AXON-TARGET INTERACTIONS IN THE DEVELOPING CEREBELLUM

After entering target regions, afferent growth cones grow among putati ve target cells, stop extending upon meeting target cells, and transit into a synaptic ending. During these events, signals are transmitted to and from target cells to stimulate programs of differentiation. Her e we describe three approaches to unraveling mechanisms of these phase s of synaptogenesis. First, dye-labeling in the intact cerebellum has revealed the orchestration of afferent ingrowth and contacts with targ et cells. Second, an in vitro model based on purified granule neurons has shed light on the role of target cells in the arrest of afferent e xtension. Third, coculture of purified granule neurons (parallel fiber afferents) with purified Purkinje cells has demonstrated facets of af ferent regulation of target cell differentiation. These analyses have suggested molecular mechanisms that mediate maturation of afferents an d their targets.