Myelin sheath survival after guanethidine-induced axonal degeneration

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.

Neurogenic inflammation and tooth pulp innervation pattern in sympathectomized rats

This study investigated possible collateral C-fiber innervation between the pulps of rat molars by assessing neurogenic inflammation (NI) induced by the C-fiber excitant mustard oil (MO). MO was applied to the pulp of the left mandibular first molar in two groups of rats: group 1, guanethidine sympathectomized rats (to dismiss sympathetic activation by MO); and group 2, unsympathectomized rats. A third group of unsympathectomized rats (group 3) had saline applied to the pulp of the left mandibular molar and served as a MO control. The NI-related plasma extravasation was examined in these teeth and in the remaining left mandibular teeth by a spectrophotometric analysis of extravasated plasma protein bound to Evans' Blue (EB) dye. The collateral innervation pattern was inferred from the NI pattern. EB concentrations were measured in the left mandibular teeth and the corresponding contralateral teeth, and expressed as a ratio. Statistical analysis of the data revealed significant differences in EB ratios in the first, second, and third molars between groups 1 and 3. This result suggests collateral C-fiber innervation exists within the pulps of molar teeth in the same dental quadrant. No difference in EB ratios was noted in the first and second molars between groups 1 and 2. Therefore, sympathetic efferents have no apparent effect on the degree of MO-induced NI.

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.

Morphometric analysis of axons myelinated during adult life in the mouse superior cervical ganglion

In experimental studies addressing the regulation of myelin formation and maintenance by Schwann cells, the sympathetic nervous system of young adult rodents has served a key role as an essentially nonmyelinated yet modifiable control tissue. Nevertheless there is clear evidence of substantial myelination in the superior cervical ganglion (SCG) of normal mice and rats of more advanced age. Against this background, interpretation of experimental outcomes in particular sympathetic tissues will require detailed quantitative control data taking account of animal age. To provide a baseline for future investigations on myelin remodelling, an ultrastructural morphometric study of myelinated fibres in the SCG was undertaken in 4 strains (QS, Balb/C, C57 and CBA) of adult male mice aged 32-72 wk. Numbers of myelinated fibres in SCG cross-sections varied substantially between individual animals, and the mean numbers for QS (132), Balb/C (165) and CBA (254) were significantly higher than that for C57 (32). Both axonal and fibre diameter were distributed unimodally (means for the 4 strains ranged from 2.3-2.4 microns and 3.2-3.6 microns respectively). Myelin spiral length was distributed unimodally and skewed to the right (range of means = 227-357 microns) and was significantly greater in QS mice as compared with the other 3 strains. While the mean g ratio (axonal diameter/fibre diameter) was significantly lower in QS mice than in the other 3 strains, the range for mean g ratio was 0.64-0.73, indicating that myelination had proceeded appropriately even though late in onset in this tissue. The index of circularity was high in all strains, both for axons (range of means = 0.80-0.88) and fibres (range of means = 0.84-0.89). The small axonal and fibre diameter and unimodal distribution are consistent with the characteristics of autonomic myelinated fibres and it is probable that most are postganglionic sympathetic fibres arising within the SCG. In terms of providing a sufficient population of myelinated fibres for future experimental studies, the QS, Balb/C and CBA strains would be preferable to C57 mice.