Differentiation of the cerebellar granule cell: expression of a synaptic vesicle protein and the microtubule-associated protein MAP1A.

The expression of the microtubule-associated protein MAP1A and the synaptic vesicle protein p65 by rat cerebellar granule cells developing in vivo and in vitro was examined. MAP1A in common with a series of previously described cytoskeletal proteins was present in immature parallel fibres and began to disappear from parallel fibres around P15. From the earliest age (P6) at which the molecular layer could be identified parallel fibres throughout the molecular layer expressed p65 with no evidence of any gradient of expression from young to mature axons. Granule cells in culture did not express p65 until later than 1 day in vitro. By 7 days in vitro numerous p65 containing varicosities were present on granule cell processes though MAP1A was still expressed. The results show that transport of synaptic vesicles and formation of varicosities is an early event in granule cell differentiation and precedes a major reorganisation of the axonal cytoskeleton.