Cilium
Hair-like cellular protrusion enabling movement and sensory signaling.
The cilium (pl.: cilia; from Latin cilium 'eyelash') is a short hair-like membrane protrusion from many types of eukaryotic cell. Cilia are absent in bacteria and archaea. They have the shape of a slender threadlike projection extending from the surface of the much larger cell body. There are two major classes of cilia: motile and non-motile cilia, each with two subtypes, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core, called the axoneme, determines the cilium class. Most motile cilia have a 9+2 axoneme; most non-motile cilia have a 9+0 axoneme. Cilia are important for movement, sensory functions, and cellular signaling, and defects in cilia cause a group of diseases called ciliopathies.
- length
- 1 to 5 micrometers
- classes
- motile and non-motile (four subtypes total)
- axoneme types
- 9+2 (motile) and 9+0 (non-motile)
- found in
- eukaryotic cells (not bacteria or archaea)
- key functions
- motility, sensory signaling, mucociliary clearance, left-right asymmetry
Lore & Background
The cilium is assembled from a basal body on the cell surface, which is a modified mother centriole. From the basal body, the ciliary rootlet forms ahead of the transition plate and transition zone, where microtubule triplets change to doublets of the axoneme. The transition zone, also called the ciliary gate, controls entry and exit of proteins. Inside the cilium, the microtubule-based axoneme acts as a scaffold for dynein arms that enable motility and provides tracks for intraflagellar transport by kinesin and dynein. Cilia are assembled during the G1 phase and disassembled before mitosis, requiring aurora kinase A. Non-motile primary cilia serve as sensory organelles on most vertebrate cell types, functioning as cellular antennas. Olfactory neurons possess many non-motile cilia. Modified non-motile cilia called kinocilia are found on hair cells in the inner ear and have a 9+2 axoneme but lack inner dynein arms. Motile cilia are found in large numbers on respiratory epithelial cells (around 200 per cell) for mucociliary clearance, on ependymal cells to move cerebrospinal fluid, and in oviducts to move egg cells. Nodal cilia in the embryonic primitive node lack the central pair and are responsible for left-right asymmetry.
Reader's Guide
The cilium is a fundamental organelle in eukaryotic cells, with roles ranging from locomotion to sensory reception. The primary cilium is now understood as a sensory cellular antenna that coordinates many signaling pathways, sometimes coupling signaling to ciliary motility or cell division and differentiation. Defects in cilia cause ciliopathies such as polycystic kidney disease, congenital heart disease, mitral valve prolapse, and retinal degeneration. Primary cilia on pancreatic beta cells regulate function and energy metabolism; their deletion can lead to type 2 diabetes. The transition zone's role in selective protein entry is conserved across diverse organisms, and its disruption compromises Hedgehog-dependent embryonic development. Motile cilia are essential for clearing mucus from lungs, circulating cerebrospinal fluid, and moving egg cells in the reproductive tract. Nodal cilia establish left-right asymmetry in bilaterians. The cilium's structure—with its basal body, transition zone, axoneme, and ciliary membrane—enables these diverse functions, making it a key focus in cell biology and medicine.
Did You Know?
- Cilia are absent in bacteria and archaea.
- Most non-motile cilia have a 9+0 axoneme lacking the central pair of microtubules.
- Respiratory epithelial cells have around 200 motile cilia per cell.
- Nodal cilia in the embryonic primitive node are responsible for left-right asymmetry in bilaterians.
Frequently Asked Questions
What is a cilium?
A cilium is a short, hair-like membrane projection extending from the surface of a eukaryotic cell, usually measuring 1 to 5 micrometers in length. It acts as either a tiny motor for movement or a sensory antenna for detecting the cell's environment.
What are the different types of cilia?
Cilia split into two major classes—motile and non-motile—each with two subtypes, giving four types in all. The internal axoneme determines the class: motile cilia carry a 9+2 microtubule arrangement, while non-motile cilia have a 9+0 arrangement.
What does a cilium actually do for the cell?
Cilia handle a wide range of jobs, including propelling the cell or sweeping fluid across its surface, relaying chemical and mechanical sensory signals, clearing mucus from airways, and helping establish left-right body asymmetry during early development.
Do bacteria or archaea have cilia?
No—cilia are exclusive to eukaryotic cells and are completely absent in both bacteria and archaea. A typical eukaryotic cell will bear either a single primary (non-motile) cilium or a cluster of many motile cilia, but not both in the same functional role.
Why is the cilium considered so important in biology?
Without cilia, organisms would lose critical motility, their primary cellular sensory pathway, mucociliary clearance in the respiratory tract, and the mechanism that sets left-right organ asymmetry. They effectively serve as the cell's combined locomotion and long-range communication system.
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