A long-standing question is how fiber pathways in the mammalian CNS diverge to both sides of the brain. Studies from our laboratory with single-fiber dye labeling have demonstrated that retinal axons from each eye distribute to both sides of the brain by diverging near the midline of the optic chiasm, a classic example of a region where fibers from two bilateral sources meet and redistribute. Immunocytochemistry with antisera to immature glia reveals a palisade of radial glia on either side of the chiasm midline. The palisade spans the region that appears to be permissive for the growth of crossed fibers, but inhibitory to the advance of uncrossed retinal fibers. The cellular structure of the chiasm midline during the period of axon outgrowth and its antigenic profile is remarkably similar to the floor plate of the spinal cord, strengthening the notion that the chiasm midline plays a role in axon guidance in the forebrain where the floor plate is absent. This proposal seeks to identify further the cellular components of the chiasm midline and their antigenic profiles, to describe the development of the chiasm and the underlying mesoderm, and to compare these features with those of the spinal cord floor plate. Finally, genes specific for the floor plate, discussed in Projects 1 and 2, will be localized in the developing chiasm. By studying the composition and development of the optic chiasm midline and the spinal cord floor plate in parallel, we will gain insight on cell patterning and the establishment of neural pathways, especially those that cross the neuraxis.