Photoreceptor fine structure in the bobtail goanna (Tiliqua rugosa).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the bobtail goanna (Tiliqua rugosa) an Australian diurnal lizard. The photoreceptors in this species are readily divisible into rods or cones based on morphological criteria. Single cones are the dominate cell type with a cone:rod ratio of about 80:1. No multiple photoreceptors were present nor was a photoreceptor mosiac observed. Cones are large cells with a short, tapering outer segment. The inner segment contains a large apical oil droplet, an ellipsoid of mitochondria and a prominent paraboloid of glycogen. Rods are slender cells with a longer non-tapering outer segment. Rod inner segments display an ellipsoid but no oil droplet or paraboloid. The nuclei of cones are large and vesicular while rod nuclei are smaller and displayed more heterochromatin. The synaptic pedicle of cones is larger than the spherule of rods. Both rods and cones displayed both invaginated (ribbon) and conventional synapses with the large cones having more synaptic sites. As only light-adapted specimens were examined it is uncertain if either rods or cones are capable of retinomotor responses.