Developing retinotectal projection in larval goldfish.

The retinotectal projection in larval goldfish was studied with the aid of anterograde filling of optic fibers with HRP applied to the retina. The results show that optic fibers have already reached the tectum and begun to form terminal arbors in newly hatched fish. The projection is topographic in that fibers from local regions of the retina project to discrete patches of tectum, with the smallest patch covering 3.5% of the total surface area of tectal neuropil. Many fibers in young larvae have numerous short side branches along their length and only some of them show evidence of terminal sprouting. The arbors are approximately elliptical in shape and average about 1,500 microns 2. Growth cones are seen frequently. In older larvae, terminal arbors are larger and more highly branched, and they have begun to resemble those in adult fish. Fibers terminate in two strata; those in the upper layer are smaller (1,800 microns 2 on average) than those in the deeper stratum (4,000 microns 2 on average). The fraction of tectal surface area covered by individual arbors (the "tectal coverage") ranges from 1.5% to 3% of the total surface area of the tectal neuropil. In contrast, the tectal coverage of individual arbors in young adult goldfish is much smaller, ranging from 0.02% to 0.42% of tectal surface area (Stuermer, '84, and unpublished). This apparent increase in precision of the map in older animals is not due to retraction of arbors, which are slightly larger in adults, but is accounted for by overall tectal growth: the tectal neuropil in goldfish increases in area by about 250-fold during this period (Raymond, '86).