Directed outgrowth from a subset of cochlear nucleus fibers in a collagen-gel matrix

We have employed a collagen-gel co-culture system to evaluate the influence of the hindbrain floor-plate on initial axon outgrowth from the cochlear nucleus. After 2 days in vitro, cochlear nucleus explants exhibited directed outgrowth towards co-cultured floor-plate explants. Comparisons with co-cultures of cochlear nucleus/forebrain or cochlear nucleus/midbrain explants or with cochlear nucleus explants cultured alone suggest that the floor-plate has a specific chemoattractant effect on the outgrowth of the pioneer fibers of the hindbrain auditory commissure. Fiber outgrowth was not directed towards a recombinant source of the chemoattractant molecule netrin suggesting that floor-plate-directed outgrowth in the cochlear nucleus is not solely dependent on netrin. In the present report, we present evidence for the first time that the floor-plate is a chemotropic source in pathfinding of second-order auditory fibers from the cochlear nucleus in the hindbrain.