Journal of Neuroscience, Vol 1, 793-811, Copyright © 1981 by Society for Neuroscience
The growth and organization of the optic nerve and tract in juvenile and adult goldfish
SS Easter Jr, AC Rusoff and PE Kish
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.
