GROWTH AND GUIDANCE OF RETINAL AXONS

The goal of this research is to provide information on the growth and guidance of axons within the central nervous system, using the formation of crossed and uncrossed projections of the retina in the mouse visual system as a model. Recent work in our laboratory has demonstrated the pattern of retinal axon growth through the optic chiasm, the site where retinal axons sort before projecting to targets on the same and opposite sides of the brain. These studies, based on dye-labeling in fixed brain and real time studies of dye-labeled axons in living brain, indicate that whereas contralateral-projecting axons traverse the chiasm midline, fibers with an ipsilateral destination approach the midline, but turn back sharply toward the ipsilateral optic tract. An important feature of fiber divergence within the chiasm appears to be inhibition of uncrossed fiber extension and permissive advance of crossed fibers at the chiasm midline. We have localized a palisade of radial glia restricted to the midline zone, a candidate for such a dual cue. Moreover, studies based on unilateral eye removal suggest that interactions with fibers from the opposite eye are required to fully effect axon divergence of uncrossed fibers. The proposed research will provide information on the mechanism of axon guidance in the optic chiasm. First, the behavior and interactions of retinal axon growth cones with cellular constituents of the midline zone will be defined, using dye labeling in fixed and living preparations, combined with immunocytochemistry and electron microscopy. In vitro assay systems will be developed by which to test the role of cells in the chiasm midline in inhibition or extension across the midline, and whether contact and/or tropic mechanisms are involved. Second, the role of fiber-fiber interactions will be investigated as an auxilliary mechanism in the divergence. Third, this system and analysis will be used to define in a wider sense, how growth cone morphology reflects growth cone behavior and cell-cell interactions. This work should elucidate the signalling mechanisms used for axon navigation in this CNS pathway, a model for the patterning of bilateral projections, and provide a dynamic analysis of growth cone kinetics and interactions.