A new approach toward analyzing peripheral nerve fiber populations. I. Variance in sheath thickness corresponds to different geometric proportions of the internodes.

The thickness of the myelin sheath is known to increase with axon caliber, but there is also a superimposed, slight variation in sheath thickness depending on whether a fiber of a given caliber has very long or very short internodes. This relationship between myelin sheath thickness and the geometric proportion of the internode has been shown in subserial sections of isolated nerve fibers. It allows a prediction of sheath thickness from the quotient internode length/axon caliber, or conversely, a prediction of internode foreshortening from sheath thickness. We applied this new approach to the analysis of sciatic fiber populations of frogs, mice, rats and cats. The geometric proportions of these fibers were defined by the quotient internode length/fiber caliber. This quotient was compared with minor variation in sheath thickness as determined with a computer-assisted technique measuring large numbers of fibers in low-power electron micrographs. The method also calculated fiber shrinkage and recalculated all data for circular fiber profiles. The data obtained confirmed previous electron microscopic measurements showing that there is a slight reduction in sheath thickness when a fiber of a given caliber has relatively short internodes, and vice versa. A population of very thin, thinly myelinated fibers was also revealed. Sheath thickness and the geometric proportions of internodes in frogs differed markedly from those in mammals.

A new approach toward analyzing peripheral nerve fiber populations. II. Foreshortening of regenerated internodes corresponds to reduced sheath thickness.

The new approach used in this study is based on the concept that axon caliber is not the only factor affecting the thickness of the myelin sheath. It is necessary to consider the entire geometric proportions of the internode, since sheath thickness corresponds to the relationship between axon caliber and the length of the internode. This type of analysis was applied to the regenerated internodes in rat sciatic nerves. Survival periods of 4, 9, 18 and 36 weeks were studied after lesions had been placed in young adult rats. The data show significantly thinner sheaths for regenerated fibers as compared with normal nerves, consistent with previous observations. This reduction in sheath thickness, however, corresponded quantitatively to the degree of foreshortening of internodes in the regenerated nerves. An average reduction of 10 in the quotient internode length/fiber caliber corresponded to a reduction of about 0.015 in the relative thickness of the sheath (quotient axon diameter/fiber diameter). This means that regenerated myelin sheaths are not truly hypoplastic; rather, they are adapted to the reduced internode length, and have the same relationship found for normal fibers. In partially damaged nerves there was a clear distinction in terms of sheath thickness between regenerated fibers and undamaged fibers. Demonstration of this phenomenon by scatter diagrams opens new possibilities for the quantitative assessment of neuropathies.

Myelination of the human vagus nerve from 24 weeks postconceptional age to adolescence.

Significant changes in respiratory reflexes occur with maturation. The vagus nerve, the pathway for the Hering-Breuer and irritant-receptor reflexes, was studied quantitatively in 33 infants and 5 adolescents. In the infants, total myelinated vagus fibers increased linearly (r m0.682, p less than 0.001) with postconceptional age (PCA), and by 40 weeks after conception, total counts were comparable to those of adolescent group. Counts of total myelinated vagus fibers in 16 term infants (greater than 41 weeks PCA) were comparable to those in the adolescent group (p less than 0.40), whereas 17 preterm infants (less than 38 weeks PCA) showed significantly fewer total myelinated vagus fibers than term or adolescent groups (p less than 0.001). Smaller-diameter (less than 2 micrometer) myelinated vagus fibers depended upon PCA in the preterm group (p less than 0.005), but were independent of PCA in the term group (p less than 0.5). Preterm infants have a higher percentage of small to total myelinated vagus fibers than term infants (p less than 0.1).

Sympathetic postganglionic unmyelinated axons in the rat peripheral nervous system.

We determined the contribution made to the unmyelinated axon population of the rat peripheral nervous system by sympathetic paravertebral ganglion cells. Sympathectomy, achieved by administration of guanethidine to neonatal rats, led to atrophy of the sympathetic paravertebral ganglion chain, a 95% decrease in peripheral nerve norepinephrine, and loss of 20 to 26% of the unmyelinated axons in a cutaneous nerve (sural), a muscular nerve (nerve to soleus), and a mixed nerve (sciatic). These data indicate that up to a quarter of the total population of peripheral nerve unmyelinated axons are sympathetic ganglia-derived.

Unmyelinated axon subpopulations in the rat peripheral nervous system.

Using the neurotoxin capsaicin, we examined subpopulations of unmyelinated axons in mixed (sciatic), cutaneous (sural), and muscular (nerve to soleus) nerves. Administration of capsaicin to neonatal rats caused reduction of the sciatic nerve immunoreactive (IR)-substance P (by 45%) and IR-somatostatin (by 84%) contents. This correlated with a substantial reduction in unmyelinated axons in the sciatic and sural nerves (45% and 65%, respectively), although there was no significant decrease in unmyelinated axons in the nerve to soleus. In a parallel study, we have shown that sympathetic ganglia-derived unmyelinated axons account for about 20 to 25% of the total unmyelinated axon population in both the sural nerve and the nerve to soleus. Thus, in the sural nerve, the majority of unmyelinated axons are dorsal root ganglia-derived, contain either substance P or somatostatin, and are capsaicin-sensitive; whereas in the nerve to soleus, the majority of unmyelinated axons are dorsal root ganglia-derived but are insensitive to capsaicin and do not contain substance P or somatostatin. These latter unmyelinated axons presumably contain a yet to be defined neurotransmitter and may be the axons connecting with muscular ergoreceptors, a subpopulation of unmyelinated axons that are biochemically and functionally distinct from the unmyelinated axons of cutaneous nerves.

Cellular mechanisms of rejection and regeneration in peripheral nerve allografts.

A model of rejection and regeneration of peripheral nerve allografts in rats is presented. A 2.5-cm segment of 28 right sciatic nerves was transplanted orthotopically from LEW.1W to DA and from DA to LEW.1W. With a microsurgical technique, proximal and distal coaptations were performed. In an autologous control group the same surgical procedure was applied. Evaluation included clinical estimation of motor recovery and macroscopic appearance of the graft, electrophysiological examination, conventional histology, and immunohistology. The latter concentrated on demonstration of monomorphic and polymorphic determinants of MHC class I and II antigens and of macrophages. By functional, electrophysiological, and histological parameters it was demonstrated that after rejection a certain degree of regeneration took place in the allografts. Both rejection and subsequent regeneration were studied in detail by immunohistology. During the course of Wallerian degeneration MHC class I expression on myelin sheaths could be demonstrated. When the rejection response occurred, additional MHC class II expression on myelin sheaths and on vascular endothelial was observed. Recipient specific class I-positive macrophages were infiltrating the graft from the epineurium and the coaptation sites, and were later present at the sites of myelin degradation. At 6 weeks postoperatively donor-specific MHC products were no longer detectable, but recipient-specific Schwann cells were present in the allograft tissue. We conclude that a rejection response renders a peripheral nerve allograft acellular but does not destroy the nerve architecture, still enabling it to function as an axon conduit. The regeneration in the rejected allograft however lacks the positive neurotropic and -trophic influence physiologically provided by viable Schwann cells.

Development of myelin in human optic nerve and tract. A light and electron microscopic study.

The early development of myelin in human optic nerve and tract was studied in plastic-embedded material from post-mortem examinations of 18 infants and children. Specimens were examined by light and electron microscopy, and a good correlation was found between the data obtained by the two techniques. The characteristic lamellae of myelin were first seen around a few fibers of tract and intracranial optic nerve at 32 weeks of gestation. By term, these myelin sheaths had become thicker, and a majority of the nerve fibers had become myelinated. In the optic nerve near the globe, myelin was first seen at term and virtually all fibers were myelinated by 7 months of age. Significant increases in sheath thickness were seen in the first two years, and modest increases were found thereafter. These results are in agreement with earlier observations that, in the optic nerve, myelination proceeds from the brain toward the eye. The present data suggest that a significant amount of myelination in the human optic nerve occurs after a full-term (40-week) gestation, during a period of rapid postnatal visual development.

Unfolding and flattening the cortex of gyrencephalic brains.

The present paper describes a procedure for unfolding and flattening gyrencephalic brains that makes it possible to cut single tangential sections through extensive regions of originally convoluted cortex. This procedure involves incising the arachnoid, removing the white matter, and then opening up the gyri and sulci in tissue that has been fixed in such a way as to maintain much of its natural flexibility. A technique is also described for preserving the complete gyral pattern of the intact brain on the surface of a flattened cerebral hemisphere. Finally, examples are presented in which the gyral pattern in the cat brain is related to the location of identified cortical areas in myelin-stained tangential sections from flattened material.

Internodal myelin volume and axon surface area. A relationship determining myelin thickness?

Internodes from normal, remyelinated and regenerated nerve fibres have been isolated from rat spinal roots and sciatic nerve. The internodes have been examined quantitatively by light and electron microscopy to determine their internodal length, myelin thickness, and the circumference and cross-sectional area of both the axons and fibre. Comparison of these measurements of the axon and myelin sheath has revealed a close relationship between the volume of myelin comprising the internode and the area over which the Schwann cell and axon are in close proximity, i.e. the surface area of the axolemma beneath the internodal myelin sheath. The same relationship described not only the internodes on normal nerve fibres, where internodal length is proportional to axon diameter, but also the short and thinly myelinated internodes formed in the adult animal on remyelinated and on regenerated axons. Examination of data presented by Berthold (1978) revealed that a closely similar relationship is also present in feline nerve fibres. In view of the constancy of the relationship between such different types of internode it is suggested that the regulation of myelin volume, and thereby of myelin thickness, may be mediated via the area of the axolemma or of the Schwann cell membrane beneath the myelin sheath.

Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat.

Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 microns axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomotor nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibers.

[Quantitative study of the peripheral nerve in semi-thin section by the Nomarski interference contrast technic]. / Etude quantitative du nerf pêriphêrique sur coupes semi-fines par la technique du contraste interfêrentiel de Nomarski

The Normarski interference-contrast microscope is well suited to observations of transverse thick sections of epon-embedded peripheral nerves. The quantitative study of myelinated fibers is possible on photomicrographs with a final magnification of X 500. The induced distorsion is very small as compared with the results obtained by conventional bright field and positive phase contrast optics. The comparative study of the same field with the three devices allows a better interpretation of marginal pictures.