DEVELOPMENT OF RETINOTECTAL ARBORIZATIONS IN THE TROUT

In order to visualize the organization adn the morphology of developin g retinal axons in the trout (Oncorhynchus mykiss). HRP or Dil crystal s were applied on the central part of the retina at different stages r anging from 21 days postfertilization (stage 27) to one month post-hat ching (stage 36). Retinal axons and arborizations were observed on tec tal whole mounts with a light microscope. The earliest stage investiga ted (stage 27) showed two groups of axons entering the tectum by its v entro-rostral part and extending in a dorso-caudal direction. As the t ectum grows, these two groups separate to outline the dorsal and the v entral borders of the tectum. At three weeks post-hatching (stage 35) we observed three distinct brachia: the dorsal and ventral fascicles, and a small group in the middle that we called the intermediate fascic le. At hatching (stage 30), retinal axons start to arborize in the cen tre of the tectum. During the first months post-hatching, these axons migrate dorso-caudally and exhibit various morphologies. Until two wee ks post-hatching (stage 34), they sprout a few long side branches, bea ring numerous filopodial growth cones, in a phase of exploratory growt h toward their target site. At stage 36, four types of terminal arbori zations can be identified on the basis of their tangential and radial location in the tectum, and on their gross morphology. Three of these arbor types are already present at earlier stages and undergo refineme nts in their shape-reduction in their branching axes, loss of branches that are behind the terminal arborization, and the sprouting of more numerous branches at their extremities. These findings confirm that th e widely branched arborizations are transient during development.