An axoneme is the core scaffold of the eukaryotic cilium and flagellum, which are projections from the cell made up of microtubules. These organelles move either the cell itself (flagella) or substances around or over the cell (cilia). Though cilia and flagella have major differences in their lengths, the internal structure of the axoneme is common to both.

Biochemical analysis has identified more than 250 components which constitute an axoneme. The characteristic feature of the axoneme is its “9 + 2” arrangement of microtubules and associated proteins, as shown in the image at right. Nine pairs of microtubule doublets, made of the protein tubulin, form a ring around a "central pair" of single microtubules. The radial spoke, a protein made of 17 polypeptides, projects from each set of outer doublets toward the central microtubules, and is important in regulating the beating of the cilia through biochemical modification of the central microtubules as well as control of the levels of ATP within the cilia. 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 outer doublet of microtubules generate a force by sliding the doublet, which bends the cilium and enables it to beat. Defects in the dynein motors of vertebrates can result in infertility, respiratory disease, and failures in determination of the left-right body axis during embryonic development.

The axoneme structure in a non-motile primary cilium shows some variation from the canonical “9 + 2” anatomy. No dynein complex attaches to the outer doublet microtubules, and there is no pair of central microtubule singlets. This organization of axoneme is referred as “9 + 0”. In addition, “9 + 1” axonemes, with only a single central microtubule, have been found to exist.

The 9 peripheral pairs of doublets share a common wall of 2-3 protofilaments. The central pair of tubules is separated from one another and is enclosed within a single, central sheath. The doublets and central sheaths are linked by proteins known as nexins.

Further reading

The 9 + 2 Axoneme Anchors Multiple Inner Arm Dyneins and a Network of Kinases and Phosphatases that Control Motility. Porter, M.E. and Sale, W.S. (2000) The Journal of Cell Biology 151, F37-42.

An Integrative Model of Internal Axoneme Mechanics and External Fluid Dynamics in Ciliary Beating. Dillon, R.H. and Fauci, L.J. (2000) Journal Theoretical Biology 207, 415-430.

Rotation of the Central Pair Microtubules in Eukaryotic Flagella. Omoto, C.K., Gibbons, I.R., Kamiya, R., Shingyoji, C., Takahashi, K., and Witman, G.B. (1999) Molecular Biology Cell 10, 1-4.

Intraflagellar transport: the eyes have it. Rosenbaum, J.L., Cole, D.G., and D.R. Diener. (1999) Journal of Cell Biology 144, 385-388.

Organelles

Aksonema