cilium

A cilium (plural cilia) is a slender, hair-like organelle that protrudes from the surface of many eukaryotic cells. Each cilium originates from a basal body and contains an axoneme, a bundle of microtubules encased in the plasma membrane. Motile cilia typically exhibit a 9+2 microtubule arrangement and are driven by dynein motor proteins, producing whip-like motion. Non-motile cilia, or primary cilia, usually have a 9+0 arrangement and function as sensory organelles that detect chemical and mechanical signals.

Functions include moving fluids over epithelial surfaces and enabling locomotion in some single-celled organisms. In humans, cilia line the respiratory tract, the fallopian tubes, and the ventricles of the brain, where they help clear mucus, transport gametes, and circulate cerebrospinal fluid. Primary cilia act as signaling hubs, participating in pathways such as Hedgehog, Wnt, and PDGF signaling, and influencing development and tissue homeostasis.

Ciliogenesis and defects: Cilia form through ciliogenesis, guided by intraflagellar transport (IFT) systems that assemble and maintain the axoneme. Defects in ciliary structure or signaling cause ciliopathies, including primary ciliary dyskinesia (leading to chronic respiratory infections and sometimes situs inversus), polycystic kidney disease, Bardet-Biedl syndrome, and Joubert syndrome.

Cilia are broadly present across many eukaryotes, with variations in structure and function reflecting diverse biological roles.