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 o ptic chiasm into the ipsilateral or contralateral optic tracts results from axon pathfinding. Using time-lapse microscopy, we show that enco unters between axons from opposite eyes at the chiasm induce axon turn ing, but do not always aim retinal axons into the optic tracts. Follow ing removal of one eye before retinal axons have invaded the chiasm, a xons from the remaining eye are still routed into the correct optic tr acts. Ipsilaterally projecting axons make turning decisions without pa using over 10-20 min, whereas contralaterally projecting axons occasio nally pause before crossing the midline. Thus, initial pathfinding at the chiasm does not depend on binocular axon interactions, but on loca l cues that trigger differential growth cone responses.