GUIDANCE OF CEREBELLOFUGAL AXONS IN THE RAT EMBRYO - DIRECTED GROWTH TOWARD THE FLOOR PLATE AND SUBSEQUENT ELONGATION ALONG THE LONGITUDINAL AXIS

To elucidate guidance mechanisms of brain commissural axons, we examin ed the navigation of cerebellofugal axons. Axons were labeled by impla ntation of the fluorescent tracer Dil into the cerebellar plate (CP) o f fixed, flat whole-mount embryonic rat brain. Axons initially grew st raight toward the ventral midline floor plate (FP) in the rostral hind brain and then, after crossing it, made a right-angled turn to grow ei ther caudally or rostrally along the longitudinal axis. In collagen ge l culture, CP axons showed directed growth toward both FP explants and heterologous cells expressing netrin-1, a FP-derived chemoattractant for spinal commissural axons. These results suggest that CP axons are guided to the midline by FP-derived chemoattractant(s) and then reorie nted, possibly by another guidance cue, for longitudinal extension. Co nsidering that the basic structures of the neural tube, including the FP, extend up to the caudal diencephalon, these results suggest that c ommon guidance mechanisms operate for ventrally decussating commissura l axons in both the brain and spinal cord.