Axoneme
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An axoneme is the core scaffold of the eukaryotic cilia and flagella, which are projections from the cell made up of microtubules. Thus, the axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in most cases, causing it to bend. Though distinctions of function and/or length may be made between cilia and flagella, the internal structure of the axoneme is common to both.
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 "doublet" microtubules, a component of the cellular cytoskeleton, form a ring around a "central pair" of single microtubules. Ciliary dynein arms, the motor complexes which allow the axoneme to bend, are anchored to these microtubules. The interactions between the ciliary dynein proteins and outer doublet microtubules generate force by sliding the doublets parallel to each other, 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 radial spoke, a protein complex important in regulating the motion of the axoneme, is also housed in the axoneme; it projects from each set of outer doublets toward the central microtubules.
The axoneme structure in non-motile primary cilium shows some variation from the canonical “9 + 2” anatomy. No dynein arms are found on 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 doublets and central sheaths are linked by proteins known as nexins.
[edit] 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.Entrez PubMed 11086017
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. Entrez PubMed 11082310
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.Entrez PubMed 9880321
Intraflagellar transport: the eyes have it. Rosenbaum, J.L., Cole, D.G., and D.R. Diener. (1999) Journal of Cell Biology 144, 385-388.Entrez PubMed 9971734
