Morphology and Topographical Organization of the Retina of Juvenile Loggerhead Sea Turtles (Caretta caretta)
The vertical organization and spatial variation of the main features of the retina of juvenile loggerhead sea turtles (Caretta caretta) were studied. Eyes were sectioned into 5-μm slices and examined using light microscopy techniques. Within each section, photoreceptor cells and ganglion cells were identified and counted. The topographical organization of cells along the retina was examined for the presence of an area centralis. The results show that the basic organization of the loggerhead retina closely follows the general vertebrate model. The retina is duplex in nature, containing both cone and rod cells throughout the photoreceptor layer. Moreover, the organization of the neural layers indicates that this eye is adapted for both high spatial resolution and low light sensitivity. A higher concentration of both cone photoreceptor cells and ganglion cells occurs dorsal to the equator of the eye, and their numbers are positively correlated. These findings suggest that loggerhead sea turtles possess a horizontal streak in the dorsal region of the retina, an attribute that could be beneficial for the loggerhead's benthic lifestyle.Abstract
Light micrograph of the retina of a juvenile loggerhead sea turtle (Caretta caretta). Abbreviations: G = ganglion layer, IN = inner nuclear layer, IP = inner plexiform layer, ON = outer nuclear layer, OP = outer plexiform layer, PE = pigment epithelium, PR = photoreceptor layer. Scale bar equals 10 μm
Mean cell counts for the eight latitudes of the eye in both the ventral and dorsal hemispheres. All error bars denote + 1 SD. (A) Cone photoreceptor cells, (B) ganglion cells, and (C) rod photoreceptor cells
Three-dimensional spherical plot of cell counts for cone photoreceptors, ganglion cells, and rod photoreceptors. Each of the eight wedges and eight latitudes are plotted to form 64 polygons. Each polygon represents the mean cell count for all eyes and is plotted (as a gradient color of the spectrum) as a percentage of the highest cell count (Red = 0%, Violet = 100%). Orientation of each of the spheres is from the posterior of the eye; latitudes for all spheres are represented on the rod photoreceptor plot. Latitude 6, latitude 7, and cornea are not illustrated on this two dimensional view. Abbreviations: D = dorsal, N = nasal, T = temporal, V = ventral