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.     

Ciliary neurotrophic factor promotes the regrowth capacity but not the survival of intraorbitally axotomized retinal ganglion cells in adult hamsters

Ciliary neurotrophic factor has recently been shown to promote the axonal regrowth of retinal ganglion cells into a peripheral nerve graft following an intracranial transection of the optic nerve (approximately 7 mm from the optic disc). It is unclear whether the enhancement of axonal regrowth by ciliary neurotrophic factor application correlates with the enhancement of survival of retinal ganglion cells and/or the up-regulation of expression of growth-associated protein-43 messenger RNA in retinas. The present study evaluated the regenerative capacity of retinal ganglion cells following intraorbital transection of the optic nerve (approximately 1.5 mm from the optic disc) and the attachment of a peripheral nerve to the ocular stump of the optic nerve. In addition, we have determined the survival of retinal ganglion cells and the expression of growth-associated protein-43 messenger RNA in ciliary neurotrophic factor-treated retinas following optic nerve transection. The results showed that in the ciliary neurotrophic factor-treated retinas, the number of retinal ganglion cells which had regrown axons into a peripheral nerve is about four times more than the control. In the axotomized retinas, ciliary neurotrophic factor initiated sprouting of axon-like processes at 14 and 28 days post-axotomy and up-regulated the expression level of growth-associated protein-43 messenger RNA at 7, 14 and 28 days post-axotomy. However, ciliary neurotrophic factor did not prevent the death of axotomized retinal ganglion cells. We suggest that one possible mechanism for the axonal regeneration of axotomized retinal ganglion cells by ciliary neurotrophic factor could be mediated by the up-regulation of growth-associated protein-43 gene expression and not by increasing the pool of surviving retinal ganglion cells after axotomy.     

Increased intrinsic neuronal vulnerability and decreased beneficial reaction of macrophages on axonal regeneration in aged rats

Previously we showed that macrophage activation in the eye by intravitreal application of zymosan increased retinal ganglion cell (RGC) survival and axonal regeneration after optic nerve injury. It is known that the intrinsic ability of CNS neurons to survive and to regrow axons after optic nerve injury differs between developing and adult mammals. However, whether aged animals also differ in their ability to survive and regrow injured axons are not known. In this study we investigated whether the abilities of RGCs to survive and to regrow injured axons differed between rats aged 6-8, 60 and over 96 weeks, and whether macrophage responses in the eye were different at different ages. We found that the intrinsic viability of RGCs, as shown in vitro, was reduced in aged rats, but RGC viability after optic nerve injury in vivo was similar among rats of the different ages. The ability of RGCs to regrow injured axons into a peripheral nerve graft also remained similar between young and aged rats. Macrophage activation in the eye was confirmed to be beneficial and provided the basis for zymosan treatment-dependent RGC protection. However, reduced activation of macrophages in zymosan-treated eyes was seen in aged rats. Importantly, this reduced macrophage activation in aged rats led to a decreased level of RGC axonal regeneration when compared with that in young rats of the same treatment. Thus age influences the intrinsic viability of RGCs and the beneficial impact of macrophages on RGC axonal regeneration after optic nerve injury.     

Electron microscopic study of the interaction of axons and glia at the site of anastomosis between the optic nerve and cellular or acellular sciatic nerve grafts

Summary The interactions between retinal ganglion cell (RGC) axons and glia at the site of optic nerve section and at the junctional zone between optic nerve and cellular or acellular peripheral nerve (PN) grafts have been studied electron microscopically. After transection, RGC axons, accompanied by processes of astrocyte cytoplasm, grew out from the proximal optic nerve stump into the scar tissue that developed between proximal and distal stumps. However, axons failed to cross the scar, and none entered the distal stump. By 3 days post lesion (DPL), bundles of RGC axons, accompanied by astrocytes and oligodendrocytes, grew out from the proximal optic nerve stump into the junctional zone between optic nerve and either type of PN graft. The bundles of RGC axons and growth cones that grew towards acellular PN grafts degenerated within 10–20 DPL; by 30 DPL a small number of axons persisted within the end of the proximal optic nerve stump. No axons were seen within the acellular PN grafts. These results suggest that reactive axonal sprouting, axon outgrowth and glial migration from the proximal optic nerve stump are events that occur during an acute response to injury, and that they are independent of the presence of Schwann cells. However, it would appear that few axons entered either scar or junctional zone unless accompanied by glia. There was little evidence that axon outgrowth was laminin-dependent.The bundles that grew towards cellular PN grafts encountered cells that we have identified as Schwann cells within the junctional zone: the axons in these bundles survived and entered the cellular grafts. Schwann cells migrated into the junctional zone from the cellular PN graft. It is probable that Schwann cells facilitated RGC axon entry into the graft directly by both cell contact and the secretion of neuronotrophic factors, and indirectly by modifying the CNS glia in the junctional zone.     

Rescuing axons from degeneration does not affect retinal ganglion cell death

After a traumatic injury to the central nervous system, the distal stumps of axons undergo Wallerian degeneration (WD), an event that comprises cytoskeleton and myelin breakdown, astrocytic gliosis, and overexpression of proteins that inhibit axonal regrowth. By contrast, injured neuronal cell bodies show features characteristic of attempts to initiate the regenerative process of elongating their axons. The main molecular event that leads to WD is an increase in the intracellular calcium concentration, which activates calpains, calcium-dependent proteases that degrade cytoskeleton proteins. The aim of our study was to investigate whether preventing axonal degeneration would impact the survival of retinal ganglion cells (RGCs) after crushing the optic nerve. We observed that male Wistar rats (weighing 200-400 g; n=18) treated with an exogenous calpain inhibitor (20 mM) administered via direct application of the inhibitor embedded within the copolymer resin Evlax immediately following optic nerve crush showed a delay in the onset of WD. This delayed onset was characterized by a decrease in the number of degenerated fibers (P<0.05) and an increase in the number of preserved fibers (P<0.05) 4 days after injury. Additionally, most preserved fibers showed a normal G-ratio. These results indicated that calpain inhibition prevented the degeneration of optic nerve fibers, rescuing axons from the process of axonal degeneration. However, analysis of retinal ganglion cell survival demonstrated no difference between the calpain inhibitor- and vehicle-treated groups, suggesting that although the calpain inhibitor prevented axonal degeneration, it had no effect on RGC survival after optic nerve damage.     

The outgrowth response of the axons of developing and regenerating rat retinal ganglion cells in vitro to neurotrophin treatment

BDNF and NT-4 (but not NT-3 or CNTF) significantly enhanced the outgrowth of early embryonic and adult regenerating RGC axons when provided with a supportive substrate in vitro. BDNF and NT-4 treatment transiently increased RGC axon outgrowth from E15 rat retinas but not from retinas at older embryonic ages. The transient effect of BDNF and NT-4 and the inability of the neurotrophins to promote outgrowth from older embryonic retinal explants suggests a time frame of neurotrophin action and that other chemical factors (target-derived or otherwise) may be necessary for the continued maintenance of developing RGC axons. BDNF and NT-4 also enhanced the outgrowth of regenerating axons from adult retinal explants, but appeared to have a more subtle effect on axon outgrowth, in that the growth-promoting effects of BDNF and NT-4 appeared continuous throughout the incubation period. The suppression of RGC axon outgrowth from embryonic and adult retinae cultured in trkB-IgG-containing medium suggests that the response of developing and regenerating axons, to BDNF and NT-4 are likely to occur through trkB signalling.     

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.     

Optic nerve regeneration after intravitreal peripheral nerve implants: trajectories of axons regrowing through the optic chiasm into the optic tracts

We have studied axon regeneration through the optic chiasm of adult rats 30 days after prechiasmatic intracranial optic nerve crush and serial intravitreal sciatic nerve grafting on day 0 and 14 post-lesion. The experiments comprised three groups of treated rats and three groups of controls. All treated animals received intravitreal grafts either into the left eye after both left sided (unilateral) and bilateral optic nerve transection, or into both eyes after bilateral optic nerve transection. Control eyes were all sham grafted on day 0 and 14 post-lesion, and the optic nerves either unlesioned, or crushed unilaterally or bilaterally. No regeneration through the chiasm was seen in any of the lesioned control optic nerves. In all experimental groups, large numbers of axons regenerated across the optic nerve lesions ipsilateral to the grafted eyes, traversed the short distal segment of the optic nerve and invaded the chiasm without deflection. Regeneration was correlated with the absence of the mesodermal components in the scar. In all cases, axon regrowth through the chiasm appeared to establish a major crossed and a minor uncrossed projection into both optic tracts, with some aberrant growth into the contralateral optic nerve. Axons preferentially regenerated within the degenerating trajectories from their own eye, through fragmented myelin and axonal debris, and reactive astrocytes, oligodendrocytes, microglia and macrophages. In bilaterally lesioned animals, no regeneration was detected in the optic nerve of the unimplanted eye. Although astrocytes became reactive and their processes proliferated, the architecture of their intrafascicular processes was little perturbed after optic nerve transection within either the distal optic nerve segment or the chiasm. The re-establishment of a comparatively normal pattern of passage through the chiasm by regenerating axons in the adult might therefore be organised by this relatively immutable scaffold of astrocyte processes. Binocular interactions between regenerating axons from both nerves (after bilateral optic nerve transection and intravitreal grafting), and between regenerating axons and the intact transchiasmatic projections from the unlesioned eye (after unilateral optic nerve lesions and after ipsilateral grafting) may not be important in establishing the divergent trajectories, since regenerating axons behave similarly in the presence and absence of an intact projection from the other eye.     

Expression of neuronal intermediate filament proteins ON1 and ON2 during goldfish optic nerve regeneration: effect of tectal ablation

Goldfish retinal explants were used to study optic tectum participation in the regulation of intermediate filament protein synthesis in retinal ganglion cells during optic nerve regeneration. Retinas were explanted at various times after removal of the contralateral optic tectum. The synthesis of the intermediate filament proteins ON1 and ON2 in the cultures was quantitated by labeling with [35S]methionine, followed by two-dimensional gel electrophoresis, autoradiography, and densitometry. Neuritic growth from the explants was quantitated based on fiber length and density. In retinal explants placed in culture after 23 days of optic nerve regeneration, the synthesis of ON1 and ON2 was reduced when the tectum had been ablated. In contrast, synthesis of these proteins in explants placed in culture at an earlier stage of regeneration was not affected by tectal ablation. At all time points tested, neuritic outgrowth from retinal explants was stimulated by tectal ablation. These findings indicate that the synthesis of the ON1 and ON2 intermediate filament proteins during regeneration is not directly regulated by axonal volume. Further, our findings suggest that interaction between growing axons and tectum is important for sustained expression of these proteins during the later stages of optic nerve regeneration.     

Metallothionein-IIA promotes neurite growth via the megalin receptor

Metallothionein (MT)-I/II has been shown to be neuroprotective and neuroregenerative in a model of rat cortical brain injury. Here we examine expression patterns of MT-I/II and its putative receptor megalin in rat retina. At neonatal stages, MT-I/II was present in retinal ganglion cells (RGCs) but not glial or amacrine cells; megalin was present throughout the retina. Whilst MT-I/II was absent from adult RGC in normal animals and after optic nerve transection, the constitutive megalin expression in RGCs was lost following optic nerve transection. In vitro MT-IIA treatment stimulated neuritic growth: more RGCs grew neurites longer than 25 μm (P < 0.05) in dissociated retinal cultures and neurite extension increased in retinal explants (P < 0.05). MT-IIA treatment of mixed retinal cultures increased megalin expression in RGCs, and pre-treating cells with anti-megalin antibodies prevented MT-IIA-stimulated neurite extension. Our results indicate that MT-IIA stimulates neurite outgrowth in RGCs and may do so via the megalin receptor; we propose that neurite extension is triggered via signal transduction pathways activated by the NPxY motifs of megalin’s cytoplasmic tail.     

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.     

The retinal axon's pathfinding to the optic disk

Retinal ganglion cell (RGC) axons travel in radial routes unerringly toward the optic disk, their first intermediate target in the center of the eye. The path of the RGC growth cone is restricted to a narrow zone subjacent to the endfeet of Müller glial cells and the vitreal basal lamina. The present survey indicates that RGC growth cones are guided by many molecular cues along their pathway which are recognized by receptors on their surface. Growth-promoting molecules on Müller glial endfeet and in the basal lamina assist growth cones in maintaining contact with these elements. The repellant character of deeper retinal laminae discourages them from escaping the RGC axon layer. Cell adhesion/recognition proteins enable growth cones to fasciculate with preformed axons in their vicinity. It is still unclear whether the optic disk emits long range guidance components which enable the growth cones to steer toward it. Recent evidence in fish indicates the existence of an axonal receptor (neurolin) for a guidance component of unknown identity. Receptor blockade causes RGC axons to course in aberrant routes before they reach the disk. At the disk, axons receive signals to exit the retina. Contact with netrin-1 at the optic disk/nerve head encourages growth cones to turn into the nerve. This response requires the axonal netrin receptor DCC, laminin-1, beta-integrin and most likely the UNC5H netrin receptors which convert the growth encouraging signal into a repulsive one which drives growth cones into the nerve.     

Regeneration of axons in the optic nerve of the adult Browman-Wyse (BW) mutant rat

Summary We have studied the regeneration of axons in the optic nerves of the BW rat in which both oligodendrocytes and CNS myelin are absent from a variable length of the proximal (retinal) end of the nerve. In the optic nerves of some of these animals, Schwann cells are present. Axons failed to regenerate in the exclusively astrocytic environment of the unmyelinated segment of BW optic nerves but readily regrew in the presence of Schwann cells even across the junctional zone and into the myelin debris filled distal segment. In the latter animals, the essential condition for regeneration was that the lesion was sited in a region of the nerve in which Schwann cells were resident. Regenerating fibres appeared to be sequestered within Schwann cell tubes although fibres traversed the neuropil intervening between the ends of discontinuous bundles of Schwann cell tubes, in both the proximal unmyelinated and myelin debris laden distal segments of the BW optic nerve. Regenerating axons never grew beyond the distal point of termination of the tubes. These observations demonstrate that central myelin is not an absolute requirement for regenerative failure, and that important contributing factors might include inhibition of astrocytes and/or absence of trophic factors. Regeneration presumably occurs in the BW optic nerve because trophic molecules are provided by resident Schwann cells, even in the presence of central myelin, oligodendrocytes and astrocytes. All the above experimental BW animals also have Schwann cells in their retinae which myelinate retinal ganglion cell axons in the fibre layer. Control animals comprised normal Long Evans Hooded rats, BW rats in which both retina and optic nerve were normal, and BW rats with Schwann cells in the retina but with normal, i.e. CNS myelinated, optic nerves. Regeneration was not observed in any of the control groups, demonstrating that, although the presence of Schwann cells in the retina may enhance the survival of retinal ganglion cells after crush, concomitant regrowth of axons cut in the optic nerve does not take place.     

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.     

In vitro regeneration of adult rat ganglion cell axons from retinal explants

Summary The potential for regeneration of adult rat ganglion cell (RGC) axons was investigated in vitro. Explants from RITC (rhodamine-B-isothiocyanate) retrogradely labeled and in vivo axotomized retinae were placed on dishes coated with various substrates. The retinal pieces were cultivated in a serum-free medium and incubated under 50 to 80% O₂-enriched and 5% CO₂-containing atmosphere. Under these conditions, massive outgrowth of fibers was observed as early as 24 h after explantation and over a period of time extending up to 7 days in culture. By various criteria, two main types of processes could be characterized: (1) Short, thick processes emerged from either migrated flat cells or from cells inside the retinal explant, and (2) long and thin processes emerged from cells in the ganglion cell layer (GCL). By immunohistochemistry, the short processes and the cells from which they had emerged, appeared to be glial acidic fibrillary protein (GFAP)-positive Thy 1 and L 1-negative, suggesting their glial nature. The second type of long, thin processes appeared to be GFAP-negative, L1- and Thy 1-positive, indicating that they were neuronal, probably RGC processes. Proof that long fibers emerged from RGCs was provided by retrograde labeling of RGCs with RITC prior to explanation. Numerous RITC-filled RGCs survived in vitro. Regrowing axons retransported part of the accumulated fluorescent dye in the orthograde direction and thus unequivocally permitted their identification as RGC axons. The fact that adult RGC axons can reelongate in vitro might provide a useful bioassay for investigations on the factors that either support or inhibit regrowth of axons from CNS neurons.     

成体金黄地鼠神经节细胞轴突在外周神经移植到视网膜后的再生

在成体金黄地鼠的视网膜上移植一段自体坐骨神经1～4个月后,视网膜神经节细胞纤维长入移植的坐骨神经中长达2 cm。具有再生纤维的神经节细胞分布于外周神经插入处与视网膜边缘之间的扇形或带状区域内。细胞数目与插入处的位置有关,接近视神经乳头处较远离视神经乳头处标记细胞数量多。移植后的视网膜标记神经元细胞体面积分布直方图表明:具有再生纤维的神经元胞体面积范围除包括正常大小的神经节细胞外,还包括相当多的胞体增大的神经元。用荧光染料核黄施于视束、真蓝施于移植的坐骨神经后的逆行荧光双标记法实验表明,再生的轴突起源于神经节细胞的损伤轴突,而不是完整神经节细胞轴突的侧芽。     

Defective myelination in the optic nerve of the Browman-Wyse (BW) mutant rat

Summary The Browman-Wyse (BW) rat displays a spectrum of ocular abnormalities which include myelination by Schwann cells of retinal ganglion cell (RGC) axons within the retina. Immunohistochemical and ultrastructural studies of the optic nerves of adult BW rats (30–60 days of age) with myelinated intraretinal axons were performed. Although individual nerves displayed considerable morphological variability, all were characterized by an initial dysmyelinated proximal segment which was separated from a normally myelinated distal segment by a transitional junctional zone. The proximal segment contained axons which were predominantly unmyelinated: where myelination occurred, almost all sheaths were P_o-positive, proteolipid protein-negative, and the myelinating cell was a Schwann cell. In the distal segment the distribution of myelinated axons appeared to be normal, sheaths were PLP⁺, and the myelinating cell was an oligodendrocyte. Within the proximal segment, axons that were myelinated by Schwann cells were isolated by a basal lamina and expanded extracellular spaces from the bulk of other RGC axons within the optic nerve. Few carbonic anhydrase (CAII)⁺ or GalC⁺ oligodendrocytes were seen in proximal segments that contained Schwann cells: anti-CAII antibody stained atypical cells within the proximal segments which did not resemble CAII⁺ oligodendrocytes in the distal segment, and which were probably GalC^–. Astrocytes appeared normal throughout the length of the nerve, and there was no morphological specialization at the junctional zone similar to that at the lamina cribrosa. The possible source (s) of the intraneural Schwann cells, and the pathogenetic mechanisms underlying the aberrant myelination of RGC axons within the BW optic nerve are discussed.     

SD大鼠视网膜中央动脉的应用解剖学研究

目的　为建立稳定可靠的视神经损伤动物模型寻求解剖学依据。 方法　用红色乳胶灌注技术显示正常SD大鼠视网膜中央动脉的来源、分支、分布及其与视神经的关系,并采用体视显微镜摄片测量;明胶墨汁灌注技术显示距眼球后极2.0 mm或6.0 mm处横断视神经后视网膜的血供。 结果　视网膜中央动脉及其分支在视神经鞘内始终与视神经干伴行,视网膜中央动脉起始部到眼球后极的距离为(5.784±0.054)mm;距离眼球后极6.0 mm处鞘内视神经横断组大鼠视网膜单位面积血管数目高于其他部位横断组。 结论　在制备视神经损伤SD大鼠模型时,损伤视神经应在鞘内进行,损伤部位距眼球后极　6.0 mm最佳。     

Myelination and remyelination in the regenerating visual system of the goldfish

Summary The remyelination of regenerated optic axons was investigated in goldfish following either optic nerve crush or ouabain retinal intoxication. Axons grown after nerve crushing acquire thinner myelin sheaths than axons originating from reconstituted ganglion cells. If axons of reconstituted ganglion cells are crushed and allowed to regenerate, the subsequent myelination is weaker than that of control axons not interrupted by crushing, but stronger than that of axons of preexisting retinal ganglion cells.The present results suggest that a neuron is capable of inducing a normally developed myelin sheath when its axon contacts an oligodendrocyte the first time, whereas a neuron whose axon contacts an oligodendrocyte the second time is not capable of forming a normal myelin sheath in the adult animal. The present results also support the notion that the oligodendrocyte requires a neuronal signal for myelin sheath formation.Supported by the Deutsche Forschungsgemeinschaft (Wo 215/5)     

Genetically induced impairment of retinal ganglion cells at the axonal level is linked to extrastriate cortical plasticity

Leber hereditary optic neuropathy (LHON) is a maternally inherited mitochondrial disorder, which leads to initially silent visual loss due to retinal ganglion cell (RGC) degeneration. We aimed to establish a link between features of retinal progressive impairment and putative cortical changes in a cohort of 15 asymptomatic patients harboring the 11778G>A mutation with preserved visual acuity and normal ocular examination. To study plasticity evoked by clinically silent degeneration of RGC we only studied mutation carriers. We phenotyped pre-clinical silent degeneration from the psychophysical, neurophysiological and structural points of view to understand whether retinal measures could be related to cortical reorganization, using pattern electrophysiology, chromatic contrast sensitivity and high-resolution optical coherence tomography to measure macular, RGC nerve fiber layer as well as inner/outer retinal layer thickness. We then performed correlation analysis of these measures with cortical thickness estimates in functionally mapped retinotopic visual cortex. We found that compensatory cortical plasticity occurring in V2/V3 is predicted by the swelling (indicating deficits of axonal transport and intracellular edema) of the macular RGC axonal layer. Increased cortical thickness (CT) in V2 and V3 was observed in peripheral regions, like visual field loss, in these mutation carriers. CT was a very discriminative measure between carriers and controls, as revealed by ROC analysis. Importantly, the substantial cortical reorganization that occurs in the carrier state can be used to provide statistical discrimination between carriers and controls to a level that is similar to measures of retinal dysfunction. We conclude that peripheral cortical compensatory plasticity in early visual areas V2/V3 may be triggered by pathology in peripheral RGC axons in combination with potential developmental changes.