During development of the chick central nervous system, the trajectories of the descending medial and lateral longitudinal fascicles (MLF and LLF) are pioneered by axons originating from the interstitial nucleus of Cajal (INC) and the mesencephalic trigeminal nucleus (MTN), respectively. Both tracts cross rhombomere 1 at two specific locations in the basal plate. In this study, we have investigated the molecular properties of these crossing points and find that they are permissive regions situated in an otherwise inhibitory boundary region. We show that the dorsal part of rhombomere 1 is inhibitory for the growth of both MTN and INC axons. Ventrally, MLF and LLF axons are repelled from the midline by Slit proteins. Our results reveal the existence of a new repulsive/inhibitory mechanism for axons in the alar plate in addition to the ventral repulsion by Slit proteins. This suggests a model where MLF and LLF axons are channeled longitudinally within the neural tube by both dorsal and ventral constraints
