Axonal torpedoes in cerebellar Purkinje cells of two normal mouse strains during aging

The present study systematically investigated the proportional evolution of Purkinje cell (PC) axonal swellings, also termed torpedoes, during aging of the two unrelated mouse strains B6CBA and C57BL/6J. Torpedoes were identified using monoclonal antibodies against the calcium-binding protein calbindin D-28k in mice ranging in age from 8 days postnatally up to 32 months. The relative density of PCs bearing torpedoes in animals up to 6 months of age was less than 0.1%. The number increases between 6-8 months and rises further in older mice almost linearly up to 13.7% affected PCs in the oldest animal (32 months) studied. In contrast, PC loss, as indicated by parvalbumin-immunoreactive empty baskets, is only at a very moderate level (less than 0.5%) in these strains. While the proximal axonic segments often show two and occasionally up to five swellings and frequently appear to be hypertrophied as a whole, the dendritic trees and neuronal somata of the affected PCs exhibit normal morphology. On rare occasions adaptive reactions indicated by "arciform axons" and enlarged varicosities of recurrent collaterals were observed. The results demonstrate that in addition to age-related PC loss of whatever degree, axonal disturbances of PCs, indicated by torpedoes, are present, leading most probably to a graded loss of cerebellar cortico-fungal projections.