Map of retinal position onto the cross section of the optic pathway of goldfish

Abstract

The position of a retinal cell is defined by the two polar coordinates: r, the distance from the optic disc, and θ the angular (or clock-face) position. Axons of similar θ value were labeled by the punctate application of horseradish peroxidase (HRP) to optic axons in the retina, and axons of similar r-value were labeled by the application of this same marker to a tectal fascicle. Labeled axons were traced in serial transverse sections of the optic pathway from the retina to the tectum to learn the map of the retinal surface onto the cross section of the pathway. Retinas were flat-mounted and treated for HRP to show the retinal origins of the labeled axons. Axons of similar r were clustered together, and the fraction of the pathway's cross-sectional area occupied by the cluster was about the same as the fraction of the retinal area occupied by the group of labeled somata. Axons of similar θ were also clustered, but the fraction of the cross-sectional area they occupied was larger than the fraction of retinal area occupied by their somata. The geometry of the clusters of labeled axons depended on the proximodistal location in the pathway. Near the retina both were stripshaped, but the location and orientation of the strip varied. Both an r-strip and a θ-strip were labeled in some pathways by dual applications of HRP; the two strips were mutually orthogonal at all levels. Each of r and θ mapped onto a separate axis. The axons from most peripheral retina (largest r) were everywhere adjacent to the pia, and axons of progressively more central retina (smaller r) were progressively more separated from the pia (except in the nerve, where the secondary fasciculation complicates the geometry by wrapping old axons in new pia). The map of the circular variable, θ onto a line, required a discontinuity, the location of which differed, depending on the proximodistal level. From the retina to the chiasm, the discontinuity was at the ventral retinal radius (i.e., the right retinal clock-face positions were ordered 6-9-12-3-6 o'clock across the line); just central to the chiasm, the fibers reordered to put the discontinuity at the nasal radius (clock-face positions ordered 3-6-9-12-3); at the brachial bifurcation, the 3-6-9 half turned dorsally, the 9-12-3 half, ventrally. The results are discussed in the context of the growth of the pathway and compared with similar organizational plans in the optic pathways of other vertebrates.