Cerebellar heterokaryon formation increases with age and after irradiation.

Hematopoietic cells have been demonstrated to survive in many nonhematopoietic tissues after transplantation. Apparent "bone marrow-derived" cerebellar Purkinje cells in fact result from fusion events and it has been suggested that fusion may be a natural physiological phenomenon to rescue dysfunctioning cells. Here, we show that fusion of transplanted bone marrow cells with resident Purkinje cells is age-dependent and is strongly enhanced when Purkinje cells are damaged by high-dose irradiation. In addition, Purkinje heterokaryons occur in increased frequencies in the cerebellum of normal, unperturbed, aged mice compared to young animals. Our data suggest that age- and/or irradiation-induced dysfunctioning of Purkinje cells in the cerebellum is required for cell fusion.