Polyaxonal myelination in developing dystrophic and normal mouse nerves

How Schwann Cells Sort Axons: New Concepts

Peripheral nerves contain large myelinated and small unmyelinated (Remak) fibers that perform different functions. The choice to myelinate or not is dictated to Schwann cells by the axon itself, based on the amount of neuregulin I-type III exposed on its membrane. Peripheral axons are more important in determining the final myelination fate than central axons, and the implications for this difference in Schwann cells and oligodendrocytes are discussed. Interestingly, this choice is reversible during pathology, accounting for the remarkable plasticity of Schwann cells, and contributing to the regenerative potential of the peripheral nervous system. Radial sorting is the process by which Schwann cells choose larger axons to myelinate during development. This crucial morphogenetic step is a prerequisite for myelination and for differentiation of Remak fibers, and is arrested in human diseases due to mutations in genes coding for extracellular matrix and linkage molecules. In this review we will summarize progresses made in the last years by a flurry of reverse genetic experiments in mice and fish. This work revealed novel molecules that control radial sorting, and contributed unexpected ideas to our understanding of the cellular and molecular mechanisms that control radial sorting of axons.

Glia unglued: how signals from the extracellular matrix regulate the development of myelinating glia

The health and function of the nervous system relies on glial cells that ensheath neuronal axons with a specialized plasma membrane termed myelin. The molecular mechanisms by which glial cells target and enwrap axons with myelin are only beginning to be elucidated, yet several studies have implicated extracellular matrix proteins and their receptors as being important extrinsic regulators. This review provides an overview of the extracellular matrix proteins and their receptors that regulate multiple steps in the cellular development of Schwann cells and oligodendrocytes, the myelinating glia of the PNS and CNS, respectively, as well as in the construction and maintenance of the myelin sheath itself. The first part describes the relevant cellular events that are influenced by particular extracellular matrix proteins and receptors, including laminins, collagens, integrins, and dystroglycan. The second part describes the signaling pathways and effector molecules that have been demonstrated to be downstream of Schwann cell and oligodendroglial extracellular matrix receptors, including FAK, small Rho GTPases, ILK, and the PI3K/Akt pathway, and the roles that have been ascribed to these signaling mediators. Throughout, we emphasize the concept of extracellular matrix proteins as environmental sensors that act to integrate, or match, cellular responses, in particular to those downstream of growth factors, to appropriate matrix attachment.

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.

Myelination-dependent axonal membrane specializations demonstrated in insufficiently myelinated nerves of the dystrophic mouse

"Dystrophic' mice of the 129/ReJ-Dy strain have a genetic defect affecting Schwann cell proliferation. Spinal nerve roots of these animals contain myelinated and unmyelinated axons in addition to groups of large "amyelinated' axons. In affected regions of the spinal roots, myelinated axons are missing their myelin sheaths. Where the myelination terminates or begins, half-nodes are created. Freeze-fracture analysis of these half-nodes shows that only the myelinated side contains rows of dimeric particles in the axonal P-face of the paranode. The P-face on the amyelinated side of a half-node, and the remainder of the amyelinated axon. contains a dense even distribution of particles, many of which are the size of dimeric-particle subunits, but only a few of which are arranged into short rows. As the long circumferential rows are not found on the unmyelinated side of the myelinated side of the half-node we conclude that the paranodal rows of dimeric particles are dependent upon myelination for their organization.

Tellurium-induced myelinopathy in adult rats

Tellurium is known to induce a neuropathy in young rats but not in adult rats. In the present study adult rats were fed a diet containing 1.25% tellurium every day for 30 days. By the end of this period, the rats moved with difficulty but did not show paralysis. Study of teased nerve fiber preparations in conjunction with light and electron microscopy revealed myelin abnormalities consisting of myelin bubbling, segmental demyelination, and remyelination. In some animals, these abnormalities had a patchy distribution, affecting several adjoining nerve fibers in sciatic nerves, brachial plexus, and spinal roots. The intramyelinic edema sometimes was associated with splitting of the axons.

Antiserum-mediated demyelination: relationship between remyelination and functional recovery

A focal demyelinative lesion of peripheral nerve was produced by intraneural injection of either antiserum from rabbits with experimental allergic neuritis or experimental allergic encephalomyelitis or antiserum to galactocerebroside. We studied the relationship between clinical and electrophysiological recovery from this lesion and the morphological pattern of remyelination. Foot muscles on the the injected side weakened within an hour of injection and remained paralyzed for 7 days; strength gradually returned to normal by 16 days after injection. Electrophysiological conduction block, apparent within a few hours of injection, persisted for about 7 days. At 8 days we detected dispersed, very low amplitude muscle action potentials with long latency. Morphologically, demyelinated axons were surrounded by Schwann cells at 7 days after injection, but compacted myelin was not present. After 8 days, remyelinating axons became surrounded by thickening compacted myelin. The time of onset of remyelination and the rate of remyelination up to 14 days following the injection were independent of axon size. The onset of clinical and electrophysiological recovery from the lesion corresponded to the appearance of 2 to 8 myelin lamellae around each remyelinating axon. At 37 days after injection, when conduction velocities had returned to preinjection values, myelin thickness of remyelinating fibers had increased to approximately one-third that of control nerves.