The distributions of photoreceptors and ganglion cells in the California ground squirrel, Spermophilus beecheyi

Abstract

The topographical distributions of photoreceptors and ganglion cells of the California ground squirrel (Spermophilus beecheyi) were quantified in a light microscopic study. The central retina contains broad, horizontal streaks of high photoreceptor density (40–44,000/mm²) and high ganglion cell density (20–24,000/mm²). The isodensity contours of both cell types are elliptical and oriented along the nasal‐temporal axis. There are roughtly fivefold decreases in both photoreceptor and ganglion cell densities with increasing eccentricity, the lowest densities being found in the superior retina. Large transitions in cell density and retinal thickness occur across the linear optic nerve head. Rod frequency increases with increasing eccentricity, from 5 to 7% in the central retina to 15 to 20% in the periphery. Roughly 10% of the cones possess wide, dark‐staining ellipsoids. These cones are uniformly distributed across the retina which suggests that they may belong to a separate cone class, possibly blue‐sensitive cones. The ganglion cell soma size distribution is unimodal, with the majority of somata being 25–50 μm². Large ganglion cells (somata > 100 μm²) are rare in the central retina, but their frequency increases with increasing eccentricity. No evidence for separate size classes of ganglion cells was found. The gradual decrement of photoreceptor density across the ground squirrel retina suggests that there are only relatively small changes in acuity across much of the animal's visual space compared with species possessing either a narrow visual streak or fovea or area centralis.