Fiber-fiber interaction and tectal cues influence the development of the chicken retinotectal projection

The development of the retinotectal projection has been studied by a new experimental approach combining antibodies against the nerve cell adhesion molecule (NCAM), and techniques for mapping neuronal pathways using rhodamine B isothiocyanate (RITC) crystals. Anti-NCAM Fab', which specifically inhibits neurite fasciculation, was injected into the eye cup of 4-day-old chicken embryos. After 4-6 days of development, a small RITC crystal was placed on the neural retina to stain selectively axons arising from a localized region. One day later the retina, optic nerve, and tectum were examined and the paths of the fluorescent retinal ganglion cell axons were traced. These studies have led to four observations: (i) The presence of anti-NCAM Fab' causes the axons to form a disordered nerve bundle at the optic fissure. (ii) Disorder produced in the optic nerve persists throughout the optic pathway up to the tectum. (iii) Many of the misrouted fibers growing on or near the tectal surface can at least partially correct their position. (iv) Late axons grow in straight tracks along other fibers and do not correct their position. Together the results suggest that formation of the retinotectal projection involves both reading of positional cues on the tectum by growth cones of early arriving retinal axons and the tracking of growth cones along preexisting fibers that normally belong to neighboring retinal ganglion cells.