Microtubules and calibers in normal and regenerating axons of the sural nerve of the rat

Use of a Multiple Lumen Cuff for Nerve Regeneration

Initial studies using the rat sciatic nerve demonstrated the ability to adapt a multiple lumen cuff face to a nerve stump repair site. The neurons in the proximal stump grew through the individual conduits of the silicone rubber cuff, crossed a 5 mm gap, and continued into the distal stump. The effect of the cuff design on axonal regeneration was studied by comparing macroscopic and microstructural results for experimental groups of Sprague-Dawley rats with controls at 8, 12, 16, and 24 weeks post-implantation. The several individual nerve bundles which formed within the cuff lumens during these periods maintained their alignment on the distal side of the gap. The use of the multiple lumen system provided suitable scaffolding support and control of orientation and direction for fibers and established a sized, controlled environment for regeneration within each of the separate nerve cuff compartments.

Electron microscopy and morphometric analyses of microtubules in two differently sized types of axons in the protocerebral tract of a crustacean

Despite several reports on the morphology and functions associated with the morphometry of the vertebrate axoplasm cytoskeleton, the subject has not been thoroughly explored in invertebrates. In vertebrates, among many other functions, microtubules (MTs) serve as scaffolding for axon assembly, and neurofilaments (NFs) as the elements that determine the axon caliber. Intermediate filaments have never been described by electron microscopy in arthropods, although NF proteins have been revealed in the MT side-arms of the axoplasm of certain species, such as the crab Ucides cordatus. Thus, it is not known which elements of the cytoskeleton of invertebrates are responsible for determination of the axon caliber. We studied, by electron microscopy and morphometric analyses, the MT and axon area variability in differently sized axons of the protocerebral tract of the crab Ucides cordatus. Our results revealed differences in the distance between MTs, in MT density and number, and in the areas of differently sized axons. The number of MTs increases with the axon area, but this relationship is not directly proportional. Therefore, MT density is greater in smaller axons than in medium axons, similar to the morphometry of the vertebrate axon MT. The distance between MTs is, however, directly related to the axonal area. On the basis of the results shown here, and on previous reports by us and others, we suggest that MTs may be involved in the determination of the axon caliber, possibly due to the presence of NF proteins found in the side-arms.

Effects of thiocolchicine on axonal cytoskeleton of the rat peroneus nerve

Thiocolchicine is a colchicine-derivative used in the therapy of some diseases and extensively studied in the field of oncological research as antimitotic agent. Here we studied the activity of thiocolchicine on the cytoskeleton of the peroneus nerve, performing a histological and ultrastructural analysis. We observed a decrease in mean myelinated fiber area in thiocolchicine-treated rats in comparison to controls; this was due to a decrease in mean axoplasm area, while myelin thickness was constant. In the ultrastructural analysis a decrease in microtubule density and an increase in neurofilaments were found; moreover, the myelinated fibers seemed to be more affected in comparison to the unmyelinated axons. These findings are in agreement with the capability of binding to microtubule skeleton shared by all the colchicinoids.

The Microtubular Pattern Changes at the Spinal Cord-Root Junction and Reverts at the Root-Peripheral Nerve Junction in Sensory and Motor Fibres of the Rat

In the rat, we studied the microtubular content of central nervous system (CNS) axons (pyramidal tract, dorsal funiculus, and intracord domain of motor axons), of radicular axons (ventral and dorsal roots), and of peripheral axons (sural and lateral gastrocnemius nerves). The microtubular density had an inverse relationship with the size of the axon. Within the CNS, values ranged from over 120 microtubules/microm2 for axons smaller than 0.1 microm2 of the pyramidal tract and dorsal funiculus to 24 for 3-microm motor axons (area, 7 microm2) in their spinal cord domain. Peripheral nerve and CNS axons of the same size had comparable microtubular densities. In contrast, the microtubular density of dorsal and ventral root axons was one half that of CNS or peripheral nerve axons of equal calibre. Considered along the axon, the microtubular density of motor and sensory fibres is high in the CNS domain, low in the root, and high again in the peripheral nerve domain. These observations are inconsistent with the notion that the cytoskeleton moves coherently away from the perikaryon. We conclude that the axonal microtubular content accords with the calibre of the fibre and with the anatomical region where it courses. We propose that axonal microtubules are regulated by local cues.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Assembly and disassembly of axonal microtubules of the toad Xenopus laevis under the effect of temperature

In toads Xenopus laevis living at 11 degrees (winter), the microtubular density of 4-microns myelinated axons of lumbosacral nerves was assessed with the electron microscope. In controls, the density was 11.2 microtubules/microns2. In nerves incubated at 0 degrees, microtubules decreased following a simple exponential curve with a half time of 4.7 min (k = 0.149 min-1); residual microtubules were 4.5%. After rewarming, the full complement of microtubules reappeared within 60 min. In steady state, the microtubular density exhibited a linear relationship with temperature (range: 0-22 degrees; slope 0.94 microtubules/microns 2 per degree; r, 0.96). After heating the nerve by 11 degrees above the physiological temperature, microtubules increased by 83%, whereby the pool of unpolymerized tubulin was at least 2.7 mg/ml of axoplasm. A seasonal variation of the microtubular density was observed which accorded with the environmental temperature. The macroscopic kinetics of microtubule disassembly in the axoplasm is similar to that reported for purified tubulin but that of assembly is slower. Microtubules of peripheral axons of Xenopus are cold-labile and vary during the annual cycle.

Axonal transport of class II and III beta-tubulin: evidence that the slow component wave represents the movement of only a small fraction of the tubulin in mature motor axons

Pulse-labeling studies demonstrate that tubulin synthesized in the neuron cell body (soma) moves somatofugally within the axon (at a rate of several millimeters per day) as a well-defined wave corresponding to the slow component of axonal transport. A major goal of the present study was to determine what proportion of the tubulin in mature motor axons is transported in this wave. Lumbar motor neurons in 9-wk-old rats were labeled by injecting [35S]methionine into the spinal cord 2 wk after motor axons were injured (axotomized) by crushing the sciatic nerve. Immunoprecipitation with mAbs which recognize either class II or III beta-tubulin were used to analyze the distributions of radioactivity in these isotypes in intact and axotomized motor fibers 5 d after labeling. We found that both isotypes were associated with the slow component wave, and that the leading edge of this wave was enriched in the class III isotype. Axotomy resulted in significant increases in the labeling and transport rates of both isotypes. Immunohistochemical examination of peripheral nerve fibers demonstrated that nearly all of the class II and III beta-tubulin in nerve fibers is located within axons. Although the amounts of radioactivity per millimeter of nerve in class II and III beta-tubulin were significantly greater in axotomized than in control nerves (with increases of +160% and +58%, respectively), immunoassay revealed no differences in the amounts of these isotypes in axotomized and control motor fibers. We consider several explanations for this paradox; these include the possibility that the total tubulin content is relatively insensitive to changes in the amount of tubulin transported in the slow component wave because this wave represents the movement of only a small fraction of the tubulin in these motor fibers.

Effects of advancing age on peripheral nerve regeneration

Following axotomy, the regrowth of peripheral axons takes longer in older individuals than in young ones. The present study compares the crush-induced process of degeneration and regeneration in the buccal branch of the facial motor nerve in groups of rats aged 3 months and 15 months. Observations are based on qualitative and quantitative analyses of the nerve 20 mm from the site of injury in rats 1, 2, 4, 16, 21, 28, and 56 days after crush. The buccal branch is purely motor and contains a unimodal population of about 1,600 axons commonly in a single fascicle. During the first 28 days post crush (dpc) in the 3-month animals, the progression of myelin and axon degeneration, myelin clearance, regrowth of axon sprouts, and axon maturation are relatively synchronized and uniform. In the older rats, the degeneration of myelin and axons, myelin clearance, and the appearance of axon sprouts at the site of sample are all delayed. In the younger animals, axon sprouts increase in numbers from their first appearance at 4 dpc through the 2 weeks examined following the restoration of whisking behavior. The numbers of regenerating older axons increase at a rate comparable to that in the younger animals through the time that bilaterally symmetrical whisking behavior is evident, but afterwards the number of axon sprouts decreases. At 2 months after crush the young animals have 30% more fibers in the buccal branch than control nerves, while the older animals have fewer than control numbers. In the 3-month regenerated nerve, 2 months post crush, 30% of the regenerated fibers are of very small caliber, less than 3 microns2 in cross sectional area, and typically these small axons have unusually thick myelin sheaths; the older nerves do not have such a skewed distribution of axon areas. The older regenerated axons at 2 months post crush have an unusually high density of microtubules compared to the younger regenerated ones (and controls), and the ratio of neurofilaments to microtubules is very low. The conclusions are that motor neurons in older animals regenerate damaged axons after a delay not apparent in the young; the strong regenerative response apparent initially in animals of both age groups is not maintained in the older animals; and the relationship between the numerical density of cytoskeletal elements and the axon cross-sectional area deviates from normal in the regenerated axons of the older animals.

Cytoskeletal components and calibers in developing fish Mauthner axon (Salmo gairdneri Rich.)

In developing axons of many vertebrates, microtubular density is inversely correlated with fiber caliber. It is suggested that microtubules are causally related to axonal caliber. For this reason, cytoskeletal analysis during development of the fish Mauthner axon, which displays a giant caliber, is of particular interest. The Mauthner axon originates from the Mauthner cell in the medulla and runs in the fasciculus longitudinalis medialis in the spinal cord. At embryonic, larval and postlarval stages in trout (Salmo gairdneri Rich.), the following parameters were measured on conventional electron micrographs of Mauthner axon cross sections; axonal caliber, number of microtubules per axons, and microtubular and neurofilament densities. Results at each stage point to an inverse correlation between axonal caliber (x) and microtubular density (y) expressed by the equation y = axb (R = 0.932). Furthermore, three periods of Mauthner axon development are identified on the basis of the cytoskeletal content: (1) embryonic; the Mauthner axon has small caliber with a high microtubule density, (2) elongation period (larval stages); the axon enlarges and a transient peak of microtubules, corresponding to the caliber increment, is observed, and (3) postlarval; the axon enlarges still further (greater than 500 microns 2) but has the lowest microtubular content. During this period neurofilaments are the main axonal component.

Axons Sprout and Microtubules Increase After Local Inhibition of RNA Synthesis, and Microtubules Decrease after Inhibition of Protein Synthesis: A Morphometric Study of Rat Sural Nerves

Drugs that inhibit RNA or protein synthesis are known to affect some functional properties of axons. In this context, we studied the ultrastructural effects of actinomycin-D, an inhibitor of RNA synthesis, and cycloheximide and emetine, inhibitors of protein synthesis, in rat sural nerves. A silicone sleeve (4 mm long) loaded with drug was placed around the nerves and left for about a week. The ultrastructural alterations of axons and Schwann cells progressed over this period. After cycloheximide and emetine, the cytoplasm of Schwann cells was enlarged and the rough endoplasmic reticulum was prominent. After actinomycin-D, the Schwann cells reached the stage of lysis. Nonmedullated were more affected than myelinated axons. After cycloheximide and emetine, the axoplasmic matrix decreased substantially but reversibly. Microtubules of nonmedullated fibres decreased by about 50%. Actinomycin-D determined sprouting of axons and a rise of axonal microtubules; in nonmedullated axons, the normal inverse correlation between microtubular density and calibre gave way to a high and constant density for all axonal sizes. A few millimetres proximal and distal to the sleeve, the nerve tissue and the axonal microtubular content were close to normal, i.e. the effects of drugs were local. Present results suggest that the local turnover of amino acids in the axon is necessary to maintain the integrity of microtubule and neurofilament proteins. We propose that the Schwann cell down-regulates the axonal cytomatrix.

Calibers and microtubules of nerve fibers: differential effect of undernutrition in developing and adult rats

Sural nerves of 9-week-old rats undernourished since birth, and of adult rats food-restricted for 27 and 48 days, were studied to explore the effect of severe undernutrition on the caliber and microtubules of axons in growing and non-growing animals. In 9-week-old undernourished rats, the number and caliber of myelinated fibers were normal while the cross-sectional area of non-medullated fibers was 29% smaller than controls. By contrast, in adult undernourished rats the cross-sectional area of myelinated fibers was affected sooner and to a greater extent (-28%) than that of non-medullated fibers (-23%). Regardless of age, in both controls and in undernourished rats non-medullated fibers of equal caliber had similar microtubular content. The same was found in 3-microns myelinated axons. These findings indicate that food restriction affects proportionately caliber and microtubules of axons. It is proposed that the anatomy of the axon is in a dynamic equilibrium and that microtubules participate in the specification of the axonal caliber.

Alphaherpesvirus saimiri infection in rabbits. 2. Morphometric studies of cutaneous spinal nerves

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.

Microtubular packing varies along the course of motor and sensory axons: possible regulation of microtubules by environmental cues

In the toad Xenopus laevis, the microtubular density of 3-microns myelinated fibres was assessed in peripheral nerves, dorsal and ventral roots, and dorsal and ventral funiculi of the spinal cord. In the roots, the axonal microtubular density was 6 microtubules/microns 2 and twice as much at the other sampling sites. This indicates that the pattern of the microtubular packing may vary along the course of the axon. We propose that axonal microtubules are regulated by local cues.

On the asymmetry of the primary branching of vagal sensory axons: possible role of the supporting tissue

Central and peripheral nonmedullated processes of vagal nodosal neurons of the cat were studied in normal nerves and after regeneration along their anatomical course and along the hypoglossal nerve. Nonmedullated fibers above the ganglion and in the root had comparable sizes (approximately 0.37 micron2) and caliber distribution. Below the ganglion, the cross-sectional area increased to 1.0 micron2. In axons of equal caliber, supranodosal and radicular fibers had similar microtubular densities while infranodosal fibers had two- to threefold that of the former. Regenerated fibers were studied after a recovery period of 6-9 months. Regrown axons were smaller than their parent axons; in turn, these were smaller than normal axons. This holds for central and peripheral nodosal branches, for homologous and heterologous regeneration. Regrown peripheral branches, either along their anatomical pathway or along the hypoglossal nerve, showed no change in microtubular density. Central branches exhibited their characteristic microtubular content when they regenerated along their anatomical course, but when regrowth took place along the hypoglossal nerve, the original low microtubular content of these branches increased to match the high content of peripheral fibers; parent central axons also shifted their microtubular content toward the pattern of peripheral fibers. We propose that the supporting tissue participates in specifying the organization of axonal microtubules.

Calibre and Microtubule Content of the Non-Medullated and Myelinated Domains of Optic Nerve Axons of Rats

Calibres and microtubule contents of the non-medullated and myelinated domains of optic nerve axons of adult rats were studied with the electron microscope. The cross-sectional areas of the non-medullated domain was 0.25 microm2, and that of the myelinated domain 0.40 microm2, that is, greater by 59%. The increase in size was uneven across the axonal population; it was marked in fine and medium sized axons, and modest in the largest axons. The number of microtubules increased with axonal size; the density, however, decreased from 85 mirotubules/microm2 in 0.1 microm2 axons to about 20 in 1.2 microm2 axons. In axons of equal cross sectional area, the microtubular density of the myelinated and non-medullated domains was the same. Microtubular density values of optic axons resemble those of dorsal roots more than those of peripheral nerve axons of equal calibre. The facts that optic axons increase in size and gain microtubules behind the eyeball while the microtubular packing decreases suggest a local regulation of the axonal cytoskeleton.

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.

Do axons grow during adulthood? A study of caliber and microtubules of sural nerve axons in young, mature, and aging rats

Calibers and microtubules of sural nerve axons were studied in young (6-week-old), mature (14-week-old), and aging (2-year-old) rats. The mean cross-sectional area of nonmedullated fibers was about 0.50 micron 2 (range: 0.47-0.52) in the three age groups. Their caliber spectra were also similar. In contrast, myelinated axons grew from 6.6 to 16.7 micron 2 between the sixth and 14th week of age. The increase of cross-sectional area was greater, the greater the initial caliber of axon (range 44-154%). No further change of caliber was observed in the aging rat. The cross-sectional area of nerve allotted per myelinated fiber was 42, 66, and 97 micron 2 in young, mature, and aging rats, respectively. The fraction of nerve tissue occupied by the axoplasm, though, did not change substantially; it was 20, 28, and 21%, respectively. The microtubular density of 3-micron myelinated axons had a general average of 21 microtubules/micron 2. Differences between groups were not significant. In nonmedullated fibers, the microtubular density decreased as the size of the axon increased. No differences were observed between age groups. We conclude that nonmedullated fibers of the sural nerve stop growing before the sixth week whereas myelinated fibers keep growing until the 14th week of age. The correlation between microtubular content and axonal caliber is a lifelong feature of axons.

Axons grow in the aging rat but fast transport and acetylcholinesterase content remain unchanged

Caliber and microtubular density of myelinated fibers, acetylcholinesterase (AChE) content and its accumulation at a ligature were studied in the phrenic nerve of mature (3-4 months) and aging (2-year-old) rats. The number of axons remained constant. The cross-sectional area of the nerve was 67% greater in the older group; the axoplasm, though, constituted about 20% of the nerve tissue irrespective of age. The mean cross-sectional area of myelinated axons was twice as big in aging compared to mature rats. All axons grew in the same proportion irrespective of their original caliber. The microtubular density of 3-microns axons was about 22 microtubules/micron2 in mature and aging rats. The AChE activity of aging rats was half as much as that of mature rats if it was expressed per wet weight of nerve tissue but did not change if it was expressed per nerve fiber. Twenty-four hours after ligation of the nerve, total AChE activity rose in mature and aging rats by ca. 168%; the molecular forms--asymmetric and globular--accumulated in the same proportion in both age groups. We conclude that myelinated axons grow in the adult stage of life but the structure of axoplasm, content of AChE per axon, and rate of fast transport remain lifelong features of nerve fibers.

Postnatal development of microtubules and neurofilaments in the rat optic nerve: a quantitative study

In this paper the postnatal changes in the cytoskeleton of the rat optic nerve fibers are described and quantified. These changes are also compared with other parameters such as myelination and axonal caliber with the aim of describing a general pattern of optic nerve maturation from a morphological point of view. The results showed that during the first postnatal week microtubules outnumbered neurofilaments but between days 8 and 20 the neurofilaments rapidly increased and on day 20 were about twice as numerous as microtubules. This proportion remained almost unaltered from the end of the third week to the 44th postnatal day. The comparison with other parameters suggested that the cytoskeleton, and in particular the proportion between its components, may be a more reliable index for measuring optic nerve maturation than other variables commonly used.

Microtubules and calibers in developing axons

The caliber and microtubular content of growing axons were assessed with the electron microscope in the sural nerve of the rat from birth until the age of 63 days (young adult). The caliber of both nonmedullated and myelinated fibers increased throughout the period of observation. At birth, nonmedullated fibers smaller than 0.2 micron2 represented 69% of the population; at day 63 less than 7% were that small. Irrespective of the age of the rat, the number of microtubule profiles of nonmedullated fibers increased with the cross-sectional area of the axon although their packing decreased. In myelinated fibers of a given caliber, the packing of microtubules increased with time, and by day 63 the density had reached its final value. In nonmedullated fibers of a given caliber, a similar trend was observed after day 31; i.e., fibers showed a small but consistent increase in density. However, before that, and in contrast to myelinated fibers, nonmedullated fibers of defined calibers exhibited a transient increase in the microtubular density. Notwithstanding, during the history of an individual fiber the packing of its microtubules may decrease continuously until stabilizing, because the developing fiber is increasing its caliber and hence decreasing its microtubular density. In the 5-day-old rats, the caliber spectra of myelinated and nonmedullated axons overlapped in the 0.49-0.83 micron2 range and their microtubular densities were similar. We conclude that the microtubular content of developing axons correlates with caliber, an architectural feature, and is largely independent of growth and of its myelinated condition. We propose that the cytoskeletal rather than the transport function commands the organization of axonal microtubules.