Cilia are slender, hair‑like organelles that protrude from the surface of many eukaryotic cells. Each cilium contains a core of microtubules called the axoneme; motile cilia usually have nine outer doublet microtubules surrounding a central pair (a 9+2 arrangement), whereas non‑motile primary cilia lack the central pair (a 9+0 arrangement). Cilia beat in coordinated waves to produce movement or function as sensory antennae on cells.
Explanation
A cilium originates from a basal body derived from a centriole. Within the axoneme, microtubule doublets serve as tracks for motor proteins; dynein arms generate sliding between adjacent microtubules, causing the cilium to bend, while kinesin drives intraflagellar transport to deliver building materials to the tip. The 9+2 arrangement of motile cilia allows rhythmic beating and is shared with eukaryotic flagella, whereas primary cilia with a 9+0 pattern lack the dynein arms required for active motility and instead serve as sensory structures that detect mechanical or chemical signals. Most vertebrate cells have a single non‑motile primary cilium that acts as a cellular antenna for signalling pathways such as Hedgehog and Wnt. Motile cilia occur in large numbers on epithelial cells lining the respiratory tract, reproductive organs and brain ventricles, where their coordinated motion moves fluids or particles. Protozoan ciliates like Paramecium use thousands of motile cilia for locomotion and feeding. The structure and function of cilia are highly conserved across eukaryotes; mutations affecting ciliary proteins can lead to ciliopathies, a group of disorders involving developmental anomalies and chronic respiratory or fertility problems.
Examples and Significance
In the human respiratory tract, motile cilia sweep mucus and trapped pathogens toward the throat, playing a key role in innate defence. In the fallopian tubes, cilia propel the ovum toward the uterus. Ependymal cells in the brain ventricles use cilia to circulate cerebrospinal fluid. In protozoa, cilia enable locomotion, feeding and environmental sensing; Paramecium and Tetrahymena are classic examples of ciliated microorganisms. Primary cilia on kidney tubule cells detect fluid flow and contribute to regulation of cell proliferation; defects in these cilia cause polycystic kidney disease. Ciliated protozoa also contribute to nutrient cycling in aquatic ecosystems by grazing on bacteria. Understanding ciliary structure and movement informs studies of motility, sensory perception and human diseases.
Cilia are ubiquitous microtubule‑based projections that provide motility and sensory capabilities to eukaryotic cells. Their diverse roles range from driving locomotion in unicellular organisms to mediating fluid transport and signalling in tissues.
Related Terms: flagellum, axoneme, basal body, protozoa, dynein