RETINAL AXON DIVERGENCE IN THE OPTIC CHIASM - UNCROSSED AXONS DIVERGEFROM CROSSED AXONS WITHIN A MIDLINE GLIAL SPECIALIZATION

A long-standing question is how fiber pathways in the mammalian CNS pr oject to both sides of the brain. Static and real-time analyses of dye -labeled retinal axons (Godement et al., 1990, 1994) have demonstrated that at embryonic day 15-17 in the mouse, crossed and uncrossed axons from each eye diverge in a zone 100-200 mu m proximal to the midline of the optic chiasm. In this study, we identify cellular specializatio ns in this zone that might serve as cues for retinal axon divergence. Second, using growth cone morphology as an indicator of growth cone de stination, we analyzed how crossed and uncrossed retinal growth cones related to these cellular components. Monoclonal antibody RC2, a marke r for radial glia in embryonic mouse CNS, revealed a palisade of radia l glia straddling the midline, At the midline, a thin raphe of cells t hat appear morphologically distinct from the radial glia express a fre e carbohydrate epitope, stage-specific embryonic antigen 1 (SSEA-1). S ections containing Dil-labeled axons and immunolabeled cells indicated that all axons enter the radial glial palisade. Uncrossed axons turn within the palisade, but never beyond the raphe of SSEA-1-positive cel ls, In addition, ultrastructural analysis indicated that all growth co nes contact radial glia, with projections of the growth cone interdigi tating with glial fibers. These results demonstrate that retinal axons diverge within a cellular specialization centered around the midline of the developing optic chiasm, consistent with the hypothesis that cu es for divergence are located in this zone.