Axoneme (source code)
= Axoneme
An \b[axoneme] is the organized microtubule-based core of a <eukaryotic> <cilium> or flagellum. It supplies a structural framework and, in motile forms, carries motor and linking components that turn relative <microtubule> sliding into movement. The <axoneme> is an internal assembly, not the entire membrane-covered projection.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 16, Cilia and Flagella: microtubule organization and sliding motors.]{ref}
The familiar 9+2 arrangement contains nine outer doublets surrounding two central singlets. It describes spatial organization, not eleven identical independent tubules: each outer doublet has two associated tubules. Other axonemal arrangements occur, so the term does not require every cilium to have a central pair or to be motile.
An important mechanical test used isolated sea-urchin sperm <axonemes>. After brief treatment with trypsin, the preparations initially retained the characteristic organization, but addition of <ATP> caused active separation into individual tubules and groups. <K. E. Summers> and <I. R. Gibbons> interpreted these observations as evidence for sliding between the structural elements.<Adenosine triphosphate-induced sliding of tubules in trypsin-treated flagella of sea-urchin sperm by K. E. Summers and I. R. Gibbons>[1971 abstract: preserved starting architecture and ATP-induced active disintegration after digestion.]{ref}
The experiment separates two roles that can otherwise be conflated. Motors such as <dynein> generate relative motion, while structural constraints normally keep sliding components in an organized assembly. Weakening the constraints reveals sliding as separation; preserving them permits bending. The disintegration experiment does not imply that a normally beating <motile cilium> repeatedly falls apart and rebuilds itself.
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