cilia

Cilia are slender, hair-like organelles that extend from the surface of many eukaryotic cells. They are classified as motile cilia, which beat in coordinated waves to move fluid or cells, and non-motile primary cilia, which primarily serve as sensory antennas that detect environmental cues and regulate cellular signaling.

Structurally, a cilium is anchored by a basal body derived from a centriole and extends a shaft called the axoneme. Motile cilia typically exhibit a 9+2 microtubule arrangement (nine outer doublets and a central pair) with dynein motor arms that generate bending motion through microtubule sliding, assisted by radial spokes and other links. Primary cilia usually have a 9+0 arrangement and lack dynein-driven motion. Assembly and maintenance rely on intraflagellar transport, driven anterogradely by kinesin-2 and retrogradely by dynein-2.

Functions vary by type. Motile cilia line the respiratory tract and portions of the reproductive system, where their coordinated beating clears mucus, debris, and moves gametes. They are also important in certain single-celled organisms for locomotion. Primary cilia act as sensory organelles that detect mechanical and chemical signals and participate in signaling pathways such as Hedgehog, Wnt, and PDGFR, influencing development and tissue homeostasis.

Clinical relevance is seen in ciliopathies caused by structural or functional defects. Primary ciliary dyskinesia (Kartagener syndrome) involves impaired motile cilia, leading to recurrent respiratory infections and sometimes situs inversus. Other ciliopathies include polycystic kidney disease, Bardet-Biedl syndrome, Joubert syndrome, and Meckel syndrome.

Cilia are conserved features across many eukaryotes, reflecting diverse roles in movement, sensation, and development. They are distinct from flagella in mammals mainly by function and, in some contexts, by structural details.