Fine retinotopic organization of optic terminal arbors in the tectum of normal goldfish

Abstract

Although the retinotectal projection of goldfish has long been known to have a high degree of retinotopic order, the structural basis for this in terms of the precise positioning of axonal arbors from neighboring retinal ganglion cells has not been determined. In studying this, a small number of neighboring retinal ganglion cells was selectively labeled by a microinjection of DiI into the retina. Following axoplasmic transport for several days, the tectum was removed and flat-mounted for fluorescence microscopy. The injection labeled a small number of axons and their terminal arbors which ranged in size from 108 × 134 μm to 394 × 331 μm with a mean of 233 × 219 μm. This mean size corresponds to about 1/15 of the length of one tectal axis. Although individual arbors labeled from one small retinal injection were always observed near the same retinotopic position, they were almost never coextensive. Overlap between pairs of arbors along the lines of projection perpendicular to the tectal surface averaged 57% of the area of a single arbor. These results indicate that neighboring retinal ganglion cells do not converge onto the same locus but instead project as a continuous retinotopic array of partially overlapping terminal fields.