Human fetal tissues grafted to rodent hosts: structural and functional observations of brain, adrenal and heart tissues in oculo

The Influence of Donor Age on the Survival of Solid and Suspension Intraparenchymal Human Embryonic Nigral Grafts

In many species, graft survival and graft-derived behavioral recovery are affected by the embryonic donor age. We compared the ability of solid and suspension grafts of human embryonic mesencephalic dopaminergic (DA) neurons at different embryonic stages to survive intra-parenchymal transplantation into 6-OHDA lesioned immunosuppressed rats. Suspension grafts survived best when donor age was between postconception (PC) days 34 and 56. Transplants displayed numerous healthy tyrosine hydroxylase immunoreactive (TH-IR) neurons which sent extensive neuritic processes into the host striatum. Suspension grafts survived poorly when donor age was greater than 65 days. Solid implants displayed comparable viability of TH-IR neurons when donor age was between 44 and 65 days. No solid grafts contained TH-IR cells when donor tissue was older than 72 days. The suspension and solid methods of transplantation resulted in comparable survival of robust grafts, but solid grafts resulted in more intergraft variability than suspension grafts, particularly among the more marginal implants. Our results demonstrate that the upper limit for survival of human embryonic DA suspension grafts correlates well with the period of development of the human nigrostriatal pathway. The “window” for donor age of solid human embryonic DA grafts appears to be extended by about 9 days in comparison to suspension grafts. These data suggest that the upper age limit for grafting human mesencephalic DA neurons should be PC day 56 for suspension grafts, and PC day 65 for solid implants. Older donors are likely to produce grafts with fewer surviving DA neurons.

Comparison of Fetal Rabbit Brain Xenografts to Three Different Strains of Athymic Nude Rats: Electrophysiological and Immunohistochemical Studies of Intraocular Grafts

Interest in the use of neural tissue transplantation for the study of CNS development and maturation and the potential use of this technique for the treatment of certain degenerative CNS disorders has led to our use of transplantation of neural tissue across species lines. Prior to extensive transplantation studies using athymic rats as recipients, we wished to evaluate the currently available strains of athymic rat for their suitability as host animals for xenografts of neural tissue. Fetal cerebellar and cerebral cortex tissue from rabbit brain of gestational age 20-25 days was dissected and transplanted to the anterior chamber of the eye of Harlan Wisconsin, Fisher 344 Jnu, or NCI-Harlan athymic nude rat strains. The brain tissue grafts were allowed to mature for 3 mo during which time the size and vascularity of each graft was monitored through the cornea of anesthetized hosts. In each group all of the transplants survived and grew to varying extents in the anterior chamber of the eye. Following the growth study in vivo extracellular recording of single neuronal activity was performed. Spontaneous neural activity was found in most transplants in all three groups with no difference in the viability or discharge rates of neurons between the groups. Illumination of the ipsilateral eye increased the firing rate of neurons in all three groups, suggesting excitatory cholinergic innervation of the grafted neurons from the host parasympathetic iris ground plexus. Antibodies directed against neurofilament protein, glial fibrillary acidic protein, synapsin, and tyrosine hydroxylase were used to characterize the transplants immunocytochemically and revealed no differences between the grafts in the three groups of recipients. All transplants contained significant numbers of glial and neuronal elements with the distribution resembling that in adult brain tissue. Some of the transplants contained a sparse innervation of tyrosine hydroxylase–positive fibers from the sympathetic plexus of the host iris. Furthermore, synapsin-immunoreactivity suggested that synaptogenesis had taken place within the grafts. Histological examination of the grafts revealed that 67% of the grafts had been infiltrated, to varying extents, by lymphocytes which led to areas of cell lysis and necrosis. All host animals had populations of T-cell receptor positive cells, most of which also expressed the T-cell surface antigens CD4 and CD8. However, no transplants were overtly rejected over the 15 wk period of study. Our investigation demonstrates that all of the athymic strains used in this study are able to mount an immune response against grafted fetal tissue, despite the absence of rejection, and that none of these strains is superior to the others with respect to suitability as a host for the long-term study of fetal CNS xenografts in oculo.

Neural control of the endocrine pancreas

The autonomic nervous system affects glucose metabolism partly through its connection to the pancreatic islet. Since its discovery by Paul Langerhans, the precise innervation patterns of the islet has remained elusive, mainly because of technical limitations. Using 3-dimensional reconstructions of axonal terminal fields, recent studies have determined the innervation patterns of mouse and human islets. In contrast to the mouse islet, endocrine cells within the human islet are sparsely contacted by autonomic axons. Instead, the invading sympathetic axons preferentially innervate smooth muscle cells of blood vessels. This innervation pattern suggests that, rather than acting directly on endocrine cells, sympathetic nerves may control hormone secretion by modulating blood flow in human islets. In addition to autonomic efferent axons, islets also receive sensory innervation. These axons transmit sensory information to the brain but also have the ability to locally release neuroactive substances that have been suggested to promote diabetes pathogenesis. We discuss recent findings on islet innervation, the connections of the islet with the brain, and the role islet innervation plays during the progression of diabetes.

Novel approaches to studying the role of innervation in the biology of pancreatic islets

The autonomic nervous system helps regulate glucose homeostasis by acting on pancreatic islets of Langerhans. Despite decades of research on the innervation of the pancreatic islet, the mechanisms used by the autonomic nervous input to influence islet cell biology have not been elucidated. This article discusses how these barriers can be overcome to study the role of the autonomic innervation of the pancreatic islet in glucose metabolism. It describes recent advances in microscopy and novel approaches to studying the effects of nervous input that may help clarify how autonomic axons regulate islet biology.

Transplantation of autologous dorsal root ganglia into the peroneal nerve of adult rats: uni- and bidirectional axonal regrowth from the grafted DRG neurons

Previous studies have demonstrated that transplanted dorsal root ganglion neurons (DRGNs) can survive and differentiate in a variety of orthotopic and heterotopic locations. In order to develop strategies aimed at restoring the sensory function following traumatic injury to the spinal cord and to its peripheral sensory connections, we have transplanted adult autologous dorsal root ganglia (DRGs) into the peroneal nerve of adult rats. Twelve female Sprague-Dawley rats were used. A segment of the peroneal nerve was isolated by double transection and ligature to prevent undesirable reinnervation. The left fifth cervical (C5) DRG was removed from its normal location and inserted into the midportion of the isolated nerve segment. One month after the grafting procedure, a morphological study included axonal retrograde labeling with True Blue (TB) and/or Diamidino Yellow (DY) applied on each cut end of the nerve segment, cell counting, and cell measurement after staining with cresyl violet. Compared to the C5 ganglion maintained in situ, the mean number of surviving DRGNs in the transplant was 1381, corresponding to a survival rate of 20%. Both singly (TB or DY) and doubly (TB + DY) stained DRGNs were encountered. The proportion of surviving neurons that appeared to be doubly labeled was 23%. These neurons were considered as having grown two opposite axonal projections, one into the "central" part of the nerve segment and a second one into its "peripheral" part. The present results give new insights and interesting prospects concerning the possibilities of reconstructing the sensory circuitry after central and/or peripheral injuries.

Regional distribution of nicotinic receptors during prenatal development of human brain and spinal cord

The development of nicotinic acetylcholine receptors (nAChRs) in brains from human fetuses of 4-12 weeks gestational age was studied. The expression of nAChR subunit mRNAs was analyzed using reverse transcriptase-polymerase chain reaction. Expression of alpha 3, alpha 4, alpha 5, alpha 7, beta 2, beta 3 and beta 4 mRNA were all detected in the prenatal spinal cord, medulla oblongata, pons, cerebellum, mesencephalon, subcortical forebrain and cortex during first trimester development. Relative quantification of mRNA showed that the highest levels for alpha 3, alpha 4 and alpha 7 were expressed in the spinal cord, alpha 5 was most abundant in the cortex and beta 3 was highest in the cerebellum. beta 4 seemed to be equally distributed in all regions whereas beta 2 was high in the cortex and cerebellum. A comparison of expression of nAChR subunit mRNAs in the cortex and cerebellum of prenatal and aged (54-81 years) brain showed that mRNA levels for alpha 4, alpha 5, alpha 7, beta 2 and beta 4 were significantly higher in the prenatal cortex and cerebellum than in aged brain, whereas the level of alpha 3 transcript was similar, and beta 3 significantly higher in aged cortex. Specific binding of [3H]-epibatidine to prenatal brain membranes was detected as early as 4-5 weeks of gestation in the spinal cord, medulla oblongata, pons and subcortical forebrain. A positive correlation between gestational age and [3H]-epibatidine and [3H]-cytisine binding was found in several brain regions. The highest specific binding of [3H]-epibatidine and [3H]-cytisine was detected in the spinal cord, pons and medulla oblongata and the lowest in the cortex. Saturation analysis of [3H]-cytisine binding in both prenatal and aged brain were best fit by a model for a single site, whereas binding data for [3H]-epibatidine revealed two classes of binding sites. The early presence of nAChR proteins and gene transcripts shown in the present study suggests an important role for nAChRs in modulating dendritic outgrowth, establishment of neuronal connections and synaptogenesis during development.

Human embryonic spinal cord grafts in adult rat spinal cord cavities: survival, growth, and interactions with the host

The ability of solid pieces of transplanted human embryonic spinal cord to survive, grow, and integrate with adult rat host spinal cord tissue was investigated. Unilateral cavities were surgically created at vertebral level T12-T13 in 10 athymic nude rats and 5 regular Sprague-Dawley rats. Seven of the athymic rats acutely received a human spinal cord graft, while the remaining 8 rats served as controls, with cavities alone. After 6 months the morphological outcome was evaluated with cresyl violet and with immunohistochemistry using antibodies toward human-specific neurofilament (hNF), human-specific Thy-1 (Thy-1), neurofilament, glial fibrillary acidic protein, serotonin (5-HT), and tyrosine hydroxylase (TH). The in situ morphology of the human embryonic spinal cord was also investigated and compared with grafts that were six months older. Solid human embryonic spinal cord grafts showed a 100% survival rate, grew to fill the volume of the cavity in a noninvasive manner, and expressed human specific antigens 6 months postgrafting. Thy-1 immunoreactivity (IR) was demonstrated up to 8 mm rostral to the graft suggestive of graft-derived fiber outgrowth. hNF-IR fibers and 5-HT- and TH-IR fibers traversed the graft-host border for a few hundred micrometers, respectively. Finally, our findings suggest that grafted solid pieces of human embryonic spinal cord minimize cystic deformations seen in the adult rat spinal cord with a unilateral cavity.

Human fetal cortical tissue fragments survive grafting following one week storage at +4 degrees C

Grafting of human fetal tissue fragments has been used successfully in experimental and clinical trials. The development of techniques to store human fetal tissue fragments for longer time periods would allow to establish temporary tissue banks. We dissected several human cortical tissue fragments from one fetus and tested different storage conditions (cooling, freezing, culturing). After storage, the tissue fragments were transplanted into cavities in the cortex of host rats and the volume of the surviving grafts calculated. We report that human cortical tissue fragments grafted immediately after dissection (control group) or grafted after storage for 3 h in cryopreservation medium at room temperature survived grafting and resulted in graft sizes of 102 +/- 26 mm3 and 242 +/- 210 mm3, respectively, however, statistically not different. When the human cortical tissue fragments were slowly frozen and stored for 1 wk and/or when the fragments were cultured for 1 week in culture medium using a roller tube technique, grafts did not survive under our conditions. However, when the human cortical tissue fragments were stored for 1 week at +4 degrees C in cryopreservation medium, the graft size (48 +/- 24 mm3) was reduced but statistically not different from the control group. We conclude that human cortical tissue fragments can be stored at +4 degrees C for at least 1 wk without major loss of ability to survive grafting.

Low-pressure aspiration abortion for obtaining embryonic and early gestational fetal tissue for research purposes

Successful transplantation of cadaver embryonic neural tissue is highly dependent on the method used to obtain the tissue. It is important that the tissue not be contaminated bacteriologically by vaginal flora during the procedure, and that it not be disrupted mechanically. A low-pressure aspiration abortion technique has been developed that allows for the safe, effective obtainment of embryonic tissue and reduces the risk of transmitting infection. Tissue was cultured in vitro in 102 cases with minimal evidence of contamination by vaginal flora. Transplants of neural tissue into over 300 rodents have resulted in no intracranial abscesses, even in the setting of immunosuppression. The suction apparatus and low-pressure aspiration minimize disruption of the embryonic tissue. Use of low pressure adds no significant additional risk to the patient. In over 300 cases, there have been no medical complications specifically attributable to the technique. Because local anesthetic is used and sonography is not routinely required, the procedure can easily be performed in an outpatient setting during routine elective abortions, with minimal slowing or disruption of the clinic's surgical schedule. In conclusion, the low-pressure aspiration abortion technique can be safely and effectively used to obtain embryonic and early gestational fetal tissue that is almost always free from bacterial, fungal, and yeast contamination and that is frequently structurally intact. It requires no significant alteration in indications for abortion, risks, methodology, timing of the abortion, or patient management.

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.

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.

Initial studies of embryonic transplants of human hippocampus and cerebral cortex derived from schizophrenic women

Human fetal brain tissue was obtained from first-trimester elective abortions of two women who also had schizophrenia. Portions of the embryonic hippocampus or cerebral cortex were transplanted into the anterior eye chamber of immunologically compromised athymic nude rats. In this environment, embryonic brain tissue derived from normal women generally continues organotypic growth and development for many months. Although initial survival after transplantation was normal, the tissue derived from schizophrenic women manifested less robust growth. However, cells in the transplants showed typical neuronal differentiation, with development of different neuronal types, such as pyramidal cells, granule cells, and gamma-aminobutyric acid (GABA)-containing interneurons. Rhythmic electrical activity was also observed, indicative of some local synaptic organization. The presence of messenger RNA (mRNA) for brain-derived neuronotrophic factor (BDNF) was observed using in situ hybridization. The reason for the decreased rate of growth of these transplants remains unknown and the significance of the finding cannot be assessed from only two fetuses. However, these preliminary findings suggest that fetal transplants may be a useful model system for the detection of developmental pathogenic processes in the expression and transmission of schizophrenia.

Human fetal xenografts of brainstem tissue containing locus coeruleus neurons: functional and structural studies of intraocular grafts in athymic nude rats

Fetal human brainstem tissue including the nucleus locus coeruleus was transplanted to the anterior eye chamber of athymic nude rats. Most transplants survived and grew in the anterior chamber of the eye. After 9-15 months, the host animals were anesthetized and electrophysiological or in vivo electrochemical recordings were performed. The brainstem transplants contained spontaneously active neurons with regular single-spike firing patterns. The neurons responded to ipsilateral light stimulation with an increase in firing rate and to the alpha 2-receptor agonist clonidine with significantly decreased firing rates. In vivo electrochemical studies demonstrated reproducible noradrenergic overflow after local application of potassium. Immunohistochemical evaluation of the brainstem transplants showed an abundance of tyrosine hydroxylase-positive neurons and neurites in all transplants and a dense network of neurofilament-, synapsin-, and glial fibrillary acidic protein-positive profiles throughout the grafts. Taken together, the present physiological and histochemical data indicate that it is possible to obtain transplants containing a specific monoaminergic population within the brainstem from human fetal fragments and to maintain these transplants in oculo in athymic nude rats for at least 15 months, during which time noradrenergic neurons develop.

Centrifugal growth in orthotopic grafts of allogeneic dorsal root ganglia in adult rats: evidence for possible central ingrowth?

Fetal allogeneic dorsal root ganglion (DRG) transplants from 13-15 day rat embryo's (E13-E15) survived and differentiated when grafted orthopically (within the capsules of the excised 4th and 5th lumbar (L4-L5) ganglia) in adult rats. Survival of grafted neurones was established by prelabeling the grafts with a fluorescent vital dye (DiI) and visualizing the retained fluorescent marker 3 to 9 months later. Simultaneous retrograde tracing using fluorescent tracers applied in the spinal cord and peripheral nerve, respectively, yielded double-labeled dorsal root ganglion neurons, some of which were prelabeled. These findings demonstrate that prelabeled E13-E15 ganglia survive orthopic grafting, organotypically differentiate into mature DRG neurones, and can be double-labeled with fluorescent dyes applied to their peripherally and centrally directed processes. The presence of DiI containing cells which were retrogradely labeled from the spinal cord suggests that fetal (E13-E15) ganglia may have the capability of growing into a mature spinal cord.

Development of dopaminergic neurons in the human substantia nigra

A series of 29 human embryonic brains were examined in order to characterize the ontogeny of dopaminergic neurons within the developing substantia nigra. Embryos from Postconception Weeks 4.0 to 11.2 (last menstrual period 6.0-13.2) were studied. Immunohistochemical staining was performed using a polyclonal antibody to tyrosine hydroxylase. Tyrosine hydroxylase-like immunoreactivity was first seen in cells of the ventral mesencephalon at 6.5 weeks adjacent to the ventricular zone. Ventral migration of TH-positive cells began at 6.7 weeks. Neural process extension was first identified in tyrosine hydroxylase-positive neurons at 8.0 weeks. The ascending nigrostriatal bundle was also first demonstrated at 8.0 weeks. Tyrosine hydroxylase containing neurites were seen initially in the developing putamen at 9.0 weeks. Only a few tyrosine hydroxylase-positive cells remained adjacent to the ventricular zone at Week 10.0 and all had disappeared from the ventricular zone by 11.2 weeks. At this latter stage, a large number of dopaminergic cells had elaborated neural processes. The sequence of developmental events of human mesencephalic dopaminergic neurons is similar to the equivalent period of ontogeny in other mammals. The duration of the developmental period is, however, significantly protracted.

Transplantation of adrenal tissue into the central nervous system

Adrenal medullary tissue can survive transplantation to the central nervous system. Such survival has been obtained experimentally with grafts to the anterior eye chamber, to the brain and to the spinal cord, using medullary tissue from the recipient animal or unrelated animals of the same or, in some cases, different species. Appropriately placed grafts have been shown, under certain conditions, to interact with the host nervous system, exerting behavioral effects including amelioration of experimentally-induced parkinsonian symptoms. Such effects may be enhanced by administration of nerve growth factor to the grafts. On the basis of such findings, adrenal medullary tissue has been grafted to the brain of Parkinson's disease patients. Both animal and human experiments raise important questions about mechanisms of graft action and about factors that influence the outcome of these procedures.

Prolonged survival and vascularization of xenografted human glioblastoma cells in the central nervous system of cyclosporine A treated rats

Glioblastoma cells from three established lines were transplanted in oculo and in cerebrum to rat hosts. A very low dose of Cyclosporine A was found sufficient to allow graft survival whereas grafts in non-immunosuppressed animals did not survive. Moderate immunosuppression permitted long term graft survival without aggressive growth of glioblastoma cells, creating a protracted course during which neither cell rejection nor tumor proliferation occurred. A tumor reminiscent of a glioblastoma was only seen in one animal on high immunosuppression. Phenotypic changes such as the induction of glial fibrillary acidic protein (GFAP) production and an astrocytic morphology were observed in the cells growing in oculo but not in cerebrum. Vascularization was easily demonstrated with laminin immunofluorescence but the endothelial proliferation typical of glioblastomas was not seen.

Human ventral mesencephalic xenografts to the catecholamine-depleted striata of athymic rats: ultrastructure and immunocytochemistry

On the basis of animal studies, grafts of fetal human dopaminergic cells have been suggested as a therapy for Parkinson's disease. The purpose of this study was to characterize the ultrastructure and immunocytochemistry of human ventral mesencephalic xenografts placed into the catecholamine-depleted striata of athymic "nude" rats. Human fetal tissue was obtained from tissue fragments derived from elective abortions during the first trimester of pregnancy. Small pieces of the basal mesencephalon were grafted into the catecholamine-depleted striata of four athymic nude rats. The rats were allowed to survive from 3 to 6 months after grafting; following fixation, the striatal tissue containing the grafts was labeled with antibodies against tyrosine hydroxylase and serotonin. Immunocytochemistry revealed tyrosine-hydroxylase-like-immunoreactive (THLI) and serotoninlike-immunoreactive (5HTLI) cell bodies within the human grafts. Both 5HTLI and THLI fibers crossed the graft-host interface and innervated the previously lesioned striatum. Both types of fibers also entered the host cortex from the adjacent human graft. At the ultrastructural level, THLI and 5HTLI fibers and synaptic terminals were observed in the host neuropil. THLI and 5HTLI dendrites and axon terminals were also observed in the neuropil of the grafts themselves. THLI axon terminals are not normally present in the substantia nigra. The results of our study indicate that human xenografts can survive in the neuropil of the host striatum and form morphologically appropriate synapses within the host brain.

Thyrotropin releasing hormone augments growth of spinal cord transplants in oculo

The effects of thyrotropin releasing hormone (TRH) on spinal cord growth were evaluated using the in oculo transplant model. The growth of fetal spinal cord allografts, placed into the anterior eye chamber of Sprague-Dawley rats, was markedly augmented by acute exposure of the graft and host animal to TRH at the time of transplantation. No significant growth augmentation was seen after equimolar administration of a mixture of the amino acids that comprise the TRH molecule. It is concluded that acutely administered TRH, at the time of grafting, elicits a significant stimulation of the growth of spinal cord tissue. Our data strengthen the rationale for continued clinical trials of this peptide in spinal cord injury.

Human fetal dopamine neurons grafted in a rat model of Parkinson's disease: ultrastructural evidence for synapse formation using tyrosine hydroxylase immunocytochemistry

Human fetal mesencephalic dopamine (DA) neurons, obtained from 6.5-9 week old aborted fetuses, were grafted to the striatum of immunosuppressed rats with 6-hydroxydopamine lesions of the ascending mesostriatal DA pathway. The effects on amphetamine-induced motor asymmetry were studied at various timepoints after grafting. At eight weeks, functional graft effects were not evident but after 11 weeks small effects on motor asymmetry could be monitored and rats tested 19-21 weeks after grafting exhibited full reversal of the lesion-induced rotational behaviour. Four rats were sacrificed at different timepoints between 8 and 20 weeks and the grafted DA neurons were studied in tyrosine hydroxylase (TH) immunocytochemically stained sections at the light and electronmicroscopic level. The grafts contained a total of 500-700 TH-positive neurons in each rat. In one rat sacrificed 8 weeks after grafting the grafted neurons were TH-positive but exhibited virtually no fiber outgrowth. In another rat, sacrificed after 11 weeks, a sparse TH-positive fiber plexus was seen to extend into the adjacent host neostriatum. Two rats sacrificed after 20 weeks both contained TH-positive neurons that gave rise to a rich fiber network throughout the entire host neostriatum, and this fiber network was also seen to extend into the globus pallidus and nucleus accumbens. Very coarse TH-positive processes, identified as dendrites in the electron microscope, projected up to 1.5-2.0 mm from the graft into the host striatum. Ultrastructural analysis revealed that the grafted neurons had formed no TH-positive synaptic contacts with host striatal neurons after 8 weeks, and at 11 weeks some few TH-positive synapses were identified. Twenty weeks after transplantation, abundant TH-positive synaptic contacts with host neurons were seen throughout the neostriatum, and such contacts were identified in the globus pallidus as well. Thus, the present study provides tentative evidence for a time-link between the development of synaptic contacts and the appearance of functional graft effects. Similar to the normal mesostriatal DA pathway, ingrowing TH-positive axons formed symmetric synapses and were mainly seen to contact dendritic shafts and spines. However, in comparison to the normal rat striatum there was a higher incidence of TH-immunoreactive boutons forming synapses onto neuronal perikarya. The TH-positive dendrites that extended into the host striatum were seen to receive non-TH-immunoreactive synaptic contacts, presumably arising from the host neurons.(ABSTRACT TRUNCATED AT 400 WORDS)