Distribution and trajectory of uncrossed axons in the optic nerves of pigmented and albino rats.

Ipsilaterally projecting axons in the optic nerve of the pigmented rat are limited to a roughly retinotopic location within the intraorbital segment of the nerve. However, immediately rostral to the chiasm they are widely dispersed. Here, the way in which this change in distribution arises is analysed by tracing individual fibers retrogradely labelled from the optic tract with horseradish peroxidase (HRP). A comparison is made between albino and pigmented animals. It is demonstrated that the change in this distribution occurs as a consequence of two types of shift in axon trajectory in the intracranial segment. Many axons change their location in the nerve gradually throughout this segment. However, in the proximal half of this region a number of axons also make abrupt changes in their trajectory by travelling at right angles across segments of the mediolateral axis of the nerve. These were seen in both the pigmented and albino animals. Although the albino has an abnormally small ipsilateral retinofugal pathway, the distribution of ipsilateral axons in the optic nerve is very similar to that seen in pigmented animals. Consequently, it is unlikely that position in the prechiasmatic nerve is related to the chiasmatic choice made by axons in this population.