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

Although the retinotectal projection of goldfish has long been known to hav e a high degree of retinotopic order, the structural basis for this in term s of the precise positioning of axonal arbors from neighboring retinal gang lion cells has not been determined. In studying this, a small number of nei ghboring retinal ganglion cells was selectively labeled by a microinjection of DiI into the retina. Following axoplasmic transport for several days, t he tectum was removed and flat-mounted for fluorescence microscopy. The inj ection labeled a small number of axons and their terminal arbors which rang ed in size from 108 X 134 mum to 394 X 331 mum with a mean of 233 X 219 mum . 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 ar bor. These results indicate that neighboring retinal ganglion cells do not converge onto the same locus but instead project as a continuous retinotopi c array of partially overlapping terminal fields.