Local connections in transplanted and normal cerebral cortex of rats.

Injections of the fluorescent tracer Fluoro-Gold were made in transplanted and normal cerebral cortex of rats in order to investigate and compare the local connectivities of both. In the normal somatosensory cortex, small injections in superficial layers (I to III) produced retrograde cell labeling below the injection site in two bands: in layer V and in the deep part of layer VI. Pieces of embryonic rat neocortical tissue were transplanted into a cavity made in the somatosensory cortex of young adult rats. After a survival period of 2-3 months, small injections of Fluoro-Gold were made in the superficial part of the grafts. These injections revealed multiple clusters of intratransplant-projecting cells. No callosal or thalamic neurons were labeled in these experiments. On occasion, a bilaminated pattern of retrograde cell labeling was observed inside the transplants. In both transplanted and normal cortices, pyramidal and non-pyramidal cells were retrograde-labeled. We conclude that in the neocortical transplants there is a pattern of local connectivity that is reminiscent of the pattern of intracortical connectivity in the normal neocortex in at least two aspects: first, the retrograde-labeled cells tended to form clusters or bands; second, both pyramidal and non-pyramidal cells were labeled.