Specific guidance and modulation of growth cone motility during in vivo development

The hemidecussation of retinal fibers that occurs in mammals offers the opportunity to study several aspects of growth cone guidance in a single model system. Recent studies suggest that growth cones of crossed and uncrossed retinal fibers respond in differential manners when they contact cells at the optic chiasm midline, and that such contact interactions are the main event involved in their divergence. Observations of the in situ behaviors of these growth cones disclose that their guidance in this decision region involves two different processes: an orientational response, mediated by the selective guidance of the filopodia of frowth cones away or towards the midline of the optic chiasm, and a dynamic response, in which growth cones go through cycles of advance and pauses while in the optic chiasm. We hypothesize that these two aspects of growth cone motility represent two different aspects of the biology of growth cones in response to extrinsic cues, which are both used in their guidance during development.