Photoreceptor fine structure in the goldeye (Hiodon alosoides) (teleost).

The retinal photoreceptors of the goldeye (Hiodon alosoides) are arranged in large bundles of 40-50 cells optically isolated from other bundles by the retinal epithelial cells. Within each bundle are found both rods and cones in roughly equal numbers. Rod photoreceptors show marked retinomotor responses to project beyond the photoreceptor bundle in light-adaptation and to lie entirely within the bundle in dark-adaptation. In all stages of the light cycle cone outer segments remain at the apex of the photoreceptor bundle. In light-adaptation, rod inner segments display an apical ellipsoid separated from a basal ellipsoid by the greatly elongated myoid. In dark-adaptation the rod inner segment is much the same diameter throughout its length. In both rods and cones, profiles of rough and smooth endoplasmic reticulum and Golgi zones are present in a supranuclear location. The nuclei of rods display little heterochromatin and are located vitreal to the external limiting membrane in light-adaptation, whereas in dark-adaptation more heterochromatin is noted and the nuclei lie scleral to the external limiting membrane. Cone nuclei display the same changes in chromatin pattern as rods but they show changes in nuclear location opposite that of the rods. Throughout its length, the rod photoreceptor cytoplasm is more electron dense than that of the cone. The synaptic spherule of rods displays 2-3 invaginated synaptic sites while the cone pedicle is larger and presents 8-10 invaginated synaptic sites. Both rods and cones also appear to have superficial synaptic sites. Membrane specializations are found along the length of the inner segments where rods and cones are contiguous. These may act as sites of intercellular communication and the whole photoreceptor bundle may therefore be considered as a macroreceptor.