Functional Morphology of Beta Cells in the Area Centralis of the Cat's Retina: A Model for the Evolution of Central Retinal Specializations

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Abstract
The dendritic morphology of .beta. cells in and around the area centralis of the retinae of normally pigmented and Siamese cats is described. Individual central .beta. cells in the Siamese cat do not differ morphologically from central .beta. cells in normally pigmented cats, and in both groups of animals, there is a clear morphological continuity between central and peripheral .beta. cells. On the basis of systematic patterns of .beta. cell dendritic orientation, the area centralis of the normal cat can be divided into a central region, .apprx. 200 .mu.m in diameter, and a pericentral region, .apprx. 1400 .mu.m in diameter. In the central region, nearly all .beta. cells have a single large primary dendrite, which descends perpendicular to the plane of the inner plexiform layer, and gives rise to a dendritic tree, which is displaced laterally from the position of the soma. For most of the cells the trajectory of the dendrite is systematically related to the location of the cell relative to the area centralis such that the somas are displaced away from its center, presumably to minimize the thickness of the ganglion cell layer in the high acuity region. Many .beta. cells outside the pericentral region also have oriented single primary dendrites, but their orientation seems fairly random with respect to the location of the area centralis. In the Siamese area centralis, this systematic pattern of .beta. cell dendritic orientation is markedly reduced, suggesting that the pattern is under genetic control. A model for the evolution of the area centralis and fovea is presented which involves selection for systematic patterns of dendritic orientation in regions of high ganglion cell density.