Time-lapse video analysis of retinal ganglion cell axon pathfinding at the mammalian optic chiasm: growth cone guidance using intrinsic chiasm cues.

The specific routing of retinal ganglion cell axons at the mammalian optic chiasm into the ipsilateral or contralateral optic tracts results from axon pathfinding. Using time-lapse microscopy, we show that encounters between axons from opposite eyes at the chiasm induce axon turning, but do not always aim retinal axons into the optic tracts. Following removal of one eye before retinal axons have invaded the chiasm, axons from the remaining eye are still routed into the correct optic tracts. Ipsilaterally projecting axons make turning decisions without pausing over 10-20 min, whereas contralaterally projecting axons occasionally pause before crossing the midline. Thus, initial pathfinding at the chiasm does not depend on binocular axon interactions, but on local cues that trigger differential growth cone responses.