Local positional cues in the neuroepithelium guide retinal axons in embryonic Xenopus brain

Growing retinal axons home to their distant target, the tectum, even when they are displaced from their normal pathway. This argues for long-range guidance mechanisms in the embryonic brain. Growth cones may orientate to diffusible attractants released from the target, as proposed in other systems, or they may use a stable distribution of positional information in the neuroepithelium. To distinguish between these possibilities, small pieces of the presumptive optic tract, through which retinal axons will normally grow, were rotated by approximately 90 degrees either clockwise or counterclockwise. When the retinal axons later encountered the rotated neuroepithelium, they also turned clockwise or counterclockwise, in correspondence with the direction of rotation. This demonstrates that long-range navigation of retinal axons in the vertebrate brain is based partly on stable, local positional factors, rather than on remote diffusible factors.