Ciliogenesis

Ciliogenesis refers to the cellular process by which cilia, small hair-like projections, are assembled and matured on the surface of most animal cells. Cilia are membrane-bound organelles containing a microtubule-based axoneme and a cylindrical transition zone that regulates entry and exit of proteins. Ciliogenesis begins with the generation or maturation of a basal body from a centriole, followed by docking of this basal body to the plasma membrane. The axoneme extends from the basal body through the ciliary pocket and out of the cell surface, a process driven by intraflagellar transport (IFT). IFT uses an anterograde motor complex, kinesin-2, to move structural and signaling components toward the growing tip, and a retrograde dynein-2 motor to recycle materials back to the cell body.

Two major forms exist: primary (non-motile) cilia, which typically have a 9+0 microtubule arrangement and function in signaling (notably Hedgehog pathway), and motile cilia, usually 9+2, that beat to move fluid and mucus. Nodal cilia, present during embryogenesis, regulate left-right asymmetry. In multiciliated cells, ciliogenesis can produce hundreds of basal bodies via deuterosomes, enabling extensive ciliation.

Cell cycle and timing: In most cells, ciliogenesis occurs when cells exit the cell cycle and enter G0 or G1, after mitosis; basal bodies derive from centrioles and docking occurs at the apical membrane. The transition zone forms a selective barrier that shapes the ciliary proteome. Ciliary membrane proteins are transported by IFT and vesicular pathways to the cilium.

Clinical relevance: Defects in ciliogenesis or ciliary function cause ciliopathies, including primary ciliary dyskinesia, polycystic kidney disease, Bardet-Biedl syndrome, Joubert syndrome, and Meckel-Gruber syndrome.