cili

Cili is a term used in some languages and older texts as a plural form of cilium. In modern English scientific usage, the standard plural is cilia. Cilia are hair-like projections that extend from the surface of many eukaryotic cells and can be motile or sensory.

The cilium is anchored to the cell by a basal body derived from a centriole. The core structure, the axoneme, typically has a 9+2 arrangement in motile cilia, with nine outer microtubule doublets and two central tubules, powered by dynein motors that produce bending waves. Non-motile, or primary, cilia usually exhibit a 9+0 pattern and function as sensory antennae, hosting various receptors and signaling proteins.

Motile cilia line many epithelia, including the respiratory tract, oviduct, and brain ventricles, where they move mucus, eggs, or cerebrospinal fluid. Primary cilia act as hubs for signaling pathways important in development and tissue homeostasis, including Hedgehog and PDGF signaling.

Defects in ciliary structure or function compromise motility or signaling, giving rise to ciliopathies such as primary ciliary dyskinesia, polycystic kidney disease, and Bardet–Biedl syndrome.

Cilia are evolutionarily ancient, present in most kingdoms of eukaryotes, and studied with advanced imaging and genetic tools. They play crucial roles in development, left-right body patterning, and sensory perception, making them a focus of cell biology and medicine.