A scanning electron microscopic study of peripheral nerve degeneration and regeneration

Guinea pigs as an animal model for sciatic nerve injury

The overwhelming use of rat models in nerve regeneration studies is likely to induce skewness in treatment outcomes. To address the problem, this study was conducted in 8 adult guinea pigs of either sex to investigate the suitability of guinea pig as an alternative model for nerve regeneration studies. A crush injury was inflicted to the sciatic nerve of the left limb, which led to significant decrease in the pain perception and neurorecovery up to the 4^th weak. Lengthening of foot print and shortening of toe spread were observed in the paw after nerve injury. A 3.49 ± 0.35 fold increase in expression of neuropilin 1 (NRP1) gene and 2.09 ± 0.51 fold increase in neuropilin 2 (NRP2) gene were recorded 1 week after nerve injury as compared to the normal nerve. Ratios of gastrocnemius muscle weight and volume of the experimental limb to control limb showed more than 50% decrease on the 30^th day. Histopathologically, vacuolated appearance of the nerve was observed with presence of degenerated myelin debris in digestion chambers. Gastrocnemius muscle also showed degenerative changes. Scanning electron microscopy revealed loose and rough arrangement of connective tissue fibrils and presence of large spherical globules in crushed sciatic nerve. The findings suggest that guinea pigs could be used as an alternative animal model for nerve regeneration studies and might be preferred over rats due to their cooperative nature while recording different parameters.

Ultrastructural arrangement of collagen fibrils in the rat facial nerve

We studied the collagen fibril arrangement in the connective tissue sheaths (epineurium, perineurium, and endoneurium) of the intratemporal and extra-temporal portion of the rat facial nerve by transmission electron microscopy after fixing the nerves with tannic acid, and by scanning electron microscopy after digesting cellular elements by sodium hydroxide treatment. These studies revealed that the epineurium consists of thick bundles of collagen fibrils and a coarse meshwork of collagen fibrils, the perineurium consists of a lacework sheet of bundles of collagen fibrils, and the endoneurium consists of a meshwork of fine collagen fibrils in the inner layer and of longitudinally oriented bundles of collagen fibrils in the outer layer. There was little difference in the three-dimensional organization of collagen fibrils between the extra-temporal segment and vertical and horizontal segments of the facial nerve. However, the facial nerve lost the epineurium and perineurium in the central portion of the horizontal segment near the geniculate ganglion and in the labyrinthine portion, i.e. the proximal portion of the intratemporal segment. In these segments, the endoneurium consisted of a meshwork of fine collagen fibrils.

Three-dimensional organization of the collagen fibrils in the rat sciatic nerve as revealed by transmission- and scanning electron microscopy

The organization of collagen fibrils in the rat sciatic nerve was studied by scanning electron microscopy after digestion of cellular elements by sodium hydroxide treatment, and by conventional transmission electron microscopy. The epineurium consisted mainly of thick bundles of collagen fibrils measuring about 10-20 microns in width; they were wavy and ran slightly obliquely to the nerve axis. Between these collagen bundles, a very coarse meshwork of randomly oriented collagen fibrils was present. In the perineurium, collagen fibrils occupied the interspaces between the concentrically arranged perineurial cells; in each interspace, they formed a sheet of characteristic lacework elaborately interwoven by thin (about 3 microns or less in width) bundles of collagen fibrils. In the subperineurial region, there was a distinct sheet of densely woven collagen fibrils between the perineurium and underlying endoneurial fibroblasts. In the endoneurium, collagen fibrils surrounded individual nerve fibers in two layers as scaffolds: the inner layer was made up of a delicate meshwork of very fine collagen fibrils, and the outer one consisted of longitudinally oriented bundles of about 1-3 microns in width. The collagen fibril arrangement described above may protect the nerve fibers against external forces.

Scanning electron microscopic studies of the myelinated nerve fibres of the mouse sciatic nerve with special reference to the Schwann cell cytoplasmic network external to the myelin sheath

The three-dimensional morphology of the surface of myelinated nerve fibres in the mouse sciatic nerve was studied by scanning electron microscopy after combined potassium hydroxide treatment and collagenase digestion (to remove the surrounding collagen fibrils and basal laminae from nerve fibres) as well as by transmission electron microscopy. The myelinated nerve fibre appeared as a long cylinder with sporadic annular constrictions corresponding to the nodes of Ranvier. Slight swellings of the surface due to Schwann cell nuclei were usually found at the middle of each internode. The surface of the nerve fibre clearly exhibited a network of bulges, which consisted of longitudinal bands extending from the nuclear swelling to the nodes of Ranvier through the internode, and transverse trabeculae bridging between these longitudinal bands. These bulges on the surface of nerve fibres were the site of the retained Schwann cell cytoplasm external to the myelin lamellae. These cytoplasmic networks on myelinated fibres presumably corresponded to the networks described by Cajal following silver impregnation. In addition, other thin elevations and focal round swellings were also found associated with these longitudinal bands and transverse trabeculae. These networks of Schwann cell cytoplasm are considered to be cytoplasmic channels for nutrition. The two apposing paranodal bulbs of nodes of Ranvier were often asymmetrical in their structure. The networks of the paranodal region were more complicated than those in the internode. The networks of Schwann cell cytoplasm converged into a continuous circumferential collar toward the node, which in turn gave rise to finger-like projections into the nodal gap.

Gene expression in the presence or absence of myelin assembly

Regulation of myelin protein gene expression in the presence and absence of myelin assembly can be assessed using crushed or permanently transected adult sciatic nerves of rats. The P0 glycoprotein and the myelin basic protein (MBP) are the major myelin-specific proteins of the peripheral nervous system. The steady-state level of P0 and MBP messenger RNA was determined by dot-blot analysis of poly(A)+ RNA from crushed and transected nerves of rats at 35 days post operation. The rat P0-specific cDNA clone, pSN63c, and mouse MBP-specific cDNA clone, pHF43, were used as probes. The level and quality of the poly(A)+ RNA was assessed by in vitro translation and immunoprecipitation of the translation products with anti-chick P0 antibody. Comparison of the steady-state level of P0 and MBP transcripts and the level of anti-P0 immunoprecipitated translation products from RNA extracts of permanently transected, crushed, adult control and 21-day-old control rat nerves indicated that the level of P0 and MBP messages was significantly reduced in the permanently transected model, whereas it was restored to normal in the crushed sciatic nerve 35 days post injury. These results suggest that regulation of P0 and MBP gene expression most likely occurs at the transcriptional or post-transcriptional level in the two models of peripheral neuropathies. Northern blot analysis indicated the absence of differential splicing of the message in crushed or transected nerves. The experiments also indicate that these two important gene products required for myelin synthesis and assembly seem to be co-regulated. However, the data do not rule out the possibility that regulation of gene expression may also occur at the level of translation or post-translational processing.

Structural properties of spinal nerve roots: biomechanics

The biomechanics of spinal nerve roots obtained from normal and nerve-crushed mice were evaluated. Photographs and longitudinal force measurements were taken as nerve roots were elongated through mechanical failure. Proportional limit stress and strain as well as the apparent modulus were calculated from photographic and force measurements to characterize nerve root strength, elasticity, and stiffness, respectively. Resulting mechanical data were indicative of an extremely weak material. Comparisons of nerve and nerve root mechanical properties revealed major differences. While nerve root elasticity was comparable to nerve, nerve root strength was only 10% that of nerve and root stiffness was only 20% of nerve values. Differences in nerve and root mechanics are attributed to the large discrepancies in relative amounts of connective tissue. Also in sharp contrast with peripheral nerve, unilateral nerve crush produced no significant alterations in root mechanics. Comparisons of nerve and nerve root strengths suggested possible pathways for dissipation of peripherally applied forces through epineurial and dural structures.

Synthesis of myelin proteins and ultrastructural investigations in regenerating rat sciatic nerve

Myelin protein synthesis, as well as ultrastructural and morphometric changes in regenerating peripheral nerve, was studied. Sciatic nerves of rats were crushed unilaterally; sham-operated nerves of the contralateral side served as controls. For the in vivo experiments, rats were killed at selected periods after the nerves were crushed (30, 60, 90, and 120 days); seven days prior to killing, the animals were injected intravenously with L-[4,5-3H]leucine. For the in vitro experiments, proximal and distal segments of sciatic nerve and equivalent sham-operated nerves were labeled with 3H-amino acid mixture 90 days after axotomy. Purified myelin was isolated from nerve segments; specific radioactivity and gel electrophoretic patterns of proteins were analyzed. Cross-sectional electron microscope (EM) preparations of proximal, distal, and contralateral segments of nerves also were examined. Results showed that the incorporation of labeled amino acids into total myelin proteins was enhanced significantly in the distal segment of sciatic nerves at all of the periods of regeneration studied. The yield of myelin protein per mm distal nerve segment increased as regeneration proceeded. The remyelination of fibers early after nerve crush was weak, whereas it gradually attained the normal range 90-120 days after axotomy. Morphometric analysis of myelin sheath thickness of regenerating axons was consistent with the data obtained for myelin protein synthesis.

Ultrasonic microdissection of the mouse mandible: exposure of the vasculature of alveolar bone and myelinated axons of the pulp

The present scanning electron microscopic study describes a new method for the exposure of blood vessels of mouse alveolar bone and myelinated nerves of the dental pulp. This technique differs from others because it does not remove the periodontal ligament, allowing study of the vascular continuity between alveolar bone and periodontal ligament. Fixed and demineralized mandibles are digested with bacterial collagenase (1 mg/ml) at 37 degrees C for 12 hours, exposed to buffered osmium tetroxide for 24 hours, and ultrasonicated at 80 kHz for 5 minutes. The technique demonstrates that the vascular distribution of the interdental and interradicular septa is different. Vessels pass horizontally through the interdental septum and are continuous with vessels of the adjacent periodontal ligament. Vessels of the interradicular septum branch from a central vessel, pass toward the adjacent periodontal ligament, and become continuous with its vessels. Thus, the pattern of vessel distribution of the interdental septum of the mouse has little similarity to that of man or of research animals. The present study provides an improved method for demonstration of bone vasculature and pulpal axons while retaining valuable anatomical landmarks.

Evaluation of the use of the argon laser in repairing rat and primate nerves

The argon laser coagulates blood selectively, making it an adherent material. Argon laser energy is almost completely absorbed by red blood cells and does not seem to affect white nerve tissue. To demonstrate the technical feasibility of laser repair for severed nerves, we cut the sciatic nerves of rats and the median nerves of nonhuman primates and then repaired them by use of an argon laser beam delivered through a 400 microns optical fiber and handpiece that was developed in our laboratory. Autogenous blood was spread around the group of fascicles at the repair site and was then coagulated with the laser to form a minitubule around each fascicle group. Transmission and scanning electron microscopy showed that the repairs appeared technically superior to control sutured nerves and that the laser apparently had no untoward effects on either the repair site or on the control nerve. The minitubules seemed to channel the axon sprouts into the distal tubules extremely well and to prevent ingrowth of scar tissue at the juncture site.

Alterations in the mechanical properties of peripheral nerve following crush injury

The mechanical properties of injured nerves have been studied. At specific times following unilateral nerve crush, the sciatic nerves of mice were tested mechanically. Photographs and longitudinal force measurements were obtained as nerve segments were elongated to mechanical failure. Stress and strain at the proportional limit and apparent elastic modulus were used as indicators of strength, elasticity and stiffness. Injury led to time-dependent increases in strength and stiffness and decreases in elasticity. These changes were apparent in both damaged and contralateral, undamaged nerves. Many of the changes appear to be related to the epineurium. Some mechanical changes in nerve could have important consequences for the integrity and function of nerves and mechanically interfaced structures.

Scanning electron microscopic observations of normal and regenerating nerve roots in the cat

The surface morphology of normal and regenerated nerve roots was studied using correlated scanning and transmission electron microscopic methods. Nerve roots of the cauda equina were either cut and rejoined or crossed from a segment above to a segment below. Good regeneration was observed in both experimental procedures. The regenerated nerve root sheath had alterations in surface structure created by extensive growth of collagen. Despite this collagen formation, regenerated axons crossed the anastomotic site with relative ease. Surface features of the regenerated axons were similar in appearance to those of the normal axon. Schwann cells were easily recognized, as were the collagen fibers of the endoneurium, although the endoneurium was more prominent and occupied more of the interaxonal space. Macrophages were identified as round structures with a laminated surface or as a honeycomb structure. Internal features of the regenerating axons were more difficult to identify, but mitochondria and a fibrous network were observed. These studies have demonstrated the application of scanning electron microscopic methods to visualize surface structures and cells in regenerated nerve roots.

Myelination of regenerating sciatic nerve of the rat: lipid components and synthesis of myelin lipids

Changes of lipid, free fatty acid, protein, DNA, and RNA content in proximal and distal segments of regenerating sciatic nerve, from 14 to 120 days after crush, were determined. During the early stage of Wallerian degeneration, a marked decrease of phospholipid, cerebroside and sulfatide content and, in contrast, a marked increase of protein, DNA, RNA, and free fatty acid content, in the distal segment of crushed nerve compared to control, was observed. A gradual increase of phospholipid, cerebroside, and sulfatide levels, approaching normal values, and a gradual slope in the increase of protein, DNA, RNA, and free fatty acid levels over the ensuing time periods of regeneration was seen. Total cholesterol content was relatively constant during regeneration, slightly increasing at day 120. The activity of 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) of myelin fraction purified from distal segment of regenerating sciatic nerve showed a significant increase in the 30-120 day regenerating period. A marked increase of the incorporation of [2-3H]glycerol and of [Me-14C]choline into myelin lipids of distal segment of regenerating nerve, was found. Labeling of myelin lipids with [3H]oleic acid (injected intravenously seven days before crush) support the evidence that a similar pattern of degeneration exists between two different types of trauma, i.e. nerve crush or cut. The findings suggest that, in the distal segment of crushed nerve, the lipid content as well as the myelin lipid synthesis increase as the regeneration period proceeds.

Enhanced visualization of peripheral nerve and sensory receptors in the scanning electron microscope using cryofracture and osmium-thiocarbohydrazide-osmium impregnation

Two methods of specimen preparation for the scanning electron microscope (SEM) have been combined for the reliable exposure and examination of nervous system tissue. When the specimen is postfixed with OSO4 prior to aqueous cryofracturing, large internal surfaces of nervous tissue are exposed, with minimal distortion to the cytoarchitecture. All tissue surfaces and interstices are subsequently impregnated with a conductive, metallic layer of osmium using a modified osmium-thiocarbohydrazide-osmium technique (OTOTO). This OTOTO technique permits SEM examination without any additional vacuum evaporated or ion-sputtered metallic layers, and has been found to eliminate specimen charging reliably. Nervous tissue has been examined in the secondary electron mode of the SEM with unrestricted use of beam currents varying from 1.3 to 60 microamperemeter, at accelerating voltages ranging from 2.5 to 80 kV, and at both low (10 X) and high (80 000 X) magnifications. In addition, a differential deposition of osmium in the tissue after the OTOTO technique has been identified using both transmission electron microscopy and energy dispersive X-ray microanalysis. The enhanced mass-density of myelin resulting from the amplification of osmium's natural affinity for unsaturated lipids was best demonstrated by the backscatter electron mode of the SEM. This mode of imaging was found useful in the identification of myelin sheaths.

Schwann cell plasma membrane changes induced by nerve crush. A freeze-fracture study

Twenty-four hours after nerve crush, the Schwann cell plasma membrane and subjacent outer layer of Schwann cell cytoplasm were examined by freeze-fracture in myelinated fibres from the rat and rabbit sciatic nerves. The irregular circumferential bands and longitudinal columns of cytoplasm which characterise the surface of the normal Schwann cell were diminished or had disappeared. Concomitantly, the membrane pores (of micropinocytotic vesicles or caveolae) which are normally present on the bands and columns were also lost. The loss of these specialised features is discussed in terms of the onset of cell dedifferentiation in association with cell division. The Schwann cell plasma membrane acquired a new feature: deep impressions of adjacent collagen fibres, present particularly on the cytoplasmic bands and columns and presumed to be related to a transient increase in fibre diameter.