The growth and organization of the optic nerve and tract in juvenile and adult goldfish.

The optic nerves, tracts, and tecta of goldfish, 1 to 5 years old, have been studied anatomically using light and electron microscopy, horseradish peroxidase (HRP), and tritiated proline radioautography. The aims were to document an earlier inference that fibers are added to the nerve continually and to describe the growth and organization of the pathway. (1) The numbers of optic fibers were counted in electron micrographs of the nerve. There were about 120,000, 165,000 and 180,000 in 1-, 3-, and 5-year-old fish, respectively. (2) In young fish, there are a few thousand nonmyelinated fibers which exit the retina together and cluster together in the nerve and tract. When the axons of only the newest (peripheral) ganglion cells were cut intraretinally, fibers in and around the bundles of nonmyelinated fibers degenerated. The nonmyelinated fibers are, therefore, the new ones. (3) Fibers from ventral or dorsal hemiretinas were backfilled selectively with HRP introduced into one of the brachia of the optic tract. Behind the optic papilla, where the cross-section of the optic nerve was trapezoidal, the new fibers were found in a strip along the narrow base of the two flanking zones. Closer to the brain, the fibers from the two hemiretinas intermingled before being segregated again at the origin of the brachia. (4) Small groups of ganglion cells were labeled by intraretinal injection of HRP and their fibers were traced in sections of the nerve and tract. The labeled fibers were clustered, but the positions of the fibers in the cross-section of the nerve were defined less precisely than the positions of the somata in the retina. (5) Hemisection of the nerve in the orbit, followed by intraocular injection of tritiated proline, produced radioautographs with an unlabeled annular zone of tectum. Since the retina projects topographically to the tectum, the severed fibers must have originated from an annular region of the retina. We infer that new fibers are added to the nerve continually and that the retinal origins of fibers are correlated with their positions in the cross-section of the nerve. These rules of order change with distance from the retina; the strict order at the optic papilla changes gradually to an equally strict but different, order at the level of the brachia.