Integration of hippocampal suspension grafts with host neocortex

The possibility of histological and functional integration of nervous tissue heterotopically grafted into the adult host brain was investigated. Suspensions of embryonic (E17-18) rat hippocampus with dentate fascia were placed into acute cavities in the barrel field of young adult rats (n = 25). Golgi-Cox silver impregnation and Cresyl Violet stain were used for histological analysis 3-4 months postgrafting. The surviving grafts were present in 80% of the grafted animals. Only three out of 20 surviving grafts were completely isolated from the surrounding host brain; other grafts had areas of direct confluence with the host neuropil. Extracellular recording of neuronal activity revealed normal spontaneous activity typical of the hippocampus in the majority of the grafts. Electrical stimulation of the posterior nucleus of the thalamus, homolateral motor neocortex, contralateral barrel field, and sensory stimulation of the host evoked responses in 50-60% of the grafted neurons. This did not differ significantly from the responsiveness of the similarly tested neurons of homotopic neocortical suspension grafts. The latencies of the responses in the hippocampal grafts were consistently longer (by about 10 ms) than in the neocortical ones. Comparison of the hippocampal suspension grafts with other types of hippocampal and neocortical grafts suggests that under certain conditions heterotopic tissue can be successfully integrated into the host brain. Development of the host-graft interconnections depends on topical proximity, the presence of denervated synaptic loci in both tissues, elimination of the intragraft neuronal targets and disruption of the intrinsic connections between them.