Axonemal

Axonemal refers to the axoneme, the core microtubule-based structure of motile cilia and flagella in many eukaryotes. In most motile cilia and flagella, the canonical axoneme exhibits a 9+2 arrangement: nine outer microtubule doublets encircle two central single microtubules. Each outer doublet bears dynein motor proteins that generate sliding between adjacent doublets, which is converted into bending by the surrounding linking proteins (nexin links) and the constraint of the microtubule array. Radial spokes connect outer doublets to the central pair, helping regulate the beat pattern; the central pair and radial spokes coordinate dynein activity to produce coherent motility.

Axonemal structures are constructed and maintained by intraflagellar transport; the base of the axoneme anchors to a basal body derived from a centriole. In many animals, the axoneme powers the beating of sperm flagella and cilia in the respiratory tract, the oviduct, and lining the brain’s ventricles.

Some cilia are non-motile (primary cilia) and typically have a 9+0 arrangement, lacking dynein-driven motility. Nonetheless, axonemal components can participate in sensory signaling and other specialized functions in these structures.

Disruption of axonemal components, particularly axonemal dynein, causes ciliary dyskinesias, including primary ciliary dyskinesia, leading to chronic respiratory infections, reduced fertility, and situs abnormalities in some cases (Kartagener syndrome). The term axonemal is widely used to describe features tied to the axoneme, such as axonemal dynein or axonemal microtubules.