Age-related fiber order in the optic tract of the ferret

In the ferret's optic tract, the position of retinofugal fibers has been related to their relative age by preferentially labeling the oldest fibers. It has been found that fiber order perpendicular to the pial surface represents the sequence of axon arrivals in the optic tract, the fibers furthest from the pia being the oldest. Immature ferrets, less than 34 days post-conception (E34), were given intravitreal injections of lectin-bound horseradish peroxidase or of 3H-amino acids. After survival times of 24 hr or less, the contralateral optic tract was uniformly labeled, whereas with longer survival times unlabeled zones were seen next to the pial border. These unlabeled zones were largest after injections into the youngest animals but were never seen when injections were made after E34, no matter what the survival time. The unlabeled zones were seen in the tracts of animals having unlabeled regions in the periphery of the retina. The unlabeled retinal ganglion cells are interpreted as ganglion cells formed after the injection was made, and unlabeled axons next to the pia in the tract are interpreted as the axons of these youngest cells. Some irregularities in the layered arrangement of the tract are described, and the fiber order in the tract is contrasted with that in the optic nerve behind the eye. It is shown that tract order is partially independent of order in the intraorbital part of the optic nerve. Between the retina and the optic tract, a sorting of fibers occurs not only in terms of the pattern of decussation, but also in terms of fiber age.