Simple and complex retinal ganglion cell axonal rearrangements at the optic chiasm

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Abstract
The rearrangements that retinal ganglion cell (RGC) axons undergo near the optic chiasm were determined by ablating either the nasal, temporal, dorsal, ventral, or peripheral retina. The axons of the remaining intact RGCs were then labelled with cobaltous lysine. RGC axons change their relationship with respect to the axes of the brain and with respect to one another. Toward the caudal end of the optic chiasm, the optic tract begins to rotate axially such that its rostral edge ultimately becomes located medially. Thereby, the column of ventronasal RGC axons shifts from a rostral to a medial position. In addition, columns of axons from other retinal sectors move with respect to one another. Ventrotemporal RGC axons, located initially at the caudal edge of the tract, move toward and come to be positioned laterally to, the column of ventronasal RGC axons. The column of dorsonasal RGC axons moves from the rostral to the lateral side of the column of dorsotemporal RGC axons. Concurrently, the axons within each column reorganize internally. Each chronological lamina of axons within a column twists such that the nasal and temporal axons within each column invert their positions with respect to the edges of the column. All of these reorganizations take place between the caudal end of the optic chiasm and the division of the main optic tract into the optic brachia. Furthermore, the rearrangements that occur do not involve any alterations in the positions of central and peripheral RGC axons with respect to the surface of the diencephalon. The results are discussed with respects to mechanisms that might influence the organization of the visual pathways.