[The Schmidt-Lantermann incisures of the myelin sheath of Mauthner axons: site of longitudinal myelin growth]

Phylogeny of proteolipid proteins: divergence, constraints, and the evolution of novel functions in myelination and neuroprotection

The protein composition of myelin in the central nervous system (CNS) has changed at the evolutionary transition from fish to tetrapods, when a lipid-associated transmembrane-tetraspan (proteolipid protein, PLP) replaced an adhesion protein of the immunoglobulin superfamily (P0) as the most abundant constituent. Here, we review major steps of proteolipid evolution. Three paralog proteolipids (PLP/DM20/DMalpha, M6B/DMgamma and the neuronal glycoprotein M6A/DMbeta) exist in vertebrates from cartilaginous fish to mammals, and one (M6/CG7540) can be traced in invertebrate bilaterians including the planktonic copepod Calanus finmarchicus that possess a functional myelin equivalent. In fish, DMalpha and DMgamma are coexpressed in oligodendrocytes but are not major myelin components. PLP emerged at the root of tetrapods by the acquisition of an enlarged cytoplasmic loop in the evolutionary older DMalpha/DM20. Transgenic experiments in mice suggest that this loop enhances the incorporation of PLP into myelin. The evolutionary recruitment of PLP as the major myelin protein provided oligodendrocytes with the competence to support long-term axonal integrity. We suggest that the molecular shift from P0 to PLP also correlates with the concentration of adhesive forces at the radial component, and that the new balance between membrane adhesion and dynamics was favorable for CNS myelination.

Effects of nucleus pulposus on nerve root neural activity, mechanosensitivity, axonal morphology, and sodium channel expression

STUDY DESIGN

This study analyzed the effects of autografted nucleus pulposus on nerve root axon morphology, neurophysiologic function, and sodium channel expression.

OBJECTIVES

To investigate the chronic effects of the epidural implantation of nucleus pulposus on nerve root morphology, neural activity, ectopic discharge, mechanosensitivity, and sodium channel expression.

SUMMARY OF BACKGROUND DATA

It has been reported that ectopic discharges were recorded antidromically from sural nerve on compressing nucleus pulposus exposed spinal nerves. However, it is not clear what the effects of nucleus pulposus are on ectopic discharges recorded directly from the spinal nerve roots. It is also not clear what the effects of nucleus pulposus are on the threshold pressure to provoke ectopic discharges in the spinal nerves. Sodium channel content increases in remodeling axons after nerve injury, but it is not clear what the effects of nucleus pulposus are on sodium channel expression in spinal nerve.

METHODS

Forty-six male Sprague-Dawley rats were used, 20 in a nucleus pulposus-implanted group, 18 in a fat-implanted group, and 8 in a normal group. Fresh autografted nucleus pulposus or fat tissue was implanted into the dorsal epidural space at the L4-L5 disc level. On the 7th, 21st, or 42nd day, neurophysiologic recordings were made to determine nerve root response to compression. Nerve roots were then harvested to determine sodium channel protein concentration and histologic changes in the nerve root. The correlations between sodium channel density and neural activity and mechanosensitivity of dorsal root were analyzed statistically.

RESULTS

Ectopic discharge rate was higher in nucleus pulposus 7-day group. Threshold pressure to evoke ectopic discharges was lower in the nucleus pulposus 7-day group, and higher in the nucleus pulposus 42-day group compared to the normal group. Sodium channel protein density increased in the nucleus pulposus 7-day and nucleus pulposus 21-day group compared to normal nerve. Sodium channel density changes were not correlated to threshold pressure. Ectopic discharge rate increased with increase of sodium channel density in the nerve roots. The number of axons with neuropathy increased in the nucleus pulposus 7-day and 21-day groups.

CONCLUSIONS

Acute exposure of nerve root to nucleus pulposus resulted in increased number of axons with neuropathy, higher intensity of ectopic discharges on compression, and nerve mechanosensitization. Chronic exposure resulted in mechanical desensitization. Changes of sodium channel density were correlated to ectopic discharge rate.

Peripheral nerve elongation by laser Doppler flowmetry controlled expansion: morphological aspects

Peripheral nerve elongation by a tissue expander may offer an alternative to nerve grafting in the management of segmental nerve loss. We investigated the morphological changes in peripheral nerve following slow nerve elongation by laser Doppler flowmetry controlled expansion in a rabbit sciatic nerve model. The animals were randomly assigned to one of four groups, with an expander volume of 0, 5, 10 or 15 cm3, respectively. An elongation of up to 40% was possible with preservation of clinical function. Nerve conduction velocity decreased in relation to elongation. Paranodal widening, followed by remyelination of the node, were early and constant morphological features. Demyelination and remyelination of whole internodes, and axonal degeneration occurred sporadically and did not correlate with elongation, rate of elongation or neurophysiological parameters. The model of laser Doppler flowmetry controlled nerve expansion provides a method for remodelling of myelin sheaths and lengthening of nerve fibers without axonal damage.

Fine structural evaluation of altered Schmidt-Lanterman incisures in human sural nerve biopsies

Fine structural alterations of Schmidt-Lanterman incisures (SLI) were investigated in a series of 242 unselected sural nerve biopsies that had been examined for diagnostic purposes. The series included cases with Friedreich's ataxia, HSAN I, HMSN I-III, HMSN VI, tomaculous neuropathy, metachromatic leukodystrophy, ceroidlipofuscinosis, dysproteinemic neuropathies, and myotonic dystrophy, in addition to several neuropathies less-specifically classified as either of a predominantly demyelinating, axonal, or neuronal type. The following classification of SLI alterations is proposed: (A) abnormal inclusions; (B) changes in shape and dimension; and (C) modes of disintegration. Abnormal inclusions comprised membranous whorls, uniform and pleomorphous lysosome-like bodies, and accumulation of granular substances at the site of the major dense line, or granular deposits at the site of the intraperiod line of the myelin sheath. Variations of incisural shape and dimension included folding, dilatation, and pocket formation (compartmentalization). Disintegration at incisures comprised a fine, vesicular and a gross, vacuolar type. Various combinations of these changes were observed. The most frequent change consisted of membranous whorls, detected in SLI of 89 biopsies. They were most prominent in chloroquine neuropathy where they occurred in SLI as well as in the adaxonal and abaxonal cytoplasm of Schwann cells. Compartmentalization of the myelin sheath at incisures associated with formation of myelin loops was a frequent feature in myotonic dystrophy. It is concluded, that changes of incisural ultrastructure are sensitive indicators of human neuropathies offering clues to the type of the underlying pathomechanism.

The lateral giant fibers of the tubificid worm, Branchiura sowerbyi: structural and functional asymmetry in a paired interneuronal system

Neuroanatomical and ultrastructural studies of the paired lateral giant nerve fibers (LGFs) in posterior segments of the tubificid worm, Branchiura sowerbyi, demonstrate that the fibers are 1) segmental in origin (two cell bodies per segment), 2) joined longitudinally and transversely to form an intersegmental syncytial network, and 3) surrounded by a myelinlike sheath. The LGFs are unique among paired giant fiber systems because of their extreme asymmetry, the diameter of the left fiber being several times greater than that of the right. Electrophysiological studies demonstrate that the small, right fiber has a high input resistance and, during mechanosensory stimulation, functions as the locus for LGF spike initiation. The larger, left fiber contributes by enhancing the speed of LGF spike conduction along the animal. One physiological benefit of this asymmetric arrangement may be optimization of escape reflex sensitivity to mechanosensory inputs.

Measurement of myelin sheath resistances: implications for axonal conduction and pathophysiology

As commonly understood, the myelin sheath of axons insulates the internodal axolemma and essentially restricts transmembrane currents to nodal regions. However, recordings obtained from within the myelin sheath showed that its apparent resistance to current generated by action potentials is similar in magnitude to that of the internodal axolemma. This suggests that the sheath does not appreciably limit transmembrane current flow, presumably because there is a longitudinal shunt under the myelin and through the paranodal region. Thus, in some demyelinating diseases and other axonopathies, the safety factor for impulse conduction may be lowered by a loosening or a reduction in the number of paranodal axoglial junctions.

The structural correlate of saltatory conduction along the Mauthner axon in the tench (Tinca tinca L.): identification of nodal equivalents at the axon collaterals

The spiny collaterals of the Mauthner axon were reinvestigated in the tench (Tinca tinca L.) with the electron microscope and special staining procedures. These collaterals, as demonstrated by intraaxonal labelling with lucifer yellow, are more or less regularly spaced (100-300 micrometers) and make synaptic contacts with processes of spinal motoneurons and interneurons. The unmyelinated tips of the collaterals are further characterized by the following structural features: (1) an electron-dense undercoating of the axolemma, (2) a positive Prussian blue reaction of the inner surface of the axolemma following ferric ion-ferrocyanide staining (Waxman and Quick, '78a), (3) expanded extracellular spaces which react specifically to inorganic phosphate, metallic ions, and diaminobenzidine. All these properties are known to be shared by the axolemma of central and peripheral nodes of Ranvier. Previous studies from this laboratory have shown that the nerve impulse is propagated along the Mauthner axon in a saltatory mode. Since classical nodal gaps could not be identified within the myelin sheath of this giant fiber, it is concluded on the basis of the present findings that the unmyelinated tips of the spiny collaterals represent nodal equivalents, and thus provide the morphological substrate for the saltatory propagation of the nerve impulse along the Mauthner axon. The typical latency steps, as demonstrated in the latency plot of the longitudinal current signals (Greeff and Yasargil, '80), and the distances between the identified membrane specializations at the axon collaterals are consistent with this conclusion.

Mauthner axon diameter and impulse conduction velocity decrease with growth of goldfish

Conduction velocities of antidromically evoked impulses along the goldfish Mauthner axon were found to be inversely correlated with body length. To test the hypothesis that such a relation is accompanied by a reduction in axonal diameter with increasing fish size, Mauthner axon diameters were measured. A parabolic relationship with respect to body length was obtained, axonal diameter being maximal in 9.5 cm fish. To our knowledge, this is the first report of a decrease in conduction velocity and axonal size during growth of an organism were functioning of the cell is maintained.

Experimental evidence for saltatory propagation of the Mauthner axon impulse in the tench spinal cord

External longitudinal current recording was applied in situ to the exposed spinal cord of the tench (Tinca tinca) for the study of impulse propagation in the Mauthner axon, a giant nerve fibre whose myelin sheath is not interrupted by nodes of Ranvier. Impulses in the antidromically excited Mauthner axon were recorded from the dorsal surface of the spinal cord; the time-lag between the main peaks of the bipolarly recorded current signal and unipolarly recorded reference signal was measured at regular intervals along the cord. Using a slot width of the bipolar electrodes, d = 0.65 mm and electrode displacement s = 0.5 mm, the latency plotted as a function of distance showed small fluctuations, but no clear-cut steps over a 12.5 mm stretch of spinal cord. However, with improved spatial resolution (d = 0.24 mm, s = 0.1 mm) and electrical insulation of the spinal cord from the underlying tissues, it was possible to demonstrate steps in the latency plot occurring at 0.5--0.3 mm intervals and indicating a saltatory propagation of the Mauthner axon impulse. The distances between the latency steps and their distribution was comparable to the known distribution of the Mauthner axon collaterals suggesting that the myelin-free regions of the collaterals may be quivalents of Ranvier nodes.

Schmidt-Lanterman incisures. II. A light and electron microscope study of remyelinating peripheral nerve fibres

A mechanism has been proposed previously for the formation of Schmidt-Lanterman incisures during ontogeny as myelinated nerve fibres increase in diameter. In this study, the mode of formation of incisures and their development were examined by light and electron microscopy in remyelinating fibres (16-200 d) of the sural nerve of rats following lysophosphatidyl choline (LPC)-induced segmental demyelination. In remyelination, a basic number of incisures is incorporated into the myelin sheath ab initio. Apparently these incisures which are at first circumferentially incomplete, being localised to the region of the mesaxons, progressively extend around the whole circumference of the myelin sheath. As fibre diameter increases, additional incisures are added, seemingly, by the intrusion of Schwann cell cytoplasm between the compact myelin lamellae, from the abaxonal side of the myelin sheath. The progressive differentiation of incisures is described in terms of their complement of organelles. The results are discussed with reference to the possible functions of incisures.

Ribosomal RNA in Mauthner axon: implications for a protein synthesizing machinery in the myelinated axon

RNA was extracted from myelin-free Mauthner axons of the goldfish on a microscale and fractionated by microelectrophoresis. Microextracts showed the presence of nominal 26 SE, 18 SE, 5 SE and 4 SE components, which co-migrated with rRNA from fish brain. In addition, a non-ribosomal 15 SE component was present in axon microextracts, but not in RNA extracts of fish brain or of myelin sheath from Mauthner axon, indicating an unusual enrichment of a putative mRNA class. Evidence was presented to support the contention that axonal rRNA was not due to contamination from the myelin sheath. Possible reasons for the lack of ultrastructural evidence for axoplasmic ribosomes are discussed.

The fine structure of myelinated nerve cell bodies in the bulbus olfactorius of man

In the bulbus olfactorius of man numerous myelinated nerve cell bodies occur in the stratum plexiforme internum and stratum granulosum internum. In many respects they resemble the neighbouring granule cells: small chromatin clumps border on more than half of the circumference of the nucleus, the thin cytoplasmic rim contains abundant polysomes and sometimes pigment complexes with numerous light vacuoles, the cells often show a process which extends up to the stratum glomerulosum, the perikarya are devoid of synaptic contacts whereas the proximal segment of the peripheral processes display rare contacts. The myelin sheath varies in thickness consisting of 2 to 24 lamellae with distances between the major dense lines ranging from 9.3 to 11.3 nm. The myelin sheath may enclose the cell body completely or partially and accompany the proximal segment of the process arising from the perikaryon. On partially enveloped perikarya, the myelin lamellae end in formations like those of the node of Ranvier, though often less regularly. Within the compact myelin sheath all of its lamellae may be distended for a short distance by glial cytoplasm as in the Schmidt-Lanterman incisures of peripheral nerve fibres. Adjacent to the outermost myelin lamella myelinated axons and cell bodies, tentatively identified as oligodendrocytes, as well as granule cells may be closely joined.