NAVIGATIONAL ERRORS MADE BY GROWTH CONES WITHOUT FILOPODIA IN THE EMBRYONIC XENOPUS BRAIN

We have developed an exposed brain preparation for observing growth co ne pathfinding behavior while performing in vivo pharmacological manip ulations, and we used it to test whether Xenopus retinal growth cones need filopodia to navigate. Time-lapse video observation showed that c ytochalasin B acted quickly and reversibly when applied; cytochalasin B-treated growth cones lacked filopodia, but had active lamellipodia a nd continued to advance slowly. Whereas normal retinotectal axons visu alized with horseradish peroxidase turn caudally in the mid-diencephal on to reach the tectum, cytochalasin B-treated axons grew past the nor mal turning point and, instead, continued straight within the dienceph alon. In dose-response experiments, pathfinding became abnormal in the same concentration range in which filopodia disappeared. These result s suggest that filopodia are necessary for retinal growth cones to res pond to guidance signals in the diencephalon.