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Ensheathment and myelination of regenerating PNS fibres by transplanted optic nerve glia
Neuroscience Le~ters, 9 (1978) 97--104 © Elsevier/North-Holland Scient|fic Publishers Ltd.
97
E N S H E A ~ I M E ~ AND M ~ L I N A T I O N OF REGENERATING PNS FIBRES BY T R A N S P L ~ D OPTIC NERVE GLIA ~ B E R T J. AGUAYO, R I C ~ R D DICKSON, JANE TRECARTEN, MARGARET ATTIWELL, GARTH M. BRAY and PETER RICHARDSON Department of Neurology & Neurosurgery, McGiU University and The Montreal General Hospital {Received June 1st, 1978) (Revised version received June 19th, 1978) (Avvepted June 20th, 1978)
SUMMARY
Interactions between PNS axons and CNS glia were studied morphologically by transplanting optic nerves into peripheral nerves in groups of rats and mice. Four to 11 months after grafting, small numbers of axons from the peripheral nerves had penetrated the CNS grafts w~er~ they became ensheathed mid myelinated by CNS gila. Glial protuberances observed at the C]~4S-PNS interfaces suggested that there had been an active glial response to hmervation by PNS axons. These findings provide experimental evidence that; denervated CNS gila can be reinnervated and form myelin.
,_.perimental nerve grafts have proven useful for the study o:~ axon-Schwann cell interdependencies during normal myelination [ 1,2] and in certain inherited human [3,4] and animal neuropathies [5,6]. The present phase and electron microscope investigation demonstra~s that by using optic nerve transplants in rodents, it is also possible to study interactions between peripheral nerve axons and CNS glia. The intra-orbitai portion of one optic nerve and the attached eye were removed f~om each anesthetized animal. The optic nerve was then cut at its plane of entry into the eve and immediately grafted in the same animal between the stumps of a transected peripheral nerve using techniques previously described [5]. Optic nerve transplants (ON) were studied in three different groups of e x p e ~ e n t a l animals. Group I: In each of 34 adult Sprague--Dawley rots, a segment of one ON approximately 3 rum in length was grafted t,etw~m the stumps of the sciatic nerve cut at the mid-thigh level. G r ~ t e d nerves w,.~re studied at various times ranging from one to eleven months~ Group H: In ~ach of 10 C57BL/6J mice, an ON was ~rs~ed into the s~:ral nerve trausected at the mid~alf level. These animals were sacrificed one to twelve months later. Group III: In 7 rots and 4 mice, ON segments were placed subcu~meousl~
98
in the lower leg. The site of these control ON transplants was marked .~ith a silk suture. All animals were sacrificed by systemic perfusion of fixative (1.5% glutea~ldehyde/0.5% formaldehyde in 0.1 M Sorensen's phosphate buffer). Nerves were processed and examined according to standard techniques [ 1]. Changes in the optic nerve grafts were compared with intact optic nerves obtained from adult anhmals. Intact ONs from mt~ and mice older than one month contained large blmdles of myelinated fibres as well as oligodendrocytes and astrocytes, the thickest myelin sheaths, composed of approximately 20 lamellae, ensheathed exons nearly 2 ~ m in diameter. A ba~al lamina-enclo~d layer of astrocyte processes, the gliallimiting membrane [7], surrounded the outer surface of intact told control optic nerves. In contrast to the rapidity with which myelin debris is cleared from peripheral nerve grafts, the changes of Wallerian degeneration were observed, as described by others [8], f~L~as long as two months after optic nerve transection. By three mom, hs after transplantation most of the myelin debris arid degenerating axon remnants had disappeered and regenerating axons could be identified in the optic nerve grafts of group I & II animals (Fig. 1). Many of these regenem~d ~ o n s within the O N glialtissue were bare but some were surrounded by C N S myelin (Fig. 2). Only one of the ONs placed subcutaneously (groNp Ill) was found to contain a single nondmyelinated axon; this axor, must have originated from nerves in adjacent tissues. In cross-sectionsof O N grafts (groups I & If) examined at several levels,it was apparent that there were many more axons in portions of the grafts near the proximal peripheral nerve stumps than near their distal stumps. However, in most grafts,a ~ew axons (no more than II in any of the ONs examined) extended the entire lengr~ of the g~aft. In serialcross~ections of the junctional zone between the host nerve stumI!m and the optic nerve grafts, there were many dome4ike protuberances which contained axons, glialprocesses and an occasional astrocyte nucleus. The perio phery of these protuberances was formed by basement membrane~nclosed layers of astrc,cyte processes which resembled the gliallimiting membrane. Axons arising from the peripheral nerve were frequently observed to enter these glialprotuberances (Fig. 3). At such junctions, Schwann cell basal laminae became conthnuous with a basal lamina of the astrocyte processes. Schwann cellsdid not penetrate into the graft or accompany axons for s~y distance within the graft but, at the PNS-glia junction, some Schwann ceU. ensheathed axons were partially encircled by astrocyte processes. By two months after grafting, the dist~lperipheral nerve stumps always con tained large numbers of axons myelinated by Schwann cells.The majority of these axons, which arose from the proximal sciatic stumps, did not grow through the optic nerve graft but bypassed the graft to reach the distal periph. eral nerve stump. ll~O N grafts of groups I & If, it was not u n c o m m o n to find that oligoden-
99
Fig. 1. Cross4ection of a mouse optic nerve (ON) 6 months after transplantation into a transected sural nerve in the same animal. The ON graft contains many astrocyte processes, numerous small axons and a few large axons, some of which are myelinated. Th~se larger axons tend to be located closer to the periphery of the ON thaan to its centre. Many axons from the peripheral nerve proximsl st!~mp have regenerated outside the ON graft; such axons are associated with Schwann cells and PNS myelin (electron micrograph montage, × 2500).
Fig. 2. Large axons regenerating in mouse ON 6 months after grafting into sciatic nerves. (a) A bare axon ~ u n d e d o _ n l F b y astrocyte processes; (b) A myelinated f i b ~ wi~h t3~S characteristics-periodicity 12 um compate~ to 14 nm for PN8 m ] e ~ outside the graft, no basal lamina and, except for the outer ton_eue (T), no cytopIamn between the outer loop of m y e ~ i ~ a d the plasma membranes of adjacent astrocyte processes (electron micrographs: (a) x 27 000. (b) x 40 000).
F-A
O O
Fig. 3. Cross4ection o f a protuberance of glia and axons in the junctional zone between the proximal sciatic nerve stump and an ON graft after 6 months. At ~he left, two obliquely sectioned axons surrounded by Sehwann cell cytoplasm appear to penetrate the glial bundle Sehwann eel] basal lamina is continuous with that of the protuberance (electron mierograph, × 13 000).
O
102
drocytes'themselves were partially or totally su~rrounded by thin myelin sheath~ containing no more than four lamellae. Similarily, some filament-filled astrocyte i processes were also surrounded by thin myelin sheaths. These unusual arrangements resembled the redundant myelin sheaths which have been observed in the normal CNS [9,10] or in association with Wallerian degeneration of CNS fibers [ 11,12]. The present experiments suggest a number of conclusions: (1) Centralnervous system glia (ON) can be transplanted into peripheral nerves where they become innervated by regenerating axons. By ana~ ~ with experiments in which Schwann cells were transplanted into periphe;al nerves [3--5], it should be possible to assess interactions between peripheral nerve axons and normal or abnormal glial cells. For example, this technique could be used to study Jimpy mice in which there is a deficit of myelination [ 13] which may be due to intrinsic astroglial abnormalities [ 14]. or an accelerated death of oligodendrocytes [15,16]. Studies of the glial cell responses in Jimpy optic nerves grafted into normal host peripheral nerves are currently in progress to assess the behaviour, proliferati and survival of glial cells for periods of ~ n e longer than is possible in the actual mouse mutants w h o ~ life span seldom exceeds one month. (2) Although it ~ known that axons entering or leaving the CNS are normally ensheathed and myelinated along their course by both glial and Schwann cells, the demonstration of myelination of regenerated PNS axons by grsfted glial cells indicates that the myelivating response can be re~iicited when denervated g!ia are contacted by 1;he regenerating axons even if, as is in the case of ON glia, this tissue had never been associated with axons from the PNS. (3) Denervated ONs are less conductive to, axonal regeneration than denerrated sciatic nerves. Although some PNS axons entered the ON t~ansplants most axons tended to bypass the ON segments before re-entering the distal peripheral nerve stump. Factors responsible for diverting most PNS axons away from the ON ~ a f t may include the astrocytic response [17] snd the relatively slow removal of myelin and axon debris that takes place in degenerating CNS tissue. (4) The territories occupied by gila and $chwann cells along individual[ axons remain sharply defined in ON grafts innc~ :ated by peripheral nerves. It has been suggested [ 18] that the glial limiting membrane may prevent Schwann cells from entering the CNS. However, i n t h e present studies the Schwann cells did not penetrate the ON grafts even if it can be assumed that a glial limiting membrane was not initially present over the cut end of the ON transplant; it is possib;e that interactions between the two contiguous populations of sheath cells (oligodendrocytes and Schwann cells) may be responsible for inhibiting the advance of Schwann cells into the graft. This hypothesis is consistent with the observation that Schwann cells do not migrate along nerves that regenerate in continuity a r e , re,suturing or nerve grafting but will do so when nerve,~ re.generate ivto areas devoid of sheath cells [1,3,4]. (5) Finally, the basal lamina.bound protuberances we describe may replresen
103
active reJnodelling of CNS tissue reinnervated by PNS ~ o n s . The appearance of these glial formations resembles the glial head seen at sites of normal confluence between the PNSanc~ CNS in cranial nerves and spinal roots [ 19,20] ~md also file glial outgrow~ that follows degeneration and regeneration of spinalroots [21].
Note c~fded in proof. Independent stud/es on the control of myelinogenesis by E.L. Weinberg and P,S, Spencer ( B ~ Research, in press), have also demonstrated rnyelination of PNS axons by oligodendrocytes from grafted optic nerves. ACKNOWLEDGEMENTS
This work was supported by the Medical Research Council of Canada, the Multiple Sclerosis Society of Canada and the Dysautonomia Foundation of America. REFERENCES
1 Aguayo, A.J., Epps, J., Charron, L. and Bray, G.M., Multipotentiality of Schwann cells in crcm-anastomosed arid grafted mye~inated and unmyelinated nerves, Brain Res., 104
(1976) 1-20. 2 Aguayo, A.J., Charron, L. and Bray, G.M., Potential of Schwann cells from unmyelinated r,erv~ to produce ~yelin-A quantitative ultrastructural and radioautographic study. J. Neuroc];tol., 5 (1976) 565--573. 3 Aguayo, A.J., Kasarjian, J., Skamene, E., Kongshavn, P. and Bray, G.M., Myelination of mottse axons by Schwmn cells transplanted from normal and abnormal human nerve~ Nature (Lond.), 268 (1977) 753--755. 4 Aguayo, A.J., Bray, G.M. and Perkins, S.C., Axon-Schwann cell relationships in neuropathies of mutant mice, Ann. N.Y. Acad. Sci. (In press). 5 Aguayo, A.J., Attiwel3, M, Trecarten, J., Perkins, S. and Bray, G.M., Abnormal mye]ination in transplanted Trembler mouse Schwann ceils, Nature (Lond), 265 (1977 ) 73--75. Ag~ayo, A.J., Perkins, S.C., Bray, G.M. and Duncan, I., Evidence from nerve transplantation studies for a primary Schwann cell d~sorder in Charcot-Marie-Tooth (CMT) disease, C ~ . J. Neurol. Sci., (1978) (in press). 7 Peters, A., Palay, S.L., and Webster, H.deF., In, The Fine Structure of the Nervous System. Harper and Row, 2nd Edition (1976). 8 Cook, R.D. and Wisniewski, H.M., The role of oligodendro[~ia and astroglia in Wal]erian degeneration of'the optic nerve, Brain Res., 61 (1973) 191--206. 9 Herndon, R.M., The fine structure of the rat cerebellum, J. Cell Biol., 23 (1964) 277-293. 10 Ros~:nbluth, J., Redundant myelin sheaths and other ult~astructural features of the toad c~rehellum J. Cell Biol., 28 (1966) 73--93. 11 Bign~ni~ A., and Kals~on, H.J., Myelination of fiduciary as~,roglial processes in long term Wsllerian degeneration. The possible rehtionship to "~-",0".~o~ _ - mo,~,,~ot-s"......_..~_, Brain ~es.~ 11 (1968) 710--713. 12 Fulc~a~d, J. and Privat, A., Neurogiial reactions secondal~y to Wallerian degeneration in the optic nerve of the postnatal rat: Ultrastructural and quantitative study, J. ~omp. Neurol., 176 (1977) 189--224. 13 Sidman, R.L., Dickie, M.M. and Appel, S.H., Mutant mice (Quaking and Jimp~r) with deficient myelination in the central nervous system, Science, 144 (1964) 309--311.
1.04; 14 Skoff, R.P., Myelin deficit in the J~np'y mous~ rosy be due to celZulsr abnormalities in astroglia, Nature (i,ond.), 264 (197~$) 560--6~ 2. 15 Privat, A., Robain, O.and Mandel, ~>.,Aspects ultrastructureux de eo~psrealleuxchez le souris Jumpy, ActaNe~Ir~path., 9.1 :~i197~),~8~--295. 16 ~eier, C. and Bisehofi, A., Oligode1~idroglial cell development il~ Jimpy mice and controis. An electron-microscopic stu(iy ill the optic nerve, J. Ne~,zrol.,8~i., 26 (1975) 517--528. 1"/ Windle, W.F., Regeneration of -~ox:~sin the ve~,tebrate cert,'a/nervo.us system,Physiol Rev., 36 (1956) 426--440. 18 BIakemore, W.F., Observations on remyelination in the rabbit spin&~cord following demyelination induced by lysoleei~hin, Neuropat,~a. & Appl. NeurObiol., 4 (1978) 47--59. 19 Fr~her, J.P., Probable glial islands i~l a rat spinal nerve root -- A h)ngitudinal study, J. Neuropath. Exp. Neurol., 33 (1974) 552--560. 20 Berthold, C.-H. snd Caristedt, T., .')Iz~ervations on the morphology at the tr~ms|tion between the peripheral and central :~ervous syslem in the eat. II. General orgsnization of the transitlonal region in S~ do~zl rootlets, Acta. Physiol. &~snd. Suppl., 446. (1977) 9.3--42. 21 Meier, C. and Sollmann, H., Glial o~stgrowth ~md c~,ntrai-type myelination of regenerating axons in spinal nerve roots foilcwing transection and suture: Light and electron microscopic study in the pig, Neuropath. and Appl. No~lrobiol., 4 (1978) 21--35.
97
E N S H E A ~ I M E ~ AND M ~ L I N A T I O N OF REGENERATING PNS FIBRES BY T R A N S P L ~ D OPTIC NERVE GLIA ~ B E R T J. AGUAYO, R I C ~ R D DICKSON, JANE TRECARTEN, MARGARET ATTIWELL, GARTH M. BRAY and PETER RICHARDSON Department of Neurology & Neurosurgery, McGiU University and The Montreal General Hospital {Received June 1st, 1978) (Revised version received June 19th, 1978) (Avvepted June 20th, 1978)
SUMMARY
Interactions between PNS axons and CNS glia were studied morphologically by transplanting optic nerves into peripheral nerves in groups of rats and mice. Four to 11 months after grafting, small numbers of axons from the peripheral nerves had penetrated the CNS grafts w~er~ they became ensheathed mid myelinated by CNS gila. Glial protuberances observed at the C]~4S-PNS interfaces suggested that there had been an active glial response to hmervation by PNS axons. These findings provide experimental evidence that; denervated CNS gila can be reinnervated and form myelin.
,_.perimental nerve grafts have proven useful for the study o:~ axon-Schwann cell interdependencies during normal myelination [ 1,2] and in certain inherited human [3,4] and animal neuropathies [5,6]. The present phase and electron microscope investigation demonstra~s that by using optic nerve transplants in rodents, it is also possible to study interactions between peripheral nerve axons and CNS glia. The intra-orbitai portion of one optic nerve and the attached eye were removed f~om each anesthetized animal. The optic nerve was then cut at its plane of entry into the eve and immediately grafted in the same animal between the stumps of a transected peripheral nerve using techniques previously described [5]. Optic nerve transplants (ON) were studied in three different groups of e x p e ~ e n t a l animals. Group I: In each of 34 adult Sprague--Dawley rots, a segment of one ON approximately 3 rum in length was grafted t,etw~m the stumps of the sciatic nerve cut at the mid-thigh level. G r ~ t e d nerves w,.~re studied at various times ranging from one to eleven months~ Group H: In ~ach of 10 C57BL/6J mice, an ON was ~rs~ed into the s~:ral nerve trausected at the mid~alf level. These animals were sacrificed one to twelve months later. Group III: In 7 rots and 4 mice, ON segments were placed subcu~meousl~
98
in the lower leg. The site of these control ON transplants was marked .~ith a silk suture. All animals were sacrificed by systemic perfusion of fixative (1.5% glutea~ldehyde/0.5% formaldehyde in 0.1 M Sorensen's phosphate buffer). Nerves were processed and examined according to standard techniques [ 1]. Changes in the optic nerve grafts were compared with intact optic nerves obtained from adult anhmals. Intact ONs from mt~ and mice older than one month contained large blmdles of myelinated fibres as well as oligodendrocytes and astrocytes, the thickest myelin sheaths, composed of approximately 20 lamellae, ensheathed exons nearly 2 ~ m in diameter. A ba~al lamina-enclo~d layer of astrocyte processes, the gliallimiting membrane [7], surrounded the outer surface of intact told control optic nerves. In contrast to the rapidity with which myelin debris is cleared from peripheral nerve grafts, the changes of Wallerian degeneration were observed, as described by others [8], f~L~as long as two months after optic nerve transection. By three mom, hs after transplantation most of the myelin debris arid degenerating axon remnants had disappeered and regenerating axons could be identified in the optic nerve grafts of group I & II animals (Fig. 1). Many of these regenem~d ~ o n s within the O N glialtissue were bare but some were surrounded by C N S myelin (Fig. 2). Only one of the ONs placed subcutaneously (groNp Ill) was found to contain a single nondmyelinated axon; this axor, must have originated from nerves in adjacent tissues. In cross-sectionsof O N grafts (groups I & If) examined at several levels,it was apparent that there were many more axons in portions of the grafts near the proximal peripheral nerve stumps than near their distal stumps. However, in most grafts,a ~ew axons (no more than II in any of the ONs examined) extended the entire lengr~ of the g~aft. In serialcross~ections of the junctional zone between the host nerve stumI!m and the optic nerve grafts, there were many dome4ike protuberances which contained axons, glialprocesses and an occasional astrocyte nucleus. The perio phery of these protuberances was formed by basement membrane~nclosed layers of astrc,cyte processes which resembled the gliallimiting membrane. Axons arising from the peripheral nerve were frequently observed to enter these glialprotuberances (Fig. 3). At such junctions, Schwann cell basal laminae became conthnuous with a basal lamina of the astrocyte processes. Schwann cellsdid not penetrate into the graft or accompany axons for s~y distance within the graft but, at the PNS-glia junction, some Schwann ceU. ensheathed axons were partially encircled by astrocyte processes. By two months after grafting, the dist~lperipheral nerve stumps always con tained large numbers of axons myelinated by Schwann cells.The majority of these axons, which arose from the proximal sciatic stumps, did not grow through the optic nerve graft but bypassed the graft to reach the distal periph. eral nerve stump. ll~O N grafts of groups I & If, it was not u n c o m m o n to find that oligoden-
99
Fig. 1. Cross4ection of a mouse optic nerve (ON) 6 months after transplantation into a transected sural nerve in the same animal. The ON graft contains many astrocyte processes, numerous small axons and a few large axons, some of which are myelinated. Th~se larger axons tend to be located closer to the periphery of the ON thaan to its centre. Many axons from the peripheral nerve proximsl st!~mp have regenerated outside the ON graft; such axons are associated with Schwann cells and PNS myelin (electron micrograph montage, × 2500).
Fig. 2. Large axons regenerating in mouse ON 6 months after grafting into sciatic nerves. (a) A bare axon ~ u n d e d o _ n l F b y astrocyte processes; (b) A myelinated f i b ~ wi~h t3~S characteristics-periodicity 12 um compate~ to 14 nm for PN8 m ] e ~ outside the graft, no basal lamina and, except for the outer ton_eue (T), no cytopIamn between the outer loop of m y e ~ i ~ a d the plasma membranes of adjacent astrocyte processes (electron micrographs: (a) x 27 000. (b) x 40 000).
F-A
O O
Fig. 3. Cross4ection o f a protuberance of glia and axons in the junctional zone between the proximal sciatic nerve stump and an ON graft after 6 months. At ~he left, two obliquely sectioned axons surrounded by Sehwann cell cytoplasm appear to penetrate the glial bundle Sehwann eel] basal lamina is continuous with that of the protuberance (electron mierograph, × 13 000).
O
102
drocytes'themselves were partially or totally su~rrounded by thin myelin sheath~ containing no more than four lamellae. Similarily, some filament-filled astrocyte i processes were also surrounded by thin myelin sheaths. These unusual arrangements resembled the redundant myelin sheaths which have been observed in the normal CNS [9,10] or in association with Wallerian degeneration of CNS fibers [ 11,12]. The present experiments suggest a number of conclusions: (1) Centralnervous system glia (ON) can be transplanted into peripheral nerves where they become innervated by regenerating axons. By ana~ ~ with experiments in which Schwann cells were transplanted into periphe;al nerves [3--5], it should be possible to assess interactions between peripheral nerve axons and normal or abnormal glial cells. For example, this technique could be used to study Jimpy mice in which there is a deficit of myelination [ 13] which may be due to intrinsic astroglial abnormalities [ 14]. or an accelerated death of oligodendrocytes [15,16]. Studies of the glial cell responses in Jimpy optic nerves grafted into normal host peripheral nerves are currently in progress to assess the behaviour, proliferati and survival of glial cells for periods of ~ n e longer than is possible in the actual mouse mutants w h o ~ life span seldom exceeds one month. (2) Although it ~ known that axons entering or leaving the CNS are normally ensheathed and myelinated along their course by both glial and Schwann cells, the demonstration of myelination of regenerated PNS axons by grsfted glial cells indicates that the myelivating response can be re~iicited when denervated g!ia are contacted by 1;he regenerating axons even if, as is in the case of ON glia, this tissue had never been associated with axons from the PNS. (3) Denervated ONs are less conductive to, axonal regeneration than denerrated sciatic nerves. Although some PNS axons entered the ON t~ansplants most axons tended to bypass the ON segments before re-entering the distal peripheral nerve stump. Factors responsible for diverting most PNS axons away from the ON ~ a f t may include the astrocytic response [17] snd the relatively slow removal of myelin and axon debris that takes place in degenerating CNS tissue. (4) The territories occupied by gila and $chwann cells along individual[ axons remain sharply defined in ON grafts innc~ :ated by peripheral nerves. It has been suggested [ 18] that the glial limiting membrane may prevent Schwann cells from entering the CNS. However, i n t h e present studies the Schwann cells did not penetrate the ON grafts even if it can be assumed that a glial limiting membrane was not initially present over the cut end of the ON transplant; it is possib;e that interactions between the two contiguous populations of sheath cells (oligodendrocytes and Schwann cells) may be responsible for inhibiting the advance of Schwann cells into the graft. This hypothesis is consistent with the observation that Schwann cells do not migrate along nerves that regenerate in continuity a r e , re,suturing or nerve grafting but will do so when nerve,~ re.generate ivto areas devoid of sheath cells [1,3,4]. (5) Finally, the basal lamina.bound protuberances we describe may replresen
103
active reJnodelling of CNS tissue reinnervated by PNS ~ o n s . The appearance of these glial formations resembles the glial head seen at sites of normal confluence between the PNSanc~ CNS in cranial nerves and spinal roots [ 19,20] ~md also file glial outgrow~ that follows degeneration and regeneration of spinalroots [21].
Note c~fded in proof. Independent stud/es on the control of myelinogenesis by E.L. Weinberg and P,S, Spencer ( B ~ Research, in press), have also demonstrated rnyelination of PNS axons by oligodendrocytes from grafted optic nerves. ACKNOWLEDGEMENTS
This work was supported by the Medical Research Council of Canada, the Multiple Sclerosis Society of Canada and the Dysautonomia Foundation of America. REFERENCES
1 Aguayo, A.J., Epps, J., Charron, L. and Bray, G.M., Multipotentiality of Schwann cells in crcm-anastomosed arid grafted mye~inated and unmyelinated nerves, Brain Res., 104
(1976) 1-20. 2 Aguayo, A.J., Charron, L. and Bray, G.M., Potential of Schwann cells from unmyelinated r,erv~ to produce ~yelin-A quantitative ultrastructural and radioautographic study. J. Neuroc];tol., 5 (1976) 565--573. 3 Aguayo, A.J., Kasarjian, J., Skamene, E., Kongshavn, P. and Bray, G.M., Myelination of mottse axons by Schwmn cells transplanted from normal and abnormal human nerve~ Nature (Lond.), 268 (1977) 753--755. 4 Aguayo, A.J., Bray, G.M. and Perkins, S.C., Axon-Schwann cell relationships in neuropathies of mutant mice, Ann. N.Y. Acad. Sci. (In press). 5 Aguayo, A.J., Attiwel3, M, Trecarten, J., Perkins, S. and Bray, G.M., Abnormal mye]ination in transplanted Trembler mouse Schwann ceils, Nature (Lond), 265 (1977 ) 73--75. Ag~ayo, A.J., Perkins, S.C., Bray, G.M. and Duncan, I., Evidence from nerve transplantation studies for a primary Schwann cell d~sorder in Charcot-Marie-Tooth (CMT) disease, C ~ . J. Neurol. Sci., (1978) (in press). 7 Peters, A., Palay, S.L., and Webster, H.deF., In, The Fine Structure of the Nervous System. Harper and Row, 2nd Edition (1976). 8 Cook, R.D. and Wisniewski, H.M., The role of oligodendro[~ia and astroglia in Wal]erian degeneration of'the optic nerve, Brain Res., 61 (1973) 191--206. 9 Herndon, R.M., The fine structure of the rat cerebellum, J. Cell Biol., 23 (1964) 277-293. 10 Ros~:nbluth, J., Redundant myelin sheaths and other ult~astructural features of the toad c~rehellum J. Cell Biol., 28 (1966) 73--93. 11 Bign~ni~ A., and Kals~on, H.J., Myelination of fiduciary as~,roglial processes in long term Wsllerian degeneration. The possible rehtionship to "~-",0".~o~ _ - mo,~,,~ot-s"......_..~_, Brain ~es.~ 11 (1968) 710--713. 12 Fulc~a~d, J. and Privat, A., Neurogiial reactions secondal~y to Wallerian degeneration in the optic nerve of the postnatal rat: Ultrastructural and quantitative study, J. ~omp. Neurol., 176 (1977) 189--224. 13 Sidman, R.L., Dickie, M.M. and Appel, S.H., Mutant mice (Quaking and Jimp~r) with deficient myelination in the central nervous system, Science, 144 (1964) 309--311.
1.04; 14 Skoff, R.P., Myelin deficit in the J~np'y mous~ rosy be due to celZulsr abnormalities in astroglia, Nature (i,ond.), 264 (197~$) 560--6~ 2. 15 Privat, A., Robain, O.and Mandel, ~>.,Aspects ultrastructureux de eo~psrealleuxchez le souris Jumpy, ActaNe~Ir~path., 9.1 :~i197~),~8~--295. 16 ~eier, C. and Bisehofi, A., Oligode1~idroglial cell development il~ Jimpy mice and controis. An electron-microscopic stu(iy ill the optic nerve, J. Ne~,zrol.,8~i., 26 (1975) 517--528. 1"/ Windle, W.F., Regeneration of -~ox:~sin the ve~,tebrate cert,'a/nervo.us system,Physiol Rev., 36 (1956) 426--440. 18 BIakemore, W.F., Observations on remyelination in the rabbit spin&~cord following demyelination induced by lysoleei~hin, Neuropat,~a. & Appl. NeurObiol., 4 (1978) 47--59. 19 Fr~her, J.P., Probable glial islands i~l a rat spinal nerve root -- A h)ngitudinal study, J. Neuropath. Exp. Neurol., 33 (1974) 552--560. 20 Berthold, C.-H. snd Caristedt, T., .')Iz~ervations on the morphology at the tr~ms|tion between the peripheral and central :~ervous syslem in the eat. II. General orgsnization of the transitlonal region in S~ do~zl rootlets, Acta. Physiol. &~snd. Suppl., 446. (1977) 9.3--42. 21 Meier, C. and Sollmann, H., Glial o~stgrowth ~md c~,ntrai-type myelination of regenerating axons in spinal nerve roots foilcwing transection and suture: Light and electron microscopic study in the pig, Neuropath. and Appl. No~lrobiol., 4 (1978) 21--35.