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. Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... cross-section of two cilia, showing 9+2 structure A cilium (plural cilia) is a fine projection from a eukaryotic cell that constantly beats in one direction. ... A flagellum (plural, flagella) is a whip-like organelle that many unicellular organisms, and some multicellular ones, use to move about. ... Cells in culture, stained for keratin (red) and DNA (green). ... Microtubules are protein structures found within cells. ... In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. ...

Cross-section of 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 "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. Image File history File links Axoneme. ... Image File history File links Axoneme. ... The eukaryotic cytoskeleton. ... Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Infertility is the inability to naturally conceive a child or to carry a pregnancy to full term. ... Diseases of the mammalian Respiratory system are classified physiologically into obstructive (i. ...

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 radial spoke is a multi-unit protein structure found in the axonemes of eukaryotic cilia and flagella. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...

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 by example in some protists.

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.Entrez PubMed 11086017 The Entrez logo The Entrez Global Query Cross-Database Search System allows access to databases at the National Center for Biotechnology Information (NCBI) website. ...

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 The Entrez logo The Entrez Global Query Cross-Database Search System allows access to databases at the National Center for Biotechnology Information (NCBI) website. ...

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 The Entrez logo The Entrez Global Query Cross-Database Search System allows access to databases at the National Center for Biotechnology Information (NCBI) website. ...

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 The Entrez logo The Entrez Global Query Cross-Database Search System allows access to databases at the National Center for Biotechnology Information (NCBI) website. ...