Axoneme

An axoneme, also called an axial filament is the microtubule-based cytoskeletal structure that forms the core of a cilium or flagellum. Cilia and flagella are found on many cells, organisms, and microorganisms, to provide motility. The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some cases, the ability to bend. Though distinctions of function and length may be made between cilia and flagella, the internal structure of the axoneme is common to both. Inside a cilium and a flagellum is a microtubule-based cytoskeleton called the axoneme. The axoneme of a primary cilium typically has a ring of nine outer microtubule doublets (called a 9+0 axoneme), and the axoneme of a motile cilium has two central microtubules in addition to the nine outer doublets (called a 9+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein complexes and provides binding sites for molecular motor proteins such as kinesin-2, that help carry proteins up and down the microtubules. The axoneme structure in non-motile primary cilia is of an outer nine microtubule doublets with no central microtubule singlets, and no dynein arms on the outer doublets. This arrangement is known as the 9+0 axoneme. Primary cilia appear to serve sensory functions. The building-block of the axoneme is the microtubule; each axoneme is composed of several microtubules aligned in a characteristic pattern known as the 9+2 axoneme as shown in the image at right. Nine sets of doublet microtubules (a specialized 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 function. The dynein arms, for example, are motor complexes that 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.