The enzymology of myelination

Role of axonal components during myelination

Myelination is a multistep ordered process whereby Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS), produce and extend membranous processes that envelop axons. Mechanisms that regulate this complex process are not well understood. Advances in deciphering the regulatory components of myelination have been carried out primarily in the PNS and although the mechanisms for triggering and directing myelination are not known, it is well established that myelination does not occur in the absence of axons or axon/neuron-derived factors. This appears to be true both in PNS and CNS. Progress in understanding CNS myelinogenesis has been relatively slow because of the unavailability of a suitable culture system, which, in turn, is partly due to complexity in the cellular organization of the CNS. Though the myelin composition differs between PNS and CNS, the regulation of myelination seems to parallel rather than differ between these two systems. This article reviews the regulatory role of axonal components during myelination. The first half consists of an overview of in vitro and in vivo studies carried out in the nervous system. The second half discusses the use of a cerebellar slice culture system and generation of anti-axolemma monoclonal antibodies to investigate the role of axonal membrane components that participate in myelination. It also describes the characterization of an axonal protein involved in myelination.

Study of O-glycan sialylation in C6 cultured glioma cells: evidence for post-translational regulation of a beta-galactoside alpha 2,3 sialyltransferase activity by N-glycosylation

We have studied the Gal beta 1-3GalNAc-R alpha 2,3 sialyltransferase from C6 glioma cells transferring Neu5Ac from CMP-Neu5Ac onto O-glycans of glycoproteins. Using synchronized C6 glioma cells, we showed that the alpha 2,3 sialyltransferase activity was inhibited by tunicamycin to a greater extend than DNA and protein biosynthesis suggesting inhibition of N-glycosylation of this enzyme. Additional demonstration of N-glycosylation of the alpha 2,3 sialytransferase was provided through ConA-Sepharose binding. Treatment of partially purified alpha 2,3 sialytransferase by peptide-N-glycosidase F showed a significative inhibition demonstrating that N-glycan moiety is required for complete activity of the C6 glioma cell alpha 2,3 sialyltransferase.

Evidence for an O-glycan sialylation system in brain. Characterization of a beta-galactoside alpha 2,3-sialyltransferase from rat brain regulating the expression of an alpha-N-acetylgalactosaminide alpha 2,6-sialyltransferase activity

We present evidence for the existence in rat brain of several sialyltransferases able to sialylate sequentially asialofetuin. [14C]Sialylated glycans of asialofetuin were analyzed by gel filtration. Three types of [14C]sialylated glycans were synthesized: N-glycans and monosialylated and disialylated O-glycans. The varying effects of N-ethylmaleimide, lysophosphatidylcholine (lysoPtdCho) and trypsin, were helpful in the identification of these different sialyltransferases. One of them, selectively inhibited by N-ethylmaleimide, was identified as the Neu5Ac alpha 2----3Gal beta 1----3GalNAc-R:alpha 2----6 sialyltransferase previously described [Baubichon-Cortay, H., Serres-Guillaumond, M., Louisot, P. and Broquet, P. (1986) Carbohydr. Res. 149, 209-223]. This enzyme was responsible for the synthesis of disialylated O-glycans. LysoPtdCho and trypsin selectively inhibited the enzyme responsible for the synthesis of monosialylated O-glycan. N-ethylmaleimide, lysoPtdCho and trypsin did not inhibit Neu5Ac transfer onto N-glycans, giving evidence for three different molecular species. To identify the enzyme responsible for monosialylated O-glycan synthesis, we used another substrate: Gal beta 1----3GalNAc--protein obtained after galactosylation of desialylated ovine mucin by a GalNAc-R:beta 1----3 galactosyltransferase from porcine submaxillary gland. This acceptor was devoid of N-glycans and of NeuAc in alpha 2----3 linkages on the galactose residue. When using N-ethylmaleimide we obtained the synthesis of only one product, a monosialylated structure. After structural analysis by HPLC on SAX and SiNH2 columns, we identified this product as Neu5Ac alpha 2----3Gal beta 1----3GalNAc. The enzyme leading to synthesis of this monosialylated O-glycan was identified as a Gal beta 1----3GalNAc-R:alpha 2----3 sialyltransferase. When using lysoPtdCho and trypsin, sialylation was completely abolished, although the Neu5Ac alpha 2----3Gal beta 1----3GalNAc-R:alpha 2----6 sialyltransferase was not inhibited. We provided thus evidence for the interpendence between the two enzymes, the alpha 2----3 sialyltransferase regulates the alpha 2----6 sialyltransferase activity since it synthesizes the alpha 2----6 sialyltransferase substrate.

Unmyelinated and myelinated axon membrane from rat corpus callosum: differences in macromolecular structure

The macromolecular structure of unmyelinated and myelinated internodal axon membrane was examined with freeze-fracture electron microscopy. Unmyelinated axons exhibited a gradient of axonal diameters, generally ranging from 0.1 to 0.5 micron, with some unmyelinated axons of up to 0.7 micron diameter. Myelinated fibers also displayed a range of axonal diameters, with axons generally 0.3-1.0 micron. The overlap in diameters, between unmyelinated and myelinated fibers, permitted a comparison of membrane structure in myelinated and unmyelinated axons of the same diameter. Small (less than 0.5 micron) diameter unmyelinated axons exhibited a moderate density (approximately 700/micron2) of P-face intramembranous particles (IMPs), while large (greater than or equal to 0.5 micron) caliber unmyelinated axons displayed a significantly greater P-face IMP density (approximately 1100/micron2). Internodal membrane of both small (less than 0.5 micron) and large (greater than or equal to 0.5 micron) diameter myelinated fibers exhibited densities of P-face particles (approximately 1400/micron2) that were similar to each other, but significantly different from unmyelinated fibers. These results demonstrate that there are differences in membrane structure between unmyelinated and myelinated axons of similar diameter. These findings also demonstrate that membrane structure of unmyelinated axons is not invariant for all unmyelinated fibers within a given CNS tract but, on the contrary, is related to diameter.

Developmental and pathophysiological aspects of the myelin-associated glycoprotein

1. A glycoslylated sulfate-containing protein known as myelin-associated glycoprotein (MAG) appears to be unique to the central and peripheral nervous systems. This component has been characterized and cDNA clones have been isolated. 2. MAG is a member of the immunoglobulin superfamily. The principal form of MAG synthesized in brain during active myelination has an apparent molecular weight of 100,000. Alternate exon splicing leads to an additional 5000-dalton smaller form with a different C terminus. 3. In patients with multiple sclerosis, MAG is rapidly lost in areas of active disease. It is immunologically reactive in patients with benign monoclonal gammopathy associated with peripheral neuropathy. 4. The role of MAG in the formation of the myelin sheath and its participation in autoimmune neurological disorders are outlined.

Cytoplasmic membrane elaborations in oligodendrocytes during myelination of spinal motoneuron axons

The ultrastructure of paranodal oligodendroglial cytoplasm, which is located in proximity to the forming myelin sheath, was studied during maturation of spinal motoneuron axons in rat. At 8 days postnatal, the paranodal oligodendroglial loops contain a network of membrane-bound tubulovesicular elements. These membrane elaborations are most common in oligodendroglial loops attached to the outermost layers of the myelin sheath, i.e., paranodal loops closest to the nodal gap. The number of oligodendroglial cytoplasmic profiles per paranodal loop falls over the course of five to ten sequential paranodal loops, and these profiles are nearly absent in paranodal oligodendroglial cytoplasm located distant from the nodal gap. Oligodendrocytes in spinal cords of 14- and 20-day-old rats and of adult rats did not exhibit networks of tubulovesicular profiles. The appearance of these membrane organelles within oligodendroglial cytoplasm during myelin maturation suggests increased membrane turnover within paranodal cytoplasm located adjacent to the axon that is being myelinated. Membrane turnover within oligodendrocytes may reflect axonal modulation of glial function during myelination.

Node of Ranvier formation along fibres regenerating through silicone tube implants: a freeze-fracture and thin-section electron microscopic study

Thin-section and freeze-fracture electron microscopy have been used to examine the morphogenesis of the node of Ranvier in peripheral nerves regenerating through silicone tubes. A major question posed by this study is whether node formation in fibres regenerating across a gap recapitulates that occurring in normal development. Node formation occurs concurrently with myelination and follows a similar spatial gradient of progression from a proximal to distal direction along the regenerated nerve. Presumptive nodal sites appear prior to myelin formation and are identified as a prominent subaxolemmal density in thin sections and axonal particle patches in freeze-fracture. Following the appearance of presumptive nodes in regenerating fibres, dimeric particles are inserted into the axolemma adjacent to the node. These particles are in close apposition to the overlying Schwann cell terminal processes and with maturity adopt the same circumferential orientation seen in adult nodes. The nodal axolemma of regenerating fibres shows a characteristic increase in the prominence of its subaxolemmal densification and number of heterogeneously sized particles. Mature regenerated nodes demonstrate a complete annulus of nodal particles indistinguishable from control nodes. The results of the present study show that the nodal architecture of regenerating fibres is a faithful reconstruction of normal mature nodes, thus indicating that the morphological correlates associated with saltatory conduction at the node are present in regenerated nodes.

Ultrastructural sequence of myelin breakdown during Wallerian degeneration in the rat optic nerve

Adult albino rats were subjected to unilateral surgical removal of the eyeball. After survival times of 7-140 days, the numerical response of the neuroglial cells, and the progressive disintegration of the myelin sheaths in the optic nerves, were studied qualitatively and quantitatively in electron-microscopic montages. The distribution density of microglia and astroglia in degenerating optic nerve increased to peaks after 35 and 56 days respectively, whereas, the oligodendroglia gradually decreased. During the early stage of degeneration, microglial cells appeared and invaded the sheath at the intraperiod line, peeling off the outer lamellae, which were then engulfed by phagocytosis. Within the microglia, myelin sheath fragments were surrounded by a membrane curled to form a myelin ring. In the intermediate stage of degeneration, the paired electron-dense lines of the ring, made up of myelin basic protein, decomposed and formed a homogeneous or heterogeneous osmiophilic layered structure, the myelin body, which, in the final stages, disintegrated and transformed into globoid lipid droplets and needle shaped cholesterol crystals.

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Dolichol deposition in developing mammalian brain: content of free and fatty-acylated dolichol and proportion of specific isoprenologues

Dolichol serves a critical role in N-linked glycoprotein synthesis as the carrier of saccharide moieties. In this study the deposition of dolichol was investigated in developing rat brain. The contents of both the free and fatty-acylated forms of dolichol as well as the specific isoprenologues of dolichol were determined. Two distinctive phases of rapid dolichol deposition were observed; the first during brain development, and the second, with aging. The predominant form of brain dolichol was shown to be the free derivative at all ages; however, during the two phases of rapid deposition of total dolichol, fatty-acylated dolichol became detectable and increased transiently. The rapid rate of increase of brain dolichol during development occurred later than that of brain cholesterol, which shares a common biosynthetic pathway with dolichol. Thus, the peak rate of increase of dolichol occurred at 25 days postnatal, and of cholesterol, at 18 days postnatal. A distinct ontogenetic alteration in the proportion of the individual molecular species (isoprenologues) of dolichol also was defined. These observations suggest that dolichol is particularly involved in developmental events of the third to sixth postnatal weeks in rat brain, e.g. myelination and synaptogenesis.

Differential expression of gangliosides on the surfaces of myelinated nerve fibers

The binding of cholera and tetanus toxins to receptors on the surfaces of teased nerve fibers was used to localize GM1 and G1b-series gangliosides, respectively, by immunocytochemical methods. Native fibers and fibers treated with various hydrolytic enzymes to degrade specific surface components were studied. With native fibers, both toxins bound abundantly to nodes of Ranvier and poorly to the most external, internodal Schwann cell surfaces. Treatment of the fibers with proteases, hyaluronidase, and chondroitin ABC lyase neither eliminated receptors at the nodes nor unmasked receptors over the internodes. The axolemma underlying the paranodal or internodal myelin, exposed by extensive treatment with protease, bound both toxins in large amounts. Neuraminidase action induced cholera toxin receptors on the Schwann cell surface; these receptors were insensitive to protease. The results indicate that GM1 and G1b-series gangliosides are predominantly localized to axonal and glial structures of the node of Ranvier and to paranodal/internodal Axolemma, and that polysialogangliosides not of the G1b-series are present on the internodal Schwann cell surface.

Ganglioside localization on myelinated nerve fibres by cholera toxin binding

GM1 ganglioside has been localized on the surfaces of myelinated, peripheral nerve fibres by using immunofluorescence to detect cholera toxin receptors. Unfixed, mouse sciatic nerves were teased into individual, intact fibres in order to expose their extracellular surfaces. Cholera toxin binding sites were abundant at all nodes of Ranvier; they were scarce on the internodal fibre surfaces. The nodal receptors were resistant to various degradative enzymes, including trypsin and proteinase K. Proteases did not unmask receptors on the internodal surfaces. Exogenous GM1 successfully competed for the toxin binding sites on the fibres. From this evidence and the specificity of cholera toxin binding, we conclude that GM1 ganglioside is abundantly present on the membrane surfaces of peripheral nodes of Ranvier and is not present on the internodal Schwann cell surfaces in an appreciable amount. The patterns of fluorescence within the node suggest that the axon and Schwann cell structures are sites where GM1 is localized. Treatment of the teased fibres with Vibrio cholerae neuraminidase, which is known to reduce polysialogangliosides to the monosialoganglioside GM1, induced cholera toxin binding on the internodal Schwann cell surfaces. The induced receptors, as well as their precursors, were resistant to trypsin and proteinase K. We conclude that the internodal Schwann cell surface is rich in an unidentified polysialoganglioside(s) that can be converted to GM1 by neuraminidase.

Cultured neonatal rat oligodendrocytes elaborate myelin membrane in the absence of neurons

We have utilized transmission electron microscopy to study oligodendrocyte-enriched cell cultures established from dissociated neonatal rat cerebra by the method of McCarthy and de Vellis [1980]. Cells were examined after 14 and 26 days in vitro. The overall morphology of the cells from cultures at both time periods was similar and consistent with previous reports of light (immature) oligodendrocyte fine structure. The cells contained an eccentrically located nucleus, prominent Golgi regions, numerous free ribosomes, and microtubules. Large numbers of processes with varying diameter were also observed. There was some indication of cytoplasmic maturation from the younger to the older cultures. The most important feature of the 26-day cultures was the large quantity of intercellular membranes which were shown to be continuous with oligodendrocyte processes. These membranes often exhibited the appearance of "loose myelin" and were therefore not normally compacted. Layers of membrane with the morphologic appearance of compact myelin were observed on an occasional oligodendrocyte perikaryon or process. This finding necessitates a reevaluation of the widely held theory that oligodendrocytes are not able to elaborate myelin in the absence of neurons.

Biochemical demonstration of the myelin-associated glycoprotein in the peripheral nervous system

Recent immunocytochemical studies indicated that the myelin-associated glycoprotein (MAG) is localized in the periaxonal region of central nervous system (CNS) and peripheral nervous system (PNS) myelin sheaths but previous biochemical studies had not demonstrated the presence of MAG in peripheral nerve. The glycoproteins in rat sciatic nerves were heavily labeled by injection of [3H]fucose in order to re-examine whether MAG could be detected chemically in peripheral nerve. Myelin and a myelin-related fraction, W1, were isolated from the nerves. Labeled glycoproteins in the PNS fractions were extracted by the lithium diiodosalicylate (LIS)-phenol procedure, and the extracts were treated with antiserum prepared to CNS MAG in a double antibody precipitation. This resulted in the immune precipitation of a single [3H]fucose-labeled glycoprotein with electrophoretic mobility very similar to that of [14C]fucose-labeled MAG from rat brain. A sensitive peptide mapping procedure involving iodination with Bolton-Hunter reagent and autoradiography was used to compare the peptide maps generated by limited proteolysis from this PNS component and CNS MAG. The peptide maps produced by three distinct proteases were virtually identical for the two glycoproteins, showing that the PNS glycoprotein is MAG. The MAG in the PNS myelin and W1 fractions was also demonstrated by Coomassie blue and periodic acid-Schiff staining of gels on which the whole LIS-phenol extracts were electrophoresed, and densitometric scanning of the gels indicated that both fractions contained substantially less MAG than purified rat brain myelin. The presence of MAG in the periaxonal region of both peripheral and central myelin sheaths is consistent with a similar involvement of this glycoprotein in axon-sheath cell interactions in the PNS and CNS.

Antiserum induced myelination inhibition in vitro without complement

Exposure of neonatal rat cerebellum cultures to antiserum to whole spinal cord or galactocerebroside inhibited myelin formation regardless of whether guinea pig serum was added fresh or after heating to 56 degrees C for 1 h in order to achieve complete removal of hemolytic complement activity. Myelination followed removal of antisera from the culture media. This suggests that the inhibition of primary myelination by anti-CNS tissue antiserum occurs through some mechanism other than as the result of a cytotoxic reaction against oligodendrocytes mediated via the complement system.

Polyaxonal myelination in developing dystrophic and normal mouse nerves

Myelin-forming Schwann cells in the peripheral nervous system characteristically surround and myelinate only single axons. Polyaxonal myelination is an anomaly of this one-to-one relationship whereby one normal-appearing Schwann cell myelinates multiple axons. We examined the ventral roots and the proximal sciatic and posterior tibial nerves of developing normal mice and of dy2J/dy2J dystrophic mice with proximal failure of myelination. Polyaxonal myelination was a rare feature in normal nerves. Examples of polyaxonal myelination were observed six times more often in dystrophic than in normal mice and were most abundant in proximal sciatic nerves. Polyaxonal myelination could result from either an axonal or a Schwann-cell abnormality, or it may be the nonspecific response of uncommitted Schwann cells to an early failure of myelination.

Disorganization of myelinogenesis in tissue culture by anti-CNS antiserum

The presence of decomplemented anti-CNS antiserum profoundly affects myelinogenesis in cultured mouse embryo spinal cord. Light and electron microscope study has shown that oligodendroglia differentiate and produce an abundance of cell processes which surround the oligodendrocytes in a chaotic, disorganized array. Where the cell processes chance to meet, they form a kind of aberrant swollen myelin. Rarely, the oligodendroglial processes ensheath axons. For the most part, the available axons remain unmyelinated. On removal of the decomplemented antiserum, oligodendroglia differentiate and form normal myelin around the available axons. Myelination of peripheral nervous system (dorsal root ganglion) axons in the same preparations is unaffected by the presence of the antiserum. Thus, under these circumstances, the message from the neuron to the oligodendrocyte to make myelin is apparently intact, yet there is interference with the ability of the oligodendroglial cell process to find, attach to and encircle CNS axons with a normal myelin sheath.

The Schwann cell: a reappraisal of its role in the peripheral nervous system

The Schwann cell is clearly essential for the maintenance of axonal integrity--yet we know little of the regulatory mechanisms governing its behaviour at any point in its life cycle, or of the nature of its interaction with the axons with which each Schwann cell is associated. In this article, the involvement of the Schwann cell in myelinogenesis, aspects of Schwann cell-axon recognition, the experimentally-demonstrable 'bipotentiality' of the Schwann cell and the possible functional significance of the proliferative response of the Schwann cell that occurs after injury are discussed. The isolation and preparation of pure populations of Schwann cells which can be injected or implanted into a damaged nerve, coupled possibly with the localized application of drugs to manipulate the cellular responses to injury within the nerve, represent interesting areas of recent research which may be applied in planning methods of therapeutic intervention in the treatment of peripheral nerve injury.

Heritable catabolic and anabolic disorders of lipid metabolism

The principal manifestations and metabolic defects in ten heritable disorders of lipid metabolism are discussed. Facile procedures have been developed for the diagnosis of patients with these conditions, the identification of heterozygous carriers, and the prenatal detection of any of these diseases. Enzyme replacement appears promising for patients with Fabry's disease and Gaucher's disease who do not have central nervous system damage. The clinical and biochemical abnormalities that occur in patients with a novel inherited disorder of ganglioside anabolism are described.

Glycoproteins are modified in the axon of R2, the giant neuron of Aplysia californica, after intra-axonal injection of [3H]N-acetylgalactosamine

We have found evidence that mechanisms exist in the axon by which proteins, originally synthesized in the cell body, can be modified. Incorporation of [3H]N-acetyl-D-galactosamine into macromolecules was studied in the axon of R2, the giant identified neuron of the abdominal ganglion of Aplysia. The precursor sugar, injected directly into the major axon in the right connective, labeled both glycoproteins and glycolipids. These macromolecules were associated with membranes and at least 90% of the incorporated radioactivity could be sedimented by centrifugation at 105,000 X g. Radiocutography of injected axons with the light microscope showed that most of the silver grains were located over the axon rather than over other tissues in the right connective. Grains appeared over a variety of axonal components, but quantitative electron microscopic radioautography revealed that vesicles were the only organells significantly labeled. Disco polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) of the membrane fraction resolved [5-(3)H]glycoprotein components. These membrane glycoproteins ranged in apparent molecular weight from 20,000 to 160,000 daltons and could be digested by pronase. Only one component had a mobility similar to that of a glycoprotein found in the axon after injection of the cell body. Incorporation of [3H]N-acetylgalactosamine into axonal glycoprotein was unaffected by anisomycin, a potent inhibitor of protein synthesis in Aplysia, and therefore presumably occurred on already existing polypeptide chains. We conclude that these were synthesized in the cell body and exported into the axon where they were modified by the addition of sugar.

Biosynthesis and function of gangliosides

Gangliosides are unique acidic glycolipids that are selectively concentrated in the plasma membrane of cells. Surface labeling studies have demonstrated that at least a portion of the oligosaccharde chain of gangliosides extends beyond the hydrophe) is imbedded in the membrane bilayer. It is becoming increasingly apparent that gangliosides participate in the internalization of environmental signals elicited by cholera toxin and glycoprotein hormones such as thyrotropic hormone and chorionic gonadotropin as well as other substances such as interferon and possibly serotonin. The mechanism by which cholera toxin binds to a specific ganglioside receptor on the celraction of trophic agents with gangliosides. We would predict that analyogous phenomena involving gangliosides will be discovered in brain. The biosynthesis of gangliosides proceeds by the ordered sequential addition of sugars to the lipid moiety. These reactions are catalyzed by a cluster of membrane-bound glycosyltransferases. Any alteration in the activity or specificity of one of these enzymes will result in a dramatic change in the ganglioside pattern of an afflicted cell or organ. The drastic consequences that accompany abnormalities of ganglioside synthesis have been documented in a heritable metabolic disorder in vivo and in tumorigenic transformation of cells in vitro. In this article, we have attempted to unify these observations and to provide a reasonable interpretation of the role of gangliosides in mediating cell surface phenomena.

Peripheral nerve glycoproteins and myelin fine structure during development of rat sciatic nerve

Developmental changes in relative amounts of peripheral nerve proteins and glycoproteins have been correlated with the degree of morphological myelination at various ages during the first 25 postnatal days in rat sciatic nerve. At birth there is virtually no major myelin glycoprotein (P0), but there is a protein which migrates to the same point on sodium dodecyl sulphate (SDS) polyacrylamide gels as the small myelin basic protein (P2). During the time myelin is being formed in the nerve, the P0 protein increases and the P2 protein appears to decrease in relative amount in the nerve. The accumulation of P0 protein in the nerve correlates extremely well with the degree of myelination in sciatic nerve. At 4-6 days postnatal, smooth membrane profiles are observed which are located within axons and in the inner Schwann cell cytoplasm. Such profiles are also observed to fuse with the axolemma-Schwann cell interface. The profiles may represent membrane material being added to or deleted from the axolemma or myelin during myelination.

Studies on the control of myelinogenesis. II. Evidence for neuronal regulation of myelin production

Tritiated thymidine has been used as a nuclear marker to trace the origin of Schwann cells, sited in the distal stump of a severed unmyelinated nerve, which are able to elaborate myelin around axons regenerating from an anastomosed proximal stump of a severed myelinated nerve. Two types of cross-anastomosis experiments were performed in young, adult rats: (1) the proximal stump of a myelinated sternohyoid nerve was labeled (5 mCi/kg body weight) selectively over a 4-day period of predetermined maximal thymidine uptake and two days later, after flushing the animal repeatedly with cold thymidine, the unmyelinated cervical sympathetic trunk was transected and its unlabeled distal stump linked to the proximal stump of the labeled sternohyoid nerve; (2) the distal stump of an unmyelinated cervical sympathetic trunk was labeled selectively over a 5-day period of predetermined maximal uptake and two days later, after flushing with cold thymidine, the myelinated sternohyoid nerve was severed and its unlabeled proximal stump linked to the labeled distal stump of the cervical sympathetic trunk. The fate of the labeled cells in each type of anastomosis was determined 3 weeks later by autoradiography and liquid scintillation spectrometry. In the first type, a small amount of label had migrated from proximal stumps but labeled Schwann cells were not found in successfully anastomosed distal stumps. In the second type, labeled Schwann cells were seen in the cervical sympathetic trunk in association with myelinated and non-myelinated axons regenerating from the sternohyoid nerve. These data suggest that the presence or absence of myelin formation by a Schwann cell is controlled by some property of the axon with which it is associated. Putative mechanisms underlying neuronal control of myelinogenesis are discussed.

Relation of axon membrane to myelin membrane in sciatic nerve during development: comparison of morphological and chemical parameters

The relationship of axon circumference to myelin area on cross-section both for myelinated and total fibres has been estimated at a mid-point in the sciatic nerve of two rabbits, one newborn and the other 20 weeks old. The slopes of the regression lines relating myelin area to axon circumference for myelinated fibres were 0.72 (standard error 0.03) with a correlation coefficient of 0.81 in the newborn nerve and 2.96 (standard error 0.05) with correlation coefficient of 0.96 in the 20-week nerve. Using the above parameters, and assuming constant thickness of axolemma and constant packing density of myelin, a correlation was made of the change in relationship of axolemma volume of myelinated fibres to myelin volume in the nerve between the two ages with the changes in content of two membrane lipids, ganglioside and cerebroside. This revealed a constant relationship of morphological to chemical parameters at the two ages although the proportion of unmyelinated fibres was much greater in the newborn. If cerebroside is regarded as a marker for myelin membrane, this observation suggests an association of gangliosides with myelination in peripheral nerve.

Multipotentiality of Schwann cells in cross-anastomosed and grafted myelinated and unmyelinated nerves: quantitative microscopy and radioautography

Cross-anastomoses and autogenous grafts of unmyelinated and myelinated nerves were examined by electron microscopy and radioautography to determine if Schwann cells are multipotential with regard to their capacity to produce myelin or to assume the configuration seen in unmyelinated fibres. Two groups of adult white mice were studied. (A) In one group, the myelinated phrenic nerve and the unmyelinated cervical sympathetic trunk (CST) were cross-anastomosed in the neck. From 2 to 6 months after anastomosis, previously unmyelinated distal stumps contained many myelinated fibres while phrenic nerves joined to proximal CSTs became largely unmyelinated. Radioautography of distal stumps indicated that proliferation of Schwann cells occurred mainly in the first few days after anastomosis but was also present to a similar extent in isolated stumps. (B) In other mice, CSTs were grafted to the myelinated sural nerves in the leg. One month later, the unmyelinated CSTs became myelinated and there was no radioautographic indication of Schwann cell migration from the sural nerve stump to the CST grafts. Thus, Schwann cell proliferation in distal stumps is an early local response independent of axonal influence. At later stages, axons from the proximal stumps cause indigenous Schwann cells in distal stumps from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously myelinated nerves become associated with unmyelinated fibres. Consequently, the regenerated distal nerve resembled the proximal stump. It is suggested that this change is possible because Schwann cells which divide after nerve injury reacquire the developmental multipotentiality which permits them to respond to aoxonal influences.

Studies on the control of myelinogenesis. I. Myelination of regenerating axons after entry into a foreign unmyelinated nerve

The proximal stump of a predominantly myelinated nerve (sternohyoid) was anastomosed in a tube to the distal stump of a largely unmyelinated nerve (cervical sympathetic) in order to explore the role of the axon in activating Schwann cells to produce myelin. The two nerves were examined after being united for periods between 3 and 15 weeks. At these times, regenerating myelinated and nonmyelinated fibres, originating from the proximal stump, were seen within the original fascicle in the distal stump of the sympathetic trunk. There was a significant (P less than 0.01) increase in the mean number of myelinated fibres in the anastomosed sympathetic nerves from the mean number of myelinated fibres in the normal sympathetic nerve. The histological and ultrastructural features of the anastomosed nerves are described and the differences between the morphology of the intratubal and extratubal regions are highlighted. The results of this study may indicate that the axon instructs the Schwann cell to produce myelin, but before this conclusion is reached, the origin of the myelinating cells has to be determined.