Numerous eukaryotic cells carry whip-like appendages (cilia or eukaryotic flagella) whose inner core consists of a cytoskeletal structure called the axoneme.
The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some 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 building block of the axonmene is the microtubule; each axomene is composed of several microtubules aligned in parallel. More specifically, the microtubules are arranged in a characteristic pattern known as the “9 + 2," as shown in the image at right. Nine sets of "doublet" microtubules (a speciallized structure consisting of two linked microtubules) form a ring around a "central pair" of single microtubules.
Besides the microtubules, the axoneme contains many proteins and protein complexes necessary for its funtion. The dynein arms, for example, are motor complexes which produce the force needed for bending. Each dynein arm is anchored to a doublet microtubule; by "walking" along an adjacent microtubule, the dynein motors can cause the microtubules to slide against each other. When this is carried out in a synchronized fashion, with the mictrotubules on one side of the axonmene being pulled 'down' and those on the other side pulled 'up,' the axoneme as a whole can bend back and forth. This process is responsible for ciliary/flagellar beating, as in the well-known example of the human sperm.
The radial spoke is another protein complex of the axoneme. Thought to be important in regulating the motion of the axoneme, this "T"-shaped complex projects from each set of outer doublets toward the central microtubules.
The doublets and central sheaths are linked by proteins known as nexins.
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. Non-motile primary cilia are expressed by many sensory cells including olfactory sensory neurons, auditory hair cells and retinal cone cells.
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