Axon regeneration across the site of injury in the optic nerve of the newt Triturus pyrrhogaster

No rapid and demarcating astroglial reaction to stab wounds in Agama and Gecko lizards and the caiman Paleosuchus - it is confined to birds and mammals

The present study proves that rapid and demarcating astroglial reactions are confined to birds and mammals. To understand the function of post-lesion astroglial reaction, the phylogenetical aspects are also to be investigated. Considering the regenerative capabilities, reptiles represent an intermediate position between the brain regeneration-permissive fishes and amphibians and the almost non-permissive birds and mammals. Damage is followed by a rapid astroglial reaction in the mammalian and avian brain, which is held as an impediment of regeneration. In other vertebrates the reactions were usually observed following long survival periods together with signs of regeneration, therefore they can be regarded as concomitant phenomena of regeneration. The present study applies short post-lesion periods comparable to those seen in mammals and birds for astroglial reactions. Two species of lizards were used: gecko (leopard gecko, Eublepharis macularius, Blyth, 1854) and agama (bearded dragon, Pogona vitticeps, Ahl, 1926). The gecko brain is rich in GFAP whereas the agama brain is quite poor in this. Crocodilia, the closest extant relatives of birds were represented in this study by Cuvier's dwarf caiman (Paleosuchus palpebrosus, Cuvier, 1807). The post-lesion astroglial reactions of crocodilians have never been investigated. The injuries were stab wounds in the telencephalon. The survival periods lasted 3, 7, 10 or 14 days. Immunoperoxidase reactions were performed applying anti-GFAP, anti-vimentin and anti-nestin reagents. No rapid and demarcating astroglial reaction resembling that of mammalian or avian brains was found. Alterations of the perivascular immunoreactivities of laminin and β-dystroglycan as indicators of glio-vascular decoupling proved that the lesions were effective on astroglia. The capability of rapid and demarcating astroglial reaction seems to be confined to mammals and birds and to appear by separate, parallel evolution in them.

Tenascin-R inhibits the growth of optic fibers in vitro but is rapidly eliminated during nerve regeneration in the salamander Pleurodeles waltl

Tenascin-R is a multidomain molecule of the extracellular matrix in the CNS with neurite outgrowth inhibitory functions. Despite the fact that in amphibians spontaneous axonal regeneration of the optic nerve occurs, we show here that the molecule appears concomitantly with myelination during metamorphosis and is present in the adult optic nerve of the salamander Pleurodeles waltl by immunoblots and immunohistochemistry. In vitro, adult retinal ganglion cell axons were not able to grow from retinal explants on a tenascin-R substrate or to cross a sharp substrate border of tenascin-R in the presence of laminin, indicating that tenascin-R inhibits regrowth of retinal ganglion cell axons. After an optic nerve crush, immunoreactivity for tenascin-R was reduced to undetectable levels within 8 d. Immunoreactivity for the myelin-associated glycoprotein (MAG) was also diminished by that time. Myelin was removed by phagocytosing cells at 8-14 d after the lesion, as demonstrated by electron microscopy. Tenascin-R immunoreactivity was again detectable at 6 months after the lesion, correlated with remyelination as indicated by MAG immunohistochemistry. Regenerating axons began to repopulate the distal lesioned nerve at 9 d after a crush and grew in close contact with putative astrocytic processes in the periphery of the nerve, close to the pia, as demonstrated by anterograde tracing. Thus, the onset of axonal regrowth over the lesion site was correlated with the removal of inhibitory molecules in the optic nerve, which may be necessary for successful axonal regeneration in the CNS of amphibians.

S-100-positive glial cells are involved in the regeneration of the visual pathway of teleosts

Glial cells in the normal and regenerating visual pathways of Tinca tinca (Cyprinid, Teleost) were studied by labelling with anti-S-100 antibody. In normal fish, S-100-positive bipolar cells were found in the optic nerve, optic tract, and in the diencephalic visual pathways. After crushing the left optic nerve, the distribution and the number of S-100-immunoreactive cells were modified. In the injured nerve, 7 to 15 days after crushing no immunoreactive cell bodies were found in the crushed area, but a greater number of S-100-positive cells were found on both sides of the injured area. Sixty days after crushing, positive cells penetrating the crushed area were observed; the normal pattern was almost restored 200 days after crushing. In the diencephalon, 25 days after crushing, the number of S-100-positive cells increased remarkably and the most intense immunostaining of glial processes was observed 60 days after crushing. The density of S-100-labelled cells decreased after 4 months postcrushing. However, in the optic tectum no changes were observed. The increase of glial cells in the lesioned visual pathway suggests that they could play an important role in axonal regeneration after crushing.

Behaviour of macroglial cells, as identified by their intermediate filament complement, during optic nerve regeneration of Xenopus tadpole

Assessment of glial cell behaviour during optic nerve (ON) regeneration in Xenopus tadpoles is hampered by the lack of classical cellular markers that distinguish different glial cells in mammals. We thus have characterized the intermediate filament (IF) complement of tadpole glial cells and used it to follow the fate of glial cell subsets during the first 10 days after ON crush. Glial cells synthesize a restricted number of cytokeratin (CK) species and vimentin. This pattern remains essentially unchanged during metamorphosis and regeneration. However, vimentin turnover is specifically enhanced after injury. The expression of CKs and vimentin has been followed immunocytochemically in situ and in isolated cells recovered from dissociated ON segments. In the normal nerve, 79% of ramified glial cells express both CK and vimentin, 1% CK and 4% vimentin only, whereas 16% express neither IF protein. We tentatively classified CK expressing cells as mature astrocytes and those without IF proteins as oligodendrocytes. In the regenerating ON, the relative number of oligodendrocytes is decreased, while the astrocytic subset becomes accordingly larger but is decreased by day 10 already in favour of cells expressing vimentin only. Astrocytes invade the lesion site soon after crush, arrange into a central core within the distal nerve segment and establish a peripheral scaffold that is readily crossed by axons. Unlike mammalian astrocytes that remain absent from the lesion site but form a scar at some distance to it, amphibian astrocytes appear to provide active guidance to axons growing through the lesion site.

Microglial cells in the brain of Pleurodeles waltl (Urodela, Salamandridae) after wallerian degeneration in the primary visual system using Bandeiraea simplicifolia isolectin B4-cytochemistry

In the brain of the salamander Pleurodeles waltl, microglial cells were investigated cytochemically with isolectin B4 (IB4) of Bandeiraea simplicifolia after optic nerve transection and during subsequent regeneration. Double-labeling with an antibody directed against the glial fibrillary acidic protein of astrocytes revealed no immunoreactivity in microglial cells and confirmed the absence of non-radial, free astroglial cells in the tectum. After two days, IB4-labeled microglial cells began to populate the rostral parts of the primary visual system. The origin of these early vimentin-immunoreactive microglial cells seemed to be mainly IB4-labeled cells in a perivascular position and meningeal macrophages. After 12 days, microglial cells infiltrated the tectum in four layers: one in the ependyma, one in the outermost periventricular grey, and two in the degenerating visual neuropil where activated microglial cells displayed a ramified morphology. After 3 weeks, microglial cells accumulated within the degenerating neuropil while reducing their processes. After 7 weeks, the number of microglial cells was still increased on the affected side. The subarachnoid space above the neuropil where regenerating retinal afferents arrived was filled with IB4-labeled macrophages. Only very few microglial cells were seen in co-existence with Müller cells in the lesioned and intact retinae, whereas microglial cells and macrophages were IB4-labeled in the optic nerve head and at the ora serrata.

Macrophage response during axonal regeneration in the axolotl central and peripheral nervous system

We have used a monoclonal antibody (5F4) and Griffonia lectin to study the recruitment of macrophages after crushing axolotl central and peripheral axons. In both cases axonal regeneration begins within one to two days and, in the CNS, proceeds at a rate of about 0.05 mm per day. However, in the spinal cord, macrophage entry is restricted to the lesion site whilst in peripheral nerves macrophages rapidly enter the distal nerve stump after injury. These results suggest that the role (if any) played by macrophages during axonal regeneration may differ in these two situations.

Regeneration in the Xenopus tadpole optic nerve is preceded by a massive macrophage/microglial response

Changes in the optic nerve following a crush lesion and during axonal regeneration have been studied in Xenopus tadpoles, using ultrastructural and immunohistological methods. Degeneration of both unmyelinated and myelinated axons is very rapid and leads to the formation, within 5 days, of a nerve which consists largely of degeneration debris and cells. Immunohistological analysis with monoclonal antibody 5F4 shows that there is a rapid and extensive microglial/macrophage response to crush of the nerve. Regenerating axons have begun to enter the distal stump by 5 days and grow along the outer part of the nerve in close approximation to the astrocytic glia limitans. Between 5 and 10 days after nerve crush, regenerating axons reach and pass the chiasma. Macrophages are seen in the nerve at the site of the lesion within 1 h, and the response peaks between 3-5 days, just before axonal regeneration gets under way.

In vitro and in vivo characterization of blastemal cells from regenerating newt limbs

To better characterize the cells involved in newt limb regeneration, blastemal cells from accumulation and differentiation phase blastemas were grown in dissociated cell culture, and their morphology and antigenic phenotype determined using a variety of antibodies directed against intermediate filaments, cell adhesion molecules, and extracellular matrix molecules. In addition to previously described blastemal cell morphologies, many of the cells in these cultures had a round cell body, with an eccentrically placed nucleus and a cytoplasm filled with autofluorescent granules. The majority of accumulation phase blastemal cells labeled with antibodies against GFAP, vimentin, 22/18 as well as with antibodies against NCAM, L-1, laminin, and fibronectin. The majority of differentiation phase blastemal cells had a similar phenotype but lacked expression of vimentin and fibronectin. Comparison of the blastemal phenotype in vitro and in vivo showed similar expression characteristics. However, in differentiation phase blastemas, laminin immunoreactivity was concentrated in specific locations. In addition, the proliferation of cultured blastemal cells is stimulated by the addition of a crude brain extract, consistent with previous studies in vivo and in vitro. Taken together, these observations suggest that dissociated cultures of newt limb blastemal cells provide a suitable model for the analysis of the cell and molecular mechanisms involved in limb regeneration.

Gliosis during optic fiber regeneration in the goldfish: an immunohistochemical study

Antisera directed against the 48 kDa and 50 kDa cytoskeletal antigens were used to examine changes in the astroglial fabric of the goldfish visual pathways following optic nerve crush. Several major observations are described. First, an optic nerve crush lesion in these animals appears to be devoid of glial cells for at least the first month after surgery. As a corollary, regenerating axons that grow across the lesion may do so over an aglial substrate. Once the axons cross the lesion, their growth is confined to the astroglial domains of the proximal nerve stump. In the optic nerve, gliosis comprises hypertrophy of astrocytic processes such that the open framework characterizing the normal nerve is obscured. In addition, during regeneration, optic nerve glia express large amounts of the 50 kDa cytoskeletal protein, which they ordinarily express at only minimal levels. In the optic tract, gliosis is reflected in a markedly increased expression of the 50 kDa protein as well as an apparent increase in the number and complexity of glial processes. In addition, optic tract glia begin to express the 48 kDa antigen during regeneration. This protein is ordinarily confined for the most part to the optic nerve and is not seen in the tract glia. Finally, no obvious changes were seen in the glia of the optic tectum. These results demonstrate many points of similarity between gliosis in the goldfish and in mammals. However, in some particulars the two responses differ, and it is possible that these differences are related to the differing ability of central axons to regenerate in the two groups of organisms.

Glial fibrillary acidic protein in the fish optic nerve

The intermediate filament glial fibrillary acidic protein (GFAP) is the predominant cytoskeletal protein of mature glial cells in the mammalian nervous system. The nervous systems of lower vertebrates, such as fish, have been examined for the presence of GFAP and several investigators have shown that goldfish (Carassius auratus) brain contains GFAP-positive astrocytes. The same studies have demonstrated that, in contrast to the brain, the optic nerve of goldfish did not show any GFAP immunoreactivity, suggesting that this intermediate filament protein is not expressed in fish optic nerve astrocytes. The present study shows, however, that the monoclonal antibodies to porcine GFAP react with the optic nerve of carp (Cyprinus carpio), another member of the goldfish family. These antibodies to porcine GFAP cross react with rat brain and carp optic nerve, yielding a band of approximately 52 kDa in both species. Northern blot analysis using mouse GFAP DNA probe revealed that carp optic nerve RNA contains two transcripts of 2.3 and 2.1 kb, which hybridize with the mouse GFAP probe. Injury to the carp optic nerve was followed by a decrease of GFAP immunoreactivity from neural tissue and a strong expression around blood vessels and connective tissues. On the basis of these observations and within the limitation of the techniques it is reasonable to conclude that the carp optic nerve expresses GFAP immunoreactivity and that the pattern of expression of this intermediate filament protein is altered after injury. Such an alteration might be relevant to the process of regeneration.

Axonal regrowth in the amyelinated optic nerve of the myelin-deficient rat: ultrastructural observations and effects of ganglioside administration

It has been postulated that myelin degradation products may inhibit regrowth of mammalian central axons and that central nervous system (CNS) myelin and oligodendrocytes may constitute a "nonpermissive substrate" for axonal growth. To address these issues, we utilized an X-linked rat mutant, myelin-deficient or md. In the optic nerve of this mutant, 40 days and more postnatally, normal myelin is absent and oligodendrocytes are few (Dentinger et al. Brain Res. 344:255-266, 1985). Twenty-eight days before sacrifice, we operated on four groups of 50-day-old md rats and age-matched normal littermates according to the following protocols: 1) unilateral intraorbital optic nerve crush; 2) beginning within 1 hour of nerve crush, daily intraperitoneal injection of GM1 ganglioside (20 mg/kg) dissolved in phosphate-buffered saline (PBS); 3) daily intraperitoneal injection of PBS alone, also begun within 1 hour of nerve crush; 4) severance of the optic nerve immediately behind the papilla 16 or 21 days after the primary crush lesions. Additionally, normal and md rats were killed 4 and 14 days after unilateral optic nerve injury. Nerves of unoperated md rats and their normal littermates were also processed. In the operated animals that did not receive GM1, ultrastructural analysis 4, 14, and 28 days after lesioning revealed that md optic nerves contained significantly greater numbers of regenerating axons, including growth cones and varicosities, than nerves of normal rats. Notably, 28 days postoperatively, (group 1), regenerating axons were still abundant in md nerve, whereas, in nerves of normally myelinated littermates, axonal numbers were diminished markedly. Regenerating optic axons of both md and normally myelinated rats were oriented by linear astrocytic arrays and often were enclosed by astrocytic cytoplasm. In normal littermates, GM1 administration (group 2) induced a significant increase in the number of axons within the operative lesion. Paradoxically, GM1 inhibited the ordinarily robust regeneration of md axons. PBS-injected md and normal rats (group 3) showed no significant differences from noninjected, operated animals. Severance of the nerve at the papilla (group 4) 7-12 days before sacrifice confirmed the origination of axonal regrowth by retinal ganglion cells. The data provide in vivo support for a role of myelin breakdown products or the secretory products of oligodendroglia in the inhibition of regenerative axonal sprouting within mammalian CNS.

Neovascularization occurs in response to crush lesions of adult frog optic nerves

The capacity of the adult frog optic nerve to regenerate following a crush lesion is well established and is in contrast to the lack of regeneration of mammalian optic nerves after similar lesions. One factor which may contribute to the enhanced regenerative capacity of amphibian optic nerves is the rapid removal of cellular debris from the nerve after injury. In this study the morphology of normal and crushed frog optic nerves has been compared. Although the intraorbital region of the normal adult frog optic nerve is avascular, new intraparenchymal blood vessels appear central to the crush site 24 h after the nerve lesion. The appearance of these blood vessels is coincident with the appearance of granulocytes and macrophages in the nerve. Successful regeneration of the adult frog optic nerve may depend on this neovascularization to facilitate the rapid removal of cellular debris and to supply regenerating axons with trophic substances.

Organization of astrocytes in the visual pathways of the goldfish: an immunohistochemical study

We have used antisera directed against glial cytoskeletal proteins to examine the distribution and organization of astrocytes in the visual pathways of the goldfish. We describe two different types of cells, which may be distinguished by their unique cytoskeletal proteins. Antibodies raised against a 48 Kd optic nerve protein react with stellate astrocytes in the optic nerve but virtually no glial cells in the brain (although blood vessels and the meninges in the brain were stained). The optic nerve astrocytes form a dense meshwork of processes through which the optic fibers pass. The intraorbital and intracranial segments of the nerve are divided into fascicles, each bounded by a glia limitans, which extend across the optic chiasm. Astroglial cells in the brain bind antibodies raised against a 50 Kd brain cytoskeletal protein. These antibodies show a very limited cross-reactivity with optic nerve cells. Brain astrocytes have filiform profiles and most appear to be deployed as radial glia. The glial fabric of the brain, as revealed by these antibodies, is far more loosely woven than that of the optic nerve. There is a sharp boundary between the two types of glial cells, immediately behind the optic chiasm. Glial processes in the optic tracts arise from cells in the preoptic area, whereas those in the optic tectum arise from cells that reside locally. In the optic tract, a glia limitans was often difficult to discern, whereas in the tectum one was always evident and composed of endfeet at the pial extremities of radial glial processes. These findings are discussed both in the context of previous observations by other workers as well as with regard to their possible functional implications.

A preliminary description of the regeneration of optic nerve fibers in a reptile, Vipera aspis

Crushing or freezing the optic nerve of the viper leads initially to the anterograde degeneration of the optic nerve fibers and to an extensive retrograde demyelination process associated with the degeneration of some retinal ganglion cells. By the 45th postoperative day, regenerating unmyelinated axons can be identified in the damaged region of the optic nerve. These fibers reach the chiasm and the marginal optic tract by the third postoperative month. The radioautographic tracing method shows that some nuclei of the primary visual system begin to be reinnervated by about the 5th postoperative month; this reinnervation was not, however, completely restored in those specimens with the longest postoperative survival of 220 days.

Prospects for axonal regrowth in spinal cord injury

Some of the evidence relating to the possible cellular relationships in regenerating mammalian central nervous tissue is reviewed. From this review it is suggested that data do exist which reveal a potential for regeneration based upon the basic properties and behavior of axons and glial cells. Models of tissue injury which optimize these intrinsic capabilities may generate significant information about the regenerative possibilities of central nervous tissue.

Studies of the early stages of optic axon regeneration in the goldfish

We have studied the early stages (4-14 days) of axonal regeneration following intraorbital optic nerve crush in the goldfish. We used 3H-proline autoradiography to anterogradely label and visualize the growing axons and wheat germ agglutinin-conjugated horseradish peroxidase (WGA:HRP) for retrograde labeling to determine the cells of origin of the earliest projections. The first retinal ganglion cells (RGCs) that could be retrogradely filled from the optic tract, following optic nerve crush, were in the central retina and were seen at 8 days postoperative. More peripheral cells were only labeled with longer postcrush survival periods. Thus, the first axons to regenerate past the lesion were from central RGCs. The axons of these cells extended into the cranial nerve stump between 4 and 5 days postcrush and entered the nerve as a fascicle, which travelled just beneath its surface. Studies of nerve cross sections from animals at 5-8 days postoperative demonstrated that initial outgrowth was not confined to any particular locale within the nerve although the early fibers appeared to avoid its temporal aspect. When the regenerating axons reached the optic tract they remained in fascicles but left the surface to run along the medial, deep portion of the tract, immediately adjacent to the diencephalon and pretectum. The positions occupied by the earliest-regenerating axons in the optic nerve were variable and not always appropriate for their central retinal origin. However, the abrupt change in growth trajectory as the fibers entered the optic tract brought them into the areas of the visual paths that are occupied by central axons in intact animals. We suggest that this change in position is related to both changes in the structural organization of the intracranial visual paths and to possible axon guidance signals in the region of the nerve-tract juncture.

Dorsal root axonal regeneration in the adult frog spinal cord. A model of vertebrate CNS regeneration

The frog dorsal root provides a useful model for the study of axonal regeneration in an adult vertebrate CNS. We have used the model to compare the regeneration of two very different types of axons within the same CNS environment and have found that regenerating dorsal root, as well as rerouted motoneuron axons, display similar growth patterns in the spinal cord. Both sensory and motor axons grow preferentially in some regions and not in others. They both regenerate effectively longitudinally as well as radially within the dorsolateral fasciculus (DLF). By contrast, fewer sensory and motor axons regenerate longitudinally or radially in the dorsal funiculus (DF). This similar preferential growth of two very different populations of axons suggests that the growth patterns reflect regional differences in the cellular environment of the cord. The DLF has fascicles of unmyelinated axons separated by radial glial processes and, after dorsal root injury, is mildly gliotic. By contrast, DF has very large myelinated axons, which widely separate the radial glial processes that traverse the region. After dorsal root injury, this region is markedly gliotic and contains myelin, debris and oligodendroglia, and microglial macrophages. Our data suggest that unmyelinated axons and radial glial processes are more preferred substrates for axonal growth than myelin debris, oligodendroglia and macrophages. It is not surprising, then, that regions of the adult mammalian CNS that are characterized by large myelinated axons fail to support axonal growth. Moreover, there is some evidence that regions of the adult mammalian CNS that are characterized by unmyelinated axons support axonal growth.

Regeneration of the frog optic nerve. Comparisons with development

Developing and regenerating frog optic axons grow within optic pathways and form connections only with optic targets. However, unlike normal development, many regenerating optic axons in the adult frog are misrouted within optic pathways, including axons that grow into the opposite retina. Many of the axons misrouted during regeneration appear to be collaterals of axons that grow in normal directions. Ganglion cell loss of up to 60% occurs after optic nerve damage, beginning prior to reinnervation of optic targets. Massive axonal collateralization also takes place near the point of nerve damage, causing the normal order found within the nerve to be lost. Collaterals are eliminated as selective reinnervation is completed, and the smaller complement of optic cell axons remaining after regeneration form an expanded projection within optic targets. Evidence is reviewed that suggests that factors involved in axonal guidance and target recognition during development remain intact in the adult frog brain. Additional conditions resulting from nerve injury causes axonal guidance to be less successful during regeneration.

Immunohistochemical localization of intermediate filament proteins of neuronal and nonneuronal origin in the goldfish optic nerve: specific molecular markers for optic nerve structures

The predominant proteins (58K) of the intermediate filament complex in the goldfish visual pathway consist of a series of isoelectric variants. Previous biochemical studies have shown that proteins ON1 and ON2 are of neuronal origin, whereas ON3 and ON4 are of nonneuronal origin. Polyclonal antibodies, purified by affinity chromatography, that are specific for ON1 and ON2 or ON3 and ON4 have been used to localize histologically the ON proteins within the normal and crushed optic nerve. Anti-ON1/ON2 antiserum presented a pattern consistent with intraaxonal staining. A nonneuronal staining pattern was observed with anti-ON3/ON4 antiserum. The two patterns were distinct from and complementary to each other. The data suggest that ON3 and ON4 represent a novel glial fibrillary acidic protein. The results are discussed in terms of the function of these proteins in development, plasticity, and regeneration.

Regeneration of peptide-containing retinofugal axons into the optic tectum with reappearance of a substance P-containing lamina

Twenty-five specimens of Rana pipiens were subjected to a unilateral crush of the optic nerve. Substance P (SP)-, leucine enkephalin (LENK)-, cholecystokinin octapeptide (CCK8)-, and bombesin (BOM)-like immunoreactivities were analyzed in the retinae, optic nerves, and optic tecta, 9 days to 9 months postoperatively, by means of immunohistochemical methods. Peptide-like immunoreactivity was observed in axons within the optic nerve stump retinal to the crush, as in previous studies (Kuljis and Karten, '83b, Kuljis et al., '84). Peptide-containing retinofugal axons began traversing the lesion site between 10 and 20 days postoperatively, in progressively increasing numbers. Ten to 20 days following crush of the optic nerve SP-, LENK-, and CCK8-containing axons could be found in the cerebral stump of the optic nerve and in the optic chiasm, advancing to the side of the brain deafferented by the crush. The number of axons displaying peptide-like immunoreactivity within the optic nerve, retinal or cerebral to the crush, and within the optic chiasm gradually decreased after 2-3 months. The optic nerve contralateral to the procedure displayed only occasional isolated peptide-containing fibers, as in normal optic nerves. The retinae ipsilateral and contralateral to the crush exhibited no change in the normal pattern of peptide-like immunoreactivity, including the absence of demonstrable peptide-like immunoreactivity in the somata of retinal ganglion cells. The optic tectum deafferented by the procedure underwent modifications in the pattern of peptide-like immunoreactivity identical to those reported following unilateral eye enucleation (Kuljis and Karten, '82a, '83a). The patterns of LENK-, CCK8-, and BOM-like immunoreactivities in the tectum were identical to those following irreversible retinal deafferentation as long as 9 months postoperatively. SP-like immunoreactivity, however, was gradually restored in layer 11 of Ramón y Cajal ('46; layer D of Potter, '69) of the superficial (retinorecipient) neuropil 4-6 months postoperatively. The persistence of lamina-specific depletion patterns of LENK-, CCK8-, and BOM-like immunoreactivities in reafferented tecta represents a puzzling observation. The latter findings contrast sharply with the recovery of SP-like immunoreactivity, which occurs long after apparently complete restitution of the retinofugal projection, as shown by anatomical (Stelzner et al., '81), physiological (Maturana et al., '59), and behavioral (Sperry, '44) methods.(ABSTRACT TRUNCATED AT 400 WORDS)

The response of optic tract glia during regeneration of the goldfish visual system. I. Biosynthetic activity within different glial populations after transection of retinal ganglion cell axons

We monitored biosynthetic activity of optic tract glia during regeneration of retinal ganglion cell axons in the goldfish and found that the greatest level of incorporated [3H]thymidine and [3H]leucine occurred in glia by 10-15 days after axotomy. During this period there was a marked increase in the number of oligodendroglia and multipotential glia near the site of injury with no change occurring in the astroglial population. Electron microscopic autoradiography showed that oligodendroglia and multipotential cells incorporated 5-7-fold more thymidine than did cells of intact control preparations. Though all glial cell types incorporated more [3H]leucine during axonal regeneration, oligodendroglia and multipotential cells together accounted for more than 90% of measured radioactivity. In order to characterize glial-stimulating events specific to axonal regeneration, we produced axonal degeneration in the optic tract by removal of the retina. Optic tract glia during axonal degeneration incorporated less amino acid when compared to glia associated with regenerating axons. The degenerating optic tract also had less 2',3'-cyclic nucleotide 3'-phosphohydrolase, an enzyme produced by oligodendroglia, than that found in the regenerating visual system. Our results suggest that in response to ganglion cell axotomy oligodendroglia and multipotential glia of the goldfish optic tract proliferate. Moreover, regenerating axons provide one type of stimulant for glial protein biosynthesis.

Basal lamina at the site of spinal cord injury in normal, immunotolerant and immunosuppressed rats

The cut ends of a rat spinal cord are capped with basal lamina (BL) within 20 days. This BL may block regenerating axons. BL at the transection site in rats made immunologically unresponsive to central nervous system antigens is not significantly different from that of control rats, but rats treated with cyclophosphamide show a less complete BL cap during the first 25 days. This may account for the increased axonal regeneration found in cyclophosphamide-treated rats.

Basal lamina at the site of spinal cord transection in the rat: an ultrastructural study

This electron microscopic study confirms that basal lamina (BL) begins to cap the cut end of the spinal cord 15 days after spinal cord transection. BL is first seen immediately adjacent to reactive glial cells but only when there is collagen in the nearby interstitial space. This finding suggests that collagen may provide a trigger to initiate the production of BL by reactive glia. We found no direct evidence that BL in this injury area impeded the outgrowth of regenerating neurites.

Degenerative and regenerative changes in the trochlear nerve of goldfish

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Astrocytic membrane morphology: differences between mammalian and amphibian astrocytes after axotomy

Previous studies have shown that astrocytes in some nonmammalian species provide a favorable environment for axonal elongation, whereas mammalian astrocytes are thought to inhibit fiber outgrowth. The present study was performed to determine whether any plasma membrane differences exist between these glial elements which could account for their contrasting effects upon axonal outgrowth. Astrocytic scars were formed in optic nerves of rats, newts, and frogs by enucleation. Subsequently, the astrocytic membranes were examined with the freeze-fracture technique. Orthogonal arrays of small intramembranous particles (IMPs) are a prominent component of the plasma membranes of normal mammalian astrocytes; these arrays are most numerous in astrocytic membranes that form an interface between the CNS and nonneural tissue. Astrocytic membranes within the normal CNS parenchyma, however, possess much lower densities of arrays. Following axotomy and Wallerian degeneration, the density of arrays increased threefold within the parenchyma of the optic nerve, while remaining constant at the glia limitans. In striking contrast, only a few aggregates of IMPs that resembled orthogonal arrays could be found in normal and reactive astrocytes of amphibians, although the cytology of these glial cells and density of the scars are otherwise similar to those of their mammalian counterparts. These findings suggest (1) that a proliferation of orthogonal arrays in astrocytic plasma membranes is a prominent feature of gliosis in the mammalian CNS and (2) that differences in the composition of reactive mammalian and amphibian astrocytic membranes may account for variations in axonal-glial interactions within the injured CNS.

Evidence for an orderly arrangement of optic axons within the optic nerves of the major nonmammalian vertebrate classes

The pathways of selected optic axons were traced in representative urodele, anuran, teleost, reptile, and avian species by filling the fibers with HRP or by tracing, at the light and electron microscopic (EM) level, the degeneration caused by focal retinal or optic nerve lesions. In all species it was shown that fibers retain retinotopic neighborhood relationships throughout their transit of the optic nerve. Additionally, in anurans, it was found that a subset of large diameter, myelinated fibers take up a random arrangement in the nerve. It is argued that retinotopic fiber organisation is a reflection of contact guidance of axons during fiber outgrowth in the embryo and that this organisation could account for the arrival of fibers in orderly arrays at central nuclei during normal embryonic development.

Axonal interactions with connective tissue and glial substrata during optic nerve regeneration in Xenopus larvae and adults

Axonal elongation through connective tissue and glial environments was compared following resection of the optic nerves in Xenopus tadpoles and frogs. During initial stages of fiber outgrowth, axons encountered connective-tissue matrices of varying degrees of complexity in the ablation gaps. Many of the neuritic sprouts were randomly directed after leaving the retinal stump, and a neuroma-like swelling ultimately formed at the cut edge. Although a large number of axons managed to traverse the lesion and associate with the cranial stump, many other fibers were less appropriately directed, especially in the frog where a greater infiltration of dense collagen occurred between the separated segments of the optic nerve. Axons often deviated from their cranially oriented pattern of outgrowth after entering the lesion and invaded surrounding extraocular muscles; others advanced along neighboring blood vessels and cranial nerve branches. In more extreme circumstances, fibers were completely misdirected at the cut end of the retinal stump and ultimately extended adjacent to the retinal segment back toward the eye. A more organized pattern of axonal elongation was observed in the presence of the glial substratum of the central stump, and growth cones appeared to associate preferentially with astrocyte endfeet in both tadpoles and frogs. These observations show that axons in the regenerating optic nerve of the amphibian can interact with a variety of cells and tissues and that the general direction of their outgrowth, at least in more peripheral regions of the visual pathway, appears to be dependent upon the orientation and, possibly, molecular properties of the terrain which they contact. In general, the basic environmental factors which either foster or impede axonal elongation in this regenerating system appear analogous to those influencing fiber outgrowth during regeneration in the peripheral nervous system of various species.

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

The retina and optic nerve head have been examined by light and electron microscopy in adult Xenopus laevis 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 separated 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.

Histopathological reactions an axonal regeneration in the transected spinal cord of Hibernating squirrels

The failure of axonal regeneration in the transected spinal cord of mammals has been attributed to many factors, including an intrinsic lack of regenerative capacity of mature CNS neurons, mechanical obstruction of axonal elongation by glial-connective tissue scars, necrosis of spinal tissue resulting in cavitation, lack of trophic influences sufficient to sustain outgrowth, and contact inhibition resulting from the formation of aberrant synapses. Assessment of te relative importance of each of these factors requires animal models in which one or more of these pathological processes can be eliminated. We therefore examined the effects of spinal transection in the hibernating animal because, during hibernation, collagen formation is depressed while nerve regeneration and slow axonal transport are maintained. Midthoracic spinal transections were performed in hibernating ground squirrels and the spinal cords were examined histologically 1-6 months later. The lesion site was composed primarily of a loose accumulation of macrophages and showed minimal glial and collagenous scarring, or cavitation. There was extensive regeneration of intrinsic spinal cord and dorsal root fibers. These axons grew to the margin of the lesion where they turned abruptly and continued growing along the interface between the lesion and the spinal cord. We conclude (1) that mammalian spinal-cord neurons have considerable regenerative potential; (2) that such mechanical impediments as collagenous and glial scarring, cyst formation, and cavitation cannot provide the sole explanation of why regeneration in the mammalian CNS is abortive; and (3) that specific physical and chemical properties of the cells in the environment of the growth cone regulate the extent and orientation of regenerative axonal outgrowth.

Early stages of axonal regeneration in the goldfish optic tract: an electron microscopic study

Two hours after the goldfish optic tract was cut, the severed axons in the retinal stump of the tract showed ballooning of the axoplasm and myelin sheath in the region of the cut, with accumulation in the swollen axon of various organelles, including dense cored vesicles. By day 1 the myelin sheath had degenerated back to a node of Ranvier and the tip of the severed axon had formed a myelin-free terminal bulb with a well-organized core of 9-10 nm filaments. By 2 days, such terminal bulbs were often seen to be extended on a neck of cytoplasm a few micrometers in length, presumably indicating axonal outgrowth. In addition, occasional small bundles of axon sprouts were first seen at this time. The sprouts had a diameter of about 2 micrometers and contained a central core of 9-10 nm filaments surrounded by a mantle of cell organelles (smooth endoplasmic reticulum, mitochondria and diverse vesicles), with few if any microtubules. Sprouts within a bundle were separated by fairly uniform 10-15 nm spaces. Beginning at 3 days, significant numbers of microtubules appeared in the sprouts, and there was an increasing proportion of small diameter (greater than or equal to 0.3 micrometer) sprouts. Thus it was not until 3 days that the sprouts took on the appearance usually considered to be typical of regenerating axons. By 6 days a dense layer of glial cells or macrophages formed a cap over the cut surface of the tract. Penetrating this layer were bundles containing up to 20-30 axon sprouts and also single axons which may have been serving as 'pioneering' fibres to which later-emerging axons would attach. There was no evidence that the regenerating axons were guided by the glial cells. At 6 days astroglia began to separate individual axons within the bundles but oligodendrocytes were still inactive at this time.

Basal lamina formation at the site of spinal cord transection

The pia-glial basal lamina (BL) at the site of spinal cord injury could be an important physical impediment to central nervous system regeneration. We used an epithelial BL-specific immunohistochemical stain to determine the location of the pia-glial BL after spinal cord transection. Small segments of BL were found at the margin of the lesion 5 days after transection. After 10 days, longer and more numerous segments were seen. At 20 days, the entire transected end of the spinal cord was capped by a layer of BL.

Penetration of grafted astrocytic scars by regenerating optic nerve axons in Xenopus tadpoles

The following study was performed in order to determine the effect of a dense glial scar upon the outgrowth of neurites in the regenerating optic nerves of Xenopus tadpoles. Glial scars, primarily comprised of hypertrophic astrocytes, were formed in the optic nerves of postmetamorphic, juvenile Xenopus by unilateral enucleation. After 25--40 days, segments of glial scar tissue were then grafted near the cut retinal stumps of the transected optic nerves in stage 54--56 tadpoles. Within 7--10 days bundles of unmyelinated axons were seen among the cytoplasmic processes of the implanted astrocytes, and many of the fibers had traversed the entire extent of the graft by 7--10 days. The results indicate that in this regenerating system an extremely dense glial scar, formed by mature, hypertrophic astrocytes, does not represent a major obstacle to axonal outgrowth. These observations are discussed in relation to the problem of glial scarring and the general failure of regeneration in the mammalian central nervous system.