Regenerating optic fibers correct large-scale errors by random growth: Evidence from in vivo imaging

Regenerating optic fibers in goldfish make large-scale errors when they inv ade tectum. and subsequently correct these to generate a projection with mo derate retinotopic order by I month. The behavior of fibers underlying thes e extensive rearrangements is not well understood. To clarify this, we have imaged optic fibers in living adult goldfish at 2-4 weeks of regeneration. A small number of neighboring retinal ganglion cells were labeled with mic roinjections of DiI and imaged in the dorsal tectum with a cooled CCD camer a on a fluorescence microscope for 5 to 8 hours. Nearly all fibers were sim ple unbranched processes and had endings that were highly dynamic showing b oth growth and retraction. Fibers from dorsal retina that normally innervat e ventral tectum were frequently observed in dorsal tectum. These ectopic f ibers oscillated more frequently between growth and retraction and retracte d more often than ventral optic fibers. Like retinotopic fibers, ectopic fi bers exhibited net growth but they showed no apparent directional preferenc e toward their retinotopic position. In contrast, large errors along the an terior-posterior axis corresponding to nasal-temporal retina were rare and there was no differential behavior that distinguished these fibers. (C) 200 1 Wiley-Liss, Inc.