Cilium

Cilia (singular: cilium, from the Latin for "eyelash") are short, hair-like extensions of the cell membrane found on many eukaryotic cells, but not on bacteria or archaea. These slender, threadlike projections stick out from the cell surface, and their core structure—called the axoneme—determines which of the four types they belong to. There are two main classes: motile cilia and non-motile cilia, each with two subtypes. A cell usually has either a single primary cilium or many motile cilia. Most motile cilia have a 9+2 axoneme, with a central pair of single microtubules surrounded by nine pairs of double microtubules, while most non-motile cilia have a 9+0 axoneme, lacking that central pair and the associated motility components like dynein arms and radial spokes. However, some motile cilia lack the central pair, and some non-motile cilia have it, giving the four distinct types.

Most non-motile cilia, called primary or sensory cilia, act solely as sensory organelles. Most vertebrate cell types have one non-motile primary cilium, which works like a cellular antenna. Olfactory neurons, in contrast, have many non-motile cilia. Non-motile cilia with a central microtubule pair are called kinocilia, found on hair cells. Motile cilia appear in large numbers—around 200 per cell—on respiratory epithelial cells, where they help clear mucus and also sense mechanical and chemical signals. They also line the brain’s ependymal cells to move cerebrospinal fluid through the ventricular system, and in the oviducts of female therian mammals, they help move egg cells from the ovary to the uterus. Motile cilia without the central microtubule pair are found on nodal cells in the embryonic primitive node, where they establish left-right asymmetry in bilaterians.

A cilium is 1 to 5 micrometers long and is built from a basal body on the cell surface. The basal body, a modified mother centriole, consists of a barrel of nine triplet microtubules with subdistal and distal appendages that anchor it to the membrane. From the basal body, the ciliary rootlet—a cytoskeleton-like structure 80–100 nm in diameter with regular cross striae—forms ahead of the transition plate and transition zone. The transition zone, or ciliary gate, controls protein entry and exit, with Y-shaped structures linking the ciliary membrane to the axoneme. Defects in transition zone components cause ciliopathies like Joubert syndrome, and disruption reduces membrane-associated ciliary proteins, affecting Hedgehog signaling and embryonic development. Inside the cilium, the axoneme is a microtubule-based core: a 9+0 ring for primary cilia, or a 9+2 arrangement for motile cilia (the same as in flagella). In motile cilia, the axoneme scaffolds inner and outer dynein arms for movement and provides tracks for kinesin and dynein motor proteins. Intraflagellar transport, similar to axonal transport, moves ciliary components bidirectionally along these tracks—kinesin toward the tip, dynein back to the cell body. The cilium has its own membrane enclosed within the cell membrane.