Figure 3. Primary cilia dynamics.

Primary cilia assembly occurs when cells exit the cell cycle or in response to developmental signals. (i) Golgi-derived pre-ciliary vesicles dock with the mother centriole and fuse to become the ciliary vesicle. The mother centriole differentiates into the basal body by the acquisition of accessory structures and microtubule capping protein CP110 is removed to enable microtubule elongation. (ii) A nascent cilium is formed within the cytosol by microtubule assembly from the basal body which is covered by the double membrane ciliary vesicle. (iii) Finally the ciliary vesicle fuses with the plasma membrane to reveal the mature cilium projecting into the extracellular space. (iv) Primary cilia disassemble as cells progress through the cell cycle in response to growth factor stimulation. Disassembly occurs via microtubule destabilisation and disassembly. Cilia decapitation also contributes to disassembly by removing positive regulators of cilia maintenance. (v) Once the cilium has disassembled the mother centriole is released for mitotic spindle formation.