THE FIRST RETINAL AXON GROWTH IN THE MOUSE OPTIC CHIASM - AXON PATTERNING AND THE CELLULAR ENVIRONMENT

The retinofugal pathway is a useful model for axon guidance because fi bers from each eye project to targets on both sides of the brain. Stud ies using static and real time analyses in mice at E15-17 demonstrated that uncrossed axons from ventrotemporal retina diverge from crossed axons in the optic chiasm, where specialized resident cells may direct divergence. Other studies, however, suggest that pioneering uncrossed retinal axons derive from a different retinal region, take a differen t course, and enter the ipsilateral optic tract independent of fiber-f iber interactions. We examine these differences by dye-labeling the ea rliest optic axons and immunocytochemically identifying cells in their path. The first optic axons arising from dorsocentral retina, enter t he diencephalon at E12.5. AII axons initially grow caudally, lateral t o a radial glial palisade. in contrast to later growing axons, early u ncrossed axons enter the ipsilateral optic tract directly. Crossed axo ns enter the glial palisade and course medially, then anteriorly, in a pathway corresponding to the border of an early neuronal population t hat expresses SSEA-1, CD44, and beta-tubulin. Axon patterning occurs i ndependent of fiber-fiber interactions from both eyes, as the first un crossed axons enter the optic tract before crossed ones from the oppos ite eye. These analyses, in conjunction with our previous studies duri ng the principal period of retinal axon growth in the diencephalon, su ggest that the adult visual projection arises from age-dependent varia tions in the types and relative contribution of cues along the path th rough the emerging optic chiasm.