axonemes

An axoneme is the core cytoskeletal structure of most eukaryotic cilia and flagella, providing the scaffold for movement and signaling. It is a highly conserved, microtubule-based assembly that extends from the basal body, which anchors the organelle to the cell.

In motile cilia and flagella, the axoneme typically exhibits a nine-plus-two (9+2) arrangement: nine outer doublets of microtubules surrounding a central pair of single microtubules. Each outer doublet consists of a complete A tubule and a partial B tubule. Dynein motor proteins, arranged as outer and inner dynein arms attached to the A tubules, hydrolyze ATP to drive sliding between adjacent doublets. This sliding motion is converted into bending by cross-linking nexin links and radial spokes that connect the doublets to the central pair. The central pair complex coordinates dynein activity and helps determine the plane and pattern of beat.

In contrast, primary, non-motile cilia generally contain a 9+0 axonemal arrangement, lacking the central pair. The axoneme is extended and organized by the basal body, which derives from a centriole and anchors the axoneme to the cell. Disruptions in axonemal components can impair motility and are associated with diseases such as primary ciliary dyskinesia (PCD), which affects mucus clearance in the respiratory tract and left-right axis development in the body.