Crossing the floor plate triggers sharp turning of commissural axons

During development of the vertebrate CNS, commissural axons initially grow circumferentially toward the ventral midline floor plate. After crossing th e floor plate, they abruptly change their trajectory from the circumferenti al to the longitudinal axis. Although recent studies have unraveled the mec hanisms that control navigation of these axons along the circumferential ax is those that result in the transition from circumferential to longitudinal trajectory remain unknown. Here, we examined whether an interaction with t he floor plate is a prerequisite for the initiation of trajectory transitio n of commissural axons, using in vitro preparations of the rat metencephalo n. We found that commissural axons in the metencephalon, once having crosse d the floor plate, turned sharply to grow longitudinally. In contrast, axon s extending in floor plate-deleted preparations, continued to grow circumfe rentially, ignoring the hypothetical turning point. These results suggest t hat a prior interaction of commissural axons with floor plate cells is a ke y step for these axons to activate a navigation program required for their change in axonal trajectory from the circumferential to the longitudinal ax is. (C) 2001 Academic Press.