Target-specific outgrowth from human mesencephalic tissue grafted to cortex or ventricle of immunosuppressed rats

Adult rat myelin enhances axonal outgrowth from neural stem cells

Axon regeneration after spinal cord injury (SCI) is attenuated by growth inhibitory molecules associated with myelin. We report that rat myelin stimulated the growth of axons emerging from rat neural progenitor cells (NPCs) transplanted into sites of SCI in adult rat recipients. When plated on a myelin substrate, neurite outgrowth from rat NPCs and from human induced pluripotent stem cell (iPSC)-derived neural stem cells (NSCs) was enhanced threefold. In vivo, rat NPCs and human iPSC-derived NSCs extended greater numbers of axons through adult central nervous system white matter than through gray matter and preferentially associated with rat host myelin. Mechanistic investigations excluded Nogo receptor signaling as a mediator of stem cell-derived axon growth in response to myelin. Transcriptomic screens of rodent NPCs identified the cell adhesion molecule neuronal growth regulator 1 (Negr1) as one mediator of permissive axon-myelin interactions. The stimulatory effect of myelin-associated proteins on rodent NPCs was developmentally regulated and involved direct activation of the extracellular signal-regulated kinase (ERK). The stimulatory effects of myelin on NPC/NSC axon outgrowth should be investigated further and could potentially be exploited for neural repair after SCI.

Xenografts of MHC-Deficient Mouse Embryonic Mesencephalon Improve Behavioral Recovery in Hemiparkinsonian Rats

The limited availability of human embryonic tissue for dopamine cell transplants in Parkinson's patients has led to an increased interest in using xenogeneic donor tissue. Unfortunately, without aggressive immunosup-pression, such brain xenografts are rejected by the host immune system. Chronic brain xenograft rejection is largely mediated by helper T cells, which require presentation of xenoantigens by major histocompatability complex (MHC) class II for their activation. We examined survival and function of xenografts of E13 mouse mesencephalon deficient in either MHC class I, class II, or both after transplantation into adult hemiparkinsonian rats without immunosuppression. Recipients received grafts from C57BL/6 mice that were either: 1) wild-type (wt), 2) MHC class I knockout (KO), 3) MHC class II KO, 4) MHC class I and II double KO, or 5) saline sham transplants. At 6 weeks after transplantation, recipients of MHC class I KO, class II KO, and double KO xenografts significantly reduced methamphetamine-induced circling rate while rats with wt xenografts and sham-operated rats showed no improvement. MHC class II KO grafts had the greatest number of surviving dopamine neurons. All transplants, including saline sham controls, contained infiltrating host MHC class II-positive cells. Saline sham grafts and MHC class II KO xenografts contained significantly fewer infiltrating host MHC class II-positive cells than did wt grafts. Our results show that MHC class II-deficient xenografts survive transplantation for at least 6 weeks in the absence of immunosup-pression, reduce rotational asymmetry, and provoke lesser immune reaction than wt grafts.

Morphological and Functional Evidence for Enhanced Growth and Potassium-Evoked Dopamine Release in Striatal Grafts Innervated with a Patchy Growth Pattern. an in Oculo Nigrostriatal Cograft Study

During development of the nigrostriatal dopamine system, a patchy and a diffuse type of striatal innervation pattern can be seen. It has been suggested that when fetal dopaminergic neurons, obtained from the ventral mesencephalon (VM), are grafted adjacent to mature striatal tissue, only the diffuse growth is induced. Intraocular grafting studies have indicated that the dopaminergic growth pattern might be influenced by the age of the target area, the lateral ganglionic eminence (LGE). In this study VM grafts were allowed to innervate LGE grafts of different ages. Fetal VM was implanted next to 2-wk-old or 26-day-old striatal in oculo grafts, and the resulting dopaminergic innervation of the striatal grafts was studied using tyrosine hydroxylase (TH) immunohistochemistry. In striatal grafts receiving innervation at the age of 2 wk in oculo, a patchy TH-immunoreactive growth pattern was found, while in striatal grafts innervated at the age of 26 days mainly the diffuse growth pattern was seen. This implies that grafted striatum reached maturity at approximately 1 mo of age. The age of the dopaminergic neurons at dissection and grafting was also studied concerning the ability to induce patchy growth into mature striatum. Thus, VM dissected from 13- and 18-mm fetuses was implanted to either 4-mo-old LGE (grafted in sequence) or to LGE from the same fetus (grafted simultaneously) as controls. TH-positive innervation of striatal tissue, evaluated 4 wk after implantation of VM, revealed a patchy growth pattern in LGE grafted simultaneously with 13- and 18-mm VM. However, when the striatum was mature at the time of innervation, diffuse growth was observed in striatum innervated by VM dissected from 13-mm fetuses. Interestingly, patchy growth was noted in striatal areas close to VM grafts when the dopaminergic neurons were derived from older fetuses (CRL 18 mm). Furthermore, potassium-induced dopamine release was greater in striatal grafts exhibiting the patchy growth than those showing the diffuse pattern of innervation. In conclusion, patchy dopaminergic growth can be induced in mature striatal tissue by grafting VM from older fetuses. Functionally, potassium-evoked dopamine release is enhanced in dopaminergic patches. These results have implications in terms of finding ways to induce patchy growth when grafting to the mature striatum of patients suffering from Parkinson's disease.

The Age of Striatum Determines the Pattern and Extent of Dopaminergic Innervation: a Nigrostriatal Double Graft Study

In animal models of Parkinson's disease, transplanted fetal mesencephalic dopaminergic neurons can innervate the dopamine-depleted host brain, but it is unclear why large portions of the host striatum are left uninnervated. During normal development, the dopaminergic innervation first occurs in the form of a dense patchy pattern in the striatum, followed by a widespread nerve fiber network. Using intraocular double grafts we have investigated dopaminergic growth patterns initiated when ventral mesencephalic grafts innervate striatal targets. The fetal lateral ganglionic eminence was implanted into the anterior eye chamber. After maturation in oculo, fetal ventral mesencephalon was implanted and placed in contact with the first graft. In other animals the two pieces of tissue were implanted simultaneously. Tyrosine hydroxylase (TH) immunohistochemistry revealed a pattern of dense TH-positive patches throughout the total volume of the striatal grafts in simultaneously transplanted cografts, while a widespread, less dense, pattern was found when mature striatal transplants were innervated by fetal dopaminergic grafts. To investigate which type or types of growth patterns that developed after grafting to striatum in situ of an adult host, fetal ventral mesencephalic tissue was implanted into the lateral ventricle adjacent to the dopamine-lesioned striatum. After maturation of the mesencephalic graft, the fetal lateral ganglionic eminence was implanted into the reinnervated part of the host striatum. TH immunohistochemistry revealed a few nerve fibers within the striatal graft and the growth pattern was of the widespread type. In conclusion, grafted dopaminergic neurons preferably innervate mature striatum with a widespread sparse nerve fiber network, while the innervation of the immature striatum occurs in the form of dense patches. Furthermore, when the patchy pattern is formed, the total volume of the striatal target is innervated while growth of the widespread type terminates prior to reaching distal striatal parts. Thus, the growth pattern seems essential to the final volume that is innervated. Once the widespread growth pattern is initiated, the presence of immature striatum does not change the dopaminergic growth pattern.

Transplantation site influences the phenotypic differentiation of dopamine neurons in ventral mesencephalic grafts in Parkinsonian rats

Foetal midbrain progenitors have been shown to survive, give rise to different classes of dopamine neurons and integrate into the host brain alleviating Parkinsonian symptoms following transplantation in patients and animal models of the disease. Dopamine neuron subpopulations in the midbrain, namely A9 and A10, can be identified anatomically based on cell morphology and ascending axonal projections. G protein-gated inwardly rectifying potassium channel Girk2 and the calcium binding protein Calbindin are the two best available histochemical markers currently used to label (with some overlap) A9- and A10-like dopamine neuron subtypes, respectively, in tyrosine hydroxylase expressing neurons both in the midbrain and grafts. Both classes of dopamine neurons survive in grafts in the striatum and extend axonal projections to their normal dorsal and ventral striatal targets depending on phenotype. Nevertheless, grafts transplanted into the dorsal striatum, which is an A9 input nucleus, are enriched for dopamine neurons that express Girk2. It remains to be elucidated whether different transplantation sites favour the differential survival and/or development of concordant dopamine neuron subtypes within the grafts. Here we used rat foetal midbrain progenitors at two developmental stages corresponding to a peak in either A9 or A10 neurogenesis and examined their commitment to respective dopaminergic phenotypes by grafting cells into different forebrain regions that contain targets of either nigral A9 dopamine innervation (dorsal striatum), ventral tegmental area A10 dopamine innervation (nucleus accumbens and prefrontal cortex), or only sparse dopamine but rich noradrenaline innervation (hippocampus). We demonstrate that young (embryonic day, E12), but not older (E14), mesencephalic tissue and the transplant environment influence survival and functional integration of specific subtypes of dopamine neurons into the host brain. We also show that irrespective of donor age A9-like, Girk2-expressing neurons are more responsive to environmental cues in adopting a dopaminergic phenotype during differentiation post-grafting. These novel findings suggest that dopamine progenitors use targets of A9/A10 innervation in the transplantation site to complete maturation and the efficacy of foetal cell replacement therapy in patients may be improved by deriving midbrain tissue at earlier developmental stages than in current practice.

Embryonic and mature astrocytes exert different effects on neuronal growth in rat ventral mesencephalic slice cultures

One obstacle with grafting of dopamine neurons in Parkinson’s disease is the insufficient ability of the transplant to reinnervate the host striatum. Another issue is the prospective interaction between the donor fetal tissue and the adult astrocytes of the host. To study nerve fiber growth and its interaction with immature/mature astrocytes, ventral mesencephalic (VM) organotypic rat tissue cultures from embryonic days (E) 12, E14, and E18 were studied up to 35 days in vitro (DIV), and co-cultures of E14 VM tissue and mature green fluorescent protein (GFP)-positive astrocytes were performed. Generally, nerve fibers grew from the tissue slice either in association with a monolayer of migrated astroglia surrounding the tissue (glial-associated), or distal to the astroglia as non-glial-associated outgrowth. The tyrosine hydroxylase (TH)-positive glial-associated nerve fiber outgrowth reached a plateau at 21 DIV in E12 and E14 cultures. In E18 cultures, TH-positive neurons displayed short processes and migrated onto the astrocytes. While the non-glial-associated nerve fiber outgrowth dominated the E14 cultures, it was found absent in E18 cultures. The GFP-positive cells in the VM and GFP-positive astrocyte co-cultures were generally located distal to the monolayer of migrated fetal astrocytes, a few GFP-positive cells were however observed within the astrocytic monolayer. In those cases TH-positive neurons migrated towards the GFP-positive cells. Both the non-glial- and glial-associated nerve fibers grew onto the GFP-positive cells. Taken together, the glial-associated growth has limited outgrowth compared to the non-glial-associated nerve fibers, while none of the outgrowth types were hampered by the mature astrocytes.

Reconstruction of brain circuitry by neural transplants generated from pluripotent stem cells

Pluripotent stem cells (embryonic stem cells, ESCs, and induced pluripotent stem cells, iPSCs) have the capacity to generate neural progenitors that are intrinsically patterned to undergo differentiation into specific neuronal subtypes and express in vivo properties that match the ones formed during normal embryonic development. Remarkable progress has been made in this field during recent years thanks to the development of more refined protocols for the generation of transplantable neuronal progenitors from pluripotent stem cells, and the access to new tools for tracing of neuronal connectivity and assessment of integration and function of grafted neurons. Recent studies in brains of neonatal mice or rats, as well as in rodent models of brain or spinal cord damage, have shown that ESC- or iPSC-derived neural progenitors can be made to survive and differentiate after transplantation, and that they possess a remarkable capacity to extend axons over long distances and become functionally integrated into host neural circuitry. Here, we summarize these recent developments in the perspective of earlier studies using intracerebral and intraspinal transplants of primary neurons derived from fetal brain, with special focus on the ability of human ESC- and iPSC-derived progenitors to reconstruct damaged neural circuitry in cortex, hippocampus, the nigrostriatal system and the spinal cord, and we discuss the intrinsic and extrinsic factors that determine the growth properties of the grafted neurons and their capacity to establish target-specific long-distance axonal connections in the damaged host brain.

Antioxidant-Enriched Diet Affects Early Microglia Accumulation and Promotes Regeneration of the Striatal Dopamine System after a 6-Hydroxidopamine-Induced Lesion in a Rat

Neuroinflammation is found both in the brain of humans suffering from Parkinson's disease and in animal models of disease. It is suggested to be involved in the pathogenesis of the disease. In the present study, in order to study the effects of antioxidants on neuroinflammation, microglial phenotypes were evaluated in rats fed with diets containing bilberries, blueberries, or crowberries at 1 and 4 weeks following striatal injection of 6-hydroxydopamine. The dopamine innervation was visualized using antibodies raised against tyrosine hydroxlase (TH) in the striatum and in the globus pallidus. One week post-lesion, the expression of Iba1-positive cells, a general microglial marker, was significantly increased in the striatum of all animals fed with antioxidant-enriched diets compared to control-diet fed animals, while the diameter of the TH-negative zone was similar in all animals. At four weeks post-lesion, the Iba1-positive microglia was significantly reduced in animals fed with antioxidant-enriched diets. The diameter of the TH-negative zone was significantly reduced in animals fed bilberry and crowberry. The expression and distribution of ED1-positive cells was similar to that of Iba1-positive cells found in the lesioned areas. A cell division marker Ki67 revealed that few microglia were proliferating in crowberry-treated animals. Otherwise dividing cells were associated with blood capillary cells. Although the antioxidant level should be equal in the entire brain, no regeneration was found in globus pallidus, suggesting the mechanism promoting regeneration in the striatum is not effective in the globus pallidus. In conclusion, diets rich in bilberries and crowberries and with high contents of antioxidants stimulate an early phase of accumulation of reactive migroglia that fades at longer time points i.e. promotes regeneration of the striatal dopamine system.

Glial influence on nerve fiber formation from rat ventral mesencephalic organotypic tissue cultures

Rat fetal ventral mesencephalic organotypic cultures have demonstrated two morphologically different dopamine nerve fiber growth patterns, in which the initial nerve fibers are formed in the absence of astrocytes and the second wave is guided by astrocytes. In this study, the presence of subpopulations of dopamine neurons, other neuronal populations, and glial cells was determined. We used "roller-drum" organotypic cultures, and the results revealed that beta-tubulin-positive/tyrosine hydroxylase (TH)-negative nerve fibers were present as early as 1 day in vitro (DIV). A similar growth pattern produced by TH-positive neurons was present from 2 DIV. These neurites grew to reach distances over 4 mm and over time appeared to be degenerating. Thin, vimentin-positive processes were found among these nerve fibers. As the first growth was retracted, a second outgrowth was initiated and formed on migrating astrocytes. TH- and aldehyde dehydrogenase-1 (ALDH1)-positive nerve fibers formed both the nonglia-associated and the glia-associated outgrowth. In cultures with membrane inserts, only the glia-associated outgrowth was found. Vimentin-positive cells preceded migration of NG2-positive oligodendrocytes and Iba-1-positive microglia. Oligodendrocytes appeared not to be involved in guiding neuritic growth, but microglia was absent over areas dense with TH-positive neurons. In conclusion, in "roller-drum" cultures, nerve fibers are generally formed in two sequences. The early-formed nerve fibers grow in the presence of thin, vimentin-positive processes. The second nerve fiber outgrowth is formed on astroglia, with no correlation to the presence of oligodendrocytes or microglia. ALDH1-positive nerve fibers, presumably derived from A9 dopamine neurons, participate in formation of both sequences of outgrowth.

Inhibition of proteoglycan synthesis affects neuronal outgrowth and astrocytic migration in organotypic cultures of fetal ventral mesencephalon

Grafting fetal ventral mesencephalon has been utilized to alleviate the symptoms of Parkinson's disease. One obstacle in using this approach is the limited outgrowth from the transplanted dopamine neurons. Thus, it is important to evaluate factors that promote outgrowth from fetal dopamine neurons. Proteoglycans (PGs) are extracellular matrix molecules that modulate neuritic growth. This study was performed to evaluate the role of PGs in dopamine nerve fiber formation in organotypic slice cultures of fetal ventral mesencephalon. Cultures were treated with the PG synthesis inhibitor methyl-umbelliferyl-beta-D-xyloside (beta-xyloside) and analyzed using antibodies against tyrosine hydroxylase (TH) to visualize dopamine neurons, S100beta to visualize astrocytes, and neurocan to detect PGs. Two growth patterns of TH-positive outgrowth were observed: nerve fibers formed in the presence of astrocytes and nerve fibers formed in the absence of astrocytes. Treatment with beta-xyloside significantly reduced the distance of glial-associated TH-positive nerve fiber outgrowth but did not affect the length of the non-glial-associated nerve fibers. The addition of beta-xyloside shifted the nerve fiber growth pattern from being mostly glial-guided to being non-glial-associated, whereas the total amount of TH protein was not affected. Further, astrocytic migration and proliferation were impaired after beta-xyloside treatment, and levels of non-intact PG increased. beta-Xyloside treatment changed the distribution of neurocan in astrocytes, from being localized in vesicles to being diffusely immunoreactive in the processes. To conclude, inhibition of PG synthesis affects glial-associated TH-positive nerve fiber formation in ventral mesencephalic cultures, which might be an indirect effect of impaired astrocytic migration.

Blueberry- and spirulina-enriched diets enhance striatal dopamine recovery and induce a rapid, transient microglia activation after injury of the rat nigrostriatal dopamine system

Neuroinflammation plays a critical role in loss of dopamine neurons during brain injury and in neurodegenerative diseases. Diets enriched in foods with antioxidant and anti-inflammatory actions may modulate this neuroinflammation. The model of 6-hydroxydopamine (6-OHDA) injected into the dorsal striatum of normal rats, causes a progressive loss of dopamine neurons in the ventral mesencephalon. In this study, we have investigated the inflammatory response following 6-OHDA injected into the striatum of adult rats treated with diet enriched in blueberry or spirulina. One week after the dopamine lesion, a similar size of dopamine degeneration was found in the striatum and in the globus pallidus in all lesioned animals. At 1 week, a significant increase in OX-6- (MHC class II) positive microglia was found in animals fed with blueberry- and spirulina-enriched diets in both the striatum and the globus pallidus. These OX-6-positive cells were located within the area of tyrosine hydroxylase (TH) -negativity. At 1 month after the lesion, the number of OX-6-positive cells was reduced in diet-treated animals while a significant increase beyond that observed at 1 week was now present in lesioned control animals. Dopamine recovery as revealed by TH-immunohistochemistry was significantly enhanced at 4 weeks postlesion in the striatum while in the globus pallidus the density of TH-positive nerve fibers was not different from control-fed lesioned animals. In conclusion, enhanced striatal dopamine recovery appeared in animals treated with diet enriched in antioxidants and anti-inflammatory phytochemicals and coincided with an early, transient increase in OX-6-positive microglia.

Fetal lateral ganglionic eminence attracts one of two morphologically different types of tyrosine hydroxylase-positive nerve fibers formed by cultured ventral mesencephalon

The purpose of this study was to investigate the influence of fetal lateral ganglionic eminence (LGE) on nerve fiber outgrowth formed by fetal ventral mesencephalon (VM). Organotypic tissue cultures of fetal VM and LGE plated as single or cocultures were employed. Survival time was 3-21 days in vitro. Nerve fiber outgrowth and migration of astrocytes were analyzed using immunohistochemistry for tyrosine hydroxylase (TH) and S100. In addition, cultures were labeled with the TUNEL technique and with antibodies directed against neurofilament (NF) in order to study apoptosis and retraction of nerve fibers, respectively. The results revealed two morphologically different types of TH-positive outgrowth growing into the substrate. The initially formed TH-positive outgrowth radiated continuously without changing direction, while a second wave of TH-positive outgrowth became obvious when the initial growth already had reached a distance of approximately 1000 microm. The second wave of TH-positive outgrowth radiated from the tissue, but at a certain distance changed direction and formed a network surrounding the culture. The initially formed TH-positive growth was not associated with the presence of S100-positive astrocytes and avoided to grow into the LGE. At longer time points the first wave of TH-positive nerve fibers appeared dotted, with disrupted NF-immunoreactive fibers and in most cultures these long distance growing fibers had disappeared at 21 days in vitro. The second wave of TH-positive nerve fibers was growing onto a layer of glia and never reached the distance of the first wave. LGE became innervated by TH-positive fibers at the time point for when the second wave of TH-positive growth had been initiated, and the innervation appeared in TH-dense patches that also showed a high density of S100-positive astrocytes. Significantly increased TUNEL activity within LGE portion of cocultures was observed when TH-positive fibers entered the LGE and formed patches. In conclusion, two morphologically different types of TH-positive outgrowth were found and the initially formed fibers neither targeted the LGE nor were they guided by glial cells, but their potential to grow for long distances was high.

Late-stage immature neocortical neurons reconstruct interhemispheric connections and form synaptic contacts with increased efficiency in adult mouse cortex undergoing targeted neurodegeneration

In the neocortex, the effectiveness of potential cellular repopulation therapies for diseases involving neuronal loss may depend critically on whether newly incorporated cells can differentiate appropriately into precisely the right kind of neuron, re-establish precise long-distance connections, and reconstruct complex functional circuitry. Here, we test the hypothesis that increased efficiency of connectivity could be achieved if precursors could be more fully differentiated toward desired phenotypes. We compared embryonic neuroblasts and immature murine neurons subregionally dissected from either embryonic day 17 (E17) (Shin et al., 2000) or E19 primary somatosensory (S1) cortex and postnatal day 3 (P3) purified callosal projection neurons (CPNs) with regard to neurotransmitter and receptor phenotype and afferent synapse formation after transplantation into adult mouse S1 cortex undergoing targeted apoptotic degeneration of layer II/III and V CPNs. Two weeks after transplantation, neurons from all developmental stages were found dispersed within layers II/III and V, many with morphological features typical of large pyramidal neurons. Retrograde labeling with FluoroGold revealed that 42 +/- 2% of transplanted E19 immature S1 neurons formed connections with the contralateral S1 cortex by 12 weeks after transplantation, compared with 23 +/- 7% of E17 neurons. A greater percentage of E19-derived neurons received synapses (77 +/- 1%) compared with E17-derived neurons (67 +/- 2%). Similar percentages of both E17 and E19 donor-derived neurons expressed neurotransmitters and receptors [glutamate, aspartate, GABA, GABA receptor (GABA-R), NMDA-R, AMPA-R, and kainate-R] appropriate for endogenous adult CPNs progressively over a period of 2-12 weeks after transplantation. Although P3 fluorescence-activated cell sorting-purified neurons also expressed these mature phenotypic markers after transplantation, their survival in vivo was poor. We conclude that later-stage and region-specific immature neurons develop a mature CPN phenotype and make appropriate connections with recipient circuitry with increased efficiency. However, at postnatal stages of development, limitations in survival outweigh this increased efficiency. These results suggest that efforts to direct the differentiation of earlier precursors precisely along specific desired neuronal lineages could potentially make possible the highly efficient reconstruction of complex neocortical and other CNS circuitry.

Guidance of dopaminergic neuritic growth by immature astrocytes in organotypic cultures of rat fetal ventral mesencephalon

Astrocytes, with their many functions in producing and controlling the environment in the brain, are of great interest when it comes to studying regeneration after injury and neurodegenerative diseases such as in grafting in Parkinson's disease. This study was performed to investigate astrocytic guidance of growth derived from dopaminergic neurons using organotypic cultures of rat fetal ventral mesencephalon. Primary cultures were studied at different time points starting from 3 days up to 28 days. Cultures were treated with either interleukin-1 beta (IL-1 beta), which has stimulating effects on astrocytic proliferation, or the astrocytic inhibitor cytosine arabinoside (Ara-C). Tyrosine hydroxylase (TH)-immunohistochemistry was used to visualize dopaminergic neurons, and antibodies against glial fibrillary acidic protein (GFAP) and S100 beta were used to label astrocytes. The results revealed that a robust TH-positive nerve fiber production was seen already at 3 days in vitro. These neurites had disappeared by 5 days. This early nerve fiber outgrowth was not guided by direct interactions with glial cells. Later, at 7 days in vitro, a second wave of TH-positive neuritic outgrowth was clearly observed. GFAP-positive astrocytic processes guided these neurites. TH-positive neurites arborized overlying S100 beta-positive astrocytes in an area distal to the GFAP-positive astrocytic processes. Treatment with IL-1 beta resulted in an increased area of TH-positive nerve fiber network. In cultures treated with Ara-C, neither astrocytes nor outgrowth of dopaminergic neurites were observed. In conclusion, this study shows that astrocytes play a major role in long-term dopaminergic outgrowth, both in axonal elongation and branching of neurites. The long-term nerve fiber growth is preceded by an early transient outgrowth of dopamine neurites.

Evidence for target-specific outgrowth from subpopulations of grafted human dopamine neurons

Clinical and experimental grafting in Parkinson's disease has shown the need for enhanced survival of dopamine neurons to obtain improved functional recovery. In addition, it has been suggested that a limited number of surviving dopamine neurons project to the dopamine-denervated host striatum. The aim of this study was to investigate if subpopulations of ventral mesencephalic dopamine neurons project to their normal targets, i.e., dorsal vs. ventral striatum. Following implantation of human ventral mesencepahlic tissue into the lateral ventricle of dopamine-depleted rats, human-derived dopamine reinnervation was achieved both in dorsal and ventral striatum. Treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) resulted in a degeneration of tyrosine hydroxylase (TH)-immunoreactive nerve fibers in dorsal striatum but not in ventral areas in some animals, while MPTP was without effect in other animals. TH-immunoreactive neurons were small and appeared shrunken in animals carrying grafts affected by the MPTP treatment. In conclusion, grafted dopamine neurons projected nerve fibers into areas that they normally innervate. Thus, when searching for factors that may enhance survival of grafted dopamine neurons it is important to study which subpopulation(s) of ventral mesencephalic dopamine neurons is affected, such that a proper reinnervation may be achieved.

Evidence for Target-Specific Nerve Fiber Outgrowth from Subpopulations of Grafted Dopaminergic Neurons: A Retrograde Tracing Study Using in Oculo and Intracranial Grafting

Efforts have been made to counteract the symptoms of Parkinson's disease by substituting the loss of dopaminergic neurons with fetal ventral mesencephalic grafts. One of the postulated limiting factors in this treatment is the relatively poor cell survival and limited graft-derived fiber outgrowth. Recent results documenting enhanced survival of grafted dopaminergic neurons showed no positive correlation to enhanced innervation of the striatal target. Therefore this study was undertaken to investigate whether all surviving grafted dopaminergic neurons projected to the striatal target. Hence, fetal ventral mesencephalic tissue was implanted adjacent to mature versus immature striatal tissue using in oculo and intraventricular grafting techniques. In in oculo grafting, fetal ventral mesencephalon was implanted simultaneously with fetal lateral ganglionic eminence (immature striatal target) or to already matured striatal in oculo grafts (mature striatal target). Furthermore, fetal ventral mesencephalon was implanted into the lateral ventricle adjacent to mature dopamine-depleted striatum. The retrograde tracer fluorogold was injected into the striatal portion of the in oculo cografts and into reinnervated areas of the adult brain. Immunohistochemistry revealed that a significantly larger proportion of tyrosine hydroxylase-positive neurons in the ventral mesencephalic graft was innervating in oculo immature striatal tissue, and hence was fluorogold-positive, in comparison with the number of tyrosine hydroxylase-positive neurons innervating mature striatal tissue. Moreover, intracranial transplantations showed that tyrosine hydroxylase-positive neurons were distributed within the grafts in dense clusters of cells. In most clusters tyrosine hydroxylase-positive cells were fluorogold-negative but calbindin-positive. In a few tyrosine hydroxylase-positive cell clusters, neurons were coexpressing fluorogold but were calbindin-negative. In conclusion, significantly more dopamine neurons projected to immature than to mature striatal tissue and thus, a subpopulation of grafted dopaminergic neurons was not projecting into adult striatum. Thus, the results from this study show that further attempts to enhance survival of grafted dopamine neurons in purpose to enhance graft-derived fiber outgrowth and efficacy should also consider different subtypes of dopamine neurons.

Neurobiology of human neurons (NT2N) grafted into mouse spinal cord: implications for improving therapy of spinal cord injury

Emerging data suggest that current strategies for the treatment of spinal cord injury might be improved or augmented by spinal cord grafts of neural cells, and it is possible that grafted neurons might have therapeutic potential. Thus, here we have summarized recent studies of the neurobiology of clonal human (NT2N) neurons grafted into spinal cord of immunodeficient athymic nude mice. Postmitotic human NT2N neurons derived in vitro from an embryonal carcinoma cell line (NT2) were transplanted into spinal cord of neonatal, adolescent and adult nude mice where they became integrated into the host gray and white matter, did not migrate from the graft site, and survived for > 15 months after implantation. The neuronal phenotype of the grafted NT2N cells was similar in gray and white matter regardless of host age at implantation, and some of the processes extended by the transplanted NT2N neurons became ensheathed by oligodendrocytes. However, there were consistent differences between NT2N processes traversing white versus gray matter. Most notably, NT2N processes with a trajectory in white matter extended over much longer distances (some for > 2 cm) than those confined to gray matter. Thus, NT2N neurons grafted into spinal cord of nude mice integrated into gray as well as white matter, where they exhibited and maintained the morphological and molecular phenotype of mature neurons for > 15 months after implantation. Also, the processes extended by grafted NT2N neurons differentially responded to cues restricted to gray versus white matter. Further insight into the neurobiology of grafted human NT2N neurons in the normal and injured spinal cord of experimental animals may lead to novel and more effective strategies for the treatment of spinal cord injury.

Pattern of long-distance projections from fetal hippocampal field CA3 and CA1 cell grafts in lesioned CA3 of adult hippocampus follows intrinsic character of respective donor cells

Fetal hippocampal grafts transplanted to the lesioned CA3 of adult hippocampus can extend axonal projections to many regions of the host brain. However, the identity of grafted cells that project to specific host regions is unknown. We hypothesize that the pattern of long-distance axonal projections from distinct fetal hippocampal cells grafted to lesioned CA3 is specified by the intrinsic nature of respective donor cells rather than characteristics of the host graft region. We grafted fetal hippocampal CA3 or CA1 cells into kainic acid lesioned CA3 of adult hippocampus at four days post-lesion. Neurons projecting to either the contralateral hippocampus or the ipsilateral septum were then measured in these grafts at four months post-grafting using Fluoro-Gold and DiI tract tracing. CA3 grafts located close to the degenerated CA3 cell layer showed a high propensity for establishing projections into the contralateral hippocampus (commissural projections) compared to similarly located CA1 grafts, which exhibited negligible commissural projections. Similar distinction was observed between the two graft types even when they were located only partially in the lesioned CA3. Among CA3 grafts, those placed near the degenerated CA3 cell layer established significantly greater commissural projections than those placed only partially in the CA3 region. Septal projections, in contrast, were robust from both CA3 and CA1 grafts. This differential projection pattern between CA3 and CA1 grafts resembles projections of CA3 and CA1 cells in intact hippocampus.These results demonstrate that the intrinsic character of grafted fetal cells determines the type of efferent projections from fetal grafts into different targets in the lesioned adult host brain. However, the extent of efferent projections from specific grafts is also influenced by the location of grafted cells within the host region. Thus, graft-mediated appropriate reconstruction of damaged circuitry in the lesioned brain may require grafting of homotopic donor cells. Further, the robust and specific projections observed from CA3 grafts is likely beneficial for functional recovery of hippocampus following CA3 injury and hence of significance towards developing a graft-mediated therapy for human temporal lobe epilepsy.

Differential effects of spinal cord gray and white matter on process outgrowth from grafted human NTERA2 neurons (NT2N, hNT)

To investigate host effects on grafts of pure, postmitotic, human neurons, we assessed the morphologic and molecular phenotype of purified NTera2N (NT2N, hNT) neurons implanted into the spinal cord of athymic nude mice. NT2N neurons were implanted into both spinal cord gray matter and white matter of neonatal, adolescent, and adult mice and were evaluated at postimplantation times up to 15 months. NT2N neurons remained at the implantation site and showed process integration into all host areas, and each graft exhibited similar phenotypic features regardless of location or host age at implantation. Evidence of host oligodendrocyte ensheathment of NT2N neuronal processes was seen, and grafted NT2N neurons acquired and maintained the morphologic and molecular phenotype of mature neurons. The microenvironments of host gray matter and white matter appear to exert differential effects on implanted neuronal processes, because consistent differences were noted in the morphologies of graft processes extending into white matter versus gray matter. NT2N processes extended for long distances (>2 cm) within white matter, whereas NT2N processes located within gray matter had shorter trajectories. This suggests that NT2N neurons integrate similarly into spinal cord gray matter and white matter, but they extend processes that respond differentially to gray matter and white matter cues. Further studies of the model system described here may identify the host molecular signals that support and direct integration of grafted human neurons as well as the outgrowth of their processes in the nervous system.

Co-culture with the striatum attenuates N-methyl-D-aspartate cytotoxicity in dopaminergic neurons of rat mesencephalic slice cultures

We examined the role of striatal cells in cytotoxicity induced by N-methyl-D-aspartate (NMDA) in dopamine (DA) neurons in rat mesencephalic slice culture. Coronal sections were prepared from 2- and 3-day-old rat brains and cultured using the interface culture method for 2-3 weeks before the NMDA cytotoxicity experiment. The exposure of mesencephalic cultures without striatum (single culture) to NMDA (10-300 microM) for 24 hr reduced the number of DA neurons in a concentration-dependent manner. The co-administration of the non-competitive NMDA-receptor antagonist significantly inhibited the neurotoxic effect of NMDA. When mesencephalon and striatum were kept in contact and co-cultured (contacting co-cultures), the growth of DA fibers into the striatal part was observed. In the contacting co-cultures with striatum, the minimal effective concentration for NMDA cytotoxicity was higher than that in single cultures. The contacting co-cultures with cerebellum did not alter the NMDA cytotoxicity. When the mesencephalon and striatum slices were kept apart and co-cultured, the co-cultures showed neither an outgrowth of DA fibers to the striatum nor any effect on the NMDA cytotoxicity. These results suggest that the projection of rat mesencephalic DA neurons to the striatum attenuates the NMDA cytotoxicity in DA neurons themselves.

Characteristics of human fetal spinal cord grafts in the adult rat spinal cord: influences of lesion and grafting conditions

The present study evaluated the growth potential and differentiation of human fetal spinal cord (FSC) tissue in the injured adult rat spinal cord under different lesion and grafting conditions. Donor tissue at 6-9 weeks of gestational age was obtained through elective abortions and transplanted either immediately into acute resection (solid grafts) or into chronic contusion (suspension and solid grafts) lesions (i.e., 14-40 days after injury) in the thoracic spinal cord. The xenografts were then examined either histologically in plastic sections or immunocytochemically 1-3 months postgrafting. Intraspinal grafts in acute lesions demonstrated an 83% survival rate and developed as well-circumscribed nodules that were predominantly composed of immature astrocytes. Solid-piece grafts in chronic contusion lesions exhibited a 92% survival rate and also developed as nodular masses. These grafts, however, contained many immature neurons 2 months postgrafting. Suspension grafts in chronic contusion lesions had an 85% survival rate and expanded in a nonrestrictive, diffuse pattern. These transplants demonstrated large neuronally rich areas of neural parenchyma. Extensive neuritic outgrowth could also be seen extending from these grafts into the surrounding host spinal cord. These findings show that human FSC tissue reliably survives and differentiates in both acute and chronic lesions. However, both the lesion environment and the grafting techniques can greatly influence the pattern of differentiation and degree of host-graft integration achieved.

Development of long-distance efferent projections from fetal hippocampal grafts depends upon pathway specificity and graft location in kainate-lesioned adult hippocampus

Fetal hippocampal cells grafted into the excitotoxically lesioned hippocampus of adult rats are capable of extending axonal projections into the host brain. We hypothesize that the axonal growth of grafted fetal cells into specific host targets, and the establishment of robust long-distance efferent graft projections, require placement of fetal cells in close proximity to appropriate axon guidance pathways. Intracerebroventricular administration of kainic acid in adult rats leads to a specific loss of hippocampal CA3 pyramidal neurons. We grafted 5'-bromodeoxyuridine-labeled embryonic day 19 hippocampal cells into adult hippocampus at four days post-kainic acid lesion, and quantitatively measured the projection of grafted cells into the contralateral hippocampus and the septum after three to four months survival using Fluoro-Gold and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Dil) tracing. Grafts located in or near the degenerated CA3 cell layer exhibited numerous neurons which established commissural projections with the contralateral hippocampus. However, such projection did not occur in intrahippocampal grafts located away from the CA3 cell layer. In contrast, neurons in all grafts established robust projections into the septum regardless of location within hippocampus although grafts located near the degenerated CA3 cell layer displayed more neurons with such projections. Location of grafted cells clearly influences the development of efferent graft projections into distant targets in the adult host brain, particularly access to axon guidance pathways to facilitate the formation of projections. The establishment of robust long-distance commissural projections of fetal hippocampal grafts is clearly dependent on their placement in or near the degenerated CA3 cell layer, suggesting that appropriate axon guidance pathways for commissural pathways are tightly focussed near this cell layer. However, the establishment of septal projections of these grafts was not dependent on specific location within the CA3 cell layer, suggesting that axonal guidance mechanisms to the septum are more diffuse and not limited to the CA3 dendritic layers. The results underscore that fetal hippocampal grafts are capable of partly restoring lesioned hippocampal circuitry in adult animals when appropriately placed in the host hippocampus.

Effects of graft-derived dopaminergic innervation on the target neurons of patch and matrix compartments of the striatum

Fetal dopaminergic neurons grafted into the dopamine-depleted striatum have previously been shown to normalize neurochemical and behavioural abnormalities. However, the extent of graft-induced recovery of striatal compartments, which differ in their ontogeny, neurochemical properties and function, is still not clear. The striosome and matrix compartments of the striatum provide a segregated projection to somatostatin-containing GABAergic neurons of the rostral part of the entopeduncular nucleus and somatostatin-negative GABAergic neurons of the caudal part of the entopeduncular nucleus, respectively. In the present study, preprosomatostatin and glutamate decarboxylase messenger RNA levels in the rostral and caudal parts of the entopeduncular nucleus were determined six and 18 months postgrafting in rats with complete recovery of rotational behaviour following apomorphine challenge, and in rats with unilateral 6-hydroxydopamine lesions or sham lesions and no grafts. Sections were processed for in situ hybridization using 35S-labelled cRNA probes for glutamate decarboxylase (67,000 mol. wt isoform; GAD67) and preprosomatostatin. Autoradiographs showed a marked increase in preprosomatostatin messenger RNA within the ipsilateral entopeduncular nucleus in 6-hydroxydopamine-lesioned rats, and a substantially lower increase six months postgrafting. At 18 months postgrafting, the preprosomatostatin messenger RNA levels were symmetrical within the entopeduncular nucleus. Unilateral depletion of striatal dopamine resulted in a moderate increase in GAD67 messenger RNA levels within the ipsilateral entopeduncular nucleus, along with a substantial decrease in GAD67 levels within the contralateral nucleus. By six months postgrafting, the GAD67 levels had decreased considerably within the ipsilateral entopeduncular nucleus, while the messenger RNA levels had returned to normal within the contralateral nucleus. Interestingly, at 18 months postgrafting, the GAD67 levels remained decreased within the ipsilateral entopeduncular nucleus and were significantly lower than the normal value. The results indicate that fetal nigral grafts placed within the dopamine-depleted striatum can restore the neurochemical alterations seen in striatal target areas such as the entopeduncular nucleus. This may form the neurochemical basis of graft-induced behavioural recovery, as the normalization of neurotransmitter messenger RNA levels in the entopeduncular nucleus reflects the restoration of overall activity in both direct and indirect striatal output pathways. The results also indicate that the graft-derived dopaminergic innervation restores the output of both striosome and matrix compartments of the striatum. The present results also showed a progressive recovery leading to over-compensation of neurotransmitter messenger RNA levels following grafting, perhaps indicating the importance of feedback regulation of grafted dopaminergic neurons by the host.

Extensive reinnervation of the hippocampus by embryonic basal forebrain cholinergic neurons grafted into the septum of neonatal rats with selective cholinergic lesions

Reconstruction of the septohippocampal pathways by axons extending from embryonic cholinergic neuroblasts grafted into the neuron-depleted septum has been explored in the neonatal rat by using a novel lesioning and grafting protocol. Neonatal ablation of the basal forebrain cholinergic projection neurons, accompanied by extensive bilateral cholinergic denervation of the hippocampus and neocortex, was produced at postnatal day (PD) 4 by 192 immunoglobulin (IgG)-saporin intraventricularly. Four days later, cholinergic neuroblasts (from embryonic day 14 rats) were implanted bilaterally into the neuron-depleted septum by using a microtransplantation approach. The results show that homotopically implanted septal neurons survive and integrate well into the developing septal area, extending axons caudally along the myelinated fimbria-fornix and supracallosal pathways that are able to reach the appropriate targets in the denervated hippocampus and cingulate cortex as early as 4 weeks postgrafting. Moreover, the laminar innervation patterns established by the graft-derived axons closely resembled the normal ones and remained essentially unchanged up to at least 6 months, which was the longest postoperative time studied. The reinnervating fibers restored tissue choline acetyltransferase activity (up to 50% of normal) in the dorsal hippocampus and the parietooccipital cortex. Retrograde labeling with Fluoro-Gold from the host hippocampus combined with immunocytochemistry confirmed that most of the projecting neurons, indeed, were cholinergic. The results suggest that the graft-host interactions that are necessary for target-directed axon growth are present in the septohippocampal system during early postnatal maturation. Thus, the present approach may contribute to overcome the functional limitations inherent in the use of ectopically placed intrahippocampal transplants.

Development of human fetal ventral mesencephalic grafts in rats with 6-OHDA lesions of the nigrostriatal pathway

Neuronal transplantation is an approach that can be exploited to study the development of the human central nervous system as well as being used in attempts to restore neurological function. In the present study, we have examined cellular events that appear to precede the development of dopamine nerve fiber extension by neurons from the human fetal ventral mesencephalon. These cellular events were examined using neuronal cell suspensions from human fetal ventral mesencephalic tissue (gestational ages 7-10 weeks) transplanted into the striatum of unilaterally lesioned 6-hydroxydopamine (6-OHDA) rats. Animals were sacrificed for immunohistochemistry 9-10 weeks after the transplantation prior to the manifestation of behavioral recovery. Histological analysis revealed tyrosine hydroxylase (TH) immunoreactive neurons in the grafts. The majority of these neurons had very short TH positive processes (60-70 microns), indicating that the maturation of grafted dopaminergic neurons was still incomplete. Immunostaining for the human specific intermediate neurofilament (hNF, clone: BF-10) showed dense neuronal fibers in the grafts. These fibers extended deeper into the host brain than the TH positive neuronal processes. The whole striatum, particularly the medial part of the striatum, exhibited long NF positive processes. Glial fibrillary acidic protein (GFAP) immunohistochemistry revealed fine astrocytic processes inside the grafts, which were clearly different from host reactive glial cells surrounding the grafts. These graft-derived glial processes tended to extend into the host brain deeper than the TH positive neuronal processes from the grafts. These early histological findings of the grafted human fetal ventral mesencephalon suggest that the graft-derived NF positive neuronal processes, as well as the glial processes, radiate from the grafted tissue and extend into the host brain prior to the extension of TH positive processes. These results further suggest that human-to-rat xenografts can be used to study the neural development of human fetal brain tissue.

Long-term effects of human-to-rat mesencephalic xenografts on rotational behavior, striatal dopamine receptor binding, and mRNA levels

Fetal ventral mesencephalic grafts have been used as a tool to counteract the symptoms of Parkinson's disease. In this study human fetal ventral mesencephalic xenografts were implanted into the lateral ventricle of unilaterally dopamine-depleted immunosuppressed rats. Rotational behavior elicited by low doses of apomorphine, host striatal dopamine receptor binding, and mRNA levels were investigated. Rotational behavior was reduced beginning 2 months after grafting. After 4 months only a small number of rotations, lasting approximately 30 min, were recorded. Seven months after transplantation, the rotational behavior was completely eleminated. Dopamine D2 receptor binding revealed significantly increased levels in sham-operated 6-hydroxydopamine- (6-OHDA) lesioned control striata. These increased levels decreased, and although still significantly higher at 4 months, normalized at the survival time of 7 months postgrafting. Regional differences were still obvious at 7 months in the dorsolateral quadrant of dorsal striatum. Dopamine D2 receptor mRNA revealed significantly increased levels in the lateral aspects of 6-OHDA-lesioned control striata, reversing by 4 months postgrafting. The D1 receptor binding revealed a moderately reduced signal in striata of lesioned animals. After grafting, this reduction became significantly lower than that seen in the control side, with a continous decrease over time. The same pattern was detected using in situ hybridization for dopamine D1 receptor mRNA, that is, moderate decreases after dopamine depletion and a significant decrease in the dorsomedial part of dorsal striatum 7 months postgrafting. Dopamine D3 receptor binding was increased after dopamine depletion, but reversed already by 4 months postgrafting. Taken together, human ventral mesencephalic xenografts are able to completely reverse apomorphine-induced rotational behavior, provided the grafts are left in vivo for a sufficient time. The increased striatal D2 receptors are reversed after grafting, but the human xenograft further suppressed the D1 receptor subtype both at binding and at mRNA levels. There was no strict correlation in the time courses of dopamine receptor changes and reduction of rotational behavior.

Human fetal neocortical tissue grafted to rat brain cavities survives, leads to reciprocal nerve fiber growth, and accumulates host IgG

The human-to-rat xenograft approach offers possibilities to study aspects of primate cortex development and function without monkeys. Human fetal cortical tissue was grafted to prepared cortical cavities of immunosuppressed host rats. Fetal tissue fragments were collected after routine low-pressure vacuum aspiration abortions performed in the first trimester of gestation. Human derived neurons and human nerve fiber outgrowth were visualized by immunohistochemistry with antibodies against human neurofilament protein 70 kD (hNFP70). Ingrowth from rat host striatum or cortex into the grafts was analyzed by immunohistochemistry with antibodies against tyrosine hydroxylase. Astrocytes were evaluated by immunohistochemistry with antibodies against glial fibrillary acidic protein. The grafts grew into different sizes (1-10 mm in diameter) and contained large numbers of hNFP70-positive nerve fibers. All grafts gave rise to outgrowth of hNFP70-positive fibers into the host with partly a cortical layering; layers III and IV received a majority of the human fibers. In several cases, the graft-derived nerve fibers entered the host brain at restricted areas, while there was no crossing over of nerve fibers at the rest of the graft-host interface. Tyrosine hydroxylase-positive fibers were usually not abundant in the grafts. Interestingly, cases of massive ingrowth occurred from host striatum into the graft in a pattern suggesting "permissive sites" at the graft-host interface in the same way as outgrowth from graft to host was found. Additionally, tyrosine hydroxylase-immunoreactive fibers from host cortex were found to grow into the transplant. Glial fibrillary acidic protein immunoreactivity was increased at the interfaces between graft and host cortex or host striatum. Immunohistochemistry using antibodies against rat IgG indicated the presence of rat IgG within the grafts, and in bordering areas of host brain, possibly indicating a defective graft-host barrier. Taken together, these results show that human cortical tissue pieces grafted to cortical cavities of immunosuppressed rats survive grafting and develop, and that reciprocal nerve fiber growth between grafts and hosts occur. Human cortical neurons can grow into the rat host brain in a pattern which is partly determined by host cortical architecture.

Re-initiated growth from mature ventral mesencephalon: an in oculo transplant study of nigrostriatal co-grafts

The ability of mature dopaminergic neurons derived from ventral mesencephalon to re-initiate growth after making contact with a non-innervated target was studied using the intra-ocular grafting model. Foetal ventral mesencephalic tissue or brain stem including the locus coeruleus area was grafted to the anterior chamber of the eye. Two weeks, 6 weeks or 1 year after the first implantation, foetal striatal tissue was placed in contact with the nigral graft or grafted alone. The size of the transplants was measured through the cornea. The final size of the striatal grafts was significantly larger when placed alone than when co-grafted with 1-year-old or 6-week-old dopaminergic grafts. Striatum grafted together with 2-week-old nigra was larger than when grafted adjacent to mature substantia nigra, but not significantly so. Nerve fibre outgrowth into the iris from the nigral transplants did not increase after maturation, but the re-innervated area of the host iris progressively increased around the locus coeruleus grafts. Ingrowth of tyrosine hydroxylase (TH) immunoreactive nerve fibres into the striatal grafts was studied 6 weeks after the second implantation. TH immunohistochemistry revealed innervation of the striatal piece in all cases, except for the group where striatum alone was grafted. With the short survival time for co-grafts of 6 weeks, TH-positive nerve fibres innervated a larger volume, had a patchy appearance and the density was higher in striatum grafted to 2 week-old nigral transplants than that seen in striatal transplants grafted to mature nigral grafts. The patchy pattern of TH-immunoreactive nerve terminals was also seen in striatum co-grafted with 6-week-old or 1-year-old nigral transplants. No difference in striatal innervation volume was detected between those latter two groups. When striatum was implanted adjacent to mature ventral mesencephalon and grown together for 6 months--the longer survival time--the same dense TH-positive innervation as seen in striatum co-grafted with immature nigral tissue at the shorter survival time was found. Additionally, the nigral part of the co-grafts showed increased TH-immunoreactive nerve fibre density. In conclusion, dopaminergic neurites from mature ventral mesencephalic transplants can re-initiate growth if placed in contact with non-innervated striatal tissue. The nigral grafts do not progressively re-innervate the host iris, while locus coeruleus grafts do. The intra-ocular grafting model can be used to study the in vivo effects of trophic factors on mature dopaminergic neurons.

Long fibre growth by axons of embryonic mouse hippocampal neurons microtransplanted into the adult rat fimbria

We have described a method for the microtransplantation of a suspension of a few thousand cells from mid to late embryonic mouse hippocampi into the fimbria of immunosuppressed adult rat hosts. There was close graft-to-host contact, across a non-scarred interface. The transplanted cells included CA3 type pyramids, and were enclosed within the host myelinated fibre tract, whose glial framework was largely undisturbed. Immunohistochemistry of two species-specific markers (M6 and Thy-1.2) showed that the donor mouse neurons grew fine (< 0.5 micron diameter) axons which extended singly or in fascicles through the rat host fimbria for a maximum distance of at least 10 mm. The donor axons were intimately integrated among and closely aligned to the host tract axons and to the interfascicular glial rows of the host tract. The axons travelled (i) laterally through the ipsilateral fimbria, (ii) medially across the midline in the ventral hippocampal commissure to reach the contralateral fimbria and alveus, and (iii) rostro-medially to the septum. On approaching the terminal fields appropriate to hippocampal CA3 pyramidal cell axons, the transplant axons gave rise to fine preterminal branches which were continuous with a reticular or amorphous immunoreactivity in the stratum oriens and stratum pyramidale of the ipsilateral hippocampus, and in the lateral and triangular septal nuclei. The donor axons extended along the host fimbria at a rate of approximately 1 mm per day, reaching their terminal field destinations by approximately 1-2 weeks. At 7 weeks the projections were maintained, but with little further extension. These observations indicate that the microenvironment of myelinated adult fibre tracts is permissive for an abundant and rapid growth of axons from transplanted embryonic cell suspensions. These axons can leave host tracts to invade appropriate terminal fields.