SPECIFIC GUIDANCE AND MODULATION OF GROWTH CONE MOTILITY DURING IN-VIVO DEVELOPMENT

The hemidecussation of retinal fibers that occurs in mammals offers th e opportunity to study several aspects of growth cone guidance in a si ngle model system. Recent studies suggest that growth cones of crossed and uncrossed retinal fibers respond in differential manners when the y contact cells at the optic chiasm midline, and that such contact int eractions are the main event involved in their divergence. Observation s of the in situ behaviors of these growth cones disclose that their g uidance in this decision region involves two different processes: an o rientational response, mediated by the selective guidance of the filop odia of frowth cones away or towards the midline of the optic chiasm, and a dynamic response, in which growth cones go through cycles of adv ance and pauses while in the optic chiasm. We hypothesize that these t wo aspects of growth cone motility represent two different aspects of the biology of growth cones in response to extrinsic cues, which are b oth used in their guidance during development.