Retinal decussation patterns in pigmented and albino ferrets

The decussation patterns of retinal ganglion cells in adult pigmented and albino ferrets were determined from the distribution of cells labelled after large unilateral injections of horseradish peroxidase into the visual pathway, involving the lateral geniculate nucleus and fibres of passage to the superior colliculus. About 6000 retinal ganglion cells project ipsilaterally in pigmented ferrets compared with only about 1500 in albino ferrets. In both strains, the vast majority of these cells (99 and 87% in pigmented and albino animals, respectively) are located in the temporal crescent, although we describe one albino ferret in which an aberrant uncrossed projection arises from nasal retina. In pigmented ferrets, there is a sharp nasotemporal division that runs through the area centralis; a small proportion of the ganglion cells in temporal crescent (less than 10%) does project contralaterally. In albinos, however, the majority of cells in temporal retina project contralaterally. There is no clear nasotemporal division in the albino retina; the density of uncrossed ganglion cells is reduced throughout temporal crescent and at no location exceeds the comparable density of the crossed projection. The peak density within the reduced uncrossed projection is also displaced away from the area centralis into temporal retina. Analysis of cell type on the basis of soma size indicates that whereas large horseradish peroxidase injections into the visual pathway of pigmented ferrets label all types of ganglion cell in the crossed projection, injections restricted to the superior colliculus label only those ganglion cells with large or small somata. The distribution of cell sizes in the crossed projection from temporal retina is biased towards small cells in the pigmented ferret but in albinos resembles that seen in the crossed projection from nasal retina. Thus the adult pigmented ferret has both a well developed nasotemporal division in which decussation lines are obvious in the crossed and uncrossed pathways and also, unlike rodents but like cats, a class of ganglion cell that does not project to the superior colliculus. The albino mutation both reduces the uncrossed projection throughout temporal retina, although the reduction is greatest close to the area centralis, and also commensurately increases the crossed projection from temporal retina