Figure 4. Cilium.

The axoneme stems from the mother centriole-derived basal body, anchored to the cell membrane via its distal appendages (subsequently called transition fibers). Transition fibers, connected both to microtubules and surrounding plasma membrane, contribute to formation of transition zone, which serves as a gate controlling sorting of molecules transported in the cilium. Depositioning of membrane to the mother centriole and formation of ciliary vesicle and subsequently ciliary shaft is coordinated by interactions of Rab11/Rabin8/Rab8 pathway, BBSome (Bardet-Biedl syndrome) protein complex, and components of distal appendages. The ciliary cargo is transported along the axoneme loaded on IFT particles, which are moved by action of molecular motors. Specifically, kinesin-2 family motors mediate anterograde transport along the microtubules towards the tip of the cilium, while cytoplasmic dynein-2 mediates retrograde movement of intraflagellar transport complexes from the tip to the ciliary base.