Characterization of the sprouting response of axon-like processes from retinal ganglion cells after axotomy in adult hamsters: a model using intravitreal implantation of a peripheral nerve

Methylene blue promotes survival and GAP-43 expression of retinal ganglion cells after optic nerve transection

AIMS

Neurodegeneration of the optic nerve and retinal ganglion cells (RGCs) leads to progressive vision loss. As part of the central nervous system, RGCs show limited ability to regenerate and there is extensive search for neuroprotective agents for optic nerve damage. Methylene blue (MB) exhibits beneficial effects against various neurodegenerative diseases of the central nervous system. However, the mechanisms associated with its putative protection on neuronal survival and regeneration remain obscure. This study used the optic nerve transection model to examine the effects of MB on RGC survival, the expression of regenerative marker GAP-43 in RGCs and microglial activation.

MAIN METHODS

Axons of RGCs were injured by cutting the optic nerve. MB was injected intravitreally either immediately post-injury or delayed to 3 days post-injury. Using immunohistochemical staining, surviving RGCs, GAP-43-positive RGCs and microglial cells were quantified in wholemount retinas 7 days post-injury.

KEY FINDINGS

Both immediate and delayed (a more clinically realistic situation) intravitreal injection of MB promoted RGC survival. MB also increased the number of GAP-43-positive RGCs, suggesting an enhanced ability of RGCs to regenerate. This was exemplified by the regenerative sprouting of axon-like processes from injured RGCs after MB treatment. The increase in RGC survival and GAP-43 expression correlated with an increase in the number of microglial cells.

SIGNIFICANCE

These results reveal that MB has survival-promoting and growth-promoting effects on RGCs after optic nerve injury. Together with the established safety profile of MB in humans, MB is a promising treatment for neurodegeneration and injury of the optic nerve.

Aberrant information transfer interferes with functional axon regeneration

Functional axon regeneration requires regenerating neurons to restore appropriate synaptic connectivity and circuit function. To model this process, we developed an assay in Caenorhabditis elegans that links axon and synapse regeneration of a single neuron to recovery of behavior. After axon injury and regeneration of the DA9 neuron, synapses reform at their pre-injury location. However, these regenerated synapses often lack key molecular components. Further, synaptic vesicles accumulate in the dendrite in response to axon injury. Dendritic vesicle release results in information misrouting that suppresses behavioral recovery. Dendritic synapse formation depends on dynein and jnk-1. But even when information transfer is corrected, axonal synapses fail to adequately transmit information. Our study reveals unexpected plasticity during functional regeneration. Regeneration of the axon is not sufficient for the reformation of correct neuronal circuits after injury. Rather, synapse reformation and function are also key variables, and manipulation of circuit reformation improves behavioral recovery.

Two Drosophila model neurons can regenerate axons from the stump or from a converted dendrite, with feedback between the two sites

BACKGROUND

After axon severing, neurons recover function by reinitiating axon outgrowth. New outgrowth often originates from the remaining axon stump. However, in many mammalian neurons, new axons initiate from a dendritic site when the axon is injured close to the cell body.

METHODS

Drosophila sensory neurons are ideal for studying neuronal injury responses because they can be injured reproducibly in a variety of genetic backgrounds. In Drosophila, it has been shown that a complex sensory neuron, ddaC, can regenerate an axon from a stump, and a simple sensory neuron, ddaE, can regenerate an axon from a dendrite. To provide a more complete picture of axon regeneration in these cell types, we performed additional injury types.

RESULTS

We found that ddaE neurons can initiate regeneration from an axon stump when a stump remains. We also showed that ddaC neurons regenerate from the dendrite when the axon is severed close to the cell body. We next demonstrated if a stump remains, new axons can originate from this site and a dendrite at the same time. Because cutting the axon close to the cell body results in growth of the new axon from a dendrite, and cutting further out may not, we asked whether the initial response in the cell body was similar after both types of injury. A transcriptional reporter for axon injury signaling, puc-GFP, increased with similar timing and levels after proximal and distal axotomy. However, changes in dendritic microtubule polarity differed in response to the two types of injury, and were influenced by the presence of a scar at the distal axotomy site.

CONCLUSIONS

We conclude that both ddaE and ddaC can regenerate axons either from the stump or a dendrite, and that there is some feedback between the two sites that modulates dendritic microtubule polarity.

Short-term increases in transient receptor potential vanilloid-1 mediate stress-induced enhancement of neuronal excitation

Progression of neurodegeneration in disease and injury is influenced by the response of individual neurons to stressful stimuli and whether this response includes mechanisms to counter declining function. Transient receptor potential (TRP) cation channels transduce a variety of disease-relevant stimuli and can mediate diverse stress-dependent changes in physiology, both presynaptic and postsynaptic. Recently, we demonstrated that knock-out or pharmacological inhibition of the TRP vanilloid-1 (TRPV1) capsaicin-sensitive subunit accelerates degeneration of retinal ganglion cell neurons and their axons with elevated ocular pressure, the critical stressor in the most common optic neuropathy, glaucoma. Here we probed the mechanism of the influence of TRPV1 on ganglion cell survival in mouse models of glaucoma. We found that induced elevations of ocular pressure increased TRPV1 in ganglion cells and its colocalization at excitatory synapses to their dendrites, whereas chronic elevation progressively increased ganglion cell Trpv1 mRNA. Enhanced TRPV1 expression in ganglion cells was transient and supported a reversal of the effect of TRPV1 on ganglion cells from hyperpolarizing to depolarizing, which was also transient. Short-term enhancement of TRPV1-mediated activity led to a delayed increase in axonal spontaneous excitation that was absent in ganglion cells from Trpv1(-/-) retina. In isolated ganglion cells, pharmacologically activated TRPV1 mobilized to discrete nodes along ganglion cell dendrites that corresponded to sites of elevated Ca(2+). These results suggest that TRPV1 may promote retinal ganglion cell survival through transient enhancement of local excitation and axonal activity in response to ocular stress.

Hepatocyte growth factor promotes long-term survival and axonal regeneration of retinal ganglion cells after optic nerve injury: comparison with CNTF and BDNF

AIMS

Different trophic factors are known to promote retinal ganglion cell survival and regeneration, but each had their own limitations. We report that hepatocyte growth factor (HGF) confers distinct advantages in supporting ganglion cell survival and axonal regeneration, when compared to two well-established trophic factors ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF).

METHODS

Ganglion cells in adult hamster were injured by cutting the optic nerve. HGF, CNTF, or BDNF was injected at different dosages intravitreally after injury. Ganglion cell survival was quantified at 7, 14, or 28 days postinjury. Peripheral nerve (PN) grafting to the cut optic nerve of the growth factor-injected eye was performed either immediately after injury or delayed until 7 days post-injury. Expression of heat-shock protein 27 and changes in microglia numbers were quantified in different growth factor groups. The cellular distribution of c-Met in the retina was examined by anti-c-Met immunostaining.

RESULTS

Hepatocyte Growth Factor (HGF) was equally potent as BDNF in promoting short-term survival (up to 14 days post-injury) and also supported survival at 28 days post-injury when ganglion cells treated by CNTF or BDNF failed to be sustained. When grafting was performed without delay, HGF stimulated twice the number of axons to regenerate compared with control but was less potent than CNTF. However, in PN grafting delayed for 7 days after optic nerve injury, HGF maintained a better propensity of ganglion cells to regenerate than CNTF. Unlike CNTF, HGF application did not increase HSP27 expression in ganglion cells. Microglia proliferation was prolonged in HGF-treated retinas compared with CNTF or BDNF. C-Met was localized to both ganglion cells and Muller cells, suggesting HGF could be neuroprotective via interacting with both neurons and glia.

CONCLUSION

Compared with CNTF or BDNF, HGF is advantageous in sustaining long-term ganglion cell survival and their propensity to respond to favorable stimuli.

Neuronal polarity in Drosophila: sorting out axons and dendrites

Drosophila neurons have identifiable axons and dendrites based on cell shape, but it is only just starting to become clear how Drosophila neurons are polarized at the molecular level. Dendrite-specific components including the Golgi complex, GABA receptors, neurotransmitter receptor scaffolding proteins, and cell adhesion molecules have been described. Proteins involved in constructing presynaptic specializations are concentrated in axons of some neurons. A very simple model for how these components are distributed to axons and dendrites can be constructed based on the opposite polarity of microtubules in axons and dendrites: dynein carries cargo into dendrites, and kinesins carry cargo into axons. The simple model works well for multipolar neurons, but will likely need refinement for unipolar neurons, which are common in Drosophila.

Global up-regulation of microtubule dynamics and polarity reversal during regeneration of an axon from a dendrite

Axon regeneration is crucial for recovery after trauma to the nervous system. For neurons to recover from complete axon removal they must respecify a dendrite as an axon: a complete reversal of polarity. We show that Drosophila neurons in vivo can convert a dendrite to a regenerating axon and that this process involves rebuilding the entire neuronal microtubule cytoskeleton. Two major microtubule rearrangements are specifically induced by axon and not dendrite removal: 1) 10-fold up-regulation of the number of growing microtubules and 2) microtubule polarity reversal. After one dendrite reverses its microtubules, it initiates tip growth and takes on morphological and molecular characteristics of an axon. Only neurons with a single dendrite that reverses polarity are able to initiate tip growth, and normal microtubule plus-end dynamics are required to initiate this growth. In addition, we find that JNK signaling is required for both the up-regulation of microtubule dynamics and microtubule polarity reversal initiated by axon injury. We conclude that regulation of microtubule dynamics and polarity in response to JNK signaling is key to initiating regeneration of an axon from a dendrite.

Axonal regeneration and development of de novo axons from distal dendrites of adult feline commissural interneurons after a proximal axotomy

Following proximal axotomy, several types of neurons sprout de novo axons from distal dendrites. These processes may represent a means of forming new circuits following spinal cord injury. However, it is not know whether mammalian spinal interneurons, axotomized as a result of a spinal cord injury, develop de novo axons. Our goal was to determine whether spinal commissural interneurons (CINs), axotomized by 3-4-mm midsagittal transection at C3, form de novo axons from distal dendrites. All experiments were performed on adult cats. CINs in C3 were stained with extracellular injections of Neurobiotin at 4-5 weeks post injury. The somata of axotomized CINs were identified by the presence of immunoreactivity for the axonal growth-associated protein-43 (GAP-43). Nearly half of the CINs had de novo axons that emerged from distal dendrites. These axons lacked immunoreactivity for the dendritic protein, microtubule-associated protein2a/b (MAP2a/b); some had GAP-43-immunoreactive terminals; and nearly all had morphological features typical of axons. Dendrites of other CINs did not give rise to de novo axons. These CINs did, however, each have a long axon-like process (L-ALP) that projected directly from the soma or a very proximal dendrite. L-ALPs were devoid of MAP2a/b immunoreactivity. Some of these L-ALPs projected through the lesion and formed bouton-like swellings. These results suggest that proximally axotomized spinal interneurons have the potential to form new connections via de novo axons that emerge from distal dendrites. Others may be capable of regeneration of their original axon.

Rewiring the injured CNS: lessons from the optic nerve

The optic nerve offers a number of advantages for investigating mechanisms that govern axon regeneration in the CNS. Although mature retinal ganglion cells (RGCs) normally show no ability to regenerate injured axons through the optic nerve, this situation can be partially reversed by inducing an inflammatory response in the eye. The secretion of a previously unknown growth factor, oncomodulin, along with co-factors, causes RGCs to undergo dramatic changes in gene expression and regenerate lengthy axons into the highly myelinated optic nerve. By themselves, strategies that counteract inhibitory signals associated with myelin and the glial scar are insufficient to promote extensive regeneration in this system. However, combinatorial treatments that activate neurons' intrinsic growth state and overcome inhibitory signals result in dramatic axon regeneration in vivo. Because of the ease of introducing trophic factors, soluble receptors, drugs, or viruses expressing any gene or small interfering RNA of interest into RGCs, this system is ideal for identifying intracellular signaling pathways, transcriptional cascades, and ligand-receptor interactions that enable axon regeneration to occur in the CNS.

Osteonectin is a Schwann cell-secreted factor that promotes retinal ganglion cell survival and process outgrowth

We have investigated the factors made by Schwann cells (SCs) that stimulate survival and neurite outgrowth from postnatal rat retinal ganglion cells (RGCs). These effects are preserved under K252a blockade of the Trk family of neurotrophin receptors and are not fully mimicked by the action of a number of known trophic factors. To identify novel factors responsible for this regenerative activity, we have used a radiolabelling assay. Proteins made by SCs were labelled radioactively and then fed to purified RGCs. The proteins taken up by the RGCs were then isolated and further characterized. Using this assay we have identified a major 40 kDa factor taken up by RGCs, which was microsequenced and shown to be the matricellular protein osteonectin (ON). Using an in vitro assay of purified RGCs we show that ON promotes both survival and neurite outgrowth. We conclude that ON has a potential new role in promoting CNS repair.

Factors secreted by Schwann cells stimulate the regeneration of neonatal retinal ganglion cells

The adult mammalian central nervous system (CNS) does not repair after injury. However, we and others have shown in earlier work that the neonatal CNS is capable of repair and importantly of allowing regenerating axons to re-navigate through the same pathways as they did during development. This phase of neonatal repair is restricted by the fragility of neurons after injury and a lack of trophic factors that enable their survival. Our aim is to define better the factors that sustain neurons after injury and allow regeneration to occur. We describe some of our work using Schwann cells to promote the regeneration of neurons from young postnatal rodents. We have established rapid methods for purifying Schwann cells without the use of either anti-mitotic agents to suppress contaminating fibroblasts or mitotic stimulation to generate large numbers of Schwann cells. The rapidly purified Schwann cells have been used to generate conditioned medium that we have shown stimulates axon regeneration in cultured retinal ganglion cell neurons. We also show that the positive effects of Schwann cells are still present after pharmacological blockade of the neurotrophin receptors, suggesting that novel factors mediate these effects.

Sprouting of axon-like processes from axotomized retinal ganglion cells induced by normal and preinjured intravitreal optic nerve grafts

The failure of axonal regeneration in the mammalian central nervous system (CNS) is currently attributed to the glial environment of the lesion site which elaborates a multitude of inhibitory factors. Less attention has been paid to the potential of trophic support associated with the CNS, especially in relation to the status of the damaged CNS after an injury has been evoked. Using a grafting paradigm to implant an optic nerve (ON) segment into the vitreous, we have addressed how a prior damage of the ON before grafting influences its ability to stimulate retinal ganglion cells (RGCs) to sprout axon-like processes. Our results showed that a normal noninjured ON implanted intravitreally stimulated sprouting of RGCs, as revealed by sliver staining of the sprouting cells, as well as increasing the number of RGCs which express GAP-43. A prior crush injury of the ON 7 days before its implantation into the vitreous resulted in a significant decrease in its ability to stimulate RGC sprouting when the crush lesion segment was used as the graft, whereas grafts taken from segments proximal and distal to the lesion segment had potencies similar to that of the noninjured graft. Both astrocytes and oligodendrocytes were drastically reduced in number in the lesion segment graft, suggesting their involvement in the secretion of soluble trophic factors that may play a role in the sprouting and regeneration of damaged neurons.

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

The temporal sequence of morphological and molecular changes in axotomized feline motoneurons leading to the formation of axons from the ends of dendrites

At 8-12 weeks post axotomy, unusual distal processes (UDPs) with axon-like structural (uniform diameter, tortuous) and molecular (growth-associated protein [GAP]43, absence of microtubule-associated protein [MAP]2a/b immunoreactivity) features emerge from distal motoneuron dendrites (Rose et al. [2001] Eur J Neurosci 13:1166-1176). In this study, we determine the time course of molecular and morphological changes associated with the formation of axons from dendrites. Motoneurons innervating neck muscles in the adult cat were permanently axotomized for 2, 4, 20, or 35 weeks and intracellularly stained with Neurobiotin. Computer-assisted reconstructions were used to map the location of MAP2a/b and GAP-43 immunoreactivity. At 2 and 4 weeks post axotomy, all UDPs had short appendages, giving them an arboreal appearance. They were immunoreactive for GAP-43 and lacked immunostaining for MAP2a/b. Axon-like UDPs were not seen until 8-12 weeks post axotomy. By 20 and 35 weeks post axotomy, some axon-like UDPs acquired morphological features of axons with synaptic connections (right-angled branching, bouton-like specializations). GAP-43 immunoreactivity was not detected in any axotomized motoneurons by 20 weeks post axotomy, whereas all UDPs remained devoid of MAP2a/b immunoreactivity even at 35 weeks post axotomy. These molecular changes accompanied structural modifications to proximal regions of "dendrites" giving rise to UDPs. The distance from the ends of the UDPs to the soma did not change. Thus, all UDPs begin as simple, arboreal structures with molecular features of growing axons, but over a period of 35 weeks, some UDPs slowly acquire morphological and molecular features of motoneuron axons with synaptic connections. These results suggest a new modus operandi for axonal growth and the establishment of new synaptic connections after injury.

Survival and axonal regeneration of retinal ganglion cells in adult cats

Axotomized retinal ganglion cells (RGCs) in adult cats offer a good experimental model to understand mechanisms of RGC deteriorations in ophthalmic diseases such as glaucoma and optic neuritis. Alpha ganglion cells in the cat retina have higher ability to survive axotomy and regenerate their axons than beta and non-alpha or beta (NAB) ganglion cells. By contrast, beta cells suffer from rapid cell death by apoptosis between 3 and 7 days after axotomy. We introduced several methods to rescue the axotomized cat RGCs from apoptosis and regenerate their axons; transplantation of the peripheral nerve (PN), intraocular injections of neurotrophic factors, or an antiapoptotic drug. Apoptosis of beta cells can be prevented with intravitreal injections of BDNF+CNTF+forskolin or a caspase inhibitor. The injection of BDNF+CNTF+forskolin also increases the numbers of regenerated beta and NAB cells, but only slightly enhances axonal regeneration of alpha cells. Electrical stimulation to the cut end of optic nerve is effective for the survival of axotomized RGCs in cats as well as in rats. To recover function of impaired vision in cats, further studies should be directed to achieve the following goals: (1). substantial number of regenerating RGCs, (2). reconstruction of the retino-geniculo-cortical pathway, and (3). reconstruction of retinotopy in the target visual centers.

Alterations to neuronal polarity following permanent axotomy: a quantitative analysis of changes to MAP2a/b and GAP-43 distributions in axotomized motoneurons in the adult cat

Following axotomy, morphologically unusual, distal processes (UDPs) emerge from motoneuron dendrites. These processes contain an axonal protein, growth-associated protein 43 (GAP-43) but lack immunostaining for the dendritic protein microtubule-associated protein 2a/b (MAP2a/b). Thus, it appears that neuronal polarity alters following axotomy. Our goal was to describe this change in neuronal polarity on a more detailed and quantitative level. We asked two questions: Following axotomy, where in the entire neuron does the immunoreactivity for MAP2a/b and GAP-43 change and do these changes reflect a transformation of dendrite to axon or growth from terminal dendrites? Using intracellular labeling and immunocytochemistry, changes in MAP2a/b and GAP-43 immunoreactivity were also found in processes with a morphology typical of terminal branches of intact motoneurons (called simple distal processes [SDPs]), as well as UDPs. Trajectories (the path from the soma to a single terminus) with UDPs and SDPs were longer than trajectories without these processes, and trajectories with UDPs were the longest. Trajectories without UDPs or SDPs were similar in length to trajectories from intact motoneurons. The distance from the soma to the point where MAP2a/b immunoreactivity became absent in trajectories with UDPs or SDPs was similar to the length of trajectories from intact motoneurons. Thus, following axotomy, two morphologically distinct types of axon-like processes emerge from dendrites. The formation of these processes does not involve a transformation of the original dendrite, but rather growth at the ends of dendrites.

Emergence of axons from distal dendrites of adult mammalian neurons following a permanent axotomy

The distinctive features of axons and dendrites divide most neurons into two compartments. This polarity is fundamental to the ability of most neurons to integrate synaptic signals and transmit action potentials. It is not known, however, if the polarity of neurons in the adult mammalian nervous system is fixed or plastic. Following axotomy, some distal dendrites of neck motoneurons in the adult cat give rise to unusual processes that, at a light microscopic level, resemble axons (Rose, P.K. & Odlozinski, M., J. Comp. Neurol., 1998, 390, 392). The goal of the present experiments was to characterize these unusual processes using well-established ultrastructural and molecular criteria that differentiate dendrites and axons. These processes were immunoreactive for growth-associated protein-43 (GAP-43), a protein that is normally confined to axons. In contrast, immunoreactivity for a protein that is widely used as a marker for dendrites, microtubule-associated protein (MAP)-2a/b, could not be detected in the unusual distal arborizations. At the electron microscopic level, unusual distal processes contained dense collections of neurofilaments and were frequently myelinated. These molecular and structural characteristics are typical of axons and suggest that the polarity of adult neurons in the mammalian nervous system can be disrupted by axotomy. If this transformation in neuronal polarity is common to other types of neurons, axon-like processes emerging from distal dendrites may represent a mechanism for replacing connections lost due to injury. Alternatively, the connections formed by these axons may be aberrant and therefore maladaptive.

Axonal regeneration of retinal ganglion cells: effect of trophic factors

A variety of neurotrophic factors can influence the cell functions of the developing, mature and injured retinal ganglion cells. The discovery that retinal ganglion cell loss can be alleviated by neurotrophic factors has generated a great deal of interest in the therapeutic potential of these molecules. Recently, evidence has provided valuable information on the receptors that mediate these events and the intracellular signaling cascades after the binding of these ligands. Signaling by neurotrophic factors does not seem to restrict to retrograde messenger from the target but also includes local interactions with neighbouring cells along the axonal pathways, anterograde signaling from the afferents and autocrine signaling. More insight into the mechanisms of action of neurotrophic factors and the signal transduction pathway leading to the protection and regeneration of retinal ganglion cells may allow the design of new therapeutic strategies.

A two-dimensional gel electrophoretic study of proteins synthesized and released by degenerating adult mouse sciatic nerve

Previous two-dimensional (2-D) gel electrophoretic studies of proteins secreted by degenerating mammalian peripheral nerves (Ignatius et al., 1986, Proc. Natl. Acad. Sci. USA 83: 1125-1129; Muller et al., 1986, J. Cell Biol. 102: 393-402) detected the up-regulation of two proteins of 67-70 and 34-37 kDa, although they failed to resolve proteins smaller than about 15 kDa or with isoelectric points greater than 8. In the present study, we have used 2-D gels that can resolve proteins in the molecular mass range 3.6-200 kDa and isoelectric point range 2.4-10.6. This revealed that the incorporation of radiolabel by three diffusible proteins with apparent molecular mass/isoelectric point values of 38/5-6, 27-31/4-5, and 8/>10 was increased in the distal stumps of sciatic nerves 4 days after lesion, while the radiolabel incorporation by a further two proteins (15/5.3 and 12.5-17.5/6.8-7.5) was increased in the distal nerve stump 15 days after lesion. The possible cellular sources of these proteins were assessed by comparing protein secretion from unoperated nerves with nerve segments maintained in culture for 4 days (in which the contribution from recruited macrophages would be expected to be minimal) and segments of nerve that had been frozen and then replaced in situ for 4 days (in which the contribution from nerve sheath cells would be expected to be minimal). This revealed that three of the proteins up-regulated in lesioned nerves (27-31/4-5, 15/5.3, and 12.5-17.5/6.8-7.5) are probably sheath cell products, while the other two (38/5-6 and 8/>10) may be secreted mainly by macrophages (or other cells) that infiltrate the frozen nerve segments. The identity of these proteins and their possible involvement in axonal regeneration remain to be determined.

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.

Re-establishment of visual circuitry after optic nerve regeneration

In mammals there are a few circumstances in which axotomised ganglion cell axons can regenerate. For instance, in vitro explants of retina can be encouraged to regenerate axons into appropriate culture media. Similarly, axotomised ganglion cells can regenerate into a peripheral nerve graft surgically connected to the optic nerve head, and during early development axons are able to regenerate across the retina to re-enter the optic nerve. This is certainly encouraging, but we are a long way from applying these observations to clinical practice. We need to know whether regenerating axons also retain a functional capacity for navigation. We must ask whether a regenerated projection is likely to be topographic rather than disordered. In this brief review we will look at some selected models of ganglion cell regeneration in order to examine this question of navigation in more detail. This is an important issue: the capacity to re-establish appropriate rather than random connections after ganglion cell regeneration would most likely be necessary for any meaningful return of visual function.

Why does the central nervous system not regenerate after injury?

Spinal cord injuries in humans and in other mammals are never followed by regrowth. In recent years, considerable progress has been made in analyzing mechanisms that promote and inhibit regeneration. The focus of this review is changes that occur in the transition period in development when the central nervous system (CNS) changes from being able to regenerate to the adult state of failure. In our experiments we have used the neonatal opossum (Monodelphis domestica), which corresponds to a 14-day embryonic rat or mouse. The CNS isolated from an opossum pup and maintained in culture shows dramatic regeneration. Fibers grow through and beyond lesions and reform synaptic connections with their targets. Similarly, anesthetized neonatal pups attached to the mother recover the ability to walk after complete spinal cord transection. Although the CNS isolated from a 9-day-old animal will regenerate in vitro, CNS from a 12-day-old will not. This is the stage at which glial cells in the CNS develop. Present research is devoted toward molecular screening to determine which growth-promoting molecules decrease during development, which inhibitory molecules increase, and which receptors on growing axons become altered. Despite progress in many laboratories, major hurdles must be overcome before patients can hope to be treated. Nevertheless, the picture today is not as discouraging as it was: one can think of strategies for research on spinal cord injury so as to promote regeneration and restore function.

Synergistic effect of optic and peripheral nerve grafts on sprouting of axon-like processes of axotomized retinal ganglion cells in adult hamsters

We investigated the sprouting response of retinal ganglion cells (RGCs) following the transplantation of peripheral nerve (PN) and/or optic nerve (ON) into the vitreous of the eye and the intraorbital transection of the optic nerve in hamsters. Our previous results showed that an intravitreal PN graft could induce sprouting of axon-like processes in axotomized RGCS [3] (Cho, E.Y. and So, K.F., Characterization of the sprouting response of axon-like processes from retinal ganglion cells after axotomy in adult hamsters: a model using intravitreal implantation of a peripheral nerve, J. Neurocytol., 21 (1992) 589-603). In this model, we have examined the effect of intravitreal ON graft on the sprouting of RGCs both following a co-transplantation of PN and ON into the vitreous and transplantation of ON alone. The present results show that sprouting is increased by more than two-fold in retinas having PN and ON grafts than a PN graft alone. However, the ON graft by itself rarely induced sprouting in RGCs. These results suggest that the ON graft enhance the number of RGCs to sprout axon-like processes in the presence of PN graft by exerting a synergistic rather than an additive effect, since ON graft alone did not induce sprouting. In addition, no diffusible inhibitory effect of ON graft on PN induced sprouting was observed.

Regenerative sprouting of retinal ganglion cells of adult hamsters induced by the epineurium of a peripheral nerve

Although it is known that transplantation of a peripheral nerve (PN) to the damaged central nervous system (CNS) promotes axonal regeneration, the interactions of cellular components of the PN with CNS neurons are still not well defined. Schwann cells in the PN are thought to be the major element involved in supporting CNS regeneration, but very little information exists with regard to whether other PN components also play an active role. Using our previously established model of transplanting a PN segment into the vitreous to stimulate regenerative sprouting of retinal ganglion cells (RGCs), we found that the epineurium isolated from a PN which had been pre-injured by transection was able to induce RGC sprouting when implanted intravitreally. Since the epineurium is composed mainly of connective tissue components and is devoid of Schwann cells, our results suggest that other cellular elements of the PN besides Schwann cells may have the potential to support CNS regeneration.

Ciliary neurotrophic factor is an axogenesis factor for retinal ganglion cells

Although mature mammalian retinal ganglion cells normally fail to regrow injured axons, exposure to the molecular environment of the peripheral nervous system stimulates regenerative growth. The present study used dissociated rat retinal ganglion cells purified by immunopanning to identify peripheral nervous system-derived factors that promote axonal outgrowth. Of the multiple growth factors investigated, only ciliary neurotrophic factor and the related cytokine, leukemia inhibitory factor, had striking neuritogenic activity, with half-maximal effects at 1-2 ng/ml. Brain-derived neurotrophic factor stimulated retinal ganglion cell survival nearly as well as ciliary neurotrophic factor, but had only minor effects on outgrowth. Thus, the neuritogenic effects of ciliary neurotrophic factor are not a simple consequence of increased survival. Ciliary neurotrophic factor-stimulated outgrowth was correlated with increased expression of the growth-associated membrane phosphoprotein, GAP-43, a hallmark of optic nerve regeneration in vivo. A high molecular weight fraction from media conditioned by rat optic or sciatic nerve mimicked the effect of ciliary neurotrophic factor in inducing axonal outgrowth. Ciliary neurotrophic factor was detected in the conditioned media on western blots, and the biological activity of the conditioned media was neutralized with an anti-ciliary neurotrophic factor antibody. These results indicate that ciliary neurotrophic factor has specific effects on axon outgrowth in retinal ganglion cells that are dissociable from its effects on cell survival, and that ciliary neurotrophic factor accounts for most of the axon-promoting activity for retinal ganglion cells present in either the sciatic or optic nerve.

Regeneration and transplantation of the optic nerve: developing a clinical strategy

Three separate experimental models of optic nerve regeneration have been presented--along the existing pathway in the presence of antibodies to neutralise inhibitory molecules, along peripheral nerve grafts and from retinal transplants. Each offers a theoretical clinical strategy for restoration of vision, if the mechanism of re-establishment of maps and reconnection to appropriate targets during regeneration can be determined. This is the process of axon guidance, and underlines the importance of our research into the molecular determinants that guide normal development of the visual system.

Regenerative capacity of retinal ganglion cells in mammals

Retinal ganglion cells (RGCs) and their projections in the optic nerve offer a convenient model to study survival and regeneration of mammalian central nervous system (CNS) nerve cells following injury. Possible factors affecting the death of RGCs following axotomy and various approaches to rescue the axotomized RGCs are discussed. In addition, two main strategies currently used to enhance axonal regeneration of damaged RGCs are described. The first focuses on overcoming the unfavorable extrinsic CNS environment and the second concentrates on upregulating the intrinsic growth potential of RGCs. Thus, the failure or success of RGC axonal regrowth after injury depends on the complicated interplay between the extrinsic and intrinsic factors.

Expansion of the dendritic tree of motoneurons innervating neck muscles of the adult cat after permanent axotomy

The morphologic characteristics of neck motoneurons with intact axons were compared with those of neck motoneurons that had been permanently axotomized for 11 to 17 weeks. Motoneurons were identified antidromically, intracellularly stained with horseradish peroxidase (HRP) and examined after reconstructions of their entire dendritic tree. Axotomized motoneurons differed qualitatively and quantitatively from motoneurons with intact axons. The distal branches of axotomized motoneurons exhibited two novel features: some gave rise to tangled appendages that exhibited growth cone-like specializations resembling lamellipodia and filopodia; others followed a meandering path and had unusually large diameters. These branches showed a discontinuous pattern of staining that was similar to the appearance of myelinated axons stained intra-axonally with HRP. A quantitative analysis of the dendritic trees of 13 completely reconstructed dendritic trees (five axotomized motoneurons and eight motoneurons with intact axons) showed that total dendritic surface area, total dendritic length, and total number of branches increased 38, 34, and 215%, respectively, after axotomy. These measurements were confirmed by comparing the sizes of a larger number of motoneurons (16 axotomized and 21 intact), calculated on the basis of correlations between dendritic tree size and proximal dendritic diameter. We conclude, therefore, that neck motoneurons, in contrast to other types of motoneurons, expand their dendritic trees after axotomy. It is suggested that this expansion is a consequence of two mechanisms: one involves dendritic growth, possibly leading to new synaptic connections; the other causes a conversion of some dendrites into axons.

Development and role of retinal glia in regeneration of ganglion cells following retinal injury

AIMS/BACKGROUND

Recent observations have shown that the glial scar resulting from a surgical lesion of the immature retina differs from elsewhere in the central nervous system, in that it permits the through growth and reconnection of regenerating axons. This study in the opossum examines in detail the development and reaction to injury of retinal glia at different developmental stages, and specifically examines the distribution of the gliosis related inhibitory molecule, chondroitin sulphate proteoglycan (CSPG), making comparisons with a control site of gliosis in the cerebral cortex.

METHODS

A linear slit was cut into the retina or cortex with a fine tungsten probe. After a variable time delay, immunocytochemistry of the resulting gliosis was employed to detect astrocytes with glial fibrillary acidic protein (GFAP), Müller cells with vimentin, and CSPG with CS-56 antibodies. GFAP was also used at different ages to examine the normal development of astrocytes in the retina of this species.

RESULTS

Astrocytes entered the retina 12 days after birth (P12), closely associated with blood vessels in the nerve fibre layer. In experiments at all ages studied, cellular continuity was re-established across the lesioned retina, which did not result in a significant astrocyte proliferation or CSPG expression. In contrast, cortical injury led to the development of a cystic cavity surrounded by astrocytes and CSPG. Müller cells expressed GFAP but not CSPG in the lesioned retina.

CONCLUSION

Successful regrowth of ganglion cells through a retinal lesion may be partly the result of the scarcity of astrocytes in the retina, which results in minimal gliosis, or of their apparent inability to express inhibitory molecules.

Monoclonal antibody C38 recognizes retinal ganglion cells in cats and rats

We developed monoclonal antibody C38 which specifically recognizes retinal ganglion cells (RGCs) in flatmount preparations of cat and rat retinas. We first induced immunological tolerance in Balb/c mice against axotomized rat retinas which lack most of the RGCs. Then the mice were immunized with intact rat retinas to produce antibodies against RGCs. Monoclonal antibody C38 appeared to be specific for cat RGCs based on immunoreactivities seen in flatmounts and vertical sections of the retina. In rats, we verified that over 90% of retrogradely labeled RGCs were immunoreactive for C38 antibody. In axotomized rat retinas, surviving RGCs were labeled with C38 without erroneous labeling of glial cells. The antigen that C38 recognized was 24 kDa in molecular weight and found in cerebrum, cerebellum, and spinal cord as well as retina. It is suggested that monoclonal antibody C38 is a useful label for RGCs.

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

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

The effect of damage of the brachium of the superior colliculus in neonatal and adult hamsters and the use of peripheral nerve to restore retinocollicular projections

Using horseradish peroxidase (HRP) tracing technique, we were able to confirm the critical age in hamsters as reported previously (SO et al., 1981). Thus, following transection of the retinal fibers at the brachium of the superior colliculus (BSC) on postnatal-day 4 (P4) or later, no retinocollicular projections were observed in the adult stage. However, the retinal fibers were observed to reinnervate the superior colliculus (SC) if the BSC was cut on P3 or earlier. Physiological recording showed a close to normal retinocollicular map following a BSC damage on P0. Although retinal fibers did not reinnervate the SC following a BSC cut on or after P4, they could be observed to grow along a membrane over the damaged site. Bridging the site of BSC damage in adult hamsters using a segment of peripheral nerve (PN), retinal fibers labelled with WGA-HRP were observed to reinnervate the SC along the PN graft and visual evoked responses could be recorded in the SC showing the PN graft is effective in restoring damaged central visual pathways in adult mammals.

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

We have examined the ability of axotomized retinal ganglion cells in adult hamsters, to regenerate axons into a peripheral nerve graft attached to the optic nerve and the expression of GAP-43 by these neurons. We also examined the effect on these events of transplanting a segment of peripheral nerve to the vitreous body. The left optic nerves in three groups of hamsters were replaced with a long segment of peripheral nerve attached to the proximal stump of the optic nerve approximately 2 mm from the optic disc to induce regeneration of retinal ganglion cells into the peripheral nerve. An additional segment of peripheral nerve was transplanted into the vitreous of the left eye in the second group. The animals from the first and second groups were allowed to survive for 1-8 weeks and the number of regenerating retinal ganglion cells was determined by applying the retrograde tracer, Fluoro-Gold to the peripheral nerve graft and the expression of GAP-43 was studied by immunocytochemistry in the same retinas. As a control, a segment of optic nerve was transplanted into the vitreous body of the left eye in the third group of hamsters. These animals were allowed to survive for 4 weeks and the number of regenerating retinal ganglion cells was counted as in Groups 1 and 2. The percentages of the regenerating retinal ganglion cells which also expressed GAP-43 were very high at all time points in Group 1 (with no intravitreal peripheral nerve) and Group 2 (with intravitreal peripheral nerve) and at 4 weeks for the Group 3 (with intravitreal optic nerve) animals. In addition, the number of regenerating retinal ganglion cells, the number of retinal ganglion cells expressing GAP-43 and the number of regenerating retinal ganglion cells which also expressed GAP-43 were much higher in Group 2 than in Group 1 at all the time points and it was also much higher in Group 2 than in Group 3 at 4 weeks whereas there was no significant difference between the results from Groups 1 and 3 at 4 weeks. These data suggested that there was a close correlation between the number of the axotomized retinal ganglion cells regenerating axons into the peripheral nerve graft attached to the optic nerve and the expression of GAP-43.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological plasticity of axotomized retinal ganglion cells following intravitreal transplantation of a peripheral nerve segment

During normal development of retinal ganglion cells when the axons are growing, transient dendritic spines have been observed. Similar dendritic spine-like processes are also exhibited by retinal ganglion cells undergoing axonal regeneration into a peripheral nerve grafted to the damaged optic axons. Here we show, using the intracellular injection of Lucifer Yellow, that when a segment of peripheral nerve is transplanted to the vitreous body, a procedure which induces ectopic sprouting of axon-like processes from the cell bodies and dendrites of some retinal ganglion cells, similar spine-like processes appear on the dendrites of cells with ectopic sprouts. Quantitative analysis indicated that there were significant changes with posttransplantation survival time in the distributions of spine-like processes and axon-like processes on these sprouting retinal ganglion cells following the intravitreal transplantation of a piece of peripheral nerve. The remodelling of the spine-like processes and axon-like processes correlated with one another suggesting that plastic changes can occur in certain dendritic subcompartments independent of the growth activity of the other dendritic subcompartments.

Number and dendritic morphology of retinal ganglion cells that survived after axotomy in adult cats

Retinal ganglion cells (RGCs) of adult cats were labeled by injection of diI into the proximal stump of completely transected optic nerves. Approximately 2% to 5% of the RGC population appeared viable 2 months after these axotomies, based on diI retention. The morphological type and dendritic arbor of these surviving RGCs were examined after intracellular injections of Lucifer Yellow into diI-labeled RGCs. Postaxotomy survival rate was much higher for alpha-like cells than for beta-like cells. However, in one of four retinas examined, a large number of RGCs seemed to survive axotomy, and among these, beta cells survived at an unusually high rate. Dendritic arbors of surviving RGCs were also examined after intracellular injection of horseradish peroxidase. Some dendrites of these RGCs lacked branches and were thin in caliber. Other dendrites displayed many spiny processes and bulbous swellings. Essentially, these results confirm the previous suggestion that alpha cells survive axotomy longer than beta cells. The ability of alpha cells to regenerate axons may thus be attributable to their relatively high resistance to axotomy. The atypical dendritic profiles seen after optic nerve transection may reflect either degeneration or regrowth of dendrites.

Trophic effect of collicular proteoglycan on neonatal rat retinal ganglion cells in situ

Naturally occurring neuronal death is widespread in the central nervous system of mammals. To date, the causes and mechanisms of such death are poorly understood. A major hypothesis is that developing neurons compete for limited amounts of trophic factor(s) released from their target centres as in the case of the peripheral nervous system and nerve growth factor. The present study aims to test this 'trophic hypothesis' in the mammalian central nervous system. In the rat, more than 50% of retinal ganglion cells die in the early post-natal period. Schulz and coworkers [57] purified a potential trophic agent from their major target, the superior colliculus, which was identified as a 480 kDa chondroitin sulfate proteoglycan. This proteoglycan or control solutions were injected into the eyes of rat pups during the post-natal part of the period of naturally occurring ganglion cell death. It was found that the collicular proteoglycan prevented the death of a significant number of the ganglion cells that would normally have been lost over a post-injection period of one or two days. The effect of the proteoglycan was dose- and time-dependent. These results support the notion that trophic interactions are a determining factor in the survival of retinal ganglion cells during the period of naturally occurring cell death. It is also the first time that a proteoglycan has been shown to possess neurotrophic properties in situ.

NADPH-diaphorase neurons in the retina of the hamster

NADPH-diaphorase-positive neurons have been demonstrated in the inner nuclear layer and ganglion cell layer of the retina of different mammalian species, but so far no experiments have been conducted to identify whether these cells are amacrine cells and/or retinal ganglion cells. We attempted to solve this problem by studying the NADPH-diaphorase-positive neurons in the hamster retina. From the NADPH-diaphorase histochemical reaction, two distinct types of neurons in the hamster retina were identified. They were named ND(g) and ND(i) cells. The ND(g) cells were cells with larger somata, ranging from 10 to 21 microns in diameter with a mean of 15.58 microns (S.D. = 2.59). They were found in the ganglion cell layer only. The ND(i) cells were smaller, with the somata ranging from 7 to 11 microns and having the mean diameter of 8.77 microns (S.D. = 1.24). Most of the ND(i) cells were found in the inner nuclear layer, and only very few could be observed in the inner plexiform layer. On average, there were 8,033 ND(g) and 5,051 ND(i) cells in the ganglion cell layer and inner nuclear layer, respectively. Two experiments were performed to clarify whether any of the NADPH-diaphorase neurons were retinal ganglion cells. Following unilateral optic nerve section, which leads to the retrograde degeneration of retinal ganglion cells, the numbers of both ND(g) and ND(i) cells did not change significantly for up to 4 months. In addition, when retinal ganglion cells were prelabeled retrogradely (horseradish peroxidase or fluorescent microspheres) and retinas were then stained for NADPH diaphorase, no double-labeled neurons were detected. These results indicated that the NADPH-diaphorase neurons in the hamster retina were the amacrine cells in the inner nuclear layer and displaced amacrine cells in the ganglion cell layer. Dendrites of the ND(g) and ND(i) cells were found to stratify in sublaminae 1, 3, and 5 of the inner plexiform layer, with a prominent staining in the sublamina 5. The possible importance of this arrangement in the rod pathway is also discussed.

Regeneration of adult rat CNS axons into peripheral nerve autografts: ultrastructural studies of the early stages of axonal sprouting and regenerative axonal growth

If one end of a segment of peripheral nerve is inserted into the brain or spinal cord, neuronal perikarya in the vicinity of the graft tip can be labelled with retrogradely transported tracers applied to the distal end of the graft several weeks later, showing that CNS axons can regenerate into and along such grafts. We have used transmission EM to examine some of the cellular responses that underlie this regenerative phenomenon, particularly its early stages. Segments of autologous peroneal or tibial nerve were inserted vertically into the thalamus of anaesthetized adult albino rats. The distal end of the graft was left beneath the scalp. Between five days and two months later the animals were killed and the brains prepared for ultrastructural study. Semi-thin and thin sections through the graft and surrounding brain were examined at two levels 6-7 mm apart in all animals: close to the tip of the graft in the thalamus (proximal graft) and at the top of the cerebral cortex (distal graft). In another series of animals with similar grafts, horseradish peroxidase was applied to the distal end of the graft 24-48 h before death. Examination by LM of appropriately processed serial coronal sections of the brains from these animals confirmed that up to several hundred neurons were retrogradely labelled in the thalamus, particularly in the thalamic reticular nucleus. Between five and 14 days after grafting, large numbers of tiny (0.05-0.20 microns diameter) nonmyelinated axonal profiles, considered to be axonal sprouts, were observed by EM within the narrow zone of abnormal thalamic parenchyma bordering the graft. The sprouts were much more numerous (commonly in large fascicles), smoother surfaced, and more rounded than nonmyelinated axons further from the graft or in corresponding areas on the contralateral side of animals with implants or in normal animals. At longer post-graft survival times, the number of such axons in the parenchyma around the graft declined. At five days, some axonal sprouts had entered the junctional zone between the brain and the graft. By eight days there were many sprouts in the junctional zone and some had penetrated the proximal graft to lie between its basal lamina-enclosed columns of Schwann cells, macrophages and myelin debris. Within the brain, sprouts were in contact predominantly with other sprouts but also with all types of glial cell.(ABSTRACT TRUNCATED AT 400 WORDS)