Journal of Neuroscience, Vol 5, 3061-3069, Copyright © 1985 by Society for Neuroscience
Age-related fiber order in the optic tract of the ferret
C Walsh and RW Guillery
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.