Unmyelinated nerve fiber degeneration in chronic inflammatory demyelinating polyneuropathy

To determine whether unmyelinated nerve fibers escape degeneration as one might expect in an immune response exclusively directed at myelin, we performed a morphometric examination of unmyelinated axons and myelinated nerve fibers in sural nerve biopsy specimens of 14 patients with a chronic inflammatory demyelinating polyneuropathy (CIDP) and of 12 age-matched normal controls. The numbers of unmyelinated axons, myelinated nerve fibers, denervated Schwann cell units and collagen pockets were quantified and related to the clinical and electrophysiological data of the patients with CIDP. In 4 patients with a rapid onset of the neuropathy and a highly elevated CSF protein, the numbers of both unmyelinated axons and myelinated nerve fibers were decreased equally. In 8 patients we found that the unmyelinated axons were relatively spared compared with the loss of myelinated nerve fibers. In these patients, however, the presence of denervated Schwann cell units and of collagen pockets was increased. We conclude that unmyelinated nerve fibers are affected in patients with CIDP.     

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.     

Some histological aspects of amyloid polyneuropathy

Summary Two sporadic cases of amyloid polyneuropathy with clinical features corresponding to the Portuguese type of this disease were studied. Histological examination of sural nerve demonstrated a marked loss of myelinated and unmyelinated fibers in the case 1 due to axonal degeneration, high content of fibers with segmental demyelination and the occurrence of several enlarged axons filled with the 10 nm filaments (so-called giant axons). In the case 2 there was total loss of unmyelinated axons and myelinated fibers were nearly completely lacking.In the development of changes in the myelinated fibers their direct compression by amyloid deposits seems to play an important role. It leads to the appearance of both axonal degeneration and segmental demyelination. The latter seems to be due to local compression and it may involve many fibers.In the light of observations reported by other authors the mechanism of changes developing in unmyelinated fibers is explained by the presence of changes in the cells of posterior root ganglia, however the question whether some abnormalities seen in unmyelinated axons could not be related to the pressure exerted by amyloid deposits directly to these fibers, remains open.This work was supported by project No. 10.4.2.02. Polish Academy of Sciences and MDAA Grant (1976)     

Axonal branching following crush lesions of peripheral nerves of rat

Branching of myelinated and unmyelinated nerve fibers in normal and regenerating personal and soleus nerves was studied by light and electron microscopy. There were at most 2% more myelinated and 13% more unmyelinated axons in the distal as compared with the proximal nerve segments. Two to four weeks after a crush lesion the distal axons became 2-3 times more numerous; thereafter their number decreased. The number of axons in the proximal nerve segment did not change. The number of myelinated sprouts in most regenerated nerves equalled the number of myelinated fibers in the proximal nerve, while the number of unmyelinated axons after 12-19 weeks was 18-60% higher than normal. Branching was not restricted to the crush region. The results indicate that following a crush lesion all axons branch but only branches of unmyelinated fibers persist for a prolonged period of time. It is tentatively suggested that regenerating axons branch when searching for a target and that when contact is made with the target this prevents additional branching and eliminates redundant branches. Myelinated axons are guided by existing Schwann cells, whereas unmyelinated axons do not follow predetermined pathways; this may explain their greater tendency to form permanent branches.     

Numbers of myelinated and unmyelinated axons in the dorsal, lateral, and ventral funiculi of the white matter of the S2 segment of cat spinal cord

The present work determines the numbers of myelinated and unmyelinated axons in the dorsal, lateral, and ventral funiculi of the S2 segment of the cat spinal cord. The major finding is that unmyelinated axons are almost as numerous as myelinated axons in these pathways. The myelinated axons tend to be distributed uniformly, although there is a slight concentration of these fibers in the dorsal part of the lateral funiculus. By contrast, the unmyelinated fibers, although found in significant numbers in all parts of these funiculi, concentrate in the dorsal part of the lateral funiculus and in the dorsal funiculus. Of particular note are the unmyelinated fibers in the dorsal funiculus, because it is highly likely that some of these are sensory. The findings in this study will serve as a basis for experimental studies to determine the numbers, locations, and types of unmyelinated fibers in the white matter of the mammalian cord.     

Nonmyelinating Schwann cell involvement with well-preserved unmyelinated axons in Charcot-Marie-Tooth disease type 1A

Electron microscopic examination was performed to compare morphologic changes of nonmyelinating Schwann cells and unmyelinated axons in patients with Charcot-Marie-Tooth disease type 1A (CMT1A) with peripheral myelin protein 22 duplication (n = 27) and normal control individuals (n = 14). Complete transverse sural nerve cross-sections were obtained in 16 patients and the total number of axons and Schwann cells in each cross-section was estimated. In patients with CMT1A, the number of myelinated axons was significantly decreased, whereas unmyelinated axons were well-preserved and did not show any marked changes. The numbers of nuclei, subunits, and profiles of nonmyelinating Schwann cells were all increased significantly in patients with CMT1A, whereas the numbers of axons per unmyelinated axon-containing subunit were significantly decreased. Schwann cell subunits consisted of layers of flattened cytoplasmic profiles wrapped around unmyelinated axons in the patient with CMT1A. The numbers of nonmyelinating Schwann cell profiles were increased and the numbers of axons per unmyelinated axon-containing subunit were reduced even in young patients with well-preserved myelinated fibers. In conclusion, there is marked alteration of the population and morphology of nonmyelinating Schwann cells, and axon-Schwann cell interactions seem to be regulated differently between myelinated and unmyelinated fibers in CMT1A.     

Propriospinal fibers in the rat

Segments of rat spinal cord were isolated by transecting in two places and sectioning all dorsal roots between the transections. Following this procedure, the areas of the gray and white matter are decreased by approximately 50% compared to normal. We feel, for reasons elaborated in the discussion, that the white matter of the isolated segments contains almost exclusively propriospinal axons. If this is accepted, then the axonal counts in this paper provide estimates of the numbers of propriospinal axons in rat spinal cord. In the isolated segments, the lateral funiculi contain 21,000 myelinated and 31,000 unmyelinated axons and the ventral funiculi 10,500 myelinated and 1,500 unmyelinated axons. The number of these fibers is approximately 33% of the number in unoperated spinal cords. Thus approximately one-third of the axons in rat sacral lateral and ventral funiculi are propriospinal, a lower figure than would have been predicted from classical work. The ratio of myelinated to unmyelinated fibers is higher for proprio-spinal fibers than for the other axons in these funiculi. Thus the propriospi-nal axons, as a group, are slightly larger than the other axons in these funic-uli. This is against currently accepted thinking which generally regards the propriospinal fibers as the finest in the white matter of the cord. Finally, the quantification of propriospinal systems in these funiculi allows more precision in our thinking about the organization of the spinal cord.     

Alphaherpesvirus saimiri infection in rabbits

To provide a better insight into the ultrastructural pathology of herpetic neuropathy, quantitative studies were made on cutaneous spinal nerves of normal rabbits and rabbits intradermally infected with alphaherpesvirus saimiri (alpha HVS) isolate KM 322. Marked reductions in the numbers and densities of myelinated and unmyelinated axons were found in the nerves of the rabbits killed 17 and 45 days after the infection. Abnormalities in the size distribution of unmyelinated axons were seen at 45 days post-inoculation where axonal sprouting caused a noticeable shift in the fiber population. Two years after virus inoculation reduction in unmyelinated axons and abnormalities in the fiber size distributions characterized by smaller diameters of both myelinated and unmyelinated axons were detected. In these nerves conspicuous fibrosis caused a significant increase in the endoneurial area. At this stage of the infection regenerative changes involving myelinated fibers were found. Since attempts to detect spontaneous reactivation of alpha HVS infection in rabbits have been unsuccessful, the finding of regeneration 2 years after exposure seems in agreement with the view that regenerated myelinated fibers never attain their original size. In the present study although both types of fibers were damaged, morphometric data suggest that unmyelinated axons were more severely affected. Whether this seemingly selective involvement was due to spreading of the virus between axons sharing the same Schwann cell subunit remains to be proved.     

Ascending unmyelinated primary afferent fibers in the dorsal funiculus

The primary purpose of the present study is to obtain evidence as to the destination of the recently discovered unmyelinated primary afferent fibers in the mammalian dorsal funiculus. To do this rat dorsal roots were transected unilaterally from segments T8 or T9 caudally, and the numbers of axons were determined in the C3 fasciculus gracilis in normal animals and from both sides of the rhizotomied animals. In addition, C3 fasciculus gracilis counts were done in animals that had complete T6 or T10 spinal transections. The data indicate that there is an 80% loss of unmyelinated axons ipsilaterally and a 60% loss contralaterally in the fasciculus gracilis of the rhizotomied animals. These findings are interpreted as indicating that a significant fraction of the unmyelinated fibers in the fasciculus gracilis ascend, presumably to the nucleus gracilis in the brain stem, and also that a significant number of these fibers branch. We also provide evidence for contralateral myelinated primary afferent fiber projection in the fasciculus gracilis and show that the myelinated primary afferent fibers seem to be a more diverse population than the unmyelinated primary afferent fibers in the C3 fasciculus gracilis.     

Unmyelinated sensory and preganglionic fibers in rat L6 and S1 ventral spinal roots

Approximately 30% of the axons in the L₆ and S₁ ventral roots in the rat are unmyelinated. Appropriate surgical procedures show that 30% of these unmyelinated axons arise from dorsal root ganglion cells and are therefore presumably sensory. Thus approximately 10% of the total number of axons in these ventral roots are sensory. The other 70% of the unmyelinated axons come from the spinal cord and are therefore presumably preganglionic parasympathetic fibers. Thus approximately 20% of the axons in these ventral roots are unmyelinated preganglionic fibers. The implications of these findings are discussed.     

A conus peptide blocks nicotinic receptors of unmyelinated axons in human nerves

The novel alpha-conotoxin Vc1.1 is a potential analgesic for the treatment of painful neuropathic conditions. In the present study, the effects of Vc1.1 were tested on the nicotine-induced increase in excitability of unmyelinated C-fiber axons in isolated segments of peripheral human nerves. Vc1.1 in concentrations above 0.1 microM antagonized the increase in axonal excitability produced by nicotine; the maximal inhibition was observed with 10 microM. We also demonstrate immunoreactivity for alpha 3 and alpha 5 subunits of neuronal nicotinic receptors on unmyelinated peripheral human axons. Blockade of nicotinic receptors on unmyelinated peripheral nerve fibers may be helpful in painful neuropathies affecting unmyelinated sympathetic and/or sensory axons.     

Unmyelinated primary afferent fibers in dorsal funiculi of cat sacral spinal cord

The present study tests the hypothesis that there are numerous unmyelinated primary afferent fibers in cat posterior funiculi. The animals have unilateral dorsal rhizotomies from L6 to Ca3. One week later the axons of both S2 dorsal funiculi are counted. The data indicate that there are approximately 22,500 myelinated and 8,500 unmyelinated axons on the unoperated side and 11,000 myelinated and 3,900 unmyelinated axons on the operated side. On this basis, we suggest that 51% of the myelinated and 54% of the unmyelinated axons in cat dorsal funiculi arise from dorsal root ganglion cells and thus are primary afferent axons. If this is correct, then 71% of the primary afferent axons in the cat dorsal funiculus are myelinated and 29% are unmyelinated. The function of this large group of previously unsuspected fine sensory axons remains to be determined.     

Trigeminal alveolar nerve of the lower jaw in the cichlid Tilapia mariae: evidence for continual axon generation and presence of exceptionally small myelinated axons

Cross sections from the trigeminal alveolar nerve of the lower jaw in the cichlid Tilapia mariae were examined by electron microscopy. The nerve fibers are arranged in groups with a core of unmyelinated and small myelinated axons, surrounded by myelinated axons of varying sizes. The core contains large bundles of unmyelinated axons collectively ensheathed by circumferentially located Schwann cells, as well as smaller bundles of unmyelinated axons partly separated from each other by Schwann cell processes. Among the unmyelinated axons, occasional scattered profiles resembling growth cones are seen. The total number of axons in this tooth-related nerve increases from approximately 1,500 to 5,000, as the animals grow in length from 4.5 to 21.5 cm. Some 24-49% of the axons are unmyelinated. The myelinated axons have maximum diameters of 1.0-3.0 micron, depending on body size. Most myelinated axons have diameters less than 1.0 micron and the smallest ones reach down to 0.3 micron. These results show that there is a continual addition of axons to the alveolar nerve of the lower jaw in Tilapia mariae and that the critical diameter for myelination in this peripheral nerve is similar to that typically found in the mammalian CNS.     

Unmyelinated axons show selective rostrocaudal pathology in the corpus callosum after traumatic brain injury

Axonal injury is consistently observed after traumatic brain injury (TBI). Prior research has extensively characterized the post-TBI response in myelinated axons. Despite evidence that unmyelinated axons comprise a numerical majority of cerebral axons, pathologic changes in unmyelinated axons after TBI have not been systematically studied. To identify morphologic correlates of functional impairment of unmyelinated fibers after TBI, we assessed ultrastructural changes in corpus callosum axons. Adult rats received moderate fluid percussion TBI, which produced diffuse injury with no contusion. Cross-sectional areas of 13,797 unmyelinated and 3,278 intact myelinated axons were stereologically measured at survival intervals from 3 hours to 15 days after injury. The mean caliber of unmyelinated axons was significantly reduced at 3 to 7 days and recovered by 15 days, but the time course of this shrinkage varied among the genu, mid callosum, and splenium. Relatively large unmyelinated axons seemed to be particularly vulnerable. Injury-induced decreases in unmyelinated fiber density were also observed, but they were more variable than caliber reductions. By contrast, no significant morphometric changes were observed in myelinated axons. The finding of a preferential vulnerability in unmyelinated axons has implications for current concepts of axonal responses after TBI and for development of specifically targeted therapies.     

Unmyelinated axons are more vulnerable to degeneration than myelinated axons of the cardiac nerve in Parkinson's disease

S. Orimo, T. Uchihara, T. Kanazawa, Y. Itoh, K. Wakabayashi, A. Kakita and H. Takahashi (2011) Neuropathology and Applied Neurobiology 37, 791–802 Unmyelinated axons are more vulnerable to degeneration than myelinated axons of the cardiac nerve in Parkinson's disease Aims: We recently demonstrated accumulation of α‐synuclein aggregates of the cardiac sympathetic nerve in Parkinson's disease (PD) and a possible relationship between degeneration of the cardiac sympathetic nerve and α‐synuclein aggregates. The aim of this study is to determine whether there is a difference in the degenerative process between unmyelinated and myelinated axons of the cardiac nerve. Methods: We immunohistochemically examined cardiac tissues from four pathologically verified PD patients, nine patients with incidental Lewy body disease (ILBD) and five control subjects, using antibodies against neurofilament, myelin basic protein (MBP) and α‐synuclein. First, we counted the number of neurofilament‐immunoreactive axons not surrounded by MBP (unmyelinated axons) and those surrounded by MBP (myelinated axons). Next, we counted the number of unmyelinated and myelinated axons with α‐synuclein aggregates. Results: (i) The percentage of unmyelinated axons in PD (77.5 ± 9.14%) was significantly lower compared to that in control subjects (92.2 ± 2.40%). (ii) The ratio of unmyelinated axons with α‐synuclein aggregates to total axons with α‐synuclein aggregates in ILBD ranged from 94.4 to 100 (98.2 ± 2.18%). Among axons with α‐synuclein aggregates, unmyelinated axons were the overwhelming majority, comprising 98.2%. Conclusion: These findings suggest that in PD unmyelinated axons are more vulnerable to degeneration than myelinated axons of the cardiac nerve, because α‐synuclein aggregates accumulate much more abundantly in unmyelinated axons.     

Numbers of regenerating axons in parent and tributary peripheral nerves in the rat

This study is concerned with numerical parameters of axonal regeneration in peripheral nerves. Our first finding is that the number of axons that regenerate into the distal stump of a somatic nerve at a particular time after transection is partially dependent on the type of lesion used to interrupt the axons. The second question concerns the proportion of axons that regenerate into the distal stump of a parent nerve compared to the proportions that regenerate into tributary nerves that arise from the parent. The proportions of regenerated myelinated axons in the nerve to the medial gastrocnemius muscle and myelinated and unmyelinated axons in the sural nerve are the same as the proportions of myelinated and unmyelinated axons that regenerate into the distal stump of the sciatic nerve for the crush, 0 and 4 mm gap transections. Proportionally fewer axons regenerate into the tributary nerves following the 8 mm gap transection, however. This implies that the length of the gap has an influence on whether or not axons in tributary nerves regenerate in concert with axons in the distal stump of the parent nerve. The unmyelinated fibers in the nerve to the medial gastrocnemius muscle are different because they do not regenerate in proportion to those in the distal stump of the sciatic nerve. We also provide evidence to indicate that myelinated axons branch whereas unmyelinated fibers end blindly when they enter the distal stump after crossing a sciatic nerve transection. Finally the normal arrangement of perineurial cells seems to be disrupted after the sciatic nerve regenerates across a gap.     

Primary afferent and propriospinal fibers in the rat dorsal and dorsolateral funiculi

The purpose of this study is to determine the numbers of primary afferent and propriospinal fibers in the dorsal and dorsolateral funiculi of the rat. The reason for concentrating on these areas is that they contain large numbers of unmyelinated axons. Our data are axonal numbers from the S2 segment of spinal cord in animals that had unilateral dorsal rhizotomies or spinal cord isolations. The major conclusions are 1) that 23% of the primary afferent fibers in the dorsal funiculus are unmyelinated; 2) that there are approximately 12,500 unmyelinated primary afferent fibers in the dorsolateral funiculus, which is more than the number of primary afferent fibers in the dorsal funiculus and tract of Lissauer combined, and 3) that approximately 25% of the axons in the dorsal funiculus and 44% of the axons in the dorsolateral funiculus are propriospinal. These data modify and extend previous ideas of the organization of spinal white matter.     

A quantitative ultrastructural study of the optic nerve of the chameleon.

The optic nerve of adult chameleons was investigated with an electron microscope. The total number of retinal ganglion cell axons, the proportion of myelinated axons, the frequency distributions of myelinated and unmyelinated axon diameters were estimated, together with the volume occupied by glial processes. These were distinguished from unmyelinated axons using an antibody directed against glial fibrillary acidic protein, in a post-embedding procedure. The total number of fibers was estimated to be 405,235 +/- 60,000 axons. The proportion of myelinated fibers varied with position between the eyeball and the chiasma; being 22-27% close to the eyeball, rising to 42-47% halfway along the optic nerve and to 56-62% close to the chiasma. Myelinated and unmyelinated fiber diameter distributions were unimodal and positively skewed, with modes of 0.7 microm and 0.2 microm, respectively. There was a significant regional variation in the size of optic nerve axons. Large myelinated axons were observed in the dorsal and ventral periphery, whereas smaller myelinated fibers and a high proportion of unmyelinated fibers were found in the center of the nerve.     

Length-dependent degeneration of fibers in Portuguese amyloid polyneuropathy: a clinicopathologic study

We investigated two patients with Portuguese amyloid polyneuropathy to learn more about the role played by amyloid in this condition. In sural nerves, axonal loss predominated in unmyelinated axons. Different abnormalities of single fibers near amyloid deposits included distortion of the myelin sheath, segmental demyelination, and wallerian degeneration. Electronmicroscopic studies showed degenerative changes of endoneurial cells in contact with fibrils of amyloid. Therefore, amyloid plays an important role in the length-dependent degeneration of fibers of this condition. Why unmyelinated fibers are so heavily affected remains unclear.     

Postnatal development and aging of the rat ventral root L5: Electron microscopic and immunohistochemical studies

The ventral root L5 of neonatal and adult rats has been used in many experimental studies on ganglionic C-fibers. Since the normal presence of such axons in L5 roots from animals of different ages is unknown, the results of these studies cannot be appropriately interpreted. In the present study we examine L5 ventral roots from developing and aging rats in this respect. Electron microscopic examination revealed that C-fibers occur in neonatal roots. The adult proportion has been established at day 30. Immunohistochemical analysis showed that thin ganglionic fibers with substance P/calcitonin gene-related peptide- or tyrosine hydroxylase-like immunoreactivity in the L5 root and the spinal pia mater seem to increase postnatally from low levels at birth. In roots from aged rats, myelinated fibers with a variety of aberrant features occur in normal numbers. The occurrence of unmyelinated axons is elevated. The increased presence of fibers with calcitonin gene-related peptide- or tyrosine hydroxylase-like immunoreactivity in aged roots indicates that the extra unmyelinated fibers may represent motor sprouts and sympathetic fibers, respectively. We conclude that the rat ventral root L5 contains a variable number of putative sensory and sympathetic axons at all ages. © 1996 Wiley-Liss, Inc.