Rules for retinotectal terminal arborizations in the goldfish optic tectum: A whole‐mount study

Abstract

Retinal axons were labeled in the retina and optic nerve with horseradish peroxidase and traced in tectal whole-mounts. The typical network formed by retinal fibers in the five retinorecipient layers of tectum is illustrated in camera lucida drawings. Three size classes of terminal arbors were identified in the Stratum fibrosum et griseum superficiale (SFGS)–ca. 34 × 52, ca. 103 × 150, and ca. 158 × 274 μm. Terminal arbors are flattened and occupy three sublayers of SFGS. Passing an HRP-coated needle through the optic nerve labeled ganglion cells in retina and axons and terminal arbors in tectum. Terminal arbors of axons that originated in retinal annuli lay in distinct annular regions in SFGS, with old generations from central retina lying central to younger generations from peripheral retina. The tectal annuli were concentric with one another and agreed with the retinotopic map as it had been described before. The youngest terminal arbors from peripheral retina were next to the path of their fascicle along the tectal periphery, connected to their fascicle by short, centrally directed extrafascicular axons. The oldest terminal arbors from central retina were caudally displaced from their rostral fascicle of entrance, at the end of long, caudally directed extrafascicular axons. Terminal arbors from intermediate retina occupied intermediate positions in the tectum. Rostrally, they arose from centrocaudally directed extrafascicular axons but caudally from axons of various orientations. Terminal arbors arising from those extrafascicular axons exhibited different orientations and shapes depending on their tectal position. The spatial order of intratectal paths and terminal arbor sites, and the variability of terminal arbor orientation and shape, are consistent with an earlier model on shifting retinotectal terminals (Easter and Stuermer, '84).