Treatment of neurodegenerative diseases with neural cell transplantation

Neural cell transplantation is an emerging therapy that may provide an effective treatment for neurodegenerative disorders. The most extensive work with neural transplants has been carried out for Parkinson's and Huntington's diseases. However, intensive efforts are also being made for the treatment of other neurological indications, such as spinal cord repair, stroke, epilepsy, multiple sclerosis (MS), Alzheimer's disease and amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), to name just a few. The major barrier for the successful application of cells as therapeutics is achieving long-term survival and function. The CNS has proven to be ideal for transplantation, in part because immune rejection is attenuated in the CNS compared to peripheral locations. However, some form of immunosuppression is desirable for optimal allograft survival and required for xenograft survival. This review will focus on the challenges of restoring function to something as intricate as the CNS and on the limitations imposed by this complexity on any cellular therapeutic.

Comparison of fresh and cryopreserved porcine ventral mesencephalon cells transplanted in A rat model of Parkinson's disease

To evaluate whether cryopreservation of porcine ventral mesencephalon cells influences graft survival and function in vivo, we have transplanted either freshly prepared or cryopreserved cells into the striatum of 6-hydroxydopamine-lesioned rats. A single cell suspension of porcine ventral mesencephalon cells from the same isolation either was stored at 4 degrees C and transplanted the next day or was cryopreserved for 4 weeks in liquid nitrogen vapor. The cryopreserved cells were then rapidly thawed, rinsed, and transplanted in the same manner as the fresh cells, with the same dose of viable cells. All animals received daily injections of cyclosporin A to prevent xenograft rejection. To monitor graft function, amphetamine-induced rotation was measured every 3 weeks between 6 and 15 weeks posttransplantation. After sacrifice at 15 weeks posttransplantation, histological methods were used to compare fresh cell and cryopreserved cell transplants with respect to graft survival, differentiation and integration, and host immune response. Cryopreserved cells were found to be either equivalent or in some cases superior to fresh cells with respect to rotational correction, graft survival, graft volume, numbers of graft-derived dopaminergic neurons, and host immune responses. In conclusion, the results indicate that it is feasible to cryopreserve porcine ventral mesencephalon cells for long-term storage of cells prior to transplantation in an animal model of Parkinson's disease.

Comparison of reactive processes in the rat brain elicited by xenotransplantation of nervous tissues of chicken or pulmonate snail

It is known that a histocompatibility system is not developed to the same extent in lower invertebrates as in vertebrate animals. We assumed that the xenografts from the newborn invertebrate nervous system would not exert destructive effects on the brain of the vertebrate recipient even without immunosuppressive therapy. In search of brain xenografts (XG) capable to survive in the brain of a recipient without intensive immunosuppression, we transplanted ganglia of terrestrial snails into the rat brain. We compared effects of transplantation of the XG taken from anterior brain of the 18-day embryo chicken (XGC) and from ganglia of a newborn terrestrial pulmonate snail (Helix aspersa L., XGSn). Part of the XGSn were stained by vital fluorescent dyes Bisbenzimid or Fast Blue before grafting. The XGSn were implanted into the neocortex parenchyma in each hemisphere. Rat brains with the XGC were examined 5 days after, and brains with the XGSn - 5 and 28 days after the transplantation. Nonstained sections with the XGSn labeled with fluorescent dyes prior to transplantation were investigated in fluorescent microscope and stained later with tionin and cresyl-violet. Quantitative videoimage analysis of lymphocyte aggregations, reactive gliosis, morphology of the XG areas, and implantation trace was performed. It was found that the XGSn transplantation did not elicit in the rat brain an intensive immunological conflict 5 and 28 days after transplantation. In contrast, the XGC rapidly elicited a strong immune response resulting in massive obliterations in the rat brain and were rejected in 5 days. Labeled snail glia and vessels were observed in the stained XGSn 28 days after transplantation by fluorescence imaging. Putative snail vessels grew into the rat brain from the place of snail tissue transplantation serving the humoral integration of the XG and the host brain. Migration of molluscan glial cells was observed in the brain of recipients.

Astrocyte line SVG-TH grafted in a rat model of Parkinson's disease

The present review describes gene transfer into the brain using extraneuronal cells with an ex vivo approach. The mild immunological reactions in the central nervous system to grafts provided the rationale and empirical basis for brain-transplantation, to replace dying cells, of potential clinical relevance. Fetal human astrocytes were genetically engineered to express tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines. These cells were also found to produce constitutively and secrete GDNF and interleukins. Therefore, these cells may prove as a drug-delivery system for the treatment of neurological degenerative conditions such as Parkinson's disease (PD). The field of neuronal reconstruction has reached a critical threshold and there is a need to evaluate the variables that will become critical as the field matures. One of the needs is to characterize the neurochemical alterations in the microenvironment in the context of grafted-host connectivity. This review discusses the functional effects of the pharmacologically-active construct, which consists of astrocytes producing L-DOPA and GDNF. The striatum in PD that lacks the dopaminergic projection from the substantia nigra metabolizes and releases dopamine differently from normal tissue and may react to different factors released by the grafted cells. Moreover, neurochemicals of the host tissue may effect grafted cells as well. An understanding of the way in which these neurochemicals are abnormal in PD and their role in the grafted brain is critical to the improvement of reconstructive strategies using cellular therapeutic strategies.

Long-term survival of fetal porcine lateral ganglionic eminence cells in the hippocampus of rats

Embryonic porcine brain tissue from the lateral ganglionic eminence was transplanted into the adult rat hippocampus to determine whether fetal striatal cells could survive, differentiate, and integrate in a heterotopic site. The hippocampus, a common site of epileptic seizure activity, was chosen to determine if fetal striatal cells could supply inhibitory GABAergic neurons that may serve to block seizures. Cells were either implanted with a single deposit using a standard metal cannula or by five smaller disseminated deposits with a glass micropipette. At 20-24 weeks, animals immunosuppressed with cyclosporin showed long-term survival of porcine cells in the adult hippocampus. Analysis by immunohistochemistry and in situ hybridization showed that the grafts contained glial and neuronal cell types, including GABAergic neurons within graft core and networks of porcine neuronal fibers extending from the graft into the host parenchyma. In addition, a marker of porcine presynaptic terminals, synaptobrevin, was abundant within the grafts and was found associated with hippocampal structures and cell layers suggesting functional integration of grafted cells within the host. The survival of xenografts in the hippocampus and potential integration of inhibitory components provides evidence that these grafts may serve as an internal negative feedback mechanism to quench epileptiform activity.

Neural transplantation of human neuroteratocarcinoma (hNT) neurons into ischemic rats. A quantitative dose-response analysis of cell survival and behavioral recovery

Transplantation of fetal neuronal tissue has been used successfully to ameliorate symptoms of neurodegenerative disease in animals and humans. This technique has recently been extended as an experimental treatment for ischemic brain damage. However, due to ethical issues with the use of fetal cells for the treatment of any human disease, there has been a concerted effort to find alternative graft sources for neural transplantation. The human neuroteratocarcinoma neuron cell is derived from an embryonal teratocarcinoma cell line that can be differentiated into post-mitotic neurons. Neural transplantation of human neuroteratocarcinoma neurons has recently been shown to produce behavioral amelioration of symptoms in rats with ischemia-induced injury. The present study was undertaken to: (i) determine the minimum effective number of transplanted human neuroteratocarcinoma neurons required for amelioration of ischemia-induced behavioral dysfunction; and (ii) quantify the survival of human neuroteratocarcinoma neurons in vivo. Transplants of 0, 5, 10, 20, 40, 80 or 160 x 10(3) human neuroteratocarcinoma neurons were made into rats that sustained ischemic damage. Animals that received 40, 80 or 160 x 10(3) human neuroteratocarcinoma neurons demonstrated a dose-dependent improvement in performance of both the passive avoidance and elevated body swing tests. At the conclusion of behavioral testing, human neuroteratocarcinoma neurons were identified in paraffin sections with human neural cell adhesion molecule MOC-1 and human neurofilament antibodies. Transplants of 80 or 160 x 10(3) human neuroteratocarcinoma neurons demonstrated a 12-15% survival of human neuroteratocarcinoma neurons in the graft, while transplants of 40 x 10(3) human neuroteratocarcinoma neurons demonstrated a 5% survival. Transplantation of human neuroteratocarcinoma neurons ameliorated behavioral deficits produced by ischemic damage. The human neuroteratocarcinoma neuron, additionally, showed greater survival than that reported for fetal cells when transplanted into the brain. Therefore, this readily available cell may prove to be an excellent candidate for the treatment of ischemic damage in human patients.

Clinical outcome of cotransplantation of peripheral nerve and adrenal medulla in patients with Parkinson's disease. Clínica Puerta de Hierro Neural Transplantation Group

OBJECT

Transplants of adrenal medulla (AM) and fetal ventral mesencephalon (FVM) are currently being tested as therapeutic alternatives in patients with Parkinson's disease (PD). At the Clínica Puerta de Hierro in Madrid, a controlled clinical trial is underway to establish which donor tissue, if any, is the best for open surgical implantation in patients with PD.

METHODS

Since 1987, varying degrees of clinical improvement have been achieved in Grade IV and V parkinsonian patients by implanting perfused AM and FVM into the right caudate nucleus. To investigate further whether implantation of different types of donor tissues results in qualitatively and quantitatively different degrees of recovery, four patients with Grade IV or V PD received implants of pre-coincubated autologous AM and intercostal nerve in the caudate nucleus. Four nonsurgically treated patients served as a control group. Three years posttransplantation, longer on phases (46.2%+/-10.4% of the day presurgery to 87.5%+/-10.4% of the day 36 months postsurgery) and improved symptoms in on and off phases persist in all four cases, with reduced dyskinesias (67.1%+/-9.2% of the day in on phases presurgery to 17%+/-13.8% of the day in on phases 36 months postsurgery). Progress appears to be stepwise, starting within weeks of tranplantation and becoming clinically significant in the 2nd and 3rd months (similar to our AM- and sooner than in our FVM-implanted patients), followed by a period of stability and, after a second wave of improvement 12 to 18 months posttransplantation (similar to FVM implants), has continued (87.5+/-7 points presurgery to 46+/-5.6 points 36 months postsurgery). In the experimental group, doses of levodopa have been reduced by more than 60% and dopamine agonist use has not resumed. In contrast, there have been no significant clinical changes in the control group.

CONCLUSIONS

Implantation of tissue other than fetal tissue can promote a long-term improvement in the clinical symptomatology of seriously disabled parkinsonian patients. This finding is supported by the autopsy report of a patient with PD who had undergone grafting of AM plus peripheral nerve in which it was demonstrated that a large number of tyrosine hydroxylase-positive cells survive 1 year after implantation. In addition, there was a dense network of host dopaminergic fibers around the graft.

Remyelination: cellular and gene therapy

Dysfunctional myelination or oligodendroglial abnormalities play a prominent role in a vast array of pediatric neurological diseases of genetic, inflammatory, immunological, traumatic, ischemic, developmental, metabolic, and infectious causes. Recent advances in glial cell biology have suggested that effective remyelination strategies may, indeed, be feasible. Evidence for myelin repair is accumulating in various experimental models of dysmyelinating and demyelinating disease. Attempts at remyelination have either been directed towards creating myelin de novo from exogenous sources of myelin-elaborating cells or promoting an intrinsic spontaneous remyelinating process. Ultimately, some disorders of myelin may require multiple repair strategies, not only the replacement of dysfunctional cells (oligodendroglia) but also the delivery or supplementation of gene products (i.e., growth factors, immune modulators, metabolic enzymes). Although primary oligodendrocytes or oligodendroglial precursors may be effective for glial cell replacement in certain discrete regions and circumstances and although various genetic vectors may be effective for the delivery of therapeutic molecules, multipotent neural stem cells may be most ideally suited for both gene transfer and cell replacement on transplantation into multiple regions of the central nervous system under a wide range of pathological conditions. We propose that, by virtue of their inherent biological properties, neural stem cells possess the multifaceted therapeutic capabilities that many diseases characterized by myelin dysfunction in the pediatric population may demand.

Dopamine turnover and metabolism in the striatum of parkinsonian rats grafted with genetically-modified human astrocytes

The potential of a novel therapeutic approach for treating Parkinson's disease, which involves the transplantation of a transfected human astrocyte cell line SVG-TH, that stably expresses the rate-limiting enzyme for dopamine production, tyrosine hydroxylase, was examined. SVG-TH and untransfected parent cells were grafted into the diseased striatum of rats in which Parkinson's disease had been induced by the administration of 6-hydroxydopamine. The in situ production and spillover of 3,4-dihydroxyphenylalanine (the precursor of dopamine), dopamine and their metabolites in the striatal extracellular fluid of the grafted rats was determined in conscious animals using the microdialysis technique and a high pressure liquid chromatography apparatus. Alleviation of symptoms of Parkinson's disease (abnormal movements) was evaluated by rotation tests. Upon transplantation of the SVG-TH cells into the striatum of the parkinsonian rats, the levels of dopamine in extracellular fluid of the striatum reached those of the normal rats, and correlated well with the improvement (74%) in their rotating behaviour (behavioural deficit). The levels of the two main dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, were low in the lesioned rats, even after SVG-TH transplantation. An alternative route of metabolism of dopamine may occur in the transplanted striatum, since the dopamine metabolite, 3-O-methoxy-4-hydroxy-phenylethylamine, appeared, which indicates activity of catechol-O-methyl transferase. Upon blockade of L-aromatic-amino acid decarboxylase, 3,4-dihydroxyphenylalanine accumulated in extracellular fluid of the 6-hydroxydopamine-lesioned and SVG-TH-grafted rats, which indicated that these cells produced active tyrosine hydroxylase in vivo. These findings indicate the potential of treating Parkinson's disease by the intrabrain grafting of human astrocyte cells transfected with the rate limiting enzyme for dopamine production.

Transplantation of cryopreserved human embryonal carcinoma-derived neurons (NT2N cells) promotes functional recovery in ischemic rats

This study was designed to explore the efficacy of a human clone cell line as an alternative neural graft source and to validate the practice of cryopreservation and xenografting as logistical approaches toward conducting neural transplantation. We investigated the biological effects of transplanting cultured human neurons (NT2N cells) derived from a well-characterized embryonal carcinoma cell line into the brains of rats subjected to transient, focal cerebral ischemia induced by embolic occlusion of the middle cerebral artery. At 1 month and extending throughout the 6-month posttransplantation test period, ischemic animals that were transplanted with NT2N cells and treated with an immunosuppressive drug displayed a significant improvement in a passive avoidance task as well as a normalization of asymmetrical motor behavior compared to ischemic animals that received rat fetal cerebellar cell grafts or vehicle alone. Remarkably, cryopreserved NT2N cell grafts compared with fresh NT2N cell grafts, remained viable in the immunosuppressed rat brain and effective in producing behavioral recovery in immunosuppressed ischemic animals. The long-term viability of cryopreserved NT2N cell xenografts in vivo and their sustained effectiveness in promoting behavioral recovery suggest potential utilization of xenografting and cryopreservation as useful protocols for establishing clone cell lines as graft source in neural transplantation therapies for central nervous system disorders.

Fetal pig neural cells as a restorative therapy for neurodegenerative disease

With proper immunosuppression, interspecies transplantation of porcine as well as other species of neural cells survive, mature, and integrate into the host in a manner which reconstructs much of the appropriate neural circuitry. These transplants have been shown to alleviate many of the symptoms of various disorders of the central nervous system. In this study, we addressed immunological and maturation issues with regards to intracerebral transplantation of fetal porcine neural cells. First, we compared fetal neural xenograft survival in athymic nude rats versus rats immunosuppressed with cyclosporin A and found that there is little discernible difference between porcine grafts in the 2 recipients. We also found that ectopic transplantation of cells isolated from the porcine striatal primordium can survive and develop into grafts composed of both neuronal and glial phenotypes within the rat hippocampus. This fact raises the possibility that cells of a particular neurotransmitter type (e.g., GABAergic cells) developing from the striatal precursor cells can be transplanted outside the striatum of the adult brain and have physiological effects.

Long-term improvement in patients with severe Parkinson's disease after implantation of fetal ventral mesencephalic tissue in a cavity of the caudate nucleus: 5-year follow up in 10 patients. Clinica Puerta de Hierro Neural Transplantation Group

Different groups worldwide have observed in recent years that stereotactic implantation of fetal tissue can ameliorate the clinical symptoms of Parkinson's disease. The authors therefore investigated whether implantation of fetal ventral mesencephalic (FVM) tissue via open surgery is also capable of producing an improvement and whether this improvement is transient or long lasting. The authors report their findings in a 5-year follow-up study in 10 patients with Hoehn and Yahr Grade IV or V Parkinson's disease in whom a single FVM graft was implanted in a cavity created in the right caudate nucleus. The results indicate that the implants improved motor function and that clinical recovery persisted in seven of the 10 patients 5 years after implantation. Amelioration was observed in both the on and off phases and was accompanied by a 64% reduction in the levodopa dose and withdrawal of the dopamine agonist. The on phase was prolonged from 39% of the waking day to 72%, with reduced intensity and duration of dyskinesias. All symptoms that were analyzed showed improvement, although they differed in intensity and time of onset. The course of improvement seemed to be stepwise, with significant improvement between 5 and 7 months postimplantation followed by two waves of progress peaking in Months 15 and 36. Withdrawal of cyclosporine in three patients after more than 2 years of administration produced a decline in the patients' clinical conditions. In conclusion, the results indicate that open surgery implantation of FVM tissue in the caudate nucleus improves the clinical condition of parkinsonian patients and that this improvement can persist for at least 5 years. In comparison with two earlier series reported by the authors, which involved implants of perfused adrenal medulla and coimplantation of adrenal medulla and peripheral nerve, the course and pattern of improvement in these implant recipients suggests that their recovery can be attributed to more than one factor.

Gene therapy approaches to Parkinson's disease: preclinical to clinical trials, or what steps to take to get there from here?

Do "gene therapies" offer new approaches to the treatment of Parkinson's disease? The presentations of this symposium, as well as numerous published articles, suggest that they might. This article describes a preclinical in vivo gene therapy study in parkinsonian monkeys and briefly reviews the history of studies with novel cellular implantation approaches to Parkinson's disease as a basis for evaluating when and how gene therapy trials in Parkinson's patients might be appropriate. A number of concerns are raised.

Dopamine secretion by PC12 cells microencapsulated in a hydroxyethyl methacrylate--methyl methacrylate copolymer

A rat pheochromocytoma cell line (PC12) was encapsulated in a water-insoluble hydroxyethyl methacrylate-methyl methacrylate copolymer by interfacial precipitation from a polyethylene glycol 200 solution into phosphate-buffered saline. The resulting capsules (660 +/- 44 microns in diameter; 84 +/- 27 microns wall thickness) contained viable PC12 cells in a spheroidal arrangement, much like tumour spheroids, the latter grown on surfaces unsuitable for cell attachment. In these spheroids, the viable cells formed a band approximately 100 microns thick, surrounding an inner core of necrotic cells. A similar arrangement was seen 14, 28 and 42 days after encapsulation, with capsules maintained in an in vitro tissue culture environment; the annular ring was roughly constant in size, although the packing density appeared to increase over the 6 week observation period. During the first 4 weeks, when measurements were made the encapsulated cells converted a tetrazolium dye (MTT) into an insoluble formazan product, in a time-after-encapsulation-dependent manner. This indicated that PC12 cells retained viability despite encapsulation and an ability to increase (at least in part) their metabolic capacity, presumably by a combination of proliferation and altered cellular activity. The encapsulated PC12 cells also secreted dopamine when incubated in a high potassium release medium but not in a low potassium, conventional tissue culture medium (RPMI 1640). Consistent with the MTT results, the amount of dopamine released was also dependent on the time after encapsulation, as well as the cell density at the time of encapsulation.

Transplantation of embryonic noradrenergic neurons in two models of adult rat spinal cord injury: ultrastructural immunocytochemical study

The synaptic connections established by grafted noradrenergic (NA) neurons into the lesioned adult rat spinal cord were analysed using immunocytochemistry at the electron microscopic level. An embryonic cell suspension of the locus coeruleus region from E-13 rat embryos was transplanted into the spinal cord following either: (1) spinal cord transection or (2), partial selective denervation by 6-hydroxy dopamine (6-OH DA). One month after grafting, the NA-neurons established, in the two models, an innervation pattern similar to that found in the intact spinal cord. In both models, the transplanted NA-immunoreactive neurons formed extensive synaptic contacts with dendrites, spines and perikarya. The proportion of axodendritic and axospinous contacts was inverse in the two models. The first model thus reproduced more closely the normal synaptic pattern prefering dendritic targets, which could correspond to a better integration of the graft. In the second model, a partially NA-denervated spinal cord, there existed a competition between residual intrinsic and grafted neuron-derived fibres, which presumably affects synaptogenesis. In conclusion, the present study illustrate the complexity of cell interations conducting to the formation of a specific circuitry. Recognition phenomenon are likely modulated by space constraints, which ultimately shape-up the geometry of synaptic contacts.

Clinical experience with cotransplantation of peripheral nerve and adrenal medulla in patients with Parkinson's disease

Coimplants of adrenal medulla (AM) and peripheral nerve (PN) in animal models of Parkinson's disease (PD) have shown that AM cells survive longer, tend to show neuronal phenotype, and enhance sprouting of host fibers. Since 1987, our implants of perfused AM and fetal ventral mesencephalon (FVM) in PD patients have achieved varying degrees of clinical improvement. If the donor tissue determines the improvement, different types of implants should result in qualitatively and quantitatively different degrees of improvement. The purpose of this study is to determine whether or not the clinical course, improvement slope, and reduction of medication observed in PD patients who undergo tissue transplantation (Tx) depend on the donor tissue type. In a pilot study, four grade IV-V PD patients received implants of precoincubated autologous AM and intercostal nerve in the caudate nucleus (open surgery). Clinical assessment was based on international scales (UPD) as reported for Tx of FVM and perfused AM. There were no systemic or neurologic complications. Four years post-Tx, longer On phases and improved PD symptoms (ADL and motor-UPD) in On and Off persist in four cases, with reduced dyskinesias. Progress appears to be stepwise, starting within weeks of Tx (similar to AM and sooner than our FVM implants), followed by a period of stability and, after a second wave of improvement 12-18 months post-Tx (similar to FVM implants), continues to date. L-dopa medication has been reduced by more than 60% and dopamine agonist use has not resumed. We conclude that our recipients continue to be clinically better than prior to Tx. The course of recovery after co-Tx of AM and PN differs from that of FVM or AM implants. This fact may be related to the etiological factors that produce the improvement.

Effects of complete and partial lesions of the dopaminergic mesotelencephalic system on skilled forelimb use in the rat

This study compares certain behavioural consequences of partial and complete unilateral lesions of the dopaminergic mesotelencephalic system. We investigated skilled forelimb use, rotations induced by apomorphine and amphetamine, and dopaminergic metabolism of the nigrostriatal system of rats that had received a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. The rats classified Apo(+), that rotated after the administration of apomorphine, had a complete lesion of the nigrostriatal system, whereas those classified Apo(-), that did not rotate after the administration of apomorphine, had a partial lesion of the nigrostriatal system. In the Apo(+) rats, 99.8% of the dopamine in the striatum was depleted, as was 85% of that in the substantia nigra. For the Apo(-) rats, 72% of the dopamine in the striatum was depleted as was 56% of that in the substantia nigra. When investigated with the staircase test, the animals with the most severe dopamine depletions were those most impaired in the paw reaching task. Complete and partial unilateral depletions of the dopaminergic mesotelencephalic system impaired the hierarchic phases of paw reaching differently. A complete dopamine depletion, but not a partial one, decreased the number of attempts made with the contralateral paw, and induced a bias towards the ipsilateral paw. A partial dopamine lesion impaired the sensorimotor co-ordination of both paws, whereas the complete dopamine lesion had a greater effect on the contralateral paw than on the ipsilateral paw. The mild paw reaching impairments observed in animals with moderate depletions of dopamine are proposed as a model of the early symptoms of Parkinson's disease that may be useful for the development of protective or restorative therapies.

Human neurons that constitutively secrete A beta do not induce Alzheimer's disease pathology following transplantation and long-term survival in the rodent brain

Since cultured neurons secrete beta-amyloid (A beta) peptides, and A beta forms amyloid deposits in the Alzheimer's disease (AD) brain, transplanted neurons may induce the deposition of A beta in the host brain. To assess this possibility, we studied grafted human neurons (NT2N cells) and their progenitors (NT2 cells) in the rodent brain. Although NT2N cells secrete more A beta than the NT2 cells in vitro, no A beta deposits or other AD lesions were induced in the rodent brain by grafts that survived days (NT2 and NT2N cells) to 46 weeks (NT2N cells). Thus, neither the deposition of A beta nor the induction of other AD lesions are obligatory consequences of the transplantation and long-term survival of human neurons or their progenitors in the rodent brain.

Grafting immortalized neurons to the CNS

The ability to transplant an unlimited supply of clonally related neural progenitors that, in the brain, have the capacity to differentiate into neurons and glia in an anatomically and, perhaps, functionally appropriate manner, may not only facilitate developmental inquiries, but may also circumvent the limitations of primary fetal tissue for neural transplantation. These types of transplants also make possible new strategies for gene therapy and repair of the CNS, including replacement of degenerated cells, engineering donor cells to be resistant to toxins, delivery of missing metabolic or other gene products, over-expression of molecules, and substitution of alternate metabolic pathways.

Human fetal tissue research: practice, prospects, and policy

Tissue from human fetal cadavers has long been used for medical research, experimental therapies, and various other purposes. Research within the last two decades has led to substantial progress in many of these areas, particularly in the application of fetal tissue transplantation to the treatment of human disease. As a result, clinical trials have now been initiated at centers around the world to evaluate the use of human fetal tissue transplantation for the therapy of Parkinson's disease, insulin-dependent diabetes mellitus, and a number of blood, immunological and, metabolic disorders. Laboratory studies suggest a much wider range of disorders may in the future be treatable by transplantation of various types of human fetal tissue. A combination of characteristics renders fetal tissue uniquely valuable for such transplantation, as well as for basic research, the development of vaccines, and a range of other applications. Although substitutes for human fetal tissue are being actively sought, for many of these applications there are at present no satisfactory alternatives. Important issues remain unresolved concerning the procurement, distribution, and use of human fetal cadaver tissue as well as the effects of such use on abortion procedures and incidence. These issues can be addressed by the introduction of appropriate guidelines or legislation, and need not be an impediment to legitimate research and therapeutic use of fetal tissue.