CHEMOSUPPRESSION OF RETINAL AXON GROWTH BY THE MOUSE OPTIC CHIASM

To determine whether diffusible guidance cues direct retinal axon grow th and divergence at the optic chiasm, we cocultured mouse retinal and chiasm explants in collagen gels. The chiasm reduced retinal neurite lengths and numbers, but did not affect commissural or pontine neurite growth. This reduction in growth was equal for all retinal quadrants and occurred without reorienting the direction of neurite extension. T he floor plate, another midline guidance locus, also suppressed retina l neurite outgrowth, whereas cortex or cerebellum explants did not. Gr owth suppression was not mediated by netrin-1, which instead enhanced retinal neurite extension. We propose that chemosuppression may be a g eneral guidance mechanism that acts in intermediate targets to prime g rowth cones to perceive other, more specific cues.