Responses of retinal axons in vivo and in vitro to position-encoding molecules in the embryonic superior colliculus.

We show that rat retinal ganglion cell axons exhibit no topographic specificity in growth along the rostral-caudal axis of the embryonic superior colliculus (SC). Position-related, morphological differences are not found between temporal and nasal axon growth cones. However, embryonic retinal axons respond in vitro to a position-dependent molecular property of SC membranes. In vivo, regional specificity in side branching is the earliest indication that axons make topographic distinctions along the rostral-caudal SC axis. Our contrasting in vivo and in vitro results indicate that molecules encoding rostral-caudal position in the SC neither guide nor restrict retinal axon growth, but may promote the development of topographic connections by controlling specificity in the extension or stabilization of branches.