Chiasmatic course of temporal retinal axons in the developing ferret

Recent studies on the distribution of optic axons in the mature visual pathways, as well as on the genesis of their ganglion cells of origin, suggest that the time of axonal arrival at the optic chiasm determines the side of the brain to which a temporal retinal axon will project. The present study has examined this issue directly in fetal ferrets, by determining the projection of the temporal retina at different developmental stages. Fetuses of known gestational age were fixed with paraformaldehyde and subsequently implanted with crystals of the carbocyanine dye, DiI, into either the temporal retina, or into one optic tract. The lipophilic diffusion of the dye within the plasma membrane of the axons revealed the course of temporal retinal fibers through the fetal chiasm, as well as the distribution of ganglion cells across the two retinae projecting to one optic tract. During early fetal stages, the temporal retina extends axons preferentially into the ipsilateral optic tract: the early retinal projection shows a classical partial decussation pattern. During later fetal stages, temporal retinal axons can be traced into both optic tracts, and the distribution of cells with crossed and uncrossed optic axons in the temporal retina is overlapping. These results indicate that the mature decussation patterns of retinal ganglion cell classes are not primarily the consequence of regressive phenomena such as cell death; rather, they are formed as axons navigate the chiasmatic region during development. The differences in decussation pattern between cell classes arise from the fact that the mechanisms producing the segregation of nasal and temporal retinal axons at the chiasm must change as development proceeds.