The photoreceptor cilium and its diseases.

Light sensation occurs in photoreceptor outer segments (OS), which derive from highly specialized primary cilia, based on structural and molecular similarities. Ciliary dysfunction causes ciliopathies, in which retinal degeneration is common. The connecting cilium (CC) is the obligate passage for proteins moving between ciliary and cellular compartment, controlling the correct distribution of proteins on either side of its barrier. While new mechanisms for selective entry of ciliary proteins are being elucidated, active transport out of the OS is increasingly studied. We further discuss other recent advances in the field, such as a role for the CC in docking and fusion of incoming transport vesicles, a newly proposed subcompartmentalization into proximal and distal CC, and mechanisms of OS membrane dynamics paralleling ectosome formation in other cilia.