EARLY DEVELOPMENT OF THE RETINAL LINE OF DECUSSATION IN NORMAL AND ALBINO FERRETS

This work describes the retinal origin of the crossed and uncrossed projections in newborn, 9-day-old and adult normally pigmented and albino ferrets. Horseradish peroxidase (HRP) was injected unilaterally into the thalamic and midbrain visual centers of ferrets to label retinal ganglion cells retrogradely. In normally pigmented adults, the retinal line of decussation was sharp and passed through the area centralis. Ganglion cells with uncrossed axons occupied the entire temporal retina. In albino adults, ganglion cells with uncrossed axons were distributed in the periphery of the temporal retina away from the area centralis. In the normally pigmented adults, about 11% of the retinal ganglion cells had uncrossed axons compared to about 4% in the albinos. At birth, normally pigmented ferrets had a sharp line of decussation with most (about 98%) uncrossed ganglion cells found in the temporal retina. In the newborn albinos, most uncrossed ganglion cells were in the temporal retina (about 89%), but there were many fewer than in the normal neonates and, as in the albino adults, the uncrossed ganglion cells were distributed along the temporalmost margin of the retina. In the normal neonates, about 11% of the ganglion cells had uncrossed axons, compared to about 3% in the albino neonates. The area centralis and visual streak were not evident until 9 days after birth. From these results we conclude that the retinal line of decussation is essentially mature by birth in the ferret, and the degree of the albino's abnormality is as extreme in neonates as in adults. The retinal decussation is virtually mature at a stage of development when the crossed and uncrosed retinal afferent axons are completely intermingled in their target nuclei and prior to the onset of significant retinal ganglion cell loss.