The Photoreceptor Outer Segment as a Sensory Cilium

Vertebrate rod and cone photoreceptors are members of the class of ciliated photoreceptors that have been identified in multiple phyla throughout the animal kingdom. In these cells, the phototransduction machinery is closely associated with and derived from the plasma membrane of a sensory cilium. Consequently, rods and cones have much in common with invertebrate ciliated photoreceptors and with nonvisual sensory neurons that do not detect light. This commonality includes the structural organization of the highly conserved sensory cilium cytoskeleton, called the axoneme, and microtubule-based trafficking pathways that are required for maintenance of the axoneme and the transduction machinery. This article highlights the features of rod and cone outer segments that are common to sensory cilia in general.