped myelinated nerw fibers end nuclei. (Epon embedded section. Toluidine blue 0 stain. • 330.)
Figure 3. Longitudinal section of tile inferior vestibular nerve from a patient with Meniere's disease. The cylindrical processes in the pale area appear to run parallel with myelinated nerve fibers. Note two spherical bodies withinthe pale area. (Epon embedded section. Toluidine blue 0 stain, x 580.]
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Figure 4. Superior vestibular nerve at the level of Scarpa's ganglion from a patient with tinnitus after sudden deafness. Nots two round pale areas {C) in the immediate vicinity of the ganglion cells. (Epon embedded section. Toluidine blue 0 stain, x 400.]
Electron Microscopy The round pale bundles seen under the light microscope were confirmed by electron microscopy as being numerous, roughly parallel, generally cylindrical projections with diameters
Figure 5. Superior vestibular n e r v e from a patient with vertigo of unknown origin. The section is probably at the level of the transition area from glia to ueurolemma. From the glial " d o m e " finger-like projections of glial tissue (G} are seen to penetrate between myelinated nerve fibers. (Epon embedded section. Toluidine blue 0 stain. • 220.)
ranging from 0,6 to 3 microns {usually 1,3 to 1.6 microns). Each process contained tightly packed, parallel arrays of fibrils or filaments arranged in a longitudinal fashion (Fig. "6). The diameters of the filaments varied from 7 to 9 nm. In cross sections the filaments appeared to
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Figure 6. Electron micrograph of two pale art;as. These are seen to be composed of cylindrical processes o{ fibrous astrocytes. Each process contains numerous microfibrils, and in some there are numerous glycogen-like particles (arrow). Note ensheathing basal lamina and one nonmyelinated nerve fiber within each bundle (small arrows), i x 11,750.)
JUKKA YL1KOSKI AND YR}0 COLLAN
14
American Journal oF
Otolaryngology
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have a hollow light center surrounded by a dense wall composed of several subunits with diameters of about 2 nm. (Fig. 7). Many processes contained collections of small tubuli with diameters from 20 to 30 nm. in the periphery (Fig. 7). Some processes displayed a large number of small single or clustered dark particles with diameters ranging from 20 to 30 nm. (Fig. 6). Occasionally mitochondria and endoplasmic reticulum cisternae were also present. Each process was separated from others by a double membrane with a gap of about 40 nm. between the membranes. A few junctional complexes were seen between the processes. The ultrastructure of some nuclei seen by light microscopy resembled that of astrocytic nuclei. The whole bundle was ensheathed by a basal lamina. Some unmyelinated or mye]inated nerve fibers were closed in the bundle and surrounded by Schwann cell cytoplasm (Fig. 6). The basal lamina of the Schwann cell was directly connected with the basal lamina of the whole bundle. Collagen fibers were seen surrounding the bundle but not in it. The spherical bodies noted within many glial bundles were seen to be composed of randomly oriented fibers and small dark deposits that showed no substructures (Fig. 8). 144
DISCUSSION
Astrocytes, oligodendroglial cells, and microglial cells are classified as neuroglial cells of the central nervous system. Astrocytes are star shaped cells with many cytoplasmic processes. According to the amount of filamentous elements in their cytoplasm, they are divided into fibrous and protoplasmic astrocytes. From the perikaryon of fibrous astrocytes several broad processes arise, branching to form numerous smaller elongated cylindrical processes. These combine with similar processes of the other cells to form layers that arrange nerve fibers into bundles. Some of the processes have expansions applied to the surfaces of the blood vessels ("end-feet"), and others extend to the surfaces of the central nervous system to form the glial limiting membrane." Astrocytes characteristically appear by light microscopy to be the palest part of the region, and their processes appear paler than the tissue in which they occur. 7 However, the astrocytes and in particular their processes can be recognized only by electron microscopy, ~ The characteristic ultrastructural features of fibrous astroglial cells are the presence of bundles of fine fibrils or filaments 8 to 9 nm. in diameter,
NEUROGLIAL TISSUg IN HUMAN EIGHTH NERVE
SPECIMENS
bundles of astrocytic processes studied may represent reactive proliferated fibrous astrocytes. The processes of reactive fibrous astrocytes have numerous densely packed filaments that may proliferate in pathological conditions. ~~ ~ The frequent occurrence of microtubules in the peripheral parts of particularly thin projections suggests a developing stage, for microtubules have been reported to occur mainly in immature astrocytes. ~" What induces astroglial proliferation in the eighth nerve? It has been shown in the central nervous system of the adult rat that the rate of astrocyte division increases with physiological stress, e.g., dehydration. ~4 In accordance with this theory and the hypothesis that glial cells divide throughout life, ~'~ astroglial islands in the eighth nerve could be a physiological man-
single or clustered dense particles with the dimensions and staining properties of glycogen, and collections of microtubules. ~' ~ These features were present in the bundles examined. The astrocytes are considered to have various functions: to support the neurons in the central nervous system, to act as components of the blood-brain barrier, and to isolate receptive surfaces. In addition they may be phagocytic and may possess tissue repairing capacity, s The reparative potential of the astrocytes is of particular interest. It has been known that neuroglial cells, especially astrocytes, can repair a defect in the central nervous system by proliferating to fill the spaces previously occupied by neurons and their processes."' ~0 The glial scar arises by an increase not in the number of astrocytes but in their size, ~ The
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Figure 8. Electron micrograph of a spherical body occasionally observed inside astroglial areas. The body appears to be composed of randomly oriented fibers and small dark deposits. A, astrocytic processes. (x 18,000.)
JUKKA YLIKOSKIAND YRJO COLLAN
14 5
American Journal
of Otolaryngology
ffestation of the aging process. It must be emphasized that Tarlov 3 found astroglial islands in the peripheral segment of the eighth nerve in only four of 18 individuals studied. On the other hand, islands of glial tissue in the eighth nerve m i g h t be reparative formations following various disorders, such as neuronal degeneration. An equivalent mechanism probably exists in the spinal roots of patients with Werdnig-Hoffmann disease (infantile spinal muscular atrophy), in w h i c h similar astrocytic proliferations were described by Chou and Fakadej 16 in 1971 and later b y other authors. Astroglial proliferation could even be a primary event and p a t h o g n o m o n i c for this disease. IT The general consensus, however, is that the degeneration of motor nerve fibers is the primary event and that astroglial invasion occurs secondarily, perhaps along the uni~yelinated nerve fibers.IS Since astrogliosis has been observed in different diseases, particularly in nerve specimens from patients with acoustic neuroma, it may be a nonspecific change. Its occurrence, however, has encouraged the formulation of hypotheses about causes of various diseases. Thus, the occurrence of astroglial scar tissue in the eighth nerve - - occasionally even in the immediate vicinity of the vestibular ganglion c e l l s - - i n Meniere's disease and the classic histopathological explanation that astroglial scars cause increased excitability of the central nervous system leading to focal epileptic seizures can be linked to explain the seizures in Meniere's disease as resulting from an increased neuronal excitability provoked b y astrogliotic loci. Similarly, it is k n o w n that S c h w a n n cell tumors almost always originate i n the non-neuronal elements of the distal or neurolemmal portion of the eighth nerve. It has been confirmed that there are often loci of astroglial tissue in the neurolemmal segment of the eighth nerve, and particularly in nerves r e m o v e d with Schwarm cell tumors. This suggests that astrocytic loci may also be the site of neoplasm of the eighth nerve.
Acknowledgments W e t h a n k W i l l i a m F. H o u s e , M.D., of the Ear Res e a r c h I n s t i t u t e i n Los A n g e l e s a n d T a u n o Palva, M.D., of the D e p a r t m e n t of O t o l a r y n g o l o g y , U n i v e r sity of H e l s i n k i , F i n l a n d , for p r o v i d i n g the m a t e r i a l for t h i s s t u d y . We a l s o t h a n k P r o f e s s o r I m r i c h F r i e d -
]46
m a n n , M.D., of N o r t h w i c k Park H o s p i t a l , London, E n g l a n d , for his v a l u a b l e r e v i e w of the m a n u s c r i p t .
References 1. Thomsen, R.: Uebar eigentiimliche aus ver~inderten Ganglienzellen hervorgegangene Gebilde in den St~-amen der Hirnnerven des Menschen. Virchows Arch. [Psthol. Anat.], 109:459-465, 1887, 2. Henschen, F.: Zur Histologie und Pathogenese der Kleinhimbrtickeuwinkeltumoren. Arch. Psychiat., 56:20--122, 1915. 3. Ta.rlov, M.: Structure of the nerve root, Arch. Neurol. Psychiat., 31:555-583, 1937. 4. Gacek, R. R., and Rasmussen, G. L: Fiber analysis of the statoacoustic nerve of guinea pig, cat and monkey. Anat. Rec., 139:455-463, 1961. 5. Sando, I., Black, F. O., and Hemenway, W. C.: Spatial distribution of vestibular nerve in the internal auditory canal. Ann. Otol. Rhinol. Laryngol., 81:305-314, 1972. 6. Peters, A., Palay, S. L., and Webster, H. F.: The Pine Structure of the Nervous System. Ed. 2. Philadelphia, W. B. Saunders Company, 1976. 7. Blackstad, T. W,: Cortical grey m a t t e r - - a correlation of light and electron microscopic data. In Hyden, H. (Editor]: The Neuron. Amsterdam, Elsevier, 1967, pp. 49-115. 8, Mugnaini, E., and Walberg, F.: Ultrastructure of neuroglia. Ergeb. Anat. Entwick. Gesch., 31:193-236, 1984. 9. Schultz, R. L., and Pease, D. C.: Cicatrix formation in rat cerebral cortex as revealed by electron microscopy. Am. J. Pathol., 35:1017-1041, 1959. 10. Maxwell, D. S., and Kruger, L.: The fine structure of astrocytes in the cerebral cortex and their response to focal injury produced by heavy ionizing particles. J. Cell Biol., 25:141-157, 1965. 11. Vaughn, J. E., Hinds, P. L., and Skoff, R. P.: Electron microscopic studies of Wallerian degeneration in rat optic nerves. I. The multipotential glia. J. Comp. Neurol., 140:175-206, 1970. 12. Luse, S. A.: Electron microscopic observations of the central nervous system. J. Biophys. Biechem. Cytol., 2:531-541, 1956. 13. Vaughan, D. W., and Peters, A.: Neuroglial cells in the cerebral cortex of rats from young adulthood to old age: an electron microscopic study. J. Neurocytol., 3:405--429, 1974. 14. Mm'ray, M.: Effects of dehydration on the rate of proliferation of hypothalamic glial cells. Exp. Neurol., 20:46{N-468, 1968. 15. Dalton, M. M., Heroines, O. R., and Leblond, C. P.: Correlation of glial proliferation with age in the mouse brain. J. Comp. Neurol., 134:397-400, 1968. 16. Chou, S. M., and Pakadej, A. V.: Ultrastructure of chromatolytic motor neurons and anterior spinal roots in a case of Werdnig-Hoffmann disease. J. Neuropathol. Exp. Neurol., 30:368-379, 1971. 17. Chou, S. M., and Nonaka, I.: Werdnig-Hoffmarm disease: proposal of a pathogenetic mechanism. Acta Neuropathol. (Berl.), 41:45--54, 1978. 18. Ghatak, N. R.: Spinal roots in Werdnig-Hoffmann disease. Acts Neuropathol. (Berl.), 41:1-7, 1978. The Ear Research Institute 256 South Lake Street Los Angeles, California 90057 (Dr. Ylikoskt)
NEUROGLIAL TISSUE IN H U M A N
EIGHTH NERVE SPECIMENS