Guidance of retinal fibers in the optic chiasm.

At the optic chiasm of vertebrates, there occurs a sharp divergence in the destination of retinal ganglion cell axons as they are on their way to more central targets. Recent studies in the mouse indicate that the growth cones of each set of fibers diverge close to the midline of the optic chiasm, at the borders of a palisade of immature radial glia. Static and dynamic views of retinal growth cones in this decision region reveal that extensive exploratory behavior and selective retraction of parts of the growing tips of uncrossed fibers, in response to cellular cues at the midline, is a major event in the guidance of these fibers. In vitro experiments further show that presumptive crossed and uncrossed fibers differ in their responses to contact with cells from the optic chiasm. As with other instances of selective guidance of fibers at midline structures, the divergence of crossed and uncrossed retinal fibers therefore involves a selective remodeling of their growing tips and transitory axon-cell contacts during growth at the optic chiasm, presumably due to biochemical heterogeneity among crossed and uncrossed ganglion cell fibers.