Morphological response of injured adult rabbit optic nerve to implants containing media conditioned by growing optic nerves

Adult rabbit retina can express regeneration-associated characteristics after optic nerve injury, provided it is supplied with appropriate diffusible substances originating from media conditioned by regenerating fish optic nerves or by optic nerves of a newborn rabbit [Hadani et al., Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 7965; Schwartz et al., Science, 228 (1985) 600]. This was shown by applying the active substances to the injured axons in the form of 'wrap-around' implants, consisting of collagen-coated silicone tubes which had been soaked in the conditioned media (CM). The regeneration-associated response was manifested biochemically and by sprouting of nerve fibers in culture. The present work provides morphological evidence that the implantation prolongs survival of ganglion cells and optic nerve fibers and induces new growth. Light microscopic analysis (using horseradish peroxidase (HRP) for labeling the fibers) revealed, 1 week following optic nerve injury, labeled fibers and ganglion cells in both the implanted and control (injured only or injured and implanted with collagen-coated silicone tubes free of CM) nerves. However, from the second week after the injury, distinct differences in the appearance of viable ganglion cells and labeled fibers, were seen between experimental and control preparations. In sections taken through the optic nerve, at the region distal to the site of injury, HRP-labeled fibers were seen in the experimental nerves 1 week, 2 weeks and to a significantly lesser extent 1 month after injury.(ABSTRACT TRUNCATED AT 250 WORDS)     

Axonal regeneration is associated with glial migration: Comparison between the injured optic nerves of fish and rats

The central nervous systems of mammals and fish differ significantly in their ability to regenerate. Central nervous system axons in the fish readily regenerate after injury, while in mammals they begin to elongate but their growth is aborted a the site of injury, an area previously shown to contain no glial cells. In the present study we compared the ability of glial cells to migrate and thus to repopulate the injured area in fish and rats, and used light and electron microscopy in an attempt to correlate such migration with the ability of axons to traverse this area. One week after the optic nerve was crushed, both axonal and glial responses to injury were similar in fish and rat. In both species glial cells were absent in the injured area (indicated by the disappearance of glial fibrillary acidic protein and vimentin immunoreactive cells from the site of injury in rat and fish, respectively), while at the same time axonal growth, indicated by expression of the growth-associated protein GAP-43, was restricted to the proximal part of the nerve. In fish, 2 weeks after the crush, GAP-43 staining (i.e., growing axons) was seen at the site of injury, in association with migrating vimentin-positive glial cells. One week later the site of injury in the fish optic nerve was repopulated by vimentin-positive glial cells, and GAP-43-positive axons had already traversed the site of injury and reached the distal part of the nerve. In contrast, the site of injury in the rat remained devoid of glial fibrillary acidic protein immunoreactive cells, and the expression of GAP-43 by growing axons was still restricted to the proximal part of the nerve. Double-labeling experiments and transmission electron microscopy performed 2 weeks after crush injury of the fish optic nerve revealed that the frontier of axonal growth (i.e., the leading growth cones) appeared to be 200–300 μm ahead of the nearest vimentin-positive glial cells. The leading growth cones were associated with other cells, presumably glial precursors, that seemed to have a high migratory potential. We suggest that the ability of fish glial precursor cells to migrate into the injured area may contribute to the potential of growing axons to traverse this area. The failure of glia and glial precursor cells to migrate into the injured area in the rat may partially account for the failure of rat axons to enter and traverse the injured area.     

Selective impairment of slow axonal transport after optic nerve injury in adult rats.

To investigate cellular responses of injured mammalian CNS neurons, we examined the slow transport of cytoskeletal proteins in rat retinal ganglion cell (RGC) axons within the ocular stump of optic nerves that were crushed intracranially. RGC proteins were labeled by an intravitreal injection of 35S-methionine, and optic nerves were examined by SDS PAGE at different times after injury. In one group of rats, the RGC proteins were labeled 1 week after crushing. From 14 to 67 d after axotomy, the labeling of tubulin and neurofilaments was reduced in relation to other labeled proteins and to the labeling of tubulin and neurofilaments in the intact optic nerve of controls. To determine whether this reduction in labeling was due to an alteration in axonal transport after axotomy, we prelabeled RGC proteins 1 week before crushing. In such experiments, the rate of slow axonal transport of tubulin and neurofilaments decreased approximately 10-fold from 6 to 60 d after injury. Our results cannot be due only to the retrograde degeneration of RGCs and injured axons caused by axotomy in the optic nerve, because fast axonal protein transport and the fluorescent labeling of many axons were preserved in the ocular stumps of these optic nerves. This selective failure of the slow axonal transport of tubulin and neurofilaments may affect the renewal of the cytoskeleton and contribute to the gradual degeneration of RGCs that is observed after axotomy. The alterations in slow transport we document here differ from the enhanced rates we previously reported when injured RGC axons regenerated along peripheral nerve segments grafted to the ocular stump of transected optic nerves (McKerracher et al., 1990).     

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.     

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.     

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.     

Different effects of astrocytes and Schwann cells on regenerating retinal axons

Following a crush injury of the optic nerve in adult rats, the axons of retinal ganglion cells, stimulated to regenerate by a lens injury and growing within the optic nerve, are associated predominantly with astrocytes: they remain of small diameter (0.1-0.5 microm) and unmyelinated for > or = 2 months after the operation. In contrast, when the optic nerve is cut and a segment of a peripheral nerve is grafted to the ocular stump of the optic nerve, the regenerating retinal axons are associated predominantly with Schwann cells: they are of larger diameter than in the previous experiment and include unmyelinated axons (0.2-2.5 microm) and myelinated axons (mean diameter 2.3 microm). Thus, the grafted peripheral nerve, and presumably its Schwann cells, stimulate enlargement of the regenerating retinal axons leading to partial myelination, whereas the injured optic nerve itself, and presumably its astrocytes, does not. The result points to a marked difference of peripheral (Schwann cells) and central (astrocytes) glia in their effect on regenerating retinal axons.     

Myelinated fiber regeneration after crush injury is retarded in sciatic nerves of aging mice

To compare nerve regeneration in young adult and aging mice, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two weeks later, both groups of mice were perfused with an aldehyde solution, and, after additional fixation, the sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. The same level was sectioned in unoperated contralateral nerves; these nerves served as controls. Electron micrographs and the Bioquant Image Analysis System IV were used to measure areas of posterior tibial fascicles and count the number of myelinated axons, the number of unmyelinated axons, and their frequency in Schwann cell units. In aging mice, the total number of regenerating myelinated axons was significantly reduced, but totals of regenerating unmyelinated axons in aging and young adults did not differ significantly. In aging mice, the frequency of Schwann cells that contained a single unmyelinated axon was greater, suggesting that before myelination began, Schwann cell ensheathment of axons also was slowed. After axotomy by a crush injury, the area of the posterior tibial fascicle was less than that in young adults and the distal disintegration of myelin sheath remnants also appeared to be retarded. The results indicate that responses of neurons, axons, and Schwann cells could be important in slowing the regeneration of myelinated fibers found in sciatic nerves from aging mice.     

Axonal branching following crush lesions of peripheral nerves of rat

Branching of myelinated and unmyelinated nerve fibers in normal and regenerating personal and soleus nerves was studied by light and electron microscopy. There were at most 2% more myelinated and 13% more unmyelinated axons in the distal as compared with the proximal nerve segments. Two to four weeks after a crush lesion the distal axons became 2-3 times more numerous; thereafter their number decreased. The number of axons in the proximal nerve segment did not change. The number of myelinated sprouts in most regenerated nerves equalled the number of myelinated fibers in the proximal nerve, while the number of unmyelinated axons after 12-19 weeks was 18-60% higher than normal. Branching was not restricted to the crush region. The results indicate that following a crush lesion all axons branch but only branches of unmyelinated fibers persist for a prolonged period of time. It is tentatively suggested that regenerating axons branch when searching for a target and that when contact is made with the target this prevents additional branching and eliminates redundant branches. Myelinated axons are guided by existing Schwann cells, whereas unmyelinated axons do not follow predetermined pathways; this may explain their greater tendency to form permanent branches.     

A morphometric study of optic axons regenerated in a sciatic nerve graft of adult rats

PURPOSE: In the present study we have morphometrically examined a regeneration model in which axons normally residing in CNS have regrown and are interacting with Schwann cells from the PNS. This study will not only provide morphometric data on regenerated optic fibers but also shed light on possible factors in determining the fiber morphometry. METHODS: The optic nerves of rats aged 6 weeks were cut intra-orbitally and replaced with a autologous sciatic nerve. After a survival period of 9 months, the graft or "regenerated" nerves containing the regenerated optic axons and Schwann cells were processed for morphometric measurements. RESULTS: The mean myelinated axon diameter of regenerated nerve (1.8 +/- 0.2 micro m) was significantly (P < 0.05) greater than that of the optic nerve (0.9 +/- 0.03 micro m). However, unmyelinated regenerated optic axons had a smaller mean axon diameter (0.49 +/- 0.04 micro m) than normal myelinated optic axons. This may suggest that myelinating glial cells exert an influence on axon caliber and Schwann cells seem to have greater effect than oligodendrocytes. The mean g-ratio showing the relative myelin sheath thickness was found to be the highest in the optic nerve (0.78 +/- 0.003), least in the sciatic nerve (0.6 +/- 0.009) and intermediate in the regenerated nerve (0.68 +/- 0.01). The results indicated that Schwann cells myelinating the regenerated optic axons have produced a thinner myelin sheath. Intra-axonally, no significant difference was detected in the number of axonal microtubules and neurofilaments between the regenerated and optic nerves. Therefore the disposition of microtubules and neurofilaments into axon may be intrinsically determined. CONCLUSIONS: In this study, we have identified some of the extrinsic and intrinsic factors in determining the fiber morphometry of the regen-erated nerve. The axon-size and myelination by glial cells were determined through the external axon-glial interactions, whereas the number of axonal microtubules and neurofilaments were intrinsically determined.     

Regeneration of unmyelinated and myelinated sensory nerve fibres studied by a retrograde tracer method

Regeneration of myelinated and unmyelinated sensory nerve fibres after a crush lesion of the rat sciatic nerve was investigated by means of retrograde labelling. The advantage of this method is that the degree of regeneration is estimated on the basis of sensory somata rather than the number of axons. Axonal counts do not reflect the number of regenerated neurons because of axonal branching and because myelinated axons form unmyelinated sprouts. Two days to 10 weeks after crushing, the distal sural or peroneal nerves were cut and exposed to fluoro-dextran. Large and small dorsal root ganglion cells that had been labelled, i.e., that had regenerated axons towards or beyond the injection site, were counted in serial sections. Large and small neurons with presumably myelinated and unmyelinated axons, respectively, were classified by immunostaining for neurofilaments. The axonal growth rate was 3.7 mm/day with no obvious differences between myelinated and unmyelinated axons. This contrasted with previous claims of two to three times faster regeneration rates of unmyelinated as compared to myelinated fibres. The initial delay was 0.55 days. Fewer small neurons were labelled relative to large neurons after crush and regeneration than in controls, indicating that regeneration of small neurons was less complete than that of large ones. This contrasted with the fact that unmyelinated axons in the regenerated sural nerve after 74 days were only slightly reduced.     

Success of regeneration of peripheral nerve axons in rats after injury at different postnatal ages

Electrophysiological experiment have been carried out on rats to see if the age at which a peripheral nerve injury occurs influences the success of regeneration. The assessment was made on the basis of two measures of peripheral nerve regeneration; the extent to which axons manage to grow across the injury site and into the distal stump, and their ability to resupply cutaneous structures with functional endings. Regeneration after nerve transection of both myelinated and unmyelinated axons was studied. The results showed that, apart from rats injured when 2 weeks old, the age at which injury occurred, over the range 4–40 weeks, had little bearing on the overall success of skin reinnervation. The 2-week-old rats showed significantly poorer recovery.     

Numbers of regenerating axons in parent and tributary peripheral nerves in the rat

This study is concerned with numerical parameters of axonal regeneration in peripheral nerves. Our first finding is that the number of axons that regenerate into the distal stump of a somatic nerve at a particular time after transection is partially dependent on the type of lesion used to interrupt the axons. The second question concerns the proportion of axons that regenerate into the distal stump of a parent nerve compared to the proportions that regenerate into tributary nerves that arise from the parent. The proportions of regenerated myelinated axons in the nerve to the medial gastrocnemius muscle and myelinated and unmyelinated axons in the sural nerve are the same as the proportions of myelinated and unmyelinated axons that regenerate into the distal stump of the sciatic nerve for the crush, 0 and 4 mm gap transections. Proportionally fewer axons regenerate into the tributary nerves following the 8 mm gap transection, however. This implies that the length of the gap has an influence on whether or not axons in tributary nerves regenerate in concert with axons in the distal stump of the parent nerve. The unmyelinated fibers in the nerve to the medial gastrocnemius muscle are different because they do not regenerate in proportion to those in the distal stump of the sciatic nerve. We also provide evidence to indicate that myelinated axons branch whereas unmyelinated fibers end blindly when they enter the distal stump after crossing a sciatic nerve transection. Finally the normal arrangement of perineurial cells seems to be disrupted after the sciatic nerve regenerates across a gap.     

Nerve growth factor enhances regeneration through silicone chambers

The effect of exogenous NGF on axonal growth across a gap between sectioned ends of a sciatic nerve within silicone chambers was examined in Sprague-Dawley rats. After nerve section and surgical implantation, silicone chambers were filled with either a 1 mg/ml nerve growth factor (NGF)/saline solution (experimental) or a normal saline solution (control). Four weeks after surgery, the regenerated nerves from within the silicone chambers were dissected and fixed for histological studies at both light microscopic and ultrastructural levels. Morphological analysis of the nerves showed no difference between the NGF-treated and control groups in the size of the regenerated nerves within the chambers or in the diameters of myelinated axons. Total myelinated axonal counts were determined from within the distal chamber. NGF significantly increased the number of myelinated axons that grew into the distal end of the chamber (2126 +/- 437 NGF/saline; 1064 +/- 268 saline; P less than 0.05 Student's t test). Counts of the unmyelinated axons from the distal nerve segment from the two groups were not different. Myelin sheath thickness was 58% greater in the NGF-treated group compared with that in the saline group. There was no difference between the two groups in the size-frequency spectra of the diameters of the myelinated axons in the distal segment. The NGF/saline group showed a more mature-appearing regenerated nerve based on the percentage of myelinated axons, thickness of the myelin sheaths, and development of internal organization (e.g., amount of endoneurial collagen fibers, ensheathment of unmyelinated axons by Schwann cells, and interfascicular patterns).(ABSTRACT TRUNCATED AT 250 WORDS)     

Fibre composition of the feline trochlear and abducens nerves

Cat trochlear and abducens nerves were studied by electron microscopy at two different levels. Five mm peripheral to the exit from the brainstem, the average number of myelinated axons is 965 in the trochlear nerve and 1901 in the abducens nerve. The size spectrum is unimodal and small myelinated axons predominate. Both nerves contain 16% unmyelinated axons at this level. At the PNS/CNS transition, the nerve fascicles contain few unmyelinated axons, but bundles of such axons are present in the adjacent pia mater. We suggest that the trochlear and abducens nerves may channel unmyelinated sensory and/or autonomic axons to the leptomeningeal blood vessels and the pia mater of the brainstem.     

A quantitative ultrastructural study of the optic nerve of the chameleon.

The optic nerve of adult chameleons was investigated with an electron microscope. The total number of retinal ganglion cell axons, the proportion of myelinated axons, the frequency distributions of myelinated and unmyelinated axon diameters were estimated, together with the volume occupied by glial processes. These were distinguished from unmyelinated axons using an antibody directed against glial fibrillary acidic protein, in a post-embedding procedure. The total number of fibers was estimated to be 405,235 +/- 60,000 axons. The proportion of myelinated fibers varied with position between the eyeball and the chiasma; being 22-27% close to the eyeball, rising to 42-47% halfway along the optic nerve and to 56-62% close to the chiasma. Myelinated and unmyelinated fiber diameter distributions were unimodal and positively skewed, with modes of 0.7 microm and 0.2 microm, respectively. There was a significant regional variation in the size of optic nerve axons. Large myelinated axons were observed in the dorsal and ventral periphery, whereas smaller myelinated fibers and a high proportion of unmyelinated fibers were found in the center of the nerve.     

Multipotentiality of Schwann cells in cross-anastomosed and grafted myelinated and unmyelinated nerves: quantitative microscopy and radioautography.

Cross-anastomoses and autogenous grafts of unmyelinated and myelinated nerves were examined by electron microscopy and radioautography to determine if Schwann cells are multipotential with regard to their capacity to produce myelin or to assume the configuration seen in unmyelinated fibres. Two groups of adult white mice were studied. (A) In one group, the myelinated phrenic nerve and the unmyelinated cervical sympathetic trunk (CST) were cross-anastomosed in the neck. From 2 to 6 months after anastomosis, previously unmyelinated distal stumps contained many myelinated fibres while phrenic nerves joined to proximal CSTs became largely unmyelinated. Radioautography of distal stumps indicated that proliferation of Schwann cells occurred mainly in the first few days after anastomosis but was also present to a similar extent in isolated stumps. (B) In other mice, CSTs were grafted to the myelinated sural nerves in the leg. One month later, the unmyelinated CSTs became myelinated and there was no radioautographic indication of Schwann cell migration from the sural nerve stump to the CST grafts. Thus, Schwann cell proliferation in distal stumps is an early local response independent of axonal influence. At later stages, axons from the proximal stumps cause indigenous Schwann cells in distal stumps from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously myelinated nerves become associated with unmyelinated fibres. Consequently, the regenerated distal nerve resembled the proximal stump. It is suggested that this change is possible because Schwann cells which divide after nerve injury reacquire the developmental multipotentiality which permits them to respond to aoxonal influences.     

Regional variation in the abundance of axonal cytoskeletal proteins.

The relative abundance of several axonal cytoskeletal proteins was determined by immunoassay at various sites in the peripheral and central nervous systems of adult rats. Within the peripheral nervous system, the ratio of tubulin to neurofilaments was greatest for nerves composed of unmyelinated axons and least for nerves with large myelinated axons. MAP1 protein was more prominent in unmyelinated fibers; conversely tau proteins were relatively more abundant in large myelinated axons. An immunochemical index of neurofilament phosphorylation was less for unmyelinated fibers than for myelinated ones. In the fimbria-fornix, pyramidal tract, and superior cerebellar peduncle, similar trends were observed: small axons had more MAP1, less tau, and a greater ratio of tubulin to neurofilament proteins. The phosphorylation index was greatest for the superior cerebellar peduncle, the tract with the largest axons. The immunochemical index of neurofilament phosphorylation was greater for the optic nerve than for axonal tracts in the brain proper. These results suggest that development of large myelinated axons is associated with greater neurofilament content, neurofilament phosphorylation, and with greater abundance of tau proteins in the CNS and the PNS; however, quantitative aspects of these relationships differ in the PNS and the CNS.     

Peripheral nerve-stimulated macrophages simulate a peripheral nerve-like regenerative response in rat transected optic nerve

We have previously demonstrated that the failure of the mammalian central nervous system (CNS) to regenerate following axonal injury is related to its immunosuppressive nature, which restricts the ability of both recruited blood-borne monocytes and CNS-resident microglia to support a process of repair. In this study we show that transected optic nerve transplanted with macrophages stimulated by spontaneously regenerating nerve tissue, e.g., segments of peripheral nerve (sciatic nerve), exhibit axonal regrowth at least as far as the optic chiasma. Axonal regrowth was confirmed by double retrograde labeling of the injured optic axons, visualized in their cell bodies. Transplanted macrophages exposed to segments of CNS (optic) nerve were significantly less effective in inducing regrowth. Immunocytochemical analysis showed that the induced regrowth was correlated with a wide distribution of macrophages within the transplanted-transected nerves. It was also correlated with an enhanced clearance of myelin, known to be inhibitory for regrowth and poorly eliminated after injury in the CNS. These results suggest that healing of the injured mammalian CNS, like healing of any other injured tissue, requires the partnership of the immune system, which is normally restricted, but that the restriction can be circumvented by transplantation of peripheral nerve-stimulated macrophages. GLIA 24:329–337, 1998. © 1998 Wiley-Liss, Inc.     

Growth of retinal ganglion cell axons following optic nerve crush in adult hamsters

Regrowth of retinal ganglion cell axons was examined 2 to 60 days after intraorbital optic nerve crush lesions in adult hamsters. Anterograde axonal transport of intraocularly injected wheat germ agglutinin-horseradish peroxidase conjugate was used to label the axons after specific postinjury time periods. Labeled axons were present in the region of the optic nerve lying between the eye and the crush site at all times, but their numbers appeared to decrease with increasing survival time. Labeled axons were first detected in the segment of optic nerve lying distal to the crush site 1 week after injury and had extended as far as 2.3 mm beyond the crush site by 60 days postinjury, growing at a rate similar to that at which the collateral branches of developing ganglion cell axons extend into their targets. Although most axons failed to regrow after these lesions, the slow reextention exhibited by members of a small population of axons indicates that the degenerating adult mammalian optic nerve provides an adequate environment for a particular mode of regrowth by injured axons of the central nervous system.