Ciliaalseid

Ciliaalseid are microscopic, hair-like projections that extend from the surface of many eukaryotic cells. They come in two main types: motile cilia, which beat in coordinated patterns to move fluid or mucus, and non-motile primary cilia, which primarily serve as sensory and signaling antennas for the cell. Each cilium is organized around an axoneme and anchored to the cell by a basal body derived from a centriole, with a transition zone at the base that regulates entry into the cilium. Motile cilia typically exhibit a 9+2 microtubule arrangement (nine outer doublets and a central pair) and rely on dynein motors to generate movement, while primary cilia usually have a 9+0 arrangement and lack the central pair, supporting signaling functions instead of propulsion.

Functions vary by type. Motile cilia line the respiratory tract and help move mucus and trapped particles out of the airways, participate in the movement of eggs through the fallopian tubes, and power the flagellum of sperm in many species. Primary cilia act as hubs for signaling pathways and environmental sensing, influencing cellular responses during development and tissue homeostasis. They detect mechanical stimuli and regulate signaling pathways such as Hedgehog and Wnt, among others.

Cilia are evolutionarily conserved and found across many eukaryotes, from single-celled organisms to humans. Their assembly and maintenance depend on intraflagellar transport, a bidirectional system that ferries tubulin and other components along the axoneme.

Clinical significance: defects in ciliary structure or function cause ciliopathies, a group of disorders with multi-organ involvement. Primary ciliary dyskinesia (Kartagener syndrome) affects motile cilia, leading to chronic respiratory infections, sinusitis, reduced fertility, and sometimes situs inversus. Other ciliopathies include polycystic kidney disease, Joubert syndrome, and Bardet-Biedl syndrome.