Fetal striatal neurons grafted into the ibotenate lesioned adult striatum: efferent projections and synaptic contacts in the host globus pallidus.
The efferent projections to the host brain from intrastriatal grafts have been examined at the ultrastructural level. Cell suspensions of E14 rat fetal striatal tissue were implanted into the ibotenic acid lesioned caudate-putamen of adult rats. After survival times of at least five months, the anterograde neuronal tracer Phaseolus vulgaris leucoagglutinin was injected into the grafts. Consistent with our previous light microscopical analysis, anterogradely labelled fibres could be followed from the grafts into the host globus pallidus along the normal trajectory of the striatopallidal pathway in the internal capsule, and a few fibres also reached the entopeduncular nucleus and the substantia nigra. In the electron microscope, the graft-derived efferents were seen to be myelinated as they projected caudally along the fibre bundles of the internal capsule, and they were thus similar to the neostriatal projection in normal control animals. In the host globus pallidus, the graft-derived fibres ramified into a terminal network forming morphologically normal synaptic contacts with neuronal elements in the host globus pallidus. A total of 118 synaptic contacts were identified, all of them formed symmetric membrane specializations. The major postsynaptic targets were dendritic shafts (90%), with only a few contacts with spines or small shafts (8%) and perikarya (2%). The morphology of the synaptic contacts and their distribution on different postsynaptic targets were similar to that which was found in the globus pallidus after tracer-injections into the caudate-putamen of normal control animals. The results show that grafted fetal striatal neurons can grow along the myelinated territory of the internal capsule to reinstate a seemingly normal synaptic input to the previously denervated neurons in the host globus pallidus.