Paths of axons in the visual system of perciform fish and implications of these paths for rules governing axonal growth.

The optic nerve of many perciform fish is ribbon-shaped, and axons from ganglion cells in specific parts of the retina are consistently found in specific places in this ribbon. I utilized this organization to fill selected groups of axons with horseradish peroxidase. I then traced these groups of axons through the nerve and across the tectum to their terminal arbors. The paths of the axons suggest that axons use a number of different mechanisms to guide them to their correct terminal sites. At some points they appear simply to grow along the surface created by earlier axons, but at other points they seem to be using cues more complex than simple mechanical guidance. In addition, I have demonstrated that for every anulus of ganglion cells on the retina there is an anulus of terminal arbors on the tectum. With time the terminals in a given anulus must move caudally to keep the retinotopic map centered on the tectum while the tectum continues growing nonsymmetrically . I have shown both that the anuli of terminals do remain roughly centered on the tectum and that the predicted pattern of terminal movement is visible on the tecta of perciform fish.