Light and electron microscopy of the photoreceptors in the retina of the red-eared slider, Pseudemys scripta elegans.

The six different types of photoreceptor in the retina of the turtle Pseudemys scripta elegans have been studied by light and electron microscopy. Fresh whole-mount retina, in which the oil droplet colors of the photoreceptors could be seen, was compared with fixed tissue cut in the vertical plane. The red single cone (large red oil droplet) is largest in diameter and, in electron microscopy (EM) views, has the least opaque oil droplet. It also has a vacuolated paraboloid and a cone pedicle that ends at the intermediate level of the outer plexiform layer (OPL). The green single cone (yellow medium-size oil droplet) has a thinner inner segment and uniformly vacuolated paraboloid. Its cone pedicle ends at the same level as that of red cones. The blue cone (small colorless oil droplet) has an obliquely running axon joining cell body to cone pedicle that lies innermost in the OPL. Double cones are recognizable by their adherent cell bodies and inner segments. The accessory member (green cone) lacks an oil droplet but has a distinct large paraboloid while the principal member (red cone) has an oil droplet (orange) lying more distal than oil droplets of other cone types. The axons of the double cones are separated by glia but have closely apposed pedicles ending above those of the single cones in the outermost level of the OPL. Rod photoreceptors have a large outer segment, no oil droplet, and a low-lying cell body continuous with the pedicle, which ends, like those of the double cones, at the outermost level of the OPL.