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Fig. 1. Cilia structure and intraflagellar transport. (A) A typical
cilium consists of an axoneme of nine doublet microtubules (two are shown in
B). Each doublet arises from the inner two microtubules of the basal body
microtubule triplets. The axoneme is surrounded by a specialized ciliary
membrane that is separated from the cell membrane by a zone of transition
fibers. (B) A cross-section of 9+2 and 9+0 cilium. Cilia are broadly
divided into two types based on the presence or absence of a central pair of
microtubule singlets in the axoneme (9+2 or 9+0 structure, respectively).
Inner and outer dynein arms, which are usually associated with 9+2 cilia, can
be present in either type of cilium and are important for ciliary motility.
Ciliary assembly and maintenance is accomplished by intraflagellar transport
(IFT), which relies on the microtubule motor proteins kinesin 2 and
cytoplasmic dynein to transport IFT protein complexes and their associated
cargo up and down the length of the cilium (depicted in A). Abbreviations:
Eb1, end-binding protein 1; Pc1 and Pc2, polycystin 1 and polycystin 2.
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