Characteristics of Nasal and Temporal Retina in Siamese and Normally Pigmented Cats; pp. 91–102

Abstract

Properties of ganglion cells in nasal and temporal retina of Siamese and normally-pigmented cats are described. In agreement with earlier reports on normal cats, the somas of both X and Y cells are larger in temporal than in nasal retina, although these differences are not entirely independent of ganglion cell density. Furthermore, the axons of X cells in temporal retina were distinctly faster and larger than those of X cells in nasal retina. Since, in normal cats, almost all nasal X cells project contralaterally and almost all temporal X cells project ipsilaterally, it is difficult to assess independently the effects of retinal location and laterality of projection on either soma size or axon size of nasal and temporal X cells. However, taking advantage of the abnormal pattern of decussation seen in Siamese cats, in which a substantial number of temporal X cells project contralaterally, it is possible to demonstrate that the difference in soma size is independent of laterality of projection and appears to be a function of retinal location, whereas the difference in axon size is a function of laterality of projection and is independent of retinal location. Thus, soma size and axon diameter are at least partially independent. We were unable to obtain clear support for the suggestion that Siamese cats suffer a selective loss of X-type retinal ganglion cells, and an alternative mechanism for the reduced prominence of the area centralis in Siamese cats is proposed. The distinction between developmental mechanisms influencing the prominence of the area centralis and those influencing laterality of projection, as well as the relationship between retinal specializations associated with acuity and specializations associated with binocularity are also considered. The trajectory of axons arising from temporal retina and travelling above or below the area centralis towards the optic disc is described. In normal cats, the general features of this trajectory are the same for ipsilaterally and contralaterally projecting axons, and for axons of all ganglion cell groups. This also appears to be the case in Siamese cats. However, axons from temporal retina show a greater tendency to pass directly over the area centralis in Siamese than in normal cats, and the boundary between the region of temporal retina from which axons pass superior to the area centralis and the region from which they pass inferior to the area centralis may be closer to the vertical meridian in Siamese than in normal cats. These differences in trajectory could lead to a rearrangement of ipsilaterally projecting axons within the optic nerve. In both Siamese and normal cats, a small number of ganglion cells, scattered throughout nasal retina, project ipsilaterally. On the basis of soma size and dendritic morphology, all cells observed in our material can be identified as either alpha or gamma cells. In Siamese cats, such cells are consistently more numerous than in normal cats. In Siamese cats, however, this group of ipsilaterally projecting cells contains a higher proportion of small gamma cells than in normal cats, and we did not observe any ipsilaterally projecting alpha cells in nasal retina. This suggests that the anomalous pattern of decussation in Siamese cats may be qualitatively different for small axons than for larger axons, although both anomalies could still be the result of a single mechanism.