Figure 1.

The structure of the primary cilium. (A) The primary cilium is formed on the apical surface of cells, from the basal body which is derived from the mother centriole. The daughter centriole stays associated, at roughly rightangles, surrounded by pericentriolar material. The basal body is located in a depression of membrane known as the ciliary pocket, and is connected to membrane here by transition fibers. The region where the central pair of microtubules in the basal body microtubule triplet grow to form the ciliary axoneme is called the transition zone. Here, Y-linkers connect the axoneme to the ciliary membrane. Protein and other cargos are transported from cilium base to tip by anterograde IFT particles and kinesin-2 motor. Protein and other cargos are transported from cilium tip to base by retrograde IFT particles and cytoplasmic dynein motor. (B) In cross-section the 9+0 formation of microtubule doublets can be seen in radial array, making up the ciliary axonome. It is formed of a ring of 9 pairs of post-translationally modified microtubules, with no central pair and no dynein arms.