Horizontal cell density and mosaic regularity in pigmented and albino mouse retina

The present study has examined the density and mosaic regularity of the population of horizontal cells in the pigmented and albino mouse retina. Retinal wholemounts were immunostained for calbindin, and labeled cells within sampled fields were analyzed to determine horizontal cell soma size and density. The X-Y positional coordinates of each cell were determined, from which the geometrical properties of the mosaic were examined using nearest neighbor and Voronoi domain analyses, and regularity indices were derived from those measures. 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 and analyzed in parallel. Neither retinal area, nor horizontal cell soma size, nor density differed between the pigmented and albino retinas. Mosaic regularity in pigmented and albino retinas did not differ, but each differed significantly from random simulations of identical density. Horizontal cells in the mouse retina exhibit exclusion zones extending beyond the physical size of the soma, but these were identical in size in the pigmented and albino retina. Such exclusion zones are suggested to reflect homotypic interactions between horizontal cells during early development that mediate cellular repulsion and tangential movement. The lack of any discernable effect brought about by the albino mutation, despite numerous developmental abnormalities associated with the retinal neuroepithelium in albino mice, is consistent with other results showing that homotypic interactions are sufficient for the genesis of the global patterning characteristic of mature retinal mosaics.