Retinal Photoreceptor Fine Structure in the Great Blue Heron (Ardea herodias)

Abstract

The morphology of the retinal photoreceptors of the great blue heron (Ardea herodias) has been investigated by light and electron microscopy. They consist of rods, single cones and double (unequal) cones present in a ratio of about 2:1:1 respectively. The rods are slender elongated cells with outer segments that reach to the retinal epithelial (RPE) cells and are surrounded by pigment-rich apical processes of the RPE cells in the light-adapted state. The rod inner segment displays an ellipsoid of mitochondria, an hyperboloid of glycogen, much rough ER, numerous polysomes, Golgi zones and autophagic vacuoles. The rod nucleus is located deep in the outer nuclear layer and the rod synaptic pedicle displays both invaginated and superficial synaptic sites. Single cones display a slightly tapered outer segment, a large electron lucent oil droplet and an ellipsoid of mitochondria in the apex of the inner segment. Double cones consist of a long thin chief member which shows an electron dense oil droplet and a shorter, stouter accessory cone with no oil droplet but a paraboloid of glycogen below the ellipsoid. As in the single cone, polysomes, RER and Golgi zones are present in the myoid region of both members of the double cone. All photoreceptor types have a connecting cilium joining inner and outer segments. Near the external limiting membrane, the chief and accessory cones show intercellular junctions. All cone photoreceptors are relatively small in diameter and hence tightly packed. While rods are felt to undergo retinomotor movements, cones are felt to move minimally or not at all. Both single and double cones display several invaginated (ribbon) synapses as well as numerous superficial (conventional) synaptic sites.