Retinal photoreceptor fine structure in the mallard duck (Anas platyrhynchos).

The retinal photoreceptors of the mallard duck (Anas platyrhynchos) consist of rods, single cones and double (unequal) cones present in a ratio of about 1:2:1. The rods have relatively large cylindrical outer segments which in the light-adapted state reach to the retinal epithelial (RPE) cell bodies and are surrounded by the pigment-laden apical processes of these RPE cells. The inner segment displays an apically located ellipsoid of mitochondria and plentiful polysomes, some rough ER and Golgi zones. The rod nucleus is located within the outer nuclear layer and the synaptic pedicle displays both invaginated (ribbon) and superficial (conventional) synaptic sites. Single cones display a thin tapering outer segment, a large often heterogeneous oil droplet in the apical region of the inner segment, an ellipsoid of mitochondria and a prominent paraboloid of glycogen. Double cones consist of a larger chief member which shows a thin tapering outer segment, a large heterogeneous oil droplet and a prominent paraboloid. The small accessory cone shows a thin outer segment, a small granular oil droplet and a paraboloid. As in the single cone, polysomes, RER and Golgi zones are also present within the inner segment. Near the outer limiting membrane the two members of the double cone show a membrane specialization on their contiguous surfaces. Both single and double cones display several invaginated (ribbon) synapses as well as numerous superficial synaptic sites.