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Mosaic Regularity of Horizontal Cells in the Mouse Retina Is Independent of Cone Photoreceptor Innervation

From the Neuroscience Research Institute and Department of Psychology, University of California at Santa Barbara, Santa Barbara, California.

PURPOSE. To determine whether the density and the mosaic regularity of the population of horizontal cells is dependent on innervation from the cone photoreceptors by comparing these features in wild-type and transgenic mice expressing an attenuated diphtheria toxin in the cones.

METHODS. Retinal wholemounts from coneless transgenic mice and their wild-type littermates were immunostained for calbindin or for cone opsins, and labeled cells and outer segments were counted to determine horizontal cell and photoreceptor density. The x-y positional coordinates of each horizontal cell were also determined, from which the geometrical properties of the horizontal cell mosaic were examined using nearest-neighbor and Voronoi domain analyses. Autocorrelation and density-recovery profile analyses were also conducted to identify the presence of exclusion zones within the population of horizontal cells. For each sampled field, random simulations of matched density, constrained by the physical size of the horizontal cells, were generated in parallel and analyzed with the real data.

RESULTS. Coneless mice were confirmed to contain only 3% of the normal cone photoreceptor population. Despite the loss of these afferents, horizontal cell density did not differ between the wild-type and coneless retinas. Mosaic regularity in wild-type and coneless retinas did not differ, but each differed significantly from random simulations of identical density. Horizontal cells in both the wild-type and coneless mouse retina exhibited exclusion zones extending beyond the physical size of the soma, suggested to reflect intercellular interactions during early development that drive tangential dispersion; these were slightly larger in the wild-type retina.

CONCLUSIONS. Cone innervation is not a necessary condition for horizontal cell survival during postnatal development. The resiliency of the regularity in the horizontal cell mosaic is consistent with the hypothesis that such global patterning is an emergent property of these cells as they engage in local interactions that are largely independent of their afferent innervation.

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