Double myelination of axons in the sympathetic nervous system

Biology and pathology of nonmyelinating Schwann cells

The CNS contains relatively few unmyelinated nerve fibers, and thus benefits from the advantages that are conferred by myelination, including faster conduction velocities, lower energy consumption for impulse transmission, and greater stability of point-to-point connectivity. In the PNS many fibers or regions of fibers the Schwann do not form myelin. Examples include C fibers nociceptors, postganglionic sympathetic fibers, and the Schwann cells associated with motor nerve terminals at neuromuscular junctions. These examples retain a degree of plasticity and a capacity to sprout collaterally that is unusual in myelinated fibers. Nonmyelin-forming Schwann cells, including those associated with uninjured fibers, have the capacity to act as the "first responders" to injury or disease in their neighborhoods.

Glial cell line-derived neurotrophic factor alters axon schwann cell units and promotes myelination in unmyelinated nerve fibers

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in the development and maintenance of a subset of dorsal root ganglion sensory neurons. We administered high-dose exogenous recombinant human GDNF (rhGDNF) daily to adult rats to examine its effect on unmyelinated axon-Schwann cell units in intact peripheral nerves. In rhGDNF-treated animals, there was a dramatic proliferation in the Schwann cells of unmyelinated fibers, which resulted in the segregation of many unmyelinated axons into a 1:1 relationship with Schwann cells and myelination of normally unmyelinated small axons. This study demonstrates that the administration of high doses of a growth factor to adult rats can change the phenotype of nerve fibers from unmyelinated to myelinated.

Myelination and axonal regeneration in the central nervous system of mice deficient in the myelin-associated glycoprotein

The myelin-associated glycoprotein, a member of the immunoglobulin superfamily, has been implicated in the formation and maintenance of myelin sheaths. In addition, recent studies have demonstrated that myelin-associated glycoprotein is inhibitory for neurite elongation in vitro and it has therefore been suggested that myelin-associated glycoprotein prevents axonal regeneration in lesioned nervous tissue. The generation of mice deficient in the expression of myelin-associated glycoprotein by targeted disruption of the mag gene via homologous recombination in embryonic stem cells has allowed the study of the functional role of this molecule in vivo. This review summarizes experiments aimed at answering the following questions: (i) is myelin-associated glycoprotein involved in the formation and maintenance of myelin in the CNS? and (ii) does myelin-associated glycoprotein restrict axonal regeneration in the adult mammalian CNS? Analysis of optic nerves from mutant mice revealed a delay in myelination when compared to optic nerves of wild-type animals, a lack of a periaxonal cytoplasmic collar from most myelin sheaths, and the presence of some doubly and multiply myelinated axons. Axonal regeneration in the CNS of adult myelin-associated glycoprotein deficient mice was not improved when compared to wild-type animals. These observations indicate that myelin-associated glycoprotein is functionally involved in the recognition of axons by oligodendrocytes and in the morphological maturation of myelin sheaths. However, results do not support a role of myelin-associated glycoprotein as a potent inhibitor of axonal regeneration in the adult mammalian CNS.

Multiply myelinated axons in the optic nerve of mice deficient for the myelin-associated glycoprotein

We recently reported that some retinal ganglion cell axons in mice deficient for the myelin-associated glycoprotein are concentrically surrounded by more than one myelin sheath. In the present study, we demonstrate that myelin sheaths displaced from the axon reveal a normal ultrastructure of compact myelin, with the only exception that multiple myelination of axons frequently correlates with the presence of unfused regions of major dense lines. Supernumerary sheaths terminated on other sheaths or on astrocyte cell surfaces in a pattern closely resembling the morphology of a true paranode. The thickness of compact myelin of multiply myelinated axons was significantly increased when compared with axons of similar caliber surrounded by a single myelin sheath. Our observations demonstrate that maintenance of compact myelin and paranodal regions is not dependent on direct axonal contact and that the presence of more than one concentric myelin sheath around an axon results in dysregulation of the axon-to-fiber ratio.

Myelination of two axons by a single Schwann cell

A Schwann cell can form only one internode of myelin around an axon. However, we observed the formation by a single Schwann cell of myelin around two axons of different diameters in the sural nerve of a 45-year-old man with mononeuritis multiplex. Schwann cell processes spiraled in the same direction around each axon, forming mesaxons. The findings in this case appear to be an undescribed type of aberrant myelination.

Myelin sheath survival after guanethidine-induced axonal degeneration

Membrane-membrane interactions between axons and Schwann cells are required for initial myelin formation in the peripheral nervous system. However, recent studies of double myelination in sympathetic nerve have indicated that myelin sheaths continue to exist after complete loss of axonal contact (Kidd, G. J., and J. W. Heath. 1988. J. Neurocytol. 17:245-261). This suggests that myelin maintenance may be regulated either by diffusible axonal factors or by nonaxonal mechanisms. To test these hypotheses, axons involved in double myelination in the rat superior cervical ganglion were destroyed by chronic guanethidine treatment. Guanethidine-induced sympathectomy resulted in a Wallerian-like pattern of myelin degeneration within 10 d. In doubly myelinated configurations the axon, inner myelin sheath (which lies in contact with the axon), and approximately 75% of outer myelin sheaths broke down by this time. Degenerating outer sheaths were not found at later periods. It is probably that outer sheaths that degenerated were only partially displaced from the axon at the commencement of guanethidine treatment. In contrast, analysis of serial sections showed that completely displaced outer internodes remained ultrastructurally intact. These internodes survived degeneration of the axon and inner sheath, and during the later time points (2-6 wk) they enclosed only connective tissue elements and reorganized Schwann cells/processes. Axonal regeneration was not observed within surviving outer internodes. We therefore conclude that myelin maintenance in the superior cervical ganglion is not dependent on direct axonal contact or diffusible axonal factors. In addition, physical association of Schwann cells with the degenerating axon may be an important factor in precipitating myelin breakdown during Wallerian degeneration.

P0 gene expression in cultured Schwann cells

This study examines the expression of the major myelin protein gene P0 in cultured Schwann cells, grown on their own or in association with neurons. Many freshly dissociated Schwann cells from actively myelinating nerves express Po mRNA in high abundance. If neurons are not present, signal intensity falls markedly with time so that by 7 days in culture only a basal expression is evident which is negligible compared to the level in vivo. Dorsal root ganglia from embryo day 16 (E16) rats contain no significant levels of Po mRNA but when grown in full myelinating medium (containing serum and embryo extract) increasing expression is seen from 4 to 5 days onward even though myelination does not occur until after the second week. In this intervening period the intensity of P0 mRNA expression is lower than that found in the actively myelinating cell. Neurons from sympathetic ganglia are also capable of inducing P0 mRNA expression. Schwann cells in dorsal root ganglia explants grown in serum-free defined medium do not assemble a basal lamina and will not wrap or myelinate axons. Nevertheless P0 mRNA, but not protein, is expressed in levels similar to those found in full myelinating medium prior to myelination. Such Schwann cells also exhibit galactocerebroside and the sulphatide recognised by the 04 antibody. It appears that in defined medium or in myelinating medium prior to myelination axonal signals can induce P0 mRNA expression to a certain degree. However, full up-regulation is usually associated with the rapid membrane expansion accompanying myelination. Whether this augmented up-regulation is due to further axonal signalling or events in the Schwann cell is unknown, but the results suggest that P0 expression can be regulated at several stages of synthesis.

Aberrant remyelination of axons after heat injury in the dorsal funiculus of rat spinal cord

We studied the course of demyelination and subsequent remyelination of nerve fibers after heat injury in the dorsal funiculus of the rat spinal cord. Four weeks after heat treatment, we observed, in addition to normally remyelinated axons, a few aberrantly remyelinated axons which had both CNS- and PNS-type myelin sheaths: the CNS-type myelin sheaths were always situated inside the PNS-type sheaths. This finding indicates that in some conditions Schwann cells can form myelin sheaths around those formed by oligodendrocytes.

Double myelination of axons in the sympathetic nervous system of the mouse. II. Mechanisms of formation

The phenomenon termed 'double myelination', present in sympathetic nerve of normal adult rats and mice, comprises regions of a myelinated axon which are concentrically ensheathed by additional (outer) myelinating Schwann cells. Evidence has been presented that in some instances the outer Schwann cell fails to make contact with an axon, yet its myelin sheath characteristically remains ultrastructurally intact. The present study has sought to identify and analyse configurations intermediate between single and double myelination, in order to determine the mechanism(s) underlying the formation of double ensheathment. Superior cervical ganglia from normal male mice aged 12-24 months were prepared for electron microscopy by systemic aldehyde perfusion. Regions of interest were extensively serial-sectioned for detailed electron microscopical analysis and reconstruction. The earliest evidence for alteration to the expected intimate ensheathment of axons by myelinating Schwann cells involved invasion of supernumerary Schwann cells and their processes at the node of Ranvier, resulting in displacement of the paranodal pockets from axonal contact. Similar paranodal displacement occurred at heminodes as a result of lateral extension and invasion of processes from the adjacent Schwann cell (i.e. the cell investing the unmyelinated domain of the axon). Subsequently, processes of the invading cell extended progressively into internodal regions, located at all times between the plasma membranes of the axon and displaced Schwann cell. The cytoplasmic pockets at the remaining paranode were then subject to invasion. At various stages of displacement myelin formation commenced within the invading cell, representing the first acquisition of double myelin ensheathment in the development of the configuration. Involvement of haematogenous cells in displacement was not detected. There was also evidence consistent with paranodal displacement by adjacent pre-existing myelinating cells, but this additional mechanism appeared minor relative to the involvement of (initially) non-myelinating Schwann cells. We found no evidence for the alternative possibility that Schwann cells could synthesize a myelin sheath around a pre-existing myelinated axon de novo, independent of any direct axonal contact. These results are consistent with the well-established requirement for axonal contact by Schwann cells engaging in initial myelin formation, in the sense that the myelin sheath of the outer cell was synthesized prior to its displacement, and that a myelin sheath was not formed by the invading cell until it had invested the axon in a 1:1 relationship.(ABSTRACT TRUNCATED AT 400 WORDS)

Double myelination of axons in the sympathetic nervous system of the mouse. I. Ultrastructural features and distribution

This study has examined the structural features and distribution of 'doubly myelinated' axons in normal adult and aged mice. Investigation focused on the superior cervical ganglion (SCG) and paravertebral sympathetic ganglia, which were extensively serial-sectioned for light and electron microscopy. In the SCG, the principal features of doubly myelinated regions were that an apparently normal myelinated axon was enclosed for part of its length by an additional (outer) myelinating Schwann cell. The separate nature of the inner and outer Schwann cells was emphasized by the consistent presence of individual nuclei in each, and by the presence of endoneurial space, often containing collagen fibrils, between the inner and outer cells. In some cases more than a single outer Schwann cell was present, arranged serially along the inner myelinated fibre. While double myelination forms through a mechanism involving displacement of an original myelinating Schwann cell by an interposed Schwann cell (see companion paper), we here provide evidence that in some instances the outer Schwann cell fails to retain any direct axonal contact, either with the axon centrally enclosed within the configuration or with any neighbouring axon. In contrast to the rat, delicate cytoplasmic processes often extended from the lateral extremes of outer Schwann cells. However, again no evidence for axonal contact was found, and similar processes also extended from the paranodal region of some singly myelinated non-displaced Schwann cells. Without exception the outer myelin sheath remained structurally intact, and characteristically underwent a series of conformational changes (progressive infolding of the paranodes and new areas of myelin compaction) which infer a continuing capacity of the outer Schwann cell to translocate myelin-specific components in a co-ordinated manner. A basal lamina was always present on the 'abaxonal' plasma membrane of the outer cell, but not on the 'adaxonal' surface except in areas involved in infolding, thus retaining the polarity which existed at the time of displacement from the axon. At single cross-sectional levels through the SCG, up to approximately 4% of myelinated axons were involved in double myelination. Double myelination was not detected in the sciatic nerve or in the paravertebral ganglia, thus indicating a predilection for the SCG as a site of development of these configurations. Though not challenging the role of the axon in initiating the formation of myelin, these data indicate that in this tissue myelin maintenance does not require direct contact between axonal and Schwann cell plasma membranes.

Analysis of myelin proteins in sympathetic peripheral nerve of adult rats

Biochemical analyses of myelin proteins in rat sympathetic peripheral nerve were correlated with morphological observations. Myelin proteins in superior cervical ganglia (SCG) and the paravertebral (thoraco-lumbar) chain of ganglia were quantitated by immunoassays and examined qualitatively by Western blotting. The results were compared to those obtained on sciatic nerves from the same animals. In rats aged one year, the concentrations of PO glycoprotein and myelin basic protein (MBP) in SCG were about 1% of those in sciatic nerve, consistent with the relatively low numbers of myelinated fibers in sympathetic nerve. The relative concentration of myelin-associated glycoprotein (MAG) was higher, being 6.7% of that in sciatic nerve. The latter finding is probably due to the greater proportion of MAG-containing membranes (periaxonal, paranodal, and Schmitt-Lanterman incisures) in myelinated fibers of the SCG, in which the internodes are both short and thinly myelinated. The proportion of the 21 kDa, 18 kDa and 17 kDa forms of MBP relative to the 14 kDa form was much higher in SCG than in sciatic nerve, probably reflecting the fact that myelin formation continues actively during adult life in the ganglia, whereas the deposition of myelin is complete at a much earlier age in somatic nerves. The levels of myelin proteins were 2- to 3-fold higher in the paravertebral chain ganglia. These studies indicate that quantitation of myelin proteins in sparsely myelinated sympathetic nerve tissue is feasible and provide a baseline for further studies on the control of myelination in sympathetic nerve during adult life.

Degeneration of myelinated sympathetic nerve fibres following treatment with guanethidine

The specificity and characteristics of the degeneration of myelinated axons after chronic guanethidine treatment have been investigated in sympathetic and non-sympathetic nerves. Adult male Sprague-Dawley rats aged approximately 43 weeks were treated with guanethidine sulphate (50 mg per kg body weight per day) for between ten days and six weeks. Tissues were examined by qualitative and quantitative light and electron microscopy. In the superior cervical (sympathetic) ganglion (SCG), guanethidine treatment produced a 78% decrease (P = 0.009) in the mean number of myelinated fibres at a standard level of section, compared to the contralateral control ganglion which was removed surgically prior to drug treatment. This reduction in the treated SCG was apparent after 10 days, though complete degeneration of nerve cell bodies was not widespread at this stage. Degeneration of unmyelinated axons was extensive. Degenerating myelinated fibres were consistently small in diameter (up to approximately 3 microns). In individual myelinated fibres the earliest signs of degeneration involved disruption of axonal organelles, particularly the cytoskeleton, and focal widening of the periaxonal space. Myelin breakdown followed these events; degeneration of myelin still associated with a structurally intact axon was not observed. Myelin breakdown appeared to take place initially within the Schwann cell, at least to the stage of 'loosened' membranes. However, infiltrating cells were also involved in myelin phagocytosis. At all stages of treatment some small diameter myelinated fibres remained intact, and there was no evidence of degeneration of the larger diameter fibres (up to approximately 15 microns) which are consistently present in small numbers in the SCG. In the cervical sympathetic trunk, which carries preganglionic axons to the SCG and the vagus and sciatic nerves, degeneration only of unmyelinated axons was detected. These results indicate that guanethidine does not exert a primary degenerative influence on myelin or myelinating Schwann cells and that the myelin degeneration observed in the SCG is a secondary result of the previously documented selectively destructive effect of guanethidine on postganglionic sympathetic neurons. Surviving, small diameter myelinated fibres in the SCG could be either preganglionic or processes of resistant postganglionic neurons, while the larger diameter fibres are likely to be somatic. While the cervical sympathetic trunk, vagus and sciatic nerves all contain postganglionic sympathetic fibres it appears that few of these are myelinated, at least at the levels sampled in this study.

Myelination in organotypic cultures of sympathetic ganglia

Explants of mouse superior cervical ganglion (SCG), co-cultured with dorsal spinal cord, were grown for up to 4 weeks in vitro. In such cultures, scattered internodes of peripheral nervous system (PNS) myelin were observed, apparently associated with SCG neurites. Although rare, the incidence of PNS myelination in this system might merit further experimentation to provide a model facilitating the evaluation of postganglionic sympathetic myelination, which in vivo may be both extensive and morphologically unusual.