Neo-pathway formation of dissociated neural grafts demonstrated by immunocytochemistry, fluorescent microspheres, and retrograde transport.

Embryonic neural tissues were taken from the ventral mesencephalon, the striatal anlage, the cerebellum, or the dorsal telencephalon. Prior to grafting these tissues were dissociated. This provided the opportunity to generate experimental cografts and allowed the introduction of neuronal tracers. Projections from grafts that contained dopaminergic neurons of the mesencephalon were identified immunocytochemically with antisera to tyrosine hydroxylase. Acetylcholinesterase histochemistry was employed to both stain the graft and to visualize graft efferent fibers. Moreover, dissociated tissues, with the exception of the ventral mesencephalon, were treated in vitro with rhodamine-conjugated microspheres. The immature cells incorporated the tracer in vitro. Labeled grafts were placed adjacent to large fiber tracts of the recipient brain or into the lateral ventricle and grown for one to two months. Different types of neurons within the grafts retained the rhodamine tracer. Some of the labeled neurons were positive for acetylcholinesterase. Efferent fibers from the different neural grafts followed similar routes within the host brain. Fibers containing microspheres projected commonly along the nearest host pathway or the labeled fibers followed the reactive glia along the original stab wound. Grafts that were located within the lateral ventricle projected their efferent fibers adjacent to the ventricular surface. Similar routes were followed by efferent fibers from transplanted dopaminergic neurons. One of the projection routes along the hosts corpus callosum was confirmed by horseradish peroxidase tracer transport. We conclude that elongation of graft pathways may occur along existing fiber tracts of the host brain, near structures of the ventricular surface, and alongside glia scars of a graft placement tract.