Ultrastructural study of retinal development in the turtle Trachemys scripta elegans

Abstract

The present study was conducted by using light and transmission electron microscopy to examine the morphologic development of turtle retina from embryonic stage 18 (S18) to S26 (hatching). Particular attention was paid to the formation of functional structures such as neurites, synapses, photoreceptors, among others, and the moment that chemical synapses appear in the outer and inner plexiform layers. The results show that retinal differentiation in the turtle follows the vitreal to scleral morphological differentiation of retinal cells. Moreover, the central region of the optic cup is most advanced compared to the peripheral parts. Early functional plexiform layers, based on appearance of synapses, precede the complete differentiation of photoreceptors. The first synaptic structures occur in the inner plexiform layer before the outer plexiform layer. Receptor outer segments and first synaptic ribbon in receptor synaptic terminals initiate the differentiation at the same time, but final maturation includes dendritic invaginations of bipolar and horizontal cells in the receptor terminals. We assume that at birth, the turtle retina has achieved the ability to see.