Influence of grafted glia cells and host mossy fibers on anomalously migrated host granule cells surviving in cortical transplants.

Embryonic cerebral cortex was transplanted over the cerebellum of one- to two-day-old rats. In mature rats, clusters of granule cells that failed to migrate into the internal granule cell layer were now found within the graft tissue. Immunocytochemical staining of astrocytic glial cells in the cortical transplants revealed that glial processes were distributed in an unusual polarized orientation in those regions that contained host granule cells. Other areas of the graft exhibited glia cells with processes that projected radially from their cell body, thus resembling fibrous astrocytes. Fibrous astrocytes in transplants, however, were more heavily stained than similar glial cells in the intact cerebral cortex of the host, indicating a quantitative difference in the glia fibrillary acidic protein. Acetylcholinesterase-positive fibers were observed between the clusters of granule cells in the cortical grafts. Such fibers traversed the molecular layer and, since they could be traced from the white matter of the host cerebellum, they were presumed to be mossy fibers. It is concluded that migration of external granule cells in the cerebellum can altered by placing embryonic cerebral cortex next to the developing cerebellum. Granule cells that have migrated into the grafted cerebral cortical tissue nevertheless receive afferent fibres.