Fine structure of the retinal photoreceptors of the emu (Dromaius novaehollandiae)

Abstract

The retinal photoreceptors of the emu (Dromaius novaehollandiae) consist of rods, single cones and double (unequal) cones present in a ratio of about 10:1:4 respectively. The rods are long slender cells which are felt to undergo retinomotor movements. The rod outer segment is a stack of bimembranous discs which display shallow peripheral incisures and are all of the same diameter. The rod inner segment displays an ellipsoid of mitochondria, much rough endoplasmic reticulum (ER), numerous polysomes and a prominent hyperboloid of glycogen. Single cones show a tapered outer segment, no oil droplet but a large ellipsoid with several small lipid droplets and a paraboloid of glycogen. Double cones consist of a chief member which displays a single large heterogeneous oil droplet and no paraboloid, and an accessory cone which has no oil droplet but again several small lipid droplets and a large paraboloid. All cones show a large ellipsoid, plentiful polysomes, rough ER, Golgi zones and autophagic vacuoles. Along the length of the contiguous membranes of the two members of the double cones are scattered membrane densifications. The cones are not believed to undergo retinomotor movements in the emu. Rod photoreceptors are relatively small in diameter and hence are closely packed, while the larger cones are not as abundant. Rods and cones (both types) display both invaginated (ribbon) synapses as well as numerous superficial (conventional) synaptic sites.