The reconstruction of cerebellar circuits.

Repair of adult 'point-to-point' systems by neural grafting is possible only when grafted neurons succeed in synaptically replacing the host's missing neurons, thus re-establishing the anatomical and functional integrity of the impaired circuits. Grafting experiments carried out on the cerebellum of the adult pcd (Purkinje-cell-degeneration) mutant mouse (an animal model of hereditary degenerative ataxia) reveal that embryonic Purkinje cells, by some unknown sorting mechanism, selectively invade the deprived cerebellar cortex. These neurons migrate to their proper domains and, inducing axonal sprouting of specific populations of host neurons, they become integrated synaptically within the pcd cerebellar cortex. However, the re-establishment of the corticonuclear projection is achieved only rarely, and this is the current experimental limit for the complete reconstruction of the cerebellar circuit.