Axoneme is the core scaffold of an eukaryotic cilium and flagellum. Biochemical analysis identified more than 250 components which constitute an axoneme. The characteristic feather of the axoneme is its “9 + 2” arrangement of microtubules and associated proteins. Nine pairs of microtubule protofilament doublets form a ring and surround two central single microtubules. Ciliary dynein motor protein complexes attach to one of the microtubules from each set of outer doublets. The interactions between the ciliary dynein proteins and adjacent microtubules generate the force, bend the cilium and enable it to beat. A spoke-like structure also projects from each set of the outer doublets toward the central microtubule.
The axoneme structure in a non-motile cilium shows some variation from the canonical “9 + 2” anatomy. No dynein complex attaches to the outer doublet microtubules, and there is also no central microtubule singlet. This organization of axoneme is referred as “9 + 0”.