Abstract
We have found that neurites from embryonic chick retinal and sympathetic explants do not mix in culture, while retinal neurites mix with retinal neurites, and sympathetic neurites mix with sympathetic neurites. These results confirm those obtained with embryonic rat tissues by Bray et al. (1980). We have also used video time-lapse techniques to examine the behavior of individual retinal and sympathetic growth cones as they attempt to cross retinal and sympathetic neurites. We have found that retinal growth cones cross retinal neurites without delay, retinal growth cones usually retract from sympathetic neurites but often cross them on a second advance, sympathetic growth cones repeatedly retract from retinal neurites, rarely managing to cross them at all, and sympathetic growth cones cross sympathetic neurites but are sometimes delayed. Our results indicate that retinal and sympathetic growth cones can distinguish between specific labels associated with retinal and sympathetic neurites. They also suggest that active avoidance could play an important role in growth cone navigation.