A centriole is a tiny, barrel-shaped structure inside your cells that plays a central role in cell division and helps build cilia, the hair-like projections that many cells use to move fluid or sense their environment. Each centriole measures only about 200 nanometers wide and 300 to 500 nanometers long, far too small to see without an electron microscope. Despite that size, centrioles are essential for organizing the machinery that pulls chromosomes apart when a cell divides, and defects in their number or structure are linked to cancer and developmental disorders.
Structure of a Centriole
A centriole’s most distinctive feature is its ninefold radial symmetry. If you sliced one in cross section, you’d see nine sets of microtubule “triplets” arranged in a ring, like the blades of a tiny turbine. No other structure in the cell has this pattern. Each triplet consists of three linked tubes called the A-, B-, and C-tubules. The A-tubule resembles a standard microtubule with 13 protein strands forming a ring, though it’s slightly oval rather than perfectly circular. The B- and C-tubules are incomplete, each made of 10 strands of their own while sharing four strands with the neighboring tubule. This interlocking design makes the whole assembly remarkably rigid for its size.
At the core of a forming centriole sits a “cartwheel,” a hub-and-spoke scaffold that establishes the ninefold symmetry early in assembly. A protein called SAS-6 is key to building this cartwheel. Structural proteins called tektins, which resemble the filaments that give cells their shape, also contribute to the centriole’s internal architecture.
Centrioles vs. Centrosomes
People often use “centriole” and “centrosome” interchangeably, but they’re not the same thing. A centrosome consists of two centrioles embedded in a cloud of proteins called the pericentriolar material (PCM). Under early electron microscopes, the PCM appeared as a dense, shapeless mass surrounding the highly structured centrioles. It’s now understood to be a porous protein scaffold that concentrates tubulin, the building block of microtubules, making the centrosome the cell’s primary hub for organizing its internal skeleton.
Think of the two centrioles as the structural core and the PCM as the functional platform built around them. The PCM handles microtubule assembly, protein trafficking, and spindle formation during division. Without the centrioles anchoring it, the PCM can’t organize properly.
Role in Cell Division
When a cell prepares to divide, its two centrosomes migrate to opposite sides of the cell and send out microtubules that attach to chromosomes, forming the mitotic spindle. This spindle pulls one copy of each chromosome to each side, ensuring both daughter cells get a complete set of DNA. Centrioles are critical for keeping this spindle bipolar, meaning it has exactly two poles. Without them, cells frequently form single-poled or multi-poled spindles, leading to uneven chromosome distribution and failed division.
Research in fruit flies illustrates this clearly. When centrioles are experimentally removed from sperm-producing cells, the resulting spindles are largely abnormal, causing dramatic errors in chromosome separation and in the final step where one cell pinches into two. In animal cells generally, centrioles are strongly involved in maintaining the spindle’s two-sided geometry and, in some cell types, in completing that final pinch.
How Centrioles Duplicate
Centrioles replicate once per cell cycle, in sync with DNA replication. The process begins in late G1 or early S phase, when a new “procentriole” starts forming at a right angle to each existing centriole. This means the cell goes from two centrioles to four, giving it two centrosomes for division.
The trigger for duplication is a protein kinase called PLK4, widely recognized as the master regulator of the process. PLK4 parks on the surface of the existing centriole and recruits two other proteins, STIL and SAS-6. STIL acts as a bridge: once PLK4 activates it, STIL brings in SAS-6, which assembles the cartwheel scaffold that gives the new centriole its ninefold symmetry. During the S and G2 phases, the new centrioles elongate by adding tubulin. By late G2 and into mitosis, the daughter centrioles mature into fully functional structures capable of anchoring PCM and nucleating microtubules on their own.
This “once and only once” duplication rule is important. If a cell makes too many centrioles, it can end up with extra centrosomes, which tends to produce abnormal spindles and unequal chromosome counts in the daughter cells.
Building Cilia and Flagella
Centrioles have a second major job that has nothing to do with division. When a cell exits the division cycle and differentiates, one of its centrioles can migrate to the cell surface and anchor there, becoming what’s called a basal body. From this docked position, the basal body templates the growth of a cilium, a slender, hair-like projection that extends from the cell.
The process begins when the centriole translocates to the plasma membrane and attaches via structures called transition fibers at its tip. Once anchored, it organizes the growth of the cilium’s internal microtubule core upward from the cell surface. Nearly every cell in the human body can produce a single primary cilium this way, and these cilia serve as sensory antennae, detecting chemical signals, fluid flow, and light depending on the tissue. Specialized cells like those lining the airways produce hundreds of motile cilia that beat in coordinated waves to sweep mucus.
Which Organisms Have Centrioles
Centrioles are found in most animal cells, as well as in the cells of lower plants, algae, and many protists. Flowering plants and most fungi, however, have lost centrioles entirely. These organisms still divide successfully, assembling spindles through alternative protein complexes that organize microtubules without a centrosome. Even some animal cells can form spindles without centrioles (mouse eggs do this naturally), but the process is less reliable, particularly for the rapid divisions of early embryonic development and for specialized divisions like those producing sperm.
Links to Disease
Abnormal centriole numbers show up in a wide range of human cancers, including breast, prostate, lung, colon, and brain tumors. More than a century ago, Theodor Boveri proposed that having too many centrosomes could create abnormal spindles and generate the kind of chromosome instability that drives cancer. Modern research confirms this idea. Extra centrioles can arise from overproduction of the duplication regulator PLK4 or from mutations in tumor suppressor genes like p53 and BRCA1. These extra centrosomes appear early, even in pre-malignant lesions, and correlate strongly with cells having abnormal chromosome counts.
Extra centrosomes can also promote cancer through a mechanism that doesn’t involve chromosome errors at all. In fruit fly brain stem cells, extra centrosomes disrupted the way cells divide asymmetrically, causing overproliferation and tumor formation with only minor chromosome abnormalities. Additionally, extra centrosomes can produce extra cilia, potentially altering signaling pathways implicated in cancers like pancreatic cancer.
Beyond cancer, centriole and centrosome defects are linked to developmental brain disorders. Several forms of autosomal recessive primary microcephaly, a condition where the brain is significantly smaller than normal, result from mutations in centrosome-associated proteins. Other conditions tied to centrosome dysfunction include lissencephaly, where the brain surface lacks its normal folds due to defective nerve cell migration, and a form of primordial dwarfism where overall growth is reduced but the brain is disproportionately affected. Defects in cilia formation, rooted in centriole dysfunction, underlie a broader class of conditions called ciliopathies that can affect the kidneys, eyes, and other organs.