GDNF enhances dopaminergic cell survival and fibre outgrowth in embryonic nigral grafts

Two groups of rats with unilateral 6OHDA lesions received either intrastriatal suspension grafts of embryonic ventral mesencephalon or sham grafts. Three subgroups of each of these received intrastriatal infusions of 1000 ng or 500 ng glial cell-line derived trophic factor (GDNF) or vehicle alone for 10 consecutive days. There was a highly significant dose-dependent effect of GDNF both on the number of TH-positive cells surviving in the grafts and on the density of fibre outgrowth. All grafted rats showed rapid compensation of amphetamine-induced rotation compared with rats with sham grafts. GDNF may provide a powerful tool to enhance the survival and maturation of dopaminergic neurones within mesencephalic transplants.

A comparative study of preparation techniques for improving the viability of striatal grafts using vital stains, in vitro cultures, and in vivo grafts

Cell suspension grafts from embryonic striatal primordia placed into the adult rat striatum survive well and are able to alleviate a number of behavioral deficits caused by excitotoxic lesions to this structure. However, neither the anatomical connectivity between the graft and host nor the functional recovery elicited by the grafts is completely restored. One way in which the survival and function of embryonic striatal grafts may be enhanced is by the improvement of techniques for the preparation of the cell suspension prior to implantation, an issue that has been addressed only to a limited extent. We have evaluated a number of parameters during the preparation procedure, looking at the effects on cell survival over the first 24 h from preparation using vital dyes and the numbers of surviving neurons in vitro, after 4 days in culture, in addition to graft survival and function in vivo. Factors influencing cell survival include the type of trypsinization procedure and the age of donor tissues used for suspension preparation. The presence of DNase has no effect on cell viability but aids the dissociation of the tissue to form single cells. These results have important implications for the use of embryonic striatal grafts in animal models of Huntington's disease, and in any future clinical application of this research.

Bridge grafts of fibroblast growth factor-4-secreting schwannoma cells promote functional axonal regeneration in the nigrostriatal pathway of the adult rat

Axons damaged in the adult mammalian central nervous system are able to regenerate when their inhibitory glial environment is replaced with a more permissive substrate. Here, we have used long oblique "bridge" grafts of fibroblast growth factor-4-transfected RN-22 schwannoma cells to allow mechanically lesioned nigrostriatal axons to regenerate back to their original target in the adult rat brain. Regenerated axons were able to leave the bridge graft to form terminal arborizations and increase the density of tyrosine hydroxylase-immunoreactive fibres within the striatum. Bridge grafting also resulted in an increase in the number of neurons within the substantia nigra pars compacta taking up the fluorescent retrograde tracer Fluoro-Gold from the striatum. Animals which had received RN-22 bridge grafts showed lower rates of amphetamine-induced rotation 10 weeks after a mechanical lesion of the nigrostriatal tract compared to lesioned controls, the magnitude of the behavioural effect being related to the number of regenerated axons, and this comparative reduction was reversed by mechanical section of the bridge graft. It is concluded that our bridge grafting strategy allowed the partial anatomical and functional regeneration of the mechanically lesioned nigrostriatal tract, an unmyelinated central axon bundle, and that bridge grafting therefore represents a realistic approach to the repair of central nervous system lesions involving axon tract damage.

The time course of loss of dopaminergic neurons and the gliotic reaction surrounding grafts of embryonic mesencephalon to the striatum

Grafts of embryonic ventral mesencephalic tissue placed in the striatum of 6-hydroxydopamine-lesioned rats survive, and make and receive connections to and from the host brain. The dopaminergic neurons of the graft can grow processes into the host brain, and thereby alleviate many of the behavioral deficits of this form of experimental Parkinson's disease. However, when examined some weeks after implantation, grafted substantia nigra only contains about 5% of the expected complement of dopaminergic neurons. We have examined the time course of loss of grafted neurons. We find that the majority die during the first 7 days after transplantation. However, we have shown previously that three-dimensional cultures with the same dimensions as a graft, made of identical cell suspensions, have much better dopaminergic neuronal survival. There must, therefore, be features in the environment surrounding a graft that are toxic to dopaminergic neurons. A limiting factor in the efficacy of dopaminergic grafts is the small distance over which the neurons are able to grow neurites and form connections in the host brain. We find that the growth of neurites from dopaminergic neurons into the host striatum occurs in two phases. Neurites reach their maximum length within 7 days of transplantation, and this is followed by a much slower process of branch and terminal formation. Since axon growth in the adult brain may be inhibited by a number of factors associated with reactive gliosis, we have immunostained various ages of graft for vimentin, tenascin, chondroitin sulfate proteoglycan (CS-PG) using the CS56 antibody, the DSD-1 proteoglycan, and microglia using the OX-42 antibody. We have compared this staining with that surrounding a simple stab wound. Vimentin staining was initially seen in the graft and in astrocytes immediately surrounding it. By 7 weeks staining was restricted to a ring of astrocytes surrounding the graft. Tenascin, DSD-1, and CS-PG were initially seen in and around the grafts. By 7 weeks they had disappeared from grafts, but CS-PG and tenascin persisted in small amounts around stab wounds. In general, immunostaining of these molecules persisted longer around a stab lesion than around a graft. There was also an intense local microglial reaction surrounding both grafts and stab wounds which had largely resolved by 7 weeks.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

A device for the implantation of multiple cellular deposits into a large volume of brain from a single cannula site

Current grafting techniques for the treatment of Parkinson's disease incompletely restore the dopaminergic innervation of the caudate/putamen and while, in successful cases, bradykinesia and rigidity are reduced, tremor is largely unaffected. Increasing the number of cellular deposits would allow grafted neurons to be dispersed more widely within the host brain. This might be expected to lead to a more complete reinnervation of the caudate/putamen and therefore a better clinical result. In order to increase the volume of brain accessible to reinnervation without increasing the number of needle passages through the cortex, we have designed a device which allows a Teflon tube to be extruded sideways from the end of a stainless steel cannula. Through this tube, cells can be implanted at some distance in any radial direction from the axis of the cannula. Using such a device we have made up to 12 deposits of lac-z-labeled cells via a single cannula entry into the rat brain, at distances of up to 5 mm from the axis of the cannula. We propose that a similar device could be used to graft embryonic neurons into the human brain.

Functional and anatomical reconstruction of the 6-hydroxydopamine lesioned nigrostriatal system of the adult rat

In an attempt to reconstruct the 6-hydroxydopamine lesioned nigrostriatal system of the adult rat we have combined homotopic grafting of embryonic ventral mesencephalon suspensions with the implantation of long oblique "bridge" grafts of fibroblast growth factor-4-transfected RN-22 schwannoma cells stretching from the site of the neuronal grafts to the striatum. At seven weeks after receiving both grafts, animals were killed and processed for immunohistochemistry against tyrosine hydroxylase. Tyrosine hydroxylase-immunoreactive axons were seen to extend from the nigral grafts, along the bridge graft to the striatum where terminal arborizations could be seen. The retrograde tracer Fluoro-gold was injected intrastriatally in some of the experimental animals and was taken up by grafted neurons confirming their projection to the striatum. In parallel to the anatomical reconstruction of the system, a decrease in amphetamine-induced rotation was demonstrated in those animals receiving both grafts which had received > 98% complete lesions. This decrease was greatest in those animals with the most tyrosine hydroxylase-immunoreactive axons in their bridge grafts. The presence of the bridge graft also led to an increase in neuronal graft survival with twice as many tyrosine hydroxylase-immunoreactive neurons being found in the grafts of those animals that had received both grafts compared to those that had received a neuronal graft but no bridge graft.

The neurotrophin NT4/5, but not NT3, enhances the efficacy of nigral grafts in a rat model of Parkinson's disease

The neurotrophins NT4/5 and NT3 have previously been shown to improve the survival and fibre outgrowth of embryonic dopaminergic neurons in vitro. In the present study we attempted to augment the efficacy of embryonic nigral grafts in vivo. This was done by directly infusing the neurotrophins intraparenchymally in close proximity to transplanted nigral tissue placed in the dopamine depleted striatum of 6-hydroxydopamine lesioned rats. Our results indicated that NT4/5, but not NT3, stimulated fibre growth from embryonic nigral cells and enhanced functional efficacy of the grafts as assessed by metamphetamine-induced rotation.

In vivo effects of kFGF on embryonic nigral grafts in a rat model of Parkinson's disease

Central injections of FGF have been reported to promote the survival of dopamine neurones in nigral grafts. With the goal of developing an improved delivery of trophic molecules, an immortalized RN22 Schwann cell line transfected with a secretory form of FGF, kFGF, was irradiated and co-transplanted with embryonic nigral grafts in the 6-OHDA lesioned rat striatum. Amphetamine-induced turning was alleviated by nigral grafts, but was not further improved by co-grafts, whether or not transfected to secrete kFGF. Histological analysis showed similar numbers of surviving transplanted cells and a similar extent of fibre growth from the nigral grafts whether implanted alone or co-grafted with the Schwann cells. These results suggest that kFGF does not have any clear in vivo effect on embryonic nigral grafts in this model.

Migration of oligodendrocyte precursors on astrocytes and meningeal cells

Oligodendrocytes populate developing white matter and repopulate demyelinated regions of the CNS by migration. Although little is known about their migratory routes, the environment through which these cells migrate, whether during development, disease, or injury, is packed with astrocytes infiltrated with or bounded by meningeal cells. In the present study, the migration of oligodendrocyte precursors from primary cultures and of the precursor cell lines (CG4 and Oli-neu) on astrocytes and meningeal cells was investigated using tissue culture migration assays and time lapse video microscopy. Oligodendrocyte precursors and the cell lines were found to migrate poorly on astrocytes and meningeal cells compared to migration on laminin even though both astrocytes and meningeal cells express cell surface laminin. The migration-inhibitory activity was not detected in conditioned media derived from either astrocytes or meningeal cells, nor was it detected from matrix deposited by these cells. Analyses of the events immediately following cell-cell contacts revealed that oligodendrocyte precursor-astrocyte contacts were typically long-lasting and appeared to be adhesive, whereas precursor-meningeal cell contacts usually resulted in rapid withdrawal of the precursor cell process. No correlation was found, however, between general adhesiveness and the rate of migration. Our results suggest that both astrocytes and meningeal cells retard migration of oligodendrocyte precursors, consistent with the view that they may impede the movement of oligodendrocyte precursors into CNS lesion sites.

Increased axon regeneration in astrocytes grown in the presence of proteoglycan synthesis inhibitors

We have recently reported that the critical difference between astrocytic cell lines that are good or poor promoters of axon growth lies in the extracellular matrix. We demonstrated that much of this difference between matrix produced by permissive and non-permissive cell lines could be ascribed to one or more dermatan/keratan sulphate proteoglycans and that these proteoglycans are able to block the neurite-promoting effect of laminin. These proteoglycans are also produced by cultures of primary astrocytes. In the present study, we have demonstrated that treatment of both astrocytic cell lines and primary astrocytes with inhibitors of proteoglycan synthesis, beta-D-xylosides and sodium chlorate, can strongly influence the axon growth promoting properties of both matrix and whole cells. Dorsal root ganglia grown on matrix or in conditioned medium from cultures treated with beta-D-xylosides or sodium chlorate had twice as many axons and the axons grew to twice the length as in control cultures. Following treatment of Neu7 cells with proteoglycan synthesis inhibitors there was also a significant reduction in the ability of Neu7 conditioned medium to block the neurite-promoting effect of laminin. Dorsal root ganglia grown on Neu7 cells treated with sodium chlorate extended 2 to 3 times the number of axons for approximately 300 mm longer distance than on control cultures. Treatment of Neu7 cells with beta-D-xylosides, however, did not make the cells less inhibitory to axon growth. We have also examined the effects of proteoglycan synthesis inhibitors on three-dimensional primary astrocyte cultures, which closely mimic the in vivo effects of astrocytes on axon growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic neuronal survival and the effects of bFGF in explant, three dimensional and monolayer cultures of embryonic rat ventral mesencephalon

Embryonic substantia nigra cells when transplanted into the striatum can reverse many of the defects of Parkinson's disease. The efficacy of such grafts is compromised by the poor survival of grafted dopaminergic neurones; typically, 3-10% survive transplantation. We used three tissue culture models to identify stages in the procedure for the preparation and insertion of grafts which might be responsible for this cell death and to identify environments in which survival is optimised. (1) The ventral mesencephalon was dissected from the donor brain, then placed immediately into culture contained in a collagen gel. (2) The dissected tissue fragments were enzymatically dissociated, then the cells placed into monolayer culture. (3) Enzymatically dissociated tissue was packed into 0.5-mm-diameter porous tubes, to simulate the compaction of cells into a graft deposit in the host brain. Dissociation of the tissue by itself caused the death of approximately 30% of dopaminergic neurones, as judged by the difference in cell counts between the intact embryonic day 14 (E14) mesencephalon, and cells dissociated then packed into tubes. Of the dissociated neurones approximately 60% died during the first 24 h and 87% during the first 3 days in monolayer culture, while only 7% of dopaminergic neurones in three-dimensional cultures and 11% of neurones in explant cultures died over the first 3 days. Embryonic dopaminergic neurones are clearly very vulnerable to adverse conditions during the first days after their removal from the donor brain. The excellent survival of neurones in three-dimensional and explant cultures indicates that close association with other cells, which may provide greatly improved access to trophic factors, can enable the cells to survive this period of vulnerability. In contrast to its effects in monolayer cultures, bFGF had no effect on dopaminergic neuronal survival in either explant or three-dimensional cultures.

A quantitative study of cell death in the substantia nigra following a mechanical lesion of the medial forebrain bundle

An extruding wire knife was used to give adult male CFHB rats a minimally traumatic unilateral mechanical lesion of the medial forebrain bundle. In addition, some rats received bilateral intrastriatal injections of one of three fluorescent retrograde tracers either eight days before or eight days after the lesion. Injections made after the lesion revealed that about half of the animals had complete lesions of the nigrostriatal tract, while the other half were incompletely lesioned, the mean proportion of non-axotomized neurons being 23%. Over the 10 weeks following the lesions, the number of tyrosine hydroxylase-immunoreactive cells in the lesioned substantia nigra fell linearly, reaching a mean of 29% of that of the control substantia nigra. In the animals which were completely lesioned, neuronal survival at 10 weeks varied between 6 and 12%. That the disappearance of tyrosine hydroxylase-immunoreactive neurons was due to cell death rather than the loss of tyrosine hydroxylase itself was confirmed by labelling the cells with Fluoro Gold before axotomy; the tracer was seen in survival neurons, microglia and in a few involuted neurons which continued to be tyrosine hydroxylase-immunoreactive. This percentage of neurons surviving axotomy corresponds to the proportion of substantia nigra neurons which project to the contralateral striatum, and these neurons were in the region of the substantia nigra from which the contralateral projection originated. It is concluded that following mechanical transection of the nigrostriatal tract, all truly axotomized substantia nigra neurons die over a period of about 10 weeks.

Increased survival of rat EGF-generated CNS precursor cells using B27 supplemented medium

Previous studies suggest that a population of precursor cells from the developing and adult mouse striatum can be expanded in culture using serum-free, N2-supplemented medium and mitogenic factors such as epidermal growth factor (EGF). Here we show that EGF-responsive precursor cells from embryonic rat striatum and mesencephalon can also be expanded in culture, incorporate bromodeoxy uridine (BrDU) and develop into spheres that either adhere to the surface of the culture dish or float freely in the medium. Addition of B27, a medium supplement that increases neuronal survival in primary CNS cultures, resulted in a tenfold increase in the number of proliferating cells in vitro over the first week. The effects of B27-supplemented medium on precursor cell survival were only seen when primary cultures were used, such that dividing cells grown in B27 for 1 week could then be transferred to either B27 or N2 medium and show similar survival and division rates in response to EGF. After 1, 2 or 4 weeks of growth in B27-supplemented medium, dissociated precursor cells from either striatal or mesencephalic cultures could be differentiated when exposed to a poly-l-lysine-coated substrate in serum and EGF-free medium supplemented with B27. These cells then matured into a mixed culture containing neurons (approximately 35% of cells), astrocytes (approximately 44% of cells), and oligodendrocytes (approximately 10% of cells), based on immunocytochemical staining with microtuble-associated protein (MAP2), glial fibriallary acidic protein and galactocerebrosidase. When whole spheres of precursor cells were allowed to differentiate, every one examined was found to generate neurons, astrocytes and oligodendrocytes in similar proportions.(ABSTRACT TRUNCATED AT 250 WORDS)

Regenerating sciatic nerve axons contain the adult rather than the embryonic pattern of microtubule associated proteins

Microtubule associated proteins play a central role in the control of axon growth. We have used immunohistochemical techniques to establish which microtubule-associated proteins are present in the rat hindlimb spinal cord, dorsal root ganglia and peripheral nerves during axonal growth during embryogenesis, in adulthood, and during regeneration of crushed sciatic nerves. During embryogenesis microtubule-associated protein-1b and tau are present in all neurons and axons, microtubule-associated protein-2 is present in neurons but not in axons, and there is no microtubule-associated protein-1a. In adults, microtubule-associated protein-1a and microtubule-associated protein-1b are present in all sciatic nerve axons and in motor and dorsal root ganglion neurons. Tau, in its adult form, is present in many fine probably sensory axons, but not in most larger axons, and in motor and sensory neurons. Microtubule-associated protein-2 is present only in neurons. During regeneration the pattern of microtubule-associated protein expression retains the adult pattern. All regenerating axons contain microtubule-associated protein-1a and microtubule-associated protein-1b, none contain microtubule-associated protein-2, and a subset of fine axons contain tau. There is no detectable change in microtubule-associated protein expression by motoneurons. While axons are clearly able to regenerate without either microtubule-associated protein-2 or tau, tau containing axons appear to regenerate faster than those which lack it. It is possible that the failure of neurons to recapitulate the embryonic pattern of microtubule-associated protein expression during regeneration could be a reason why regenerative axon growth is slower and less vigorous than axon growth in embryos.

An inhibitor of neurite outgrowth produced by astrocytes

We have produced a number of astrocytic cell lines, some of which promote abundant neurite outgrowth, some of which are poor promoters of neurite outgrowth. The critical difference between these lines lies in the extracellular matrix, cell lines that are good promoters of axon growth producing a matrix that promotes axon growth, cell lines that are poor promoters of axon growth producing a non-permissive matrix. We were unable to find any consistent correlations between promotion of axon growth and production of proteases, protease inhibitors, N-cadherin, growth cone collapsing activity, and several extracellular matrix molecules. In the present study we have compared the least permissive of our cell lines, Neu7, with the most permissive, A7. Medium conditioned by the cell lines has the same properties as the matrix, since dorsal root ganglia (DRGs) grown in conditioned medium from the Neu7 line grow axons poorly, while DRGs grown in medium conditioned by A7 or primary astrocytes grow many long axons. Since matrix produced by all the cell lines contains large amounts of laminin, we looked to see whether the cells were producing laminin-blocking activity. Medium from the Neu7 line blocked laminin, while that from the A7 and primary astrocytes did not. However, when the conditioned media were heat-treated to remove neurite-promoting activity, they all had laminin-blocking activity: the blocking activity is heat stable. The neurite-promoting properties of the conditioned media therefore probably reflect a balance between promoting molecules and blockers.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA receptor blockade alters the topography of naturally occurring ganglion cell death in the rat retina

During the first 10 days after birth around half the rat's retinal ganglion cells die. Previous work has shown that ganglion cells whose axons have made large topographic targeting errors are preferentially eliminated during this period and that the selection of such cells for preferential elimination is dependent on an activity-driven mechanism: this process is one way in which the postnatal refinement of the topography of the retinocollicular projection is achieved. We have given systemic injections of the NMDA channel blocker MK801 during the first 14 days of life to see whether this activity-dependent process works via the NMDA channel. We assessed the topographic pattern of retinal ganglion cell death by making localized injections of fast blue into the superior colliculus at birth and measuring the distribution of labeled ganglion cells either at Postnatal Day 2 or at Day 14. We find that overall retinal ganglion cell death, measured by optic nerve axon counts, is not prevented by MK801 treatment. However, whereas in untreated animals ganglion cells whose axons have reached the most topographically inappropriate target area are preferentially eliminated, in MK801-treated animals ganglion cell death appears to be random, in that we see no evidence of the preferential elimination of retinal ganglion cells whose axons have made large topographic targeting errors in MK801-treated animals. NMDA receptor blockade therefore has the same effect on the pattern of retinal ganglion cell death as tetrodotoxin blockade of the retina.

Basic fibroblast growth factor promotes the survival of embryonic ventral mesencephalic dopaminergic neurons--II. Effects on nigral transplants in vivo

The clinical potential of transplants of fetal dopaminergic neurons is limited by the fact that the percentage of cells surviving in such grafts is in general quite low. This report investigates the use of basic fibroblast growth factor administration (given either as a pretreatment or by repeated intrastriatal infusions) to promote the survival and behavioural efficacy of embryonic dopamine-rich nigral transplants in rats. Pretreatment of the graft tissue by brief incubation with basic fibroblast growth factor increased the survival of tyrosine hydroxylase-immunoreactive (presumed dopaminergic) neurons in the grafts in comparison to control grafts, and accelerated the recovery in the transplanted animals in tests of drug-induced rotational asymmetry. However, the clear advantage seen in the rotation test conducted three weeks after transplantation had disappeared by nine weeks. The moderate effects of pretreatment were markedly enhanced by repeated intrastriatal infusion of basic fibroblast growth factor into the host animals over 20 days following transplantation. This resulted in > 100% increase in the number of dopaminergic neurons surviving in the grafts, and was accompanied by a significantly greater recovery of the rats' rotational asymmetries which persisted over the full nine weeks of testing. However, the repeated intracerebral infusions induced an inflammatory reaction in the striatum, and the associated trauma both complicates the interpretation of the mechanism of observed recovery and compromises the utility of this route of basic fibroblast growth factor administration for promoting graft survival.

Basic fibroblast growth factor promotes the survival of embryonic ventral mesencephalic dopaminergic neurons--I. Effects in vitro

We have studied the effects of basic fibroblast growth factor on rat embryonic mesencephalic neurons in vitro. Basic fibroblast growth factor promotes the survival of dopaminergic neurons in vitro, the effect increasing with dose and reaching a maximum at 10 ng/ml. In the absence of basic fibroblast growth factor the number of tyrosine hydroxylase-stained (tyrosine hydroxylase positive) neurons declines to almost zero within 14 days, whereas in the presence of basic fibroblast growth factor numbers remain almost constant from three to 28 days in vitro. This effect of basic fibroblast growth factor is abolished by preventing non-neuronal cells from appearing in the cultures, apart from a basic fibroblast growth factor-mediated increase in the numbers of tyrosine hydroxylase-positive cells during the first two days in vitro. The presence or absence of non-neuronal cells also influences dopaminergic neuronal morphology, the neurons having more, longer, and more varicose processes in the absence of astrocytes. Survival of dopaminergic neurons in vitro in the absence of basic fibroblast growth factor is very dependent on plating cell density, but in the presence of basic fibroblast growth factor this dependency vanishes. It is also possible to make survival independent of plating density by growing the cultures on inverted coverslips, which have the effect of concentrating secreted molecules in the thin layer of medium between coverslip and dish. Our conclusions from these experiments on plating density are that astrocytes probably constitutively secrete a small amount of a trophic factor which promotes survival of dopaminergic neurons, and that the rate of production of this factor is greatly increased by basic fibroblast growth factor. If basic fibroblast growth factor is withdrawn from cultures after two or seven days the dopaminergic neurons soon die. However, if basic fibroblast growth factor is withdrawn after 14 days, after the period of naturally occurring cell death of these neurons, there is no increase in dopaminergic neuronal death compared to controls in which basic fibroblast growth factor treatment is maintained. If basic fibroblast growth factor is used to improve the survival of dopaminergic neurons grafted in vivo, it should therefore be sufficient to treat the grafts for 14 days.

Mitogenic effect of basic fibroblast growth factor on embryonic ventral mesencephalic dopaminergic neurone precursors

The effects of embryonic age and the presence of basic fibroblast growth factor (bFGF) have been examined on the survival and rate of cell division of dopaminergic neurones of the ventral mesencephalon. Cultures were produced from 7.5 mm and 11 mm rat embryos, pulsed with [3H]thymidine during the first 12 h, and the survival and labelling of cells measured after 3 and 7 days in vitro. bFGF largely prevented the decline in numbers of tyrosine hydroxylase (TH)-positive neurones that occurred in control cultures between 3 days and 1 week. In cultures derived from the younger 7.5 mm embryos there were more TH-positive neurones in the presence of exogenous bFGF than under control conditions after 3 days in vitro. No similar effect was seen in the cultures derived from the older 11 mm embryos. Combined [3H]thymidine labelling and TH immunocytochemistry suggested that this effect was attributable, at least in part, to a bFGF-associated increase in the proliferation of TH-positive neurone progenitors during the first day or so, which was seen in cultures from 7.5 mm but not 11 mm embryos. The effect of bFGF on cultures from older embryos is therefore purely on neuronal survival, while the effect on cultures from younger embryos is a mixture of survival and mitogenic actions.

Axonal growth on astrocytes is not inhibited by oligodendrocytes

Axon growth in vitro may be inhibited by contact with oligodendrocytes, but most axons grow readily on the surface of astrocyte monolayers. Since both cell types are in close contact with one another in the damaged nervous system, we have examined the growth of axons on cultures which contain both astrocytes and oligodendrocytes. Cultures derived from neonatal rat forebrain develop with a monolayer of large flat astrocytes attached to the culture dish, and with many smaller cells of the oligodendrocyte lineage on their surface. Dorsal root ganglia placed on these cultures grow axons readily, the overall extent of growth being unaffected by the presence or absence of oligodendrocytes, many of which express galactocerebroside and the inhibitory molecule janusin. A previous set of experiments had shown that growth of these axons is inhibited by oligodendrocytes by themselves. Scanning electron microscopy coupled with silver-intensified immunostaining reveals that the axons grow on the surface of the astrocytic layer, underneath the oligodendrocytes, and are therefore in contact with both cell types as they grow. The presence of astrocytes therefore alters the results of axonal contact with oligodendrocytes.

Spinal cord repair: future directions

The main cause of disability following spinal injury is failure of axons to regenerate and reconnect the spinal cord with the brain. If patients with cord lesions are ever to make a full recovery some means will have to be found to restore ascending sensory and descending motor connections. Until the last few years there has been a very limited understanding of the reasons why axons in the central nervous system (CNS) fail to regenerate, but as a result of recent work the picture is now much clearer.

Intrinsic neuronal determinants of regeneration

Axon growth and axon regeneration are co-operative processes; the speed and extent of axon growth are influenced both by the properties of the environment surrounding the axon growth cone, and the properties of the neuron itself. In recent years, the environmental influences on axon growth have received most of the attention directed towards this area of research, but the properties of the neurons themselves are likely to be just as important. Within both adults and embryos there are differences in the growth potential of different neuronal types, and there is also evidence for an overall decrease in the vigour of axon growth with neuronal age.

The role of Schwann cells in the regeneration of peripheral nerve axons through muscle basal lamina grafts

Evacuated muscle is a possible substitute for nerve autografts in the repair of damaged peripheral nerves. Previous experiments have shown that killed or evacuated muscle grafts are as effective as nerve autografts for bridging gaps of up to 4 cm between proximal and distal nerve stumps. Evacuated muscle grafts are made of extracellular matrix components, which are good substrates for axon growth in vitro. However, experiments in vivo have generally demonstrated that live Schwann cells are essential for successful axon regeneration. In the present experiments we have used immunohistochemical techniques with anti-S100 and anti-neurofilament antibodies to visualize axon growth and Schwann cell migration into muscle grafts over the first 10 days following grafting. We only saw axons growing into grafts accompanied by Schwann cells, and most though not all Schwann cells were associated with axons. Schwann cell migration from the proximal stump in association with axons was much faster and more extensive than from the distal stump. We examined muscle grafts over the first 20 days after grafting by electron microscopy. Regenerating axons were always associated with Schwann cells, which were mostly in the basal lamina-lined tubes left by the evacuated myofibrils. A comparison between evacuated muscle grafts and grafts in which the muscle had been killed but not evacuated revealed that 7 days after grafting there were more than twice as many regenerated axons in and distal to the evacuated grafts, but that by 20 days the numbers of axons were similar in the two groups.