Morphology and distribution of neurons in the retina of the American garter snake Thamnophis sirtalis

It is confirmed that cone photoreceptors observed in flatmounts of the American garter snake Thamnophis sirtalis, retina correspond to the retinal mosaic viewed in the living eye (Land and Snyder, Vision Res. 11:105-114, '85). The garter snake has three major morphological types of cones; large single cones, small single cones, and double cones. The brightly reflecting components seen in the living eye are large single cones and principle cones of double cones, whereas irregularly spaced dark regions within this mosaiac mark the positions of small single cones. The "sparkle" of the retinal mosaic originates from the ellipsoid region of the cones where microdroplets of high refractive index are densely packed. Unlike conventional oil droplets, these microdroplets reside adjacent to mitochondrial cristae within the ellipsoid. However, the microdroplets may function collectively as a single large oil droplet to increase the angular sensitivity of the inner segments, thus reducing a potentially large Stiles-Crawford effect predicted for this geometrically small eye. The ganglion cell layer of the garter snake comprises two morphologically distinct populations of presumed neurons; classical neurons and microneurons. Density distribution maps for neurons in the ganglion cell layer and the photoreceptor layer reveal the presence of a putative area centralis and a horizontal visual streak. The topography of large cones parallels that of classical neurons. Small single cones have a more circular distribution, but also peak in density at the area centralis. The convergence of cones to classical neurons is lowest at the area centralis, 2.5:1, and highest, 4:1, at the retinal edge. With its interesting structural features, the garter snake retina provides helpful insight into different strategies in eye design.