Projections from fetal neocortical transplants placed in the frontal neocortex of newborn rats. A Phaseolus vulgaris-leucoagglutinin tracing study

Summary Fetal rat neocortex grafted into lesion cavities made in the newborn rat neocortex can exchange multiple axonal connections with the host brain. Most previous studies demonstrating efferent transplant-tohost brain connections have used fluorescent retrograde tracers injected into the host brain (Castro et al. 1985, 1987; Floeter and Jones 1984; O'Leary and Stanfield 1989). Other studies have used anterograde axonal tracing with either tritium-labelled amino acids impregnating the transplant and its efferents (Floeter and Jones 1985) or horseradish peroxidase injected into the transplants (Chang et al. 1984, 1986). In the present study we used the anterograde axonal tracer Phaseolus vulgaris — leucoagglutinin (PHA-L) to examine in detail the course and termination of the efferent neocortical graft fibers. Twenty-six newborn rats had the right frontal cortex forepaw area removed by vacuum aspiration, while anesthetized by hypothermia. A piece of fetal frontal cortex 14–16 embryonic days old (E14–16) was immediately thereafter placed in the lesion, and the recipient rats allowed to survive for 5–7 months. At this time the rats were reoperated under sodium pentobarbital (Nembutal) anesthesia and the transplants iontophoretically injected with PHA-L. Two weeks later the animals were again anesthetized, perfused, and processed for PHA-L immunocytochemistry and routine histology. Analysis of acetylcholinesterase- (AChE) and Nissl-stained sections showed graft survival in 19 of the 26 animals used in this study. When these 19 brains were processed for PHA-L immunocytochemistry, 5 of them were found with certainty to have the PHA-L injection confined to the transplant. Based on these cases PHA-L-reactive fibers arising from labelled transplant neurons were traced into the ipsilateral host neocortex adjacent to the transplant and found to project through the subcortical white matter to the ipsilateral parietal neocortical area 1, and claustrum. Callosal fibers were traced to the contralateral frontal neocortical forelimb and parietal areas. Transplant fibers were also observed to descend through the caudate putamen in the dispersed fiber bundles of the internal capsule to distribute as terminal branches and varicose fibers within the mesencephalic periaqueductal gray, red nucleus, deep mesencephalic nucleus, and intermediate gray of the superior colliculus, as well as in the pontine gray. Similar fibers and terminations were present in the caudate putamen, the reticular, ventrobasal, centrolateral, posterior, and parafascicular thalamic nuclei. On the side contralateral to the transplant, fewer fibers were observed in the caudate putamen, the ventrobasal, centrolateral, and posterior thalamic nuclei, as well as more caudally in the deep mesencephalic nucleus and the intermediate gray of the superior colliculus. Our findings demonstrate that homotopic grafts of fetal rat frontal neocortex can project to the developing host brain in a manner which for most projections corresponds to the normal rat neocortical parietal area 1–2 and forelimb area. The density of these transplant-to-host projections is, however, less than in the normal rat corticofugal pathways.