Topography of the goldfish optic tracts: Implications for the chronological clustering model

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Abstract
Both the dorsal and ventral optic tracts of goldfish have similar shapes when they are sectioned perpendicularly to their longitudinal axes. Each tract is pear-shaped, consisting of a narrow and a deep apex, a wide mid-span, and a progressively curved and tapered superficial base. The tracts differ in that the apex of the dorsal optic tract points caudally, while the apex of the ventral optic tract points medially. In addition, upon segregating from the main optic tract, the dorsal optic tract courses dorsally while the ventral optic tract courses caudally. Thus, the two optic tracts have similar shapes and are orthogonal to one another. The topography of the retinal fibers within the optic tracts was determined either by ablating part of the retina and subsequently filling the axons from the intact hemiretina with cobaltous-lysine or by applying cobaltous-lysine to a slit in the retina. Both optic tracts contain a similar arrangement of optic fibers. Axons of central retinal ganglion cells (RGCs) are in the apex of each tract and optic fibers of peripheral RGCs are located along the base of each tract. Axons of temporal RGCs are located dorsally in the ventral optic tract and laterally in the dorsal optic tract, while axons of nasal RGCs are located ventrally in the ventral optic tract and medially in the dorsal optic tract. These findings indicate that the optic axons are organised as laminae. Deeper laminae contain the axons of older annuli of RGCs and superficial laminae contain the axons of younger annuli of RGCs. This type of chronological organization appears to be consistent across vertebrates. Furthermore, the organization of the optic axons across laminae is constant. Temporal RGC axons are at one end, and nasal RGC axons are at the other end of each lamina. These results are consistent with those found in other nonmammalian vertebrates. Moreover, these findings preclude the possibility that the axons of a half-annulus of RGCs in goldfish travel together as a fascicle in the optic tracts that is continuous with a fascicle in the stratum opticum of the tectum. The general position of the fibers from different retinal quadrants in the optic tracts may lead to a probabilistic determination as to which part of the tectum they innervate. Namely, temporal RGC axons, destined for the rostral tectum, are situated in the optic tracts so that they are close to the rostral tectum. Nasal RGC axons, destined for caudal tectum, are situated in the optic tracts so thay they are farthest from the rostral tectum. Thus, axons of nasal, RGCs have a lower probability of innervating the rostral tectum.