Centrioles are tiny barrel-shaped structures inside animal cells that organize the machinery of cell division and build sensory projections called cilia. Each one is barely 200–250 nanometers across and about 500 nanometers long, invisible under a standard light microscope, yet a defect in a single centriole protein can lead to anything from a brain development disorder to cancer. Their defining architectural feature is a ninefold radial symmetry: nine sets of microtubule blades arranged around a central hub, a geometry so distinctive it was one of the first subcellular patterns ever identified by electron microscopy.
The Cartwheel and Ninefold Symmetry
If you look at a centriole end-on in an electron microscope, it resembles a tiny pinwheel. In most animal cells, the outer wall is built from nine “triplet” microtubules, meaning each of the nine blades is itself a bundle of three connected tubes (called the A, B, and C tubules). Those triplets give the centriole its characteristic outer diameter of roughly 220 nanometers in mammalian cells. When researchers knocked out delta-tubulin or epsilon-tubulin in human cells, the resulting centrioles had only singlet microtubules instead of triplets, and their outer diameter shrank to about 170–175 nanometers, confirming that the triplet architecture accounts for the centriole’s full width.1PubMed Central. Centriole triplet microtubules are required for stable centriole formation and inheritance in human cells
At the core of the centriole sits the cartwheel, a stack of ring-shaped assemblies that dictates the ninefold pattern. The key cartwheel protein is SAS-6. Work in the green alga Chlamydomonas showed that when SAS-6 is completely absent, cells can still make centrioles, but many of them end up with seven, eight, ten, or eleven blades instead of the normal nine.2PubMed. SAS-6 is a cartwheel protein that establishes the 9-fold symmetry of the centriole SAS-6 essentially acts as a molecular ruler: it self-assembles into rings whose inner diameter is about 19 nanometers, matching the cartwheel hubs observed inside living cells.3eLife. Structure of the SAS-6 cartwheel hub from Leishmania major The rings stack on top of one another with their spoke-like arms interdigitating, building the cartwheel upward and locking in the ninefold geometry that the outer microtubule wall then follows.4PubMed. The centriolar cartwheel structure: symmetric, stacked, and polarized
How Centrioles Duplicate
Centrioles copy themselves once per cell cycle in a process that looks almost like budding. A new “procentriole” sprouts at a right angle from the side of each existing centriole during the transition from the G1 to the S phase of the cell cycle. This means a cell that enters S phase with two centrioles leaves it with four, each old centriole now paired with one new daughter. The process is described as semi-conservative because the original centriole is retained intact while a fresh one is assembled alongside it.5PubMed Central. Centriole Duplication at the Crossroads of Cell Cycle Control and Oncogenesis
The master switch for duplication is an enzyme called PLK4 (Polo-like kinase 4). PLK4 phosphorylates a protein called STIL at two different regions, and each phosphorylation event triggers a separate step. One modification enables STIL to bind SAS-6 and kick off cartwheel assembly; a second modification promotes STIL binding to CPAP, which links the growing cartwheel to the microtubule wall of the centriole.6eLife. PLK4 promotes centriole duplication by phosphorylating STIL to link the procentriole cartwheel to the microtubule wall Without PLK4 activity, new centrioles simply do not form.
Interestingly, cells can also build centrioles from scratch, without a pre-existing template. This “de novo” pathway is normally suppressed when template centrioles are present, but it switches on when they are experimentally removed. Researchers found that even a mutant form of SAS-6 that cannot self-assemble into rings was still capable of driving de novo centriole formation, suggesting that the C-terminal portion of SAS-6 carries the main assembly-promoting activity rather than the ring-forming domain alone.7PubMed Central. De novo centriole formation in human cells is error-prone and does not require SAS-6 self-assembly De novo assembly is error-prone, though, frequently producing centrioles with abnormal structure, which helps explain why cells prefer the templated pathway under normal circumstances.
What Controls Centriole Length
Centrioles are remarkably uniform in length within a given species, which means cells must have a way to stop building once the structure reaches its target size. Two opposing forces appear to set this length. On one side, the protein CPAP promotes microtubule growth at the centriole by recruiting tubulin dimers. Overexpressing CPAP in human cells produces dramatically elongated centriole-like structures, and mutating the region of CPAP that binds tubulin blocks this elongation, confirming that its tubulin-binding activity is what drives procentriole growth.8PubMed. CPAP is a cell-cycle regulated protein that controls centriole length
On the other side, a cap complex made of CP110 and its partner Cep97 sits on the distal end of the centriole and physically blocks further microtubule extension. Removing CP110 leads to overgrowth of centriolar microtubules.9PubMed. Control of centriole length by CPAP and CP110 More recently, researchers discovered that a kinase called Nek1 restricts centriole length through a completely separate pathway: losing Nek1 causes pronounced microtubule hyperelongation even while the CP110-Cep97 cap remains in place, meaning the cell has at least two independent braking mechanisms.10PubMed Central. Nek1 defines a branch of centriolar microtubule length control parallel to CP110-Cep97 A system with redundant length controls makes sense for a structure where even modest over- or under-growth could throw off the geometry of cell division.
Mother Versus Daughter Centrioles
Because duplication is semi-conservative, the two centrioles in a cell that has just finished dividing are always different ages. One was assembled during the previous cell cycle (the “mother”), and the other was built alongside it (the “daughter”). This age difference matters because only the older, mother centriole carries specialized protein structures called distal and subdistal appendages.11PubMed. Control of centrosome distal appendages assembly and disassembly Appendages are not decorative. They are essential for anchoring the centriole to the cell membrane, a step required before a cilium can form. The daughter centriole cannot build a cilium until it ages through at least one more round of cell division and acquires appendages of its own.
Building Cilia From Centrioles
When a cell exits the division cycle and enters a resting state, the mother centriole can convert into what is called a basal body. In this role, it migrates to the cell surface, docks with the plasma membrane, and templates the growth of a cilium, a slender projection that acts as either a sensor or a propeller depending on the cell type.12PubMed Central. Regulating the transition from centriole to basal body Nearly every cell in your body can grow a single “primary” cilium, which functions like an antenna, detecting chemical signals and mechanical forces in the environment. Specialized cells, such as those lining your airways, grow hundreds of motile cilia that beat in coordinated waves to move mucus.
The docking step is tightly controlled. The distal appendage protein CEP83 is required for the centriole to physically contact the plasma membrane. When researchers depleted CEP83, centrioles could still travel to the vicinity of the membrane but failed to actually attach, and no cilia grew.13Genes & Development. Centriole distal appendages promote membrane docking, leading to cilia initiation Docking also triggers the removal of the CP110 cap (mentioned earlier as a centriole length regulator), and that removal depends on the kinase TTBK2 being recruited to the mother centriole. Without docking, TTBK2 never arrives and CP110 stays in place, effectively keeping the gate to cilia formation locked shut.13Genes & Development. Centriole distal appendages promote membrane docking, leading to cilia initiation A separate appendage protein, Cep164, plays a complementary role by mediating the capture of small vesicles at the centriole’s distal end, a step that precedes axoneme extension.14PubMed Central. Cep164 mediates vesicular docking to the mother centriole during early steps of ciliogenesis
Organizing the Mitotic Spindle
During cell division, centrioles serve as the core of each centrosome, the structure that nucleates the microtubule fibers of the mitotic spindle. The centriole pair does not do this alone. It sits embedded in a cloud of proteins called the pericentriolar material (PCM), which concentrates tubulin and houses the complexes that actually launch new microtubules.15PubMed Central. Pericentriolar material structure and dynamics The PCM expands dramatically as cells enter mitosis, boosting microtubule nucleation several-fold.
Having two centrosomes, one at each pole, gives the spindle a bipolar geometry that pulls duplicated chromosomes apart evenly. Astral microtubules radiating from each centrosome anchor the spindle to the cell cortex, and the polo-like kinase Plk1 helps fine-tune spindle orientation so that the division plane cuts the cell correctly.16PubMed. Centrosomes in mitotic spindle assembly and orientation When researchers removed centrosomes from vertebrate cells, spindles still formed (because microtubules can self-organize to a degree), but they failed to reposition properly. Anaphase often began with the spindle off-center or sideways, leading to chromatin bridges between daughter nuclei and frequent failure of the final division step, cytokinesis.17PubMed Central. Centrosomes Enhance the Fidelity of Cytokinesis in Vertebrates and Are Required for Cell Cycle Progression So centrosomes are not strictly required for splitting chromosomes, but they are required for doing it cleanly and reliably.
Centrioles Beyond Cell Division
One of the more surprising roles for centrioles has nothing to do with mitosis or cilia. In cytotoxic T lymphocytes, the immune cells that kill virus-infected or cancerous targets, the centrosome physically migrates to the contact point between the T cell and its target (the immunological synapse) and docks directly against the plasma membrane. Secretory granules loaded with killing molecules ride along microtubules toward the centrosome and are released precisely at the synapse, focusing the lethal payload on the target while sparing neighboring cells.18PubMed. Centrosome polarization delivers secretory granules to the immunological synapse This is a fundamentally different delivery mechanism from the one used by most other secretory cells, and it was not recognized until live imaging caught centrosomes touching the membrane in real time.
Sperm Centrioles and Fertilization
Human eggs lose their centrioles during development, so the centrioles that organize the first cell divisions of an embryo come from the sperm. Each spermatozoon contributes two remodeled centrioles to the fertilized egg, where they reconstitute a centrosome, assemble the sperm aster (a radial array of microtubules), and help guide the two parental pronuclei toward each other before the first cleavage division.19PubMed Central. The Role of Sperm Centrioles in Human Reproduction – The Known and the Unknown Defective sperm centrioles are a suspected cause of some cases of male infertility and early pregnancy failure, though this remains an active area of investigation.
Because the egg eliminates its own centrioles, the cell must also have a disposal system. During normal fertilization only one sperm enters, but when extra sperm penetrate (polyspermy), the surplus centrioles need to be cleared. Evidence points to autophagy, a cellular recycling pathway, as part of this elimination process: the extra sperm components become heavily tagged with ubiquitin and surrounded by autophagosome markers before being degraded.20PubMed Central. Towards understanding centriole elimination
Cells That Divide Without Centrioles
Not all cells need centrioles to divide. Mammalian oocytes assemble meiotic spindles entirely without centrosomes, relying instead on a recently discovered phase-separated structure called the liquid-like meiotic spindle domain (LISD). The LISD permeates the spindle poles, concentrating microtubule-regulatory proteins and allowing them to diffuse rapidly to where they are needed. Disrupting the LISD leads to severe spindle defects.21PubMed Central. A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes Flowering plants and most fungi also lack centrioles entirely. Yeasts, for instance, use a structure called the spindle pole body, and some amoebae rely on a nucleus-associated body. Although these organelles look nothing like centrioles, they share some of the same molecular components, hinting at a deep evolutionary connection.22Current Biology. Centrioles: Structure, Function, and Their Role in Cell Biology
When Centrioles Go Wrong in Cancer
Cancer cells frequently have too many centrosomes, a condition called centrosome amplification. This can happen when centrioles duplicate more than once per cell cycle or when cell division fails and the daughter cells inherit extra copies. Centrosome amplification has been found in virtually all cancer types and is linked to chromosome instability, because extra centrosomes can pull chromosomes toward more than two poles, scrambling the genome of daughter cells.23PubMed. Centrosome amplification, chromosomal instability and cancer: mechanistic, clinical and therapeutic issues Multiple studies have correlated the degree of amplification with higher tumor grade and greater tendency to metastasize, suggesting it could serve as a prognostic marker.24PubMed Central. Centrosome amplification: a quantifiable cancer cell trait with prognostic value in solid malignancies The PLK4 enzyme that normally triggers centriole duplication has become a drug target for this reason: blocking PLK4 could, in theory, starve cancer cells of the extra centrosomes they depend on.
Ciliopathies and Brain Development
Because centrioles template cilia, mutations in centriole or cilia genes can produce a broad family of disorders called ciliopathies. More than a dozen distinct conditions fall under this umbrella, including polycystic kidney disease, Bardet-Biedl syndrome, Joubert syndrome, and certain forms of retinal degeneration.25Cell. The Ciliopathies: An Cellular Bridge between Diverse Phenotypes and Diseases Symptoms range widely because cilia are present on nearly every cell type, so the organ affected depends on which tissues are most sensitive to the particular mutation.
The brain is especially vulnerable to centrosome defects. A striking number of genes linked to inherited microcephaly (a condition in which the brain is abnormally small) encode centrosome-related proteins.26PubMed Central. Understanding microcephaly through the study of centrosome regulation in Drosophila neural stem cells Neural stem cells undergo rapid, precisely oriented divisions to populate the developing brain. When centrosome function is impaired, these stem cells either die during faulty mitoses or prematurely stop dividing and differentiate too early, shrinking the pool of progenitors that should be building brain tissue.27PubMed Central. Same but different: pleiotropy in centrosome-related microcephaly A recent study identified mutations in the centrosomal protein CETN3 in a child with primary microcephaly. When the same gene was knocked out in lab-grown brain organoids, the organoids were smaller, showed impaired centrosome assembly, and had reduced neural stem cell proliferation, recapitulating the patient’s condition.28PubMed Central. CETN3 deficiency induces microcephaly by disrupting neural stem/progenitor cell fate through impaired centrosome assembly and RNA splicing
Seeing Centrioles in New Ways
For decades, studying centriole architecture required electron microscopy, which offers superb resolution but demands elaborate sample preparation and captures only snapshots. A technique called ultrastructure expansion microscopy (U-ExM) has changed the landscape. The approach physically swells preserved cells by embedding them in a gel, allowing features normally below the resolution limit of light microscopes to become visible. When paired with super-resolution fluorescence imaging, U-ExM has revealed details such as centriolar chirality, the left- or right-handedness of the microtubule blade arrangement, that were previously accessible only by electron microscopy.29Nature Methods. Imaging cellular ultrastructures using expansion microscopy
Using U-ExM, researchers mapped the precise positions of 21 different centriolar proteins in human cells, assigning each to specific structural elements of the centriole and procentriole for the first time at this level of detail.30Cell. Architecture of human centriole biogenesis These molecular maps are reshaping how researchers think about assembly order and disease-causing mutations, because you can now ask exactly where a mutant protein sits, or fails to sit, relative to the rest of the structure.
Rebuilding Centrioles in a Test Tube
One of the more ambitious frontiers in centriole biology is the attempt to assemble cartwheels entirely outside of cells. Researchers developed a cell-free system using purified SAS-6 and Bld10p proteins from Chlamydomonas and showed that these two components alone are sufficient to build core cartwheel structures. Cryo-electron tomography revealed that the reconstituted cartwheels closely resembled those found in living cells, and that SAS-6 possesses an intrinsic ability to stack its rings in a self-organized fashion. Mathematical modeling of the height distribution suggested that cartwheel growth proceeds by preferential addition of pairs of SAS-6 rings rather than single rings.31PubMed Central. Cell-free reconstitution reveals centriole cartwheel assembly mechanisms Building the structure from scratch in a dish not only confirms which proteins are truly essential but opens the door to testing how mutations and drugs alter the assembly process, free from the complexity of a living cell.