The numbers of unmyelinated and myelinated axons in normal and regenerated rat saphenous nerves

Counts have been made of the numbers of unmyelinated and myelinated axons in the proximal and distal stumps of regenerated rat saphenous nerves and from equivalent sites in normal nerves. In the proximal part of normal nerves there were averages of 1 045 myelinated axons and 4 160 unmyelinated ones. Regenerated nerves contained the same number of myelinated axons in their proximal stumps but there was a 40% reduction in the unmyelinated axon count. In the distal stumps of these nerves the myelinated axon count had increased by an average of 620; this comes about because some regenerated myelinated axons support more than one process in the distal stump. In contrast, the number of unmyelinated axons was reduced further, from a mean of 2 476 in the proximal stump to one of 2 219.

The sizes of Schwann cell units in the normal and regenerated nerves were also noted. Schwann cell units in the proximal and distal stumps of the regenerated nerves were smaller than those in the normal ones.

These changes associated with unmyelinated axons in regenerated nerves are likely to contribute to the sensory, vasomotor and sudomotor abnormalities that sometimes occur after peripheral nerve injury and regeneration.     

Poor growth of Mammalian motor and sensory axons into intact proximal nerve stumps

Wallerian degeneration is very slow in the mouse strain now known as C57BL/Ola. Sensory axon regrowth following peripheral nerve lesions is very poor in these animals but motor axons succeed in reinnervating the distal nerve stump even while the majority of severed axons are still intact (Lunn et al., Eur. J. Neurosci., 1, 27 - 33, 1989). To see if motor axons could grow into a completely undegenerated portion of nerve, the proximal stumps of the peroneal and tibial nerves were sutured together in six BALB/c mice and the ability of large motor and sensory fibres from the tibial nerve to grow into the peroneal nerve was examined electrophysiologically in four of them. For the acute experiment the peroneal nerve was cut approximately 7 mm central to the point of suture to the tibial nerve. Both at 2 weeks and 7 weeks after surgery the size of the potential recorded in the ventral roots on stimulating the portion of peroneal nerve into which tibial axons were directed to grow was only approximately 8% of the potential recorded when the tibial nerve was itself stimulated. The potential recorded in the dorsal roots was only approximately 2%. Counts of axon numbers in electron micrographs showed a small but non-significant increase over normal in the number of unmyelinated axons in the peroneal nerves which had been connected to the tibial nerve in this way. It is concluded, in agreement with Langley and Anderson (J. Physiol., 31, 365 - 391, 1904), that axon growth into intact nerves is extremely limited in mammals and that the distal nerve stump of C57BL/Ola mice, although it degenerates very slowly, is not therefore equivalent to an intact peripheral nerve.     

Changes in myelinated and unmyelinated axon numbers in the proximal parts of rat sural nerves after two types of injury

Counts of myelinated and unmyelinated axon profiles have been made from normal, uninjured rat sural nerves and from nerves injured 6 months earlier in one of two ways. In one group of rats the nerve was simply cut and left to regenerate, leading to the development of a neuroma in continuity, while in the second group the nerve was cut but then ligated as well to prevent regeneration; this led to stump neuroma formation. After nerve transection and regeneration, with subsequent formation of a neuroma in continuity, there was no change in the number of myelinated axon profiles found 25 mm proximal to the old injury site when compared with control, but there was an 18% reduction (P < 0.05) in the number of unmyelinated axon profiles. Immediately proximal to the injury site the picture was similar, with there still being the same number of myelinated axon profiles as in control material but here the reduction in unmyelinated axon numbers was slightly greater at 24% (P < 0.05). In the proximal part of nerves that had been cut and stump neuroma formation induced there was a large increase (33%) in myelinated axon profiles over and above control values (P < 0.001) but the number of unmyelinated profiles was the same as in controls. Closer to the stump neuroma the number of myelinated axon profiles had increased yet further to be 88% (P < 0.001) above control while the number of unmyelinated ones remained no different from control. Our interpretation of these results is that after nerve transection and regeneration there is no loss of peripheral neurons supporting myelinated axons but some loss of those supporting unmyelinated ones. If a cut nerve is prevented from regenerating and a stump neuroma forms, however, a vigorous sprouting response is triggered in neurons with myelinated axons while those supporting unmyelinated axons are possibly prevented from dying. The reaction of peripheral neurons to injury is such that the number of axons they support varies along the nerve as one goes disto-proximally away from the injury site. Thus discrepancies in results from different laboratories have come about because material for axon counting has been taken from different points along the nerve relative to the injury site and also because the material has been taken from nerves injured in different ways.     

Regeneration of transected rat sciatic nerves after using isolated nerve fragments as distal inserts in silicone tubes

We determined blood vessel and perineurial fascicle densities as well as axonal numbers in regenerated rat sciatic nerves 8 weeks after the nerves had been transected, the proximal stumps placed into the proximal ends of silicone tubes, and isolated fragments of nerve placed into the distal ends of the same tubes. The data are compared with data from the normal nerve and from regeneration in a similar paradigm in which the distal stumps were used as the inserts into the distal end of the silicone tubes. A major difference between the two regeneration paradigms was that axons were discouraged from reaching the periphery when the distal insert was an isolated fragment and encouraged to reach the periphery when the distal insert was the distal stump. We found that fascicle and blood vessel densities were greater than normal but less than with the distal stump as the distal insert. Thus we concluded that the nature of the distal insert had a bearing on how many vessels and perineurial fascicles were formed during regeneration in these conditions. Myelinated axon numbers did not differ in the two conditions whereas there were more unmyelinated axons with the isolated distal stump as the distal insert. Thus at this regeneration time the numbers of myelinated axons were not as dependent on the nature of the distal insert as were the numbers of unmyelinated axons. Finally the length of the gap had a great influence on the numbers of axons that regenerated.(ABSTRACT TRUNCATED AT 250 WORDS)     

30 mm regeneration of rat sciatic nerve along collagen filaments

This paper describes 30 mm regeneration of peripheral nerve axons along collagen filaments; 31-mm-long collagen filaments or collagen tube were grafted to bridge a 30-mm defect of rat sciatic nerve. The mean number and the diameter of regenerated myelinated axons were 330+/-227 and 2.7+/-0.9 microm at the distal end of the collagen-filaments 12 weeks postoperatively; while at the distal end of the tube no axon was found.     

Delayed peripheral nerve repair: methods,including surgical ‘cross-bridging' to promote nerve regeneration

Despite the capacity of Schwann cells to support peripheral nerve regeneration, functional recovery after nerve injuries is frequently poor, especially for proximal injuries that require regenerating axons to grow over long distances to reinnervate distal targets. Nerve transfers, where small fascicles from an adjacent intact nerve are coapted to the nerve stump of a nearby denervated muscle, allow for functional return but at the expense of reduced numbers of innervating nerves. A 1-hour period of 20 Hz electrical nerve stimulation via electrodes proximal to an injury site accelerates axon outgrowth to hasten target reinnervation in rats and humans, even after delayed surgery. A novel strategy of enticing donor axons from an otherwise intact nerve to grow through small nerve grafts(cross-bridges) into a denervated nerve stump, promotes improved axon regeneration after delayed nerve repair. The efficacy of this technique has been demonstrated in a rat model and is now in clinical use in patients undergoing cross-face nerve grafting for facial paralysis. In conclusion, brief electrical stimulation, combined with the surgical technique of promoting the regeneration of some donor axons to ‘protect' chronically denervated Schwa nn cells, improves nerve regeneration and, in turn, functional outcomes in the management of peripheral nerve injuries.     

Sex-related difference in collateral sprouting of nociceptive axons after peripheral nerve injury in the rat

Possible sex-related differences in the extent of collateral sprouting of noninjured nociceptive axons after peripheral nerve injury were examined. In the first experiment, peroneal, tibial, and saphenous nerves were transected and ligated in female and male rats. Eight weeks after nerve injury, skin pinch tests revealed that the nociceptive area of the noninjured sural nerve in the instep skin expanded faster in females; the final result was a 30% larger increase in females than in males. In the second experiment, the end-to-side nerve anastomosis was used as a model for axon sprouting. In addition to the previous procedure, the end of an excised peroneal nerve segment was sutured to the side of the intact sural nerve. Eight weeks later, collateral sprouting of nociceptive axons into the anastomosed peroneal nerve segment was assessed by the nerve pinch test and axon counting. There was no significant difference with respect to the percentages of male and female rats with a positive nerve pinch test. The number of myelinated axons in the anastomosed nerve segment was significantly larger in female (456 +/- 217) than in male (202 +/- 150) rats, but the numbers of unmyelinated axons were not significantly different. In normal sural nerves, the numbers of either all myelinated axons or thin myelinated axons did not significantly differ between the two sexes. Therefore, the more extensive collateral axon sprouting observed in female than in male rats is probably due to the higher sprouting capacity of thin myelinated sensory axons in females.     

Nerve regeneration along collagen filament and the presence of distal nerve stump

This article describes the regeneration of severed peripheral nerve axons along collagen filaments in the absence of the distal nerve stump. 22-mm long nerve guides made of collagen filaments were sutured to the proximal ends of severed rat sciatic nerves. The distal ends of the guides were sutured to the distal stumps of the nerves in a group and not sutured in the other. Nerve autografts and collagen tubes were used as controls. At 8 weeks postoperatively, the mean number and the mean diameter of myelinated axons were 5491 +/- 617 (mean +/- SD) and 2.3 +/- 1.3 microns at the distal ends of the collagen filaments nerve guides those the distal ends were sutured to the distal stumps of the nerves, while in the nerve autografts these were 4837 +/- 604 and 3.3 +/- 1.4 microns. These were 1992 +/- 770 and 2.7 +/- 1.2 microns at the distal ends of the collagen-filaments guides those the distal ends were not sutured to the distal stumps of the nerves, while in the nerve autografts these were 3041 +/- 847 and 2.3 +/- 1.1 microns. No axon was found at the distal ends of the collagen tubes. The results suggested that the contact guidance and the chemotaxis guided regenerating axons along the collagen filaments.     

Functional aspects of the regeneration of unmyelinated axons in the rat saphenous nerve

Electrophysiological experiments have been carried out to investigate aspects of unmyelinated axon regeneration in a transected cutaneous nerve. Some comparisons with regeneration of myelinated axons in the same nerve have also been made.

By 3 months after injury approximately 80% of the unmyelinated axons that had survived in the proximal stump had regenerated into the distal stump. About the same proportion of myelinated axons had regrown into the distal stump by this time. With both groups of axons there was no marked increase in the amount of regeneration across the injury site with longer recovery times. Conduction velocities in the regenerated unmyelinated axons tended to be slower across the injury site than proximally; the proximal conduction velocities did not differ from those in control nerves. The unmyelinated axons seemed to take longer to resupply the skin than did the myelinated ones, but in both cases the extent of skin innervation had reached about 60% of control values by 6 months after the injury.     

Effects of sciatic nerve resection on L7 spinal roots and dorsal root ganglia in adult cats

The size, distribution, and number of nerve fibers and neuronal perikarya in the L7 spinal roots and ganglia of adult cats were examined 35, 90, and 190 days after ipsilateral sciatic nerve resection. With increasing survival time the size spectra of myelinated ventral root nerve fibers showed a progressive flattening of the alpha peak. In the dorsal roots the myelinated fiber size distribution exhibited a marked shift toward smaller sizes. The reduction in the proportion of large myelinated axons was particularly evident in the dorsal roots. Less clearcut changes were found in the size distribution of spinal ganglion neuronal perikarya. No significant loss of axons could be detected in ventral or dorsal roots. There was, however, a marked reduction in the number of dorsal root ganglion neurons. This discrepancy suggested the possibility that an initial loss of dorsal root axons was concealed by recurrent sprouting of axons from the proximal nerve stump. However, neuroma excision 90 days after nerve resection did not lead to any reduction in dorsal root axon numbers. Thus, any ingrowth of new axons to the dorsal root should occur from levels proximal to the neuroma. In comparison with previous findings in kittens, peripheral nerve resection in adult cats had significantly smaller effects on sizes and numbers of spinal root nerve fibers as well as of dorsal root ganglion neurons. Therefore, the potential for restitution of the peripheral innervation by axon regeneration appeared to be basically greater in mature than in immature animals.     

Selective reinnervation of distal motor stumps by peripheral motor axons

Random matching of regenerating axons with Schwann tubes in the distal nerve stump is thought to contribute to the often poor results of peripheral nerve repair. Motor axons would be led to sensory end organs and sensory axons to motor end plates; both would remain functionless. However, the ability of regenerating axons to differentiate between sensory and motor environments has not been adequately examined. The experiments reported here evaluated the behavior of regenerating motor axons when given equal access to distal sensory and motor nerve stumps across an unstructured gap. "Y"-shape silicon chambers were implanted within the rat femoral nerve with the proximal motor branch as axon source in the base of the Y. The distal sensory and motor branches served as targets in the branches of the Y, and were placed 2 or 5 mm from the axon source. After 2 months for axon regeneration, horseradish peroxidase was used to label the motoneurons projecting axons into either the motor or the sensory stump. Equal numbers of motoneurons were labeled from the sensory and motor stumps at 2 mm, but significantly more motoneurons were labeled from the motor stump at 5 mm. (P = 0.016). This finding is consistent with selective reinnervation of the motor stump. Augmentation of this phenomenon to produce specific reunion of individual motor axons could dramatically improve the results of nerve suture.     

Collateral sprouting of sensory axons after end-to-side nerve coaptation--a longitudinal study in the rat

The end-to-side nerve coaptation is able to induce collateral sprouting of axons from the donor nerve and to provide functional reinnervation of the target tissue. Sensory axon sprouting and its effects on the donor nerve up to 9 months after the end-to-side nerve coaptation were studied in the rat. Peroneal, tibial and saphenous nerves were transected and ligated, and the distal stump of the transected peroneal nerve was sutured to the side of the uninjured sural nerve. The average skin area of the residual sensitivity to pinch due to the axons sprouting through the recipient peroneal nerve did not change statistically significantly between 4 and 9 months after surgery. Axon counting, measurements of compound action potentials and retrograde neuron labeling indicate that the sprouting of the myelinated sensory axons and unmyelinated axons through the recipient nerve was largely completed by 2 months and 4 months after the end-to-side nerve coaptation, respectively, and remained stable thereafter for at least 9 months. A decrease in the amplitude and area of the CAP of myelinated fibers, observed in the donor nerve up to 4 months after surgery, was probably due to mild degeneration of nerve fibers and a tendency of the diameter of myelinated axons to decline. However, no significant changes in functional, electrophysiological or morphological properties of the donor nerve could be observed at the end of the observational period, indicating that end-to-side nerve coaptation has no detrimental effect on the donor nerve on a long-term scale.     

Surgical pathology of spinal schwannoma: has the nerve of its origin been preserved or already degenerated during tumor growth?

OBJECTIVE: This study was aimed to understand ultrastructural pathology of nerves of tumor origin of spinal schwannomas, which has not been reported so far, in order to understand the mechanism of the postoperative functional restoration after the nerve transection. METHODS: From 13 patients who underwent sacrifice of an affected nerve root at total removal of spinal schwannomas (C2 conus), the proximal (spinal cord side, n = 12) and distal (dorsal root ganglion side, n = 10) stumps of the nerves of the tumor origin were collected and examined by light and electron microscope, followed by morphometric analysis (n = 9). RESULTS: Almost all of affected nerves at both proximal and distal to the lesion were composed of well-preserved myelin sheath and axons with mild disturbance of endo- and perineurial structures at light microscopic level except one case, which showed severe fibrosis. Electron-microscopically, regenerated axons with thin myelin were found in part in the proximal and distal nerves with few macrophages in three cases. The area of nerves (mm2), density of myelinated axons (axons/mm2) and total number of myelinated axons in the proximal stump (0.552 +/- 0.430, 10,400 +/- 5,240 and 5,480 +/- 4,790) was approximately 70%, 80% and 60%, respectively, of those in the distal stump (0.765 +/- 0.333, 12,400 +/- 5,180 and 9,970 +/- 8,630). CONCLUSIONS: This data combined with no permanent deficits after nerve transection suggest that the nerves of tumor origin are in the processes of slowly progressed deterioration with repeated degeneration and regeneration/remyelination, and the postoperative rapid recovery from the transient neurological deficit may be explained by functional compensation by the adjacent non-affected nerves with slow tumor growth.     

An efficient stereological sampling approach for quantitative assessment of nerve regeneration

Aims: The aim of the present study was to find the most efficient sampling strategy for stereological analysis of peripheral nerve, including the number of myelinated nerve fibres, axon cross‐sectional area and myelin sheet thicknesses of nerve fibres. Methods: Two groups of rats underwent experimental resection of the tibial and peroneal nerves. The first group received tibial‐peroneal end to end autograft repair (n = 6). Tibial and peroneal nerves were isolated, transected, and separated 1 cm distal to the trifurcation, where they lay adjacent to each other by a 1‐cm gap, then repaired with an autologous nerve graft taken from the tibial nerve. The proximal stump of the tibial nerve and distal stump of the peroneal nerve were connected to each other by means of the nerve graft. The second group received tibial‐peroneal repair with a flexible collagen tube (n = 6). After 90 days of recovery, animals were sacrificed and nerve segments were removed and sectioned for microscopy. Three different sampling strategies, that is, small, medium and large step sizes were applied to obtain each quantitative parameter. Results: There are no significant differences between these sampling strategies with respect to total number of myelinated nerve fibres, axon cross‐sectional area and myelin sheet thicknesses of nerve fibres. Conclusion: Findings show that one can achieve the desired estimate precisely with a rather large and less time‐consuming sampling approach. In addition, it was observed that the size discrepancy of nerve regeneration can be improved by collagen tube conduit even with a 1‐cm gap.     

Sleeve bridging of the rhesus monkey ulnar nerve with muscular branches of the pronator teres: multiple amplification of axonal regeneration

Multiple-bud regeneration, i.e., multiple amplification, has been shown to exist in peripheral nerve regeneration. Multiple buds grow towards the distal nerve stump during proximal nerve fiber regeneration. Our previous studies have verified the limit and validity of multiple amplification of peripheral nerve regeneration using small gap sleeve bridging of small donor nerves to repair large receptor nerves in rodents. The present study sought to observe multiple amplification of myelinated nerve fiber regeneration in the primate peripheral nerve. Rhesus monkey models of distal ulnar nerve defects were established and repaired using muscular branches of the right forearm pronator teres. Proximal muscular branches of the pronator teres were sutured into the distal ulnar nerve using the small gap sleeve bridging method. At 6 months after suture, two-finger flexion and mild wrist flexion were restored in the ulnar-sided injured limbs of rhesus monkey. Neurophysiological examination showed that motor nerve conduction velocity reached 22.63 ± 6.34 m/s on the affected side of rhesus monkey. Osmium tetroxide staining demonstrated that the number of myelinated nerve fibers was 1,657 ± 652 in the branches of pronator teres of donor, and 2,661 ± 843 in the repaired ulnar nerve. The rate of multiple amplification of regenerating myelinated nerve fibers was 1.61. These data showed that when muscular branches of the pronator teres were used to repair ulnar nerve in primates, effective regeneration was observed in regenerating nerve fibers, and functions of the injured ulnar nerve were restored to a certain extent. Moreover, multiple amplification was subsequently detected in ulnar nerve axons.     

Effects of predegeneration on nerve regeneration through silicone Y-chambers

The effects of nerve predegeneration on the preferential growth of regenerating axons were studied using a silicone Y-chamber model. This system provided a choice for axons to grow towards two distal nerve options, either a 7-day predegenerated nerve segment (PNS) or a fresh nerve segment (FNS). The rat peroneal or tibial nerve was inserted into the proximal intlet and the PNS and FNS of the corresponding nerve were inserted into the distal outlets. At 28 days postoperative, the size of the distal regenerate was significantly greater (26%) towards the PNS for the tibial nerve group. The density and number of regenerated myelinated axons in the distal nerve segment was greater on the PNS for both the tibial (97 and 88%, respectively) and peroneal (221 and 221%, respectively) nerve groups. In contrast, the elevated density and number of nonvascular nuclei was relatively constant for both PNS and FNS. Immunocytochemical and ultrastructural evidence support the hypothesis that the early activation of Schwann cells is primarily responsible for the enhanced regeneration and maturation observed in PNS. It is suggested that PNS might improve the outcome after clinical repair of injured peripheral nerves.     

The effects of an autologous transplant on patterns of regeneration in rat sciatic nerve

Autologous transplants are often used in repair of peripheral nerve injury. Quantitative evaluation of the results of such a transplant is obviously desirable. In previous study, we determined numerical and cytologic parameters of the regeneration that followed transection of rat sciatic nerve, but no transplant was used. This work now serves as a basis for evaluating the use of an autologous transplant in the same transection paradigm. Our procedure is to remove 8 mm of sciatic nerve in the thigh. The removed segment is then put into the center of a silicone tube and the proximal and distal stumps of the severed nerve are placed into the ends of the tube. The data show: (1) a high percentage of successful regenerations; (2) a relatively large nerve in the gap; (3) a typical outer perineurium underlying the epineurium; (4) a well-developed fascicular perineurium; and (5) approximately equal numbers of myelinated and unmyelinated axons in the gap and distal stump. If a transplant is not used there are: (1) a greater number of failures of regeneration; (2) a smaller nerve in the gap; (3) a less well-developed fascicular perineurium; (4) unequal numbers of axons in the gap as compared to the distal stump; and (5) no outer perineurium forms. The presence of a typical outer perineurium after a transplant and its absence if a transplant is not used is probably the most striking cytologic difference between the two paradigms. The equal numbers of axons in the gap and distal stump following regeneration after transplantation presumably indicate that all axons in the gap enter the distal stump without branching or ending blindly, a situation that is presumably beneficial and contrasts with the findings when a transplant is not used. Both paradigms show a remarkable increase in the density of blood vessels in the regenerated nerve in the gap between the two stumps. These findings will serve as a basis for further studies on the mechanisms of peripheral nerve regeneration.     

Peripheral nerve repair across a gap studied by repeated observation in a new window implant chamber

We have developed a new peripheral nerve chamber system which allows high resolution observation of the cellular events involved in nerve regeneration. The growth into the chamber is confined to a two-dimensional sheet resembling tissue culture. An intact blood supply forms within the chamber and by 100 days the nerve bridging the chamber has a nearly normal perineurium surrounding unmyelinated and myelinated axons. Degenerating axons are very rarely seen. The early growth in the chamber has a tendency to spread widely and form a two-dimensional sheet. This results in morphologies similar to those found in tissue culture and facilitates observation of individual elements. The initially wide tissue growth gradually re-models to form a bridge with a constant width similar to the width of the peroneal nerve. Occasionally a 'side arm' containing myelinated axons was retained even though the majority of axons appeared to loop back and rejoin the main bridge. Prefilling the chamber with Matrigel did not produce a significant enhancement of growth rate over that found following prefilling with sterile saline but did result in a more normally organized structure in the long term. Proximal ingrowth occurred at a similar rate in the absence of the distal stump. The structure of the proximal stump in the absence of the distal stump was similar to the structure when both stumps were present, including the presence of myelinated axons near the proximal port by 20 days. However, at subsequent stages the absence of a distal stump led to withdrawal of the proximal growth.     

Transforming growth factor-beta and forskolin attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo

Transforming growth factor-beta (TGF-beta) plays a central role in the regulation of Schwann cell (SC) proliferation and differentiation and is essential for the neurotrophic effects of several neurotrophic factors (reviewed by Unsicker and Krieglstein, 2000; Unsicker and Strelau, 2000). However, its role in peripheral nerve regeneration in vivo is not yet understood. Our studies were carried out to characterize (1) the effects of duration of regeneration, and chronic SC denervation on the number of tibial (TIB) motor neurons that regenerated axons over a fixed distance (25 mm into distal common peroneal [CP] nerve stumps), and (2) the effect of in vitro incubation of 6-month chronically denervated sciatic nerve explants with TGF-beta and forskolin on their capacity to support axonal regeneration in vivo. TIB--CP cross-suture in Silastic tubing was used, and regeneration into 0-24-week chronically denervated CP stumps was allowed for either 1.5 or 3 months. Chronically denervated rat sciatic nerve explants (3 x 3 mm(2)) were incubated in vitro with either DMEM and 15% fetal calf serum (D-15) plus TGF-beta/forskolin or D-15 alone for 48 h and placed into a 10-mm Silastic tube that bridged the proximal and distal nerve stumps of a freshly cut TIB nerve. The number of tibial motor neurons that regenerated axons through the explants and 25 mm into the distal nerve stump after 6 months, and TIB regeneration into the CP nerve stumps, were assessed using retrograde tracers, fluorogold, or fluororuby. We found that all tibial motor neurons regenerate their axons 25 mm into 0-4-week denervated CP nerve stumps after a regeneration period of 3 months. Reducing regeneration time to 1.5 months and chronic denervation, reduced the number of motor neurons that regenerated axons over 25 mm. Exposure of 6-month denervated nerve explants to TGF-beta/forskolin increased the number of motor neurons that regenerated through them from 258 +/-13; mean +/- SE to 442 +/- 22. Hence, acute treatment of atrophic SC with TGF-beta can reactivate the growth-permissive SC phenotype to support axonal regeneration.     

Nerve Regeneration in Predegenerated Basal Lamina Graft: The Effect of Duration of Predegeneration on Axonal Extension

We used predegenerated acellular grafts to bridge proximal and distal stumps of transected nerves and studied how the duration of predegeneration might affect axonal regeneration. Predegenerated acellular grafts were prepared by transecting the tibial nerve of donor rats and, after a period of degeneration, freeze-thawing a 40-mm long segment of the distal stump. Five degeneration periods were used: 0 days (for fresh grafts), 3 days, 1 week, 4 weeks, and 8 weeks. Fresh cellular grafts not treated with freeze-thawing were also used for comparison. Each graft was then transplanted to an isogeneic recipient rat, in which it was used to bridge the proximal stump of the transected left tibial nerve and the distal stamp of the transected right tibial nerve. Six weeks were allowed for the regeneration of axons in all grafts. The regeneration was then assessed by studying transverse sections of the grafts, to determine the maximum length that the axons had regenerated, and the packing density of axons (percentage of sampled areas occupied by axons). The results show that axons had grown to the maximum length in the 4-week predegenerated grafts, and had the highest packing density in the 1-week predegenerated grafts. Regeneration in the fresh acellular (0-day predegenerated) and 8-week predegenerated grafts, especially the latter, was poor. We examine the results with reference to time-dependent events of Wallerian degeneration and propose that there are beneficial effects of multiple factors on the grafts during the first 4 weeks of predegeneration, causing a slow but significant improvement in their capability to support axonal growth. The subsequent rapid deterioration of such capability may be related to structural changes in the extracellular scaffold.