Time course of mossy fiber degeneration following pontine ablation in the rat.

The normal ultrastructure of mossy fiber terminals within the ansiform lobule of the rat was investigated in parasagittal sections of the cerebellar cortex. Four types of mossy fiber varicosities (simple and complex dispersed, simple and complex clustered) were distinguished on the basis of the presence or absence of clublike excrescences and the concentration of synaptic vesicles within the cortical zone of the terminal. Removal of the pontocerebellar input to the contralateral cerebellar hemisphere was performed electrolytically. Such lesions, when placed in the basilar pontine gray using a dorsal approach, resulted in the degeneration of nearly the entire population of mossy fiber varicosities within the contralateral ansiform lobule. As early as 3 days following pontine ablation, degenerative changes were observed within the synaptic portions of the mossy fibers. Two distinct axonal reactions were apparent within this population of degenerating varicosities. A small population of mossy fiber varicosities (12%) of the simple clustered variety underwent a rapid course of electron-dense degeneration, which was complete by the fifth day. These mossy fiber varicosities were very susceptible to phagocytosis by reactive glial elements. The second group which consisted of simple (60%) and complex (26%) varieties of dispersed terminals, underwent a very slow course of electron-dense degeneration. Reactive glial cells were rarely found in association with this second group of degenerating varicosities. As a result, most glomeruli were found to contain debris from degenerating mossy fiber varicosities throughout the first 57 days following pontine lesions. The majority of the cerebellar glomeruli, however, were denervated by 80 days.