<p>A histological and electron microscopic study was conducted on heterotopic neural transplants of the dentate fascia developing in the somatosensory region of the neocortex in adult rats. Mature neural tissue was shown to develop in the grafts, reproducing the organotypic characteristics of differentiated neurons and glial cells. Particular attention was paid to the cellular organization of the boundary between the transplanted and neocortical tissues (the interface) and the possibility that neuronal processes could grow through it. Phenotypically distinct astrocyte subpopulations were found to play a leading role in the processes of structural and functional integration. Mature protoplasmic and fibrous astrocytes, astrocytic precursors, and ependymal cells were identified in the interface. The cytological composition of the boundary zone influenced the degree of integration of grafts with the adjacent brain, ranging from complete fusion of adjacent tissues in the astrocytic boundary type to limited exchange of axonal and dendritic processes in the ependymal type. Transiting bundles of axons and dendrites were always accompanied by the processes of fibrous astrocytes. The only barrier for sprouting nerve processes were the areas of the interface into which large blood vessels from the pia mater penetrated.</p>