Disconnected optic axons persist in the visual pathway during regeneration of the retino-tectal projection in the frog

Summary In this study, we crushed one optic nerve in the frog Litoria (Hyla) moorei and at intervals thereafter anterogradely labelled optic axons with horseradish peroxidase (HRP). For one series, HRP was applied between the eye and the crush site and in a second series between the crush site and the chiasm. A tectal projection of regenerating axons was seen in both series but, in addition, up to 12 weeks post-crush, the second series displayed an additional projection. Its appearance matched that of the disconnected, but persisting, optic axon terminals which are found after enucleation or optic nerve ligation. We conclude that, in the frog, many disconnected optic axons persist throughout the period of optic nerve regeneration and of restoration of an orderly retino-tectal map.Abbreviation HRP horseradish peroxidase     

Peripheral nerve explants grafted into the vitreous body of the eye promote the regeneration of retinal ganglion cell axons severed in the optic nerve

Summary We have conducted experiments in the adult rat visual system to assess the relative importance of an absence of trophic factors versus the presence of putative growth inhibitory molecules for the failure of regeneration of CNS axons after injury. The experiments comprised three groups of animals in which all optic nerves were crushed intra-orbitally: an optic nerve crush group had a sham implant-operation on the eye; the other two groups had peripheral nerve tissue introduced into the vitreous body; in an acellular peripheral nerve group, a frozen/thawed teased sciatic nerve segment was grafted, and in a cellular peripheral nerve group, a predegenerate teased segment of sciatic nerve was implanted. The rats were left for 20 days and their optic nerves and retinae prepared for immunohistochemical examination of both the reaction to injury of axons and glia in the nerve and also the viability of Schwann cells in the grafts. Anterograde axon tracing with rhodamine-B provided unequivocal qualitative evidence of regeneration in each group, and retrograde HRP tracing gave a measure of the numbers of axons growing across the lesion by counting HRP filled retinal ganglion cells in retinal whole mounts after HRP injection into the optic nerve distal to the lesion. No fibres crossed the lesion in the optic nerve crush group and dense scar tissue was formed in the wound site. GAP-43-positive and rhodamine-B filled axons in the acellular peripheral nerve and cellular peripheral nerve groups traversed the lesion and grew distally. There were greater numbers of regenerating fibres in the cellular peripheral nerve compared to the acellular peripheral nerve group. In the former, 0.6–10% of the retinal ganglion cell population regenerated axons at least 3–4 mm into the distal segment. In both the acellular peripheral nerve and cellular peripheral nerve groups, no basal lamina was deposited in the wound. Thus, although astrocyte processes were stacked around the lesion edge, a glia limitans was not formed. These observations suggest that regenerating fibres may interfere with scarring. Viable Schwann cells were found in the vitreal grafts in the cellular peripheral nerve group only, supporting the proposition that Schwann cell derived trophic molecules secreted into the vitreous stimulated retinal ganglion cell axon growth in the severed optic nerve. The regenerative response of acellular peripheral nerve-transplanted animals was probably promoted by residual amounts of these molecules present in the transplants after freezing and thawing. In the optic nerves of all groups the astrocyte, microglia and macrophage reactions were similar. Moreover, oligodendrocytes and myelin debris were also uniformly distributed throughout all nerves. Our results suggest either that none of the above elements inhibit CNS regeneration after perineuronal neurotrophin delivery, or that the latter, in addition to mobilising and maintaining regeneration, also down regulates the expression of axonal growth cone-located receptors, which normally mediate growth arrest by engaging putative growth inhibitory molecules of the CNS neuropil.     

Anomalous axonal outgrowth at the retina caused by injury to the optic nerve or tectal ablation in adultXenopus

Summary The retina and optic nerve head have been examined by light and electron microscopy in adultXenopus laeuis after injury to optic nerve fibres. Intraorbital resection, transection or crush of the optic nerve all resulted in the appearance at the retina of a mass of actively growing axons which formed a ring around the intraretinal and adjacent choroidal portions of the optic nerve head. Formation of this heterotopic axon population was first noted at two weeks after nerve injury and fibres persisted for at least six months. The ectopic fibres were seperated from the optic nerve head by astrocytes within the retina or by blood vessels and fibroblasts of the leptomeninges at extraretinal locations. In general, the orientation of the ectopic fibres was perpendicular to the fibres of the optic nerve. Bundles of axons were found between the ring of ectopic fibres and the pigment epithelial layer of the retina or among the blood sinuses of the choroid. Similar ectopic fibres were seen following transection of the optic nerve at the chiasm and after tectal ablation although the onset of these changes was slower than that seen after nerve resection. It is concluded that damage to visual pathways in the frog induces dramatic morphological alterations in the optic nerve and retina far proximal to the site of injury in this regenerating system.     

Differential effects of intravitreal optic nerve and sciatic nerve grafts on the survival of retinal ganglion cells and the regeneration of their axons

We have investigated the effects of intravitreal sciatic nerve (SN) and/or optic nerve (ON) grafts on the survival and the axonal regeneration of retinal ganglion cells (RGCs). Following transection of the ON, approximately 40% RGCs survived at 7 days post-axotomy (dpa). Results showed that the intravitreal ON graft significantly promoted the survival of RGCs at 7 dpa (39,063 vs 28,246). Intravitreal SN graft, however, did not rescue axotomized RGCs at 5, 7 or 14 dpa. Axotomized RGCs could be induced to regenerate axons along a segment of SN graft attached to the proximal stump of ON. On average, 608 axotomized RGCs were induced to regenerate axons along the attached SN graft. The presence of intravitreal SN graft promoted about 100% increase in the number of regenerating RGCs (1,227) relative to the control groups. The intravitreal ON graft, surprisingly, also induced about 100% more regenerating RGCs (1220) than in the control group. When SN and ON grafts were co-transplanted into the vitreous, about 200% more regenerating RGCs (1916) were observed than in the control group. These findings illustrated that the intravitreal ON graft rescued axotomized RGCs and enhanced the regeneration of retinal axons. This is the first report to show that ON promotes RGC axonal regeneration. The intravitreal SN graft did not rescue RGCs but promoted axonal regeneration. The differential effects of intravitreal ON and SN grafts on the survival and the RGC regeneration suggest that these might be two independently operating events.     

Biphasic cellular response to transection in the newt optic nerve: Glial reactivity precedes axonal degeneration

Summary Morphological interactions between axons and glia within the lesioned newt optic nerve were studied at time periods prior to the onset of Wallerian degeneration. Optic nerves were transected 0.5 mm from the eye, animals were killed at 5,10,20 and 30 min post-lesion, and the intracranial half of the tract was examined with light and electron microscopy. A sequence of structural changes was observed within the time interval 5–30 min post-lesion. Over the first 20 minutes these changes primarily involved the endogenous neuroglia; there was a displacement of glial nuclei from the center to the periphery of the nerve and an increase of 50–100% in glial cytoplasmic and nuclear area. Nuclei of reactive glia were euchromatic and surrounded by a high density of Golgi, vesicles, mitochondria and filaments, the last of which extended throughout the expanded glial processes. Optic axons appearintact at 20 min post-lesion except for some separation between the axolemma and myelin sheath in some of the myelinated fibres. By 30 min post-lesion both myelinated and non-myelinated fibres were found in various stages of lysis. Many of the expanded glial processes contained a population of vesicles aggregated adjacent to the glial plasmalemma. Profiles of infolded glial membranes suggested the opening of such vesicles into the extracellular space around degenerating axons. We conclude that, after optic nerve injury, there are very rapid reactive changes in glia and axons, with the changes in glia preceding the degenerative events in axons.     

Monensin induces distortion of optic nerve crossing at the chick embryo chiasm

Summary The effects of intraocular injection of monensin, a specific inhibitor of membrane glycoprotein transport within the axon, on the development of the chick embryo optic nerve were examined. A proportion of axons growing from the monensin-injected eye were misrouted into the opposite optic nerve at the chiasm and grew to the retina of the opposite eye. These results suggest that formation of the decussation pattern at the chiasm primarily depends on neurite fasciculation mediated by membrane glycoproteins.     

Fast and slow recovery phases of goldfish behavior after transection of the optic nerve revealed by a computer image processing system.

As the goldfish is a common experimental animal for vision research, including psychophysical behavior, it is very important to quantitatively score fish behavior. We have previously developed a computer image processing system which can acquire the positional coordinates of goldfish moving freely in an aquarium and determine turning directions (go straight, right or left turn). In the present study, an algorithm to determine tilting angles of moving goldfish was constructed. We also made histograms for quantifying the interaction between pairs of goldfish (two-point distance). By using these histograms, we estimated the time-course of behavioral regeneration after optic nerve transection in goldfish. Control goldfish showed an equal percentage of right or left turns and maintained an upright position in a dorsoventral axis. When the optic nerve of a goldfish was unilaterally sectioned, the goldfish showed predominant turning and slight tilting toward the intact eye. The abnormal turning and tilting behaviors lasted for 10-14 days and then gradually decreased, returning to control behaviors by one month after the unilateral transection. When the optic nerve of a single goldfish was bilaterally sectioned, it did not show any preferential turning and tilting behavior, which is similar to what was observed in control goldfish. However, the trace maps showed that, after bilateral sectioning, fish preferred to cross the center of the tank, which was unlike control fish. In control pairs, one goldfish chased the other with a fixed small range of two-point distances. However, in pairs of goldfish with bilateral transection of the optic nerve, the blind goldfish behaved independently of each other, with a long two-point distance. The long two-point distance of the blind goldfish lasted for at least two months and then slowly returned to control two-point distance by four months after bilateral transection. Such fast and slow recovery in goldfish behaviors evoked after unilateral and bilateral transection of the optic nerve is discussed with respect to reconnection of regenerating optic nerves in the fish central nervous system. This computer image processing system is a useful tool with which we can quickly and easily quantify fish behavior.     

Regeneration and retrograde degeneration of axons in the rat optic nerve

Summary The retinal stump of the rat optic nerve was examined histologically 1–64 weeks after intracranial section of the nerve with or without grafting of autologous peripheral nerve segments. Single unmyelinated axons and bundles of unmyelinated axons appeared in cut optic nerves and were most abundant 2–4 weeks after section. With light and electron microscope radioautography after injection of tritiated amino acids into the globe, it was confirmed that many unmyelinated fibres arose from the optic nerve rather than from nearby peripheral nerves and it was estimated that some axons regenerated as far as 0.5 mm. At or near the end of retinofugal axons, structures resembling growth cones were seen at 2 weeks and vesicle-containing swellings similar to synapses were found at 1–2 months. Outgrowth from optic nerve axons was not obviously enhanced by peripheral nerve grafts although a few retinofugal axons became ensheathed by Schwann cells. Retrograde axonal degeneration was rapid in both cut and grafted optic nerves, the number of nerve fibres near the globe falling to less than 10% of normal after 4 weeks. A few myelinated and unmyelinated fibres were still present 64 weeks after nerve transection. In conclusion, some cut axons in the rat optic nerve display a transient regenerative response before undergoing retrograde degeneration.     

Retrograde degeneration of myelinated axons and re-organization in the optic nerves of adult frogs (Xenopus laevis) following nerve injury or tectal ablation

Summary The optic nerve proximal to the lesion (toward the retina) was examined by light and electron microscopy in adultXenopus laevis after various types of injury to optic nerve fibres. Intraorbital resection, transection or crush of the optic nerve or ablation of the contralateral optic tectum all resulted in marked alterations in the myelinated axon population and in the overall appearance of the nerve proximal to the site of injury. Examination of the nerves from 3 days to 6 months postoperatively indicated that a progressive, retrograde degeneration of myelin and loss of large-diameter axons occurred throughout the retinal nerve stump regardless of the type of injury or distance of the injury from the retina. The retinal stump of nerves receiving resection or transection showed a nearly complete loss of myelin and large-diameter axons while the degree of degeneration was subtotal in nerves receiving crush injury or after lesions farther from the retina (i.e. tectal ablation). In addition, the entire retinal nerve stump after all types of injury was characterized by the appearance of an actively growing axon population situated circumferentially under the glia limitans. The latter fibres are believed to represent regrowing axons which are being added onto the nerve, external to the original axon population and are suspected to modify actively the glial terrain and glia limitans.     

Regeneration in the optic nerve of adult rats: influences of cultured astrocytes and optic nerve grafts of different ontogenetic stages

Summary We have studied the effects of transplanted optic nerves of different ontogenetic stages (E19 to adult), and cultured astrocytes from P2 cerebral cortex on the regeneration of axons in the optic nerve of adult rats. Regeneration was visualized by anterograde tracing with rhodamine-iso-thiocyanate. Grafts were identified with Nuclear Yellow. Astroglia within both the cut optic nerve and the transplants were detected by anti-glial fibrillary acidic protein staining. In control animals (cut optic nerve, 2–3 mm behind the optic disc), only a few neurites were found 15 days after the operation which grew randomly for short distances into the surrounding meningeal sheaths. Perinatal (E19 to P2) optic nerves induced a massive outgrowth of RITC-filled axons from the host optic nerve. The regenerating fibres grew for up to 3 mm towards the graft, ahead of glial fibrillary acidic protein-positive astroglia emanating from the host optic nerve that seemed to follow them. Although the regenerating fibres reached the grafts, they did not penetrate them. Optic nerve grafts of increasing age elicited smaller growth responses; e.g. grafts from P8 promoted only a very limited (several 100 m) growth response, grafts from P12 and later induced outgrowth comparable with that of control animals. Grafted astrocytes from P2 donors that had previously been grown in culture, were also capable of promoting outgrowth of rhodamine-iso-thiocyanate-filled axons from the host optic nerve. These findings suggest that only astrocytes at an immature stage of differentiation are capable of inducing axon growth from the adult optic nerve. Furthermore, the absence of an obvious cellular bridge between host and graft suggests that the graft effect is probably mediated by the release of astroglia-derived diffusible neurite growth promoting factors.     

Regrowth of retinal ganglion cell axons into a peripheral nerve graft in the adult hamster is enhanced by a concurrent optic nerve crush

Summary Transplantation of a segment of peripheral nerve to the retina of the adult hamster resulted in regrowth of damaged ganglion cell axons into the graft, with the fastest regenerating axons extending at 2 mm/day after an initial delay of 4.5 days (Cho and So 1987b). In this study, the effect of making 2 lesions on the same axon (the conditioning lesion effect) on the regrowth of ganglion cell axons into the peripheral nerve graft was examined. When a conditioning lesion (first lesion) was made by crushing the optic nerve 7 or 14 days before the peripheral nerve grafting (the second lesion) to the retina, the distance of regrowth achieved by the fastest regenerating axons in the graft, measured at the 7th post-grafting day, was lower than in animals with a peripheral nerve grafted to a normal eye. This indicated that in contrast to the situation in peripheral nerve axons (Forman et al. 1980) and goldfish optic axons (Edwards et al. 1981), the conditioning lesion was unable to enhance the regrowth of mammalian retinal ganglion cell axons. However, when crushing of the optic nerve was followed immediately by peripheral nerve grafting, an enhancement in axonal regrowth could be observed. The initial delay time before the axons extended into the peripheral nerve graft was reduced by 1 day while the rate of elongation of the fastest regrowing axons in the graft apparently remained unchanged. Moreover, the shortening of the initial delay could still be observed even when the sequence of performing the 2 lesions was reversed. From these data, it was concluded that the classical conditioning lesion effect was not responsible for the enhancement observed. Rather it was suggested that changes in the intra-retinal environment brought about by crushing of the optic nerve might account for it.     

Axon diameter distributions across the monkey's optic nerve

The distribution of axons according to diameter has been examined in the optic nerve of old world monkeys. Axon diameters were measured from electron micrographs, and histograms were constructed at regular intervals across a section through the optic nerve to reveal the local axon diameter distribution. The total axon diameter distribution was also estimated. Fine-calibre optic axons (less than 2.0 micron in diameter) are found at all locations across the optic nerve. They are most frequent centrotemporally, where very few coarse optic axons can be found, but also make up the majority at the optic nerve's periphery. Coarse optic axons (greater than 2.0 microns in diameter) are increasingly common at progressively peripheral positions in the nerve. Around the nerve's circumference, these coarse optic axons are least numerous temporally, and most common dorsonasally. The axon diameter distribution peaks around 1.25 microns at most locations across the optic nerve, but there are more, slightly larger (1.5-2.0 microns), optic axons dorsally than ventrally. The estimated total axon diameter distribution is unimodal, peaking at 1.0-1.25 microns, with an extended tail towards larger diameters. This centroperipheral gradient of increasing axon diameters across the optic nerve is not substantial enough to account for the partial segregation of axons by size in the monkey's optic tract: there, coarse optic axons form a conspicuously greater proportion of the local axon diameter distribution along the tract's superficial (sub-pial) border, and fine optic axons are the only axons present near the tract's deep border. Hence, the fibre distribution in the optic tract cannot be formed by a simple combination of the fibre distributions of the two respective half-nerves, as described in the classic neuro-ophthalmologic literature. Rather, the present results, in conjunction with previous results from the optic tract, demonstrate that there must be a reorganization of axons by size in or near the optic chiasm.     

Changes in NADPH diaphorase expression in the fish visual system during optic nerve regeneration and retinal development

The various functions of nitric oxide (NO) in the nervous system are not fully understood, including its role in neuronal regeneration. The goldfish can regenerate its optic nerve after transection, making it a useful model for studying central nervous regeneration in response to injury. Therefore, we have studied the pattern of NO expression in the retina and optic tectum after optic nerve transection, using NADPH diaphorase histochemistry. NO synthesis was transiently up-regulated in the ganglion cell bodies, peaking during the period when retinal axons reach the tectum, between 20–45 days after optic nerve transection. Enzyme activity in the tectum was transiently down-regulated and then returned to control levels at 60 days after optic nerve transection, during synaptic refinement. To compare NO expression in the developing and regenerating retina, we have looked at NO expression in the developing zebrafish retina. In the developing zebrafish retina the pattern of staining roughly followed the pattern of development with the inner plexiform layer and horizontal cells having the strongest pattern of staining. These results suggest that NO may be involved in the survival of ganglion cells in the regenerating retina, and that it plays a different role in the developing retina. In the tectum, NO may be involved in synaptic refinement.     

Regrowth of PNS axons through grafts of the optic nerve of the Browman-Wyse (BW) mutant rat

Summary We have examined the behaviourin vivo of regenerating PNS axons in the presence of grafts of optic nerve taken from the Browman-Wyse mutant rat. Browman-Wyse optic nerves are unusual because a 2–4 mm length of the proximal (retinal) end of the nerve lacks oligodendrocytes and CNS myelin and therefore retinal ganglion cell axons lying within the proximal segment are unmyelinated and ensheathed by processes of astrocyte cytoplasm. Schwann cells may also be present within some proximal segments. Distally, Browman-Wyse optic nerves are morphologically and immunohistochemically indistinguishable from control optic nerves.When we grafted intact Browman-Wyse optic nerves or triplets consisting of proximal, junctional and distal segments of Browman-Wyse optic nerve between the stumps of freshly transected sciatic nerves, we found that regenerating axons avoided all the grafts which did not contain Schwann cells, i.e., proximal segments which contained only astrocytes; regions of Schwann cell-bearing proximal segments which did not contain Schwann cells; junctional and distal segments (which contained astrocytes, oligodendrocytes and CNS myelin debris). However, axons did enter and grow through proximal segments which contained Schwann cells in addition to astrocytes. Schwann cells were seen within grafts even after mitomycin C pretreatment of sciatic proximal nerve stumps had delayed outgrowth of Schwann cells from the host nerves; we therefore conclude that the Schwann cells which became associated with regenerating axons within the grafts of Browman-Wyse optic nerve were derived from an endogenous population. Our findings indicate that astrocytes may be capable of supporting axonal regeneration in the presence of Schwann cells.     

Influence of peripheral nerve grafts on the expression of GAP-43 in regenerating retinal ganglion cells in adult hamsters

Summary We have examined the ability of axotomized retinal ganglion cells in adult hamsters, to regenerate axons into a peripheral nerve graft attached to the optic nerve and the expression of GAP-43 by these neurons. We also examined the effect on these events of transplanting a segment of peripheral nerve to the vitreous body. The left optic nerves in three groups of hamsters were replaced with a long segment of peripheral nerve attached to the proximal stump of the optic nerve 2 mm from the optic disc to induce regeneration of retinal ganglion cells into the peripheral nerve. An additional segment of peripheral nerve was transplanted into the vitreous of the left eye in the second group. The animals from the first and second groups were allowed to survive for 1–8 weeks and the number of regenerating retinal ganglion cells was determined by applying the retrograde tracer, Fluoro-Gold to the peripheral nerve graft and the expression of GAP-43 was studied by immunocytochemistry in the same retinas. As a control, a segment of optic nerve was transplanted into the vitreous body of the left eye in the third group of hamsters. These animals were allowed to survive for 4 weeks and the number of regenerating retinal ganglion cells was counted as in Groups 1 and 2. The percentages of the regenerating retinal ganglion cells which also expressed GAP-43 were very high at all time points in Group 1 (with no intravitreal peripheral nerve) and Group 2 (with intravitreal peripheral nerve) and at 4 weeks for the Group 3 (with intravitreal optic nerve) animals. In addition, the number of regenerating retinal ganglion cells, the number of retinal ganglion cells expressing GAP-43 and the number of regenerating retinal ganglion cells which also expressed GAP-43 were much higher in Group 2 than in Group 1 at all the time points and it was also much higher in Group 2 than in Group 3 at 4 weeks whereas there was no significant difference between the results from Groups 1 and 3 at 4 weeks. These data suggested that there was a close correlation between the number of the axotomized retinal ganglion cells regenerating axons into the peripheral nerve graft attached to the optic nerve and the expression of GAP-43. In addition, the intravitreal peripheral nerve, probably by releasing various neurotrophic factors and by acting synergistically, can enhance the expression of GAP-43 in some of the axotomized retinal ganglion cells and promote the regeneration of retinal ganglion cells into the peripheral nerve graft.     

Preferential loss of collaterals from goldfish retinal axons in the optic tract is delayed by tetrodotoxin

Following optic nerve section (ONS) in goldfish, the right eye was repeatedly injected with tetrodotoxin (TTX) and the left eye with Ringer solution. At various survival periods after ONS, horseradish peroxidase (HRP) was applied to small groups of axons in dorsal or ventral retina in both eyes. Counts of labeled regenerating retinal axons show that 43 +/- 4.6% of regenerating axons course through the inappropriate brachium of the optic tract between 20 and 65 days after ONS. The amount of misrouted axons declines to 30 +/- 6.1% between 70 and 80 days after ONS. Under TTX blockade the reduction of misrouted axons is delayed but reaches 27 +/- 3.6% at 150 days after ONS.     

Temporal and spatial patterns of expression of laminin, chondroitin sulphate proteoglycan and HNK-1 immunoreactivity during regeneration in the goldfish optic nerve

Summary Current views suggest that the extracellular environment is critically important for successful axonal regeneration in the CNS. The goldfish optic nerve readily regenerates, indicating the presence of an environment that supports regeneration. An analysis of changes that occur during regeneration, in this model may help identify those molecules that contribute to a favourable environment for axonal regrowth. We examined the distribution and expression of two extracellular matrix molecules, laminin and chondroitin sulphate proteoglycan, and a carbohydrate epitope shared by a family of adhesion molecules (HNK-1), using immunocytochemical detection in sections from the normal adult goldfish optic nerve and in nerves from one hour to five months following optic nerve crush. We also usedin vitro preparations to determine if neurites in retinal explants could express these same molecules.The linear distributions of laminin and chondroitin sulphate proteoglycan immunoreactivity in control optic nerves are co-extensive with the glia limitans, suggesting both are expressed by non-neuronal components surrounding the axon fascicles. Between one and three weeks postoperatively when axons elongate and reach their target, laminin and chondroitin sulphate proteoglycan immunoreactivity increases around the crush site and distally. At six weeks postoperatively the pattern of immunoreactivity has returned to normal. While the temporal pattern of changes in immunoreactivity is similar, the spatial pattern of these two extracellular proteins in the regenerating nerve differs. Chondroitin sulphate proteoglycan immunoreactivity is organized in discrete columns associated with regenerating axons while laminin immunoreactivity is more diffusely distributed. Examination of retinal explants reveals growing neurites express chondroitin sulphate proteoglycan but not laminin. Our results suggest that laminin is only associated with non-neuronal cells, while chondroitin sulphate proteoglycan is associated with axons as well as non-neuronal cells.HNK-1 immunoreactivity is co-extensive with both the glia limitans and axon fascicles and is more extensively distributed in the intact nerve than either laminin or chondroitin sulphate proteoglycan immunoreactivity. In contrast to laminin and chondroitin sulphate proteoglycan, HNK-1 immunoreactivity is substantially decreased at the crush site within one week following optic nerve crush. HNK-1 immunoreactivity reappears through the crush site during the next several weeks, although non-immunoreactive regions, co-extensive with areas predominantly containing non-neuronal cells, persist both proximal and distal to the crush, up to six weeks postoperatively. The pattern suggests that HNK-1 epitope expression by these non-neuronal cells is decreased during axonal regeneration.Our results show that each of these molecules is constitutively expressed with a unique distribution in the normal goldfish optic nerve and each exhibits different patterns of change during regeneration. It thus appears that each may contribute to modifications of the environment that supports axonal regeneration. Both neurons and non-neuronal cells contribute to these changes.     

Regeneration and remyelination of Xenopus tadpole optic nerve fibres following transection or crush

Optic nerves of stage 54-56 Xenopus laevis tadpoles were either transected or crushed, and subsequent Wallerian degeneration, regeneration, and remyelination were examined. After 4 days, normal myelinated fibres were no longer present in the distal stump, and only a few unmyelinated fibres remained. After 10-13 days, the distal nerve consisted mainly of a core of reactive astrocytes with enlarged processes and scattered oligodendrocytes which persisted throughout the degenerative period. Regenerating axons traversed the site of the lesion and extended into the distal stump within 13-15 days. As regeneration progressed, astrocytic processes extended radially from the optic nerve's central cellular core and formed longitudinal compartments for regenerating axons. Between 15-19 days, a few regenerating fibres were remyelinated and by 35 days, more axons were surrounded either by thin collars of oligodendrocyte cytoplasm or by 1-3 spiral turns of myelin membrane. By 95 days, the number of myelinated fibres had increased to about 50% of those present in control nerves. Their myelin sheaths were normal in appearance and thickness relative to their respective axon diameters. The largest axons were surrounded by compact sheaths with 4-9 lamellae.     

Different multiple regeneration capacities of motor and sensory axons in peripheral nerve

Abstract After peripheral nerve injury, axons often project sprouts from the node of Ranvier proximal to the damage site. It is well known that one parent axon can sprout and maintain several regenerating axons. If enough endoneurial tubes in the distal stump are present for the regenerating axons to grow along, then the number of mature myelinated nerve fibers in the distal stump will be greater than the number in the proximal stump. "Multiple regeneration" is used to describe this phenomenon in the peripheral nerve. According to previous studies, a prominent nerve containing many axons can be repaired by the multiple regenerating axons sprouting from another nerve that contains fewer axons. Most peripheral nerves contain a mixture of myelinated motor and sensory axons as well as unmyelinated sensory and autonomic axons. In this study, a multiple regeneration animal model was developed by bridging the proximal common peroneal nerve with the distal common peroneal nerve and the tibial nerve. Differences in the multiple regeneration ratio of motor and sensory nerves were evaluated using histomorphometry one month after ablating the dorsal root ganglion (DRGs) and ventral roots, respectively. The results suggest that the motor nerves have a significantly larger multiple regeneration ratio than the sensory nerves at two different time points.