Astrocytes release a trypsin-sensitive factor that induces and increases neuronal migration.A, ACM increases the percentage of SVZ explants with migrating cells in serum-free conditions after 2 DIV (p < 0.0001; 11 independent experiments).B, Medium conditioned by astrocytes (ACM) but not by cerebellar granule cells (GCM), fibroblasts (FCM), or myotubes (MCM) increases the distance of migration from SVZ explants after 1 DIV (p< 0.012 for comparison of ACM with all other treatments; averages from 3 independent experiments). C, The migration-inducing activity present in ACM is abolished by trypsinization (p < 0.004). When STI was added before trypsin, the increase in neuronal migration from SVZ explants was equal to that in untreated ACM (p = 0.55; averages from 3 independent experiments). The error bars represent SEM.