Development of retino-tectal arborizations in the trout.

In order to visualize the organization and the morphology of developing retinal axons in the trout (Oncorhynchus mykiss), HRP or DiI crystals were applied on the central part of the retina at different stages ranging from 21 days postfertilization (stage 27) to one month post-hatching (stage 36). Retinal axons and arborizations were observed on tectal whole mounts with a light microscope. The earliest stage investigated (stage 27) showed two groups of axons entering the tectum by its ventro-rostral part and extending in a dorso-caudal direction. As the tectum grows, these two groups separate to outline the dorsal and the ventral 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 fascicle. At hatching (stage 30), retinal axons start to arborize in the centre of the tectum. During the first month post-hatching, these axons migrate dorso-caudally and exhibit various morphologies. Until two weeks post-hatching (stage 34), they sprout a few long side branches, bearing numerous filopodial growth cones, in a phase of exploratory growth towards their target site. At stage 36, four types of terminal arborizations 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 refinements 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 the widely branched arborizations are transient during development.