The position of the crossed and uncrossed optic axons, and the non-optic axons, in the optic tract of the rat

Abstract

The position of the crossed and uncrossed optic axons, and of the non-optic axons, within the optic tract was determined in the adult hooded rat. Horseradish peroxidase histochemistry and lesion-induced degeneration of axonal profiles were independently used to study the position of the three relevant populations of axons within the optic tract. The boundaries of the optic tract are distinct at all but its caudomedial border, where it abuts the supra-optic commissures running parallel to the fibres of the optic tract. Labelling the crossed population of optic axons, or inducing their degeneration, both demonstrate a clear caudomedial border of the optic tract, although a number of optic axons stray out of the optic tract and course within the supra-optic commissures immediately caudomedial to the tract. The uncrossed optic axons are, as a population, positioned relatively deep in the optic tract, towards its dorsal border. A few occur at further ventral positions, but their density is greatly reduced there. There is also a very thin region along the dorsalmost edge of the optic tract free of uncrossed optic axons. The relative position of the uncrossed to the crossed optic axons is discussed in the context of the mammalian optic tract as a chronological map: spatial position in the tract may reflect temporal order of axonal arrival during early development. A large population of non-optic axons belonging to Gudden's commissure courses within the boundaries of the optic tract at a relatively ventral position. They are most frequent caudomedially, and are absent rostrolaterally. Hence, axons of the optic tract and Gudden's commissure are substantially intermingled in the caudomedial half of the optic tract. These non-optic axons greatly outnumber the uncrossed optic axons, and will consequently distort counts of uncrossed optic axons based on intact profiles that remain after removal of the opposite eye. However, they are still a minority in comparison to the crossed optic axons in this region.