Centrioles are small cylindrical organelles in the cytoplasm of many eukaryotic cells. They consist primarily of microtubules and typically occur in pairs, forming the core of the cell’s main microtubule-organizing center. In many cells, the pair of centrioles plus surrounding proteins form the structure known as the centrosome. Centrioles play essential roles in organizing the microtubule network, facilitating cell division, and serving as basal bodies for the formation of cilia or flagella in cells that require them.
Because of their key roles in microtubule organization and cell division, centrioles are critical for the internal architecture, polarity, and proper division of many cells. At the same time, not all eukaryotic cells have centrioles, and their absence in those cells reveals that alternative mechanisms exist to organize microtubules and perform mitosis. The presence, structure, and duplication of centrioles reflect an important evolutionary and functional specialization in many eukaryotes.
Key Takeaways: Centrioles
- Centrioles are barrel-shaped microtubule-based organelles that are common in animal cells and some lower plants but absent in most higher plants and many fungi.
- A typical centriole has nine sets of microtubule triplets arranged in a cylindrical structure.
- Centrioles often exist as a pair, oriented at roughly right angles, embedded in pericentriolar material to form a centrosome.
- Their main functions include organizing microtubules, helping form the mitotic spindle during cell division, and acting as basal bodies for cilia or flagella.
- Centriole duplication occurs once per cell cycle, typically during S phase, ensuring each daughter cell inherits a centrosome.
- In cells that produce cilia or flagella, a centriole can migrate to the cell surface and function as a basal body.
- Some cells and organisms lack centrioles, yet still carry out mitosis, providing evidence that centrioles are not strictly essential for cell division, though they improve its efficiency and reliability.
What Are Centrioles?
Centrioles are non–membrane-bound organelles that consist of microtubules. They serve as structural cores for centrosomes and also function as basal bodies to nucleate cilia and flagella. They have distinctive microtubule architecture and pairing and play a key role in microtubule organization.
Location, Appearance, Size, and Structure
Because of their highly ordered and conserved architecture, centrioles are among the largest protein complexes (molecular assemblies) in the cell.
- Location: In most animal cells, centrioles reside in the cytoplasm, often near the cell nucleus. They are embedded in a dense protein matrix known as pericentriolar material (PCM). The pair of centrioles plus PCM together form the centrosome, the cell’s primary microtubule‑organizing center.
- Appearance: Under electron microscopy, centrioles appear as short hollow cylinders. A transverse (cross) section reveals a characteristic “9 + 0” arrangement. There are nine microtubule triplets in a ring around an empty center.
- Size: In human (and many animal) cells a mature centriole typically measures roughly 150–500 nanometers in length and about 200–250 nanometers in diameter.
- Structure details: A mature centriole typically measures about 150–500 nanometers in length and about 200–250 nanometers in diameter. Each of the nine microtubule triplets consists of one complete microtubule (the A tubule) and two incomplete tubules (B and C), forming a radial symmetry. The older, or “mother,” centriole often has additional structures such as distal and subdistal appendages that are important when it converts to a basal body for cilium or flagellum formation. The pair of centrioles is usually at right angles to one another.
Functions of Centrioles
Centrioles perform several critical functions that are central to cell structure and division.
- Microtubule Organization: Centrioles help nucleate and organize the microtubules that form the cytoskeleton, supporting the cell’s shape and internal transport systems.
- Cell Division: During mitosis and meiosis, centrioles play a central role in forming the spindle apparatus. Centrosomes, each containing a centriole pair, migrate to opposite poles of the cell to direct the spindle microtubules that separate chromosomes.
- Cilia and Flagella Formation: In cells that possess cilia or flagella, a centriole can become a basal body. The basal body nucleates the axoneme (the core structure of a cilium or flagellum) and anchors it to the cell surface.
- Cell Polarity and Spatial Organization: By organizing microtubules, centrioles influence the positioning of the nucleus and organelles and determine the cell’s polarity.
- Developmental and Specialized Roles: In specific tissues or during development, centrioles influence asymmetric cell division, the orientation of growth, and the construction of sensory cilia.
Which Cells Have Centrioles and Which Do Not?
Not all eukaryotic cells have centrioles.
- Cells with Centrioles: Most animal cells have centrioles. Many protists and lower plants also possess them, particularly those with motile stages or flagellated gametes.
- Cells without Centrioles: Most higher plants (such as flowering plants and conifers) and fungi lack centrioles entirely.
- Special Cases: Some specialized animal cells may lack centrioles (e.g., oocytes), while some lower plants retain them only in certain reproductive cells.
Cells that lack centrioles still organize microtubules using other mechanisms, such as microtubule-nucleating sites on the nuclear envelope. These alternative methods support cell division, though perhaps with less spatial precision than centriole-based systems. The evolutionary loss of centrioles in some lineages suggests that while they enhance cellular organization, they are not essential for all types of eukaryotic life.
Difference Between a Centrosome and a Centriole
A centriole is a single barrel-shaped organelle made of microtubule triplets, while a centrosome is a structure that contains two centrioles arranged at right angles, surrounded by pericentriolar material (PCM). The centrosome acts as the main microtubule-organizing center (MTOC) of animal cells.
During mitosis, the centrosomes duplicate and migrate to opposite poles, forming the spindle apparatus. However, in contexts such as cilia formation, a single centriole may function independently as a basal body without forming a complete centrosome.
How Centriole Duplication Occurs Before/During Cell Division
Centriole duplication is a tightly regulated process that ensures each new cell receives a pair of centrioles during division.
Duplication begins in the S phase of the cell cycle. Each mother centriole serves as a template for a daughter centriole (procentriole), which begins growing orthogonally from its base. This process starts with the formation of a cartwheel-like structure that sets the symmetry of the new centriole.
The microtubules of the new centriole assemble from the A, B, and C tubules, assisted by proteins such as γ-tubulin, STIL, and PLK4, which regulate the biogenesis and prevent overduplication. After duplication, the two pairs of centrioles (within two centrosomes) remain together until the onset of mitosis, when they move to opposite poles to guide chromosome segregation.
This precise one-duplication-per-cycle mechanism prevents defects in spindle formation and ensures genomic stability.
History of the Discovery and Study of Centrioles
The study of centrioles has evolved alongside advances in microscopy and cell biology.
- In 1875, Walther Flemming observed structures that were likely centrioles.
- Édouard Van Beneden described centrosome structures in 1876 and identified their paired arrangement in 1883.
- Theodor Boveri introduced the term “centrosome” in 1888 and coined “centriole” in 1895.
- Electron microscopy in the mid-20th century revealed the detailed triplet microtubule structure and ninefold symmetry.
- Molecular studies in the late 20th and 21st centuries identified key regulatory proteins (e.g., PLK4) and mechanisms of centriole biogenesis.
Over time, centrioles have been recognized as not just structural curiosities but as critical components of the cell cycle, development, and even diseases such as ciliopathies and cancer when centriole function is disrupted.
Centrioles and Human Disease
Centrioles are critical for both cell division and cilia formation, so errors in their number or structure can lead to major health problems. When centriole duplication becomes uncontrolled or defective, cells may form too many spindle poles during mitosis, resulting in chromosomal mis-segregation and genomic instability. This phenomenon is common in cancer cells, where centrosome amplification contributes to tumor progression.
Because basal bodies originate from centrioles, defects in centriole maturation or appendage formation can disrupt cilia or flagella. Diseases resulting from abnormal cilia are called ciliopathies. These include disorders such as Bardet–Biedl syndrome, primary ciliary dyskinesia, Joubert syndrome, and certain forms of polycystic kidney disease. In addition, mutations in specific centriole duplication proteins are associated with primary microcephaly, a developmental disorder in which the brain fails to reach a normal size due to defective neural cell division and differentiation.
Understanding how centriole abnormalities contribute to disease is an important area of biomedical research, with implications for developmental disorders, cancer diagnosis, and potential therapies targeting mitosis.
Centriole‑Associated Proteins
Many specialized proteins build and regulate centrioles. These proteins ensure ninefold symmetry, stabilize the microtubule triplets, and control duplication so each centriole forms only one daughter.
Key protein roles include:
- Cartwheel formation: SAS‑6 and related proteins set ninefold symmetry during early biogenesis.
- Microtubule stabilization: Centrin, CPAP, and other structural factors ensure correct length and support the barrel shape.
- Duplication control: PLK4, STIL, and SAS‑5 trigger and regulate procentriole formation, restricting duplication to once per cell cycle.
- Centriole maturation: ODF2 and others form distal and subdistal appendages on the mother centriole, enabling basal body function and microtubule anchoring.
Many of these proteins are conserved across eukaryotes, underscoring the evolutionary importance of centriole structure.
Evolution of Centrioles
Centrioles are thought to have evolved from ancient basal bodies associated with primitive motile structures. Their widespread presence in animals and protists, along with their absence in many higher plants and fungi, suggests that centrioles may have originally been required for flagellar or ciliary motility.
Several evolutionary trends are recognized:
- Organisms that rely on cilia or flagella tend to retain centrioles.
- Higher plants, which have largely lost motile cells, evolved alternative microtubule‑organizing strategies.
- Fungi also perform mitosis without centrioles, reflecting functional redundancy and evolutionary divergence.
These patterns highlight centriole specialization in certain lineages and reveal flexible solutions for microtubule organization in different eukaryotes.
Centrioles in Reproductive and Stem Cells
Centriole inheritance varies among organisms and cell types. In many animals, sperm contribute a centriole to the fertilized egg, establishing the zygote’s centrosome. By contrast, oocytes in various species lose centrioles during maturation, relying on the sperm’s centriole for later cell divisions.
In embryonic development and stem cell biology, centrioles also influence the balance between self‑renewal and differentiation. Asymmetric centriole inheritance can guide cell polarity and fate decisions. For example, in neural stem cells, the older mother centriole is often inherited by the self‑renewing daughter cell, while the newer centriole is passed to the differentiating neuron.
These variations reveal the centriole’s importance beyond simple organelle duplication, extending into developmental regulation.
Centriole Life Cycle and Maturation
Centrioles undergo a multi‑step life cycle that affects their structure and function.
Key stages include:
- Formation: New daughter centrioles form orthogonally to mothers during S phase.
- Engagement: A mother and daughter remain connected until late mitosis.
- Maturation: After disengagement, daughter centrioles add appendages as they age.
- Basal body conversion: Mature mother centrioles can dock at the cell surface to form cilia or flagella.
The presence of appendages, length, and protein composition distinguish mature and immature centrioles. Only fully mature mother centrioles can efficiently nucleate a primary cilium, making centriole age a key determinant of cellular behavior.
Experimental Techniques for Studying Centrioles
Advances in research tools have transformed centriole biology into a highly detailed field of study.
Common techniques include:
- Electron microscopy (TEM or cryo‑EM) to visualize triplet microtubule architecture.
- Fluorescence microscopy with labeled tubulin or centriole proteins to track location and duplication.
- Live‑cell imaging to observe centriole dynamics through the cell cycle.
- Genetic manipulation such as knockouts or RNA interference to identify essential proteins.
- Proteomic and biochemical studies to map centriole composition and assembly pathways.
Modern tools continue to reveal previously unknown components and deepen understanding of centriole structure and regulation.
Considerations and Open Questions
Despite extensive research, centrioles still pose several open questions in cell biology.
- The exact molecular mechanisms that limit centriole duplication to once per cell cycle needs more study, including how symmetry-breaking occurs to form only one daughter centriole.
- The evolutionary reasons for the presence or absence of centrioles in various eukaryotic lineages remain a topic of investigation.
- The functional differences between mother and daughter centrioles remains an area of active research.
Understanding these processes may have implications for developmental biology, stem cell research, and therapies for diseases involving cell division and ciliary function.
FAQs About Centrioles
Q: Do all cells have centrioles?
A: No. Most animal cells have centrioles, but many other eukaryotic cells (higher plants and most fungi) lack them. These organisms use alternative structures to organize their microtubules and conduct cell division.
Q: What is the difference between a centriole and a centrosome?
A: A centriole is a single cylindrical organelle consisting of microtubule triplets. A centrosome is a larger structure that includes two centrioles (usually arranged at right angles) and surrounding pericentriolar material (PCM). The centrosome acts as the main microtubule-organizing center in many animal cells.
Q: Can cells divide without centrioles?
A: Yes. Some cells divide successfully without centrioles, using alternative microtubule-organizing centers. However, centrioles increase the efficiency and spatial organization of mitosis and are important for forming cilia and flagella.
Q: What happens if a cell has too many centrioles?
A: Supernumerary centrioles can cause the formation of abnormal mitotic spindles with more than two poles. This may result in unequal chromosome segregation, genomic instability, and an increased risk of cancer.
Q: How many centrioles does a typical animal cell have?
A: A typical animal cell has a pair of centrioles within the centrosome. During cell division, this pair duplicates so each daughter cell inherits two centrioles.
Q: Do cilia and flagella require centrioles?
A: Yes. Centrioles can convert into basal bodies, which are essential for initiating the growth of cilia and flagella by organizing their microtubule structure (the axoneme).
Q: Do plant cells have centrioles?
A: Most higher plant cells do not have centrioles. Instead, they use other structures near the nuclear envelope to organize microtubules and form the mitotic spindle.
Q: How does centriole inheritance work during fertilization?
A: In many animals, the egg lacks centrioles, and the sperm contributes one or two centrioles to the zygote. This re-establishes the centrosome and enables the first cell divisions after fertilization.
Q: Why do some cells lack centrioles altogether?
A: Cells in lineages that do not need cilia or flagella (e.g., most flowering plants) may lose centrioles evolutionarily. They have developed alternative systems for organizing microtubules that do not rely on centrioles or centrosomes.
Q: What proteins control centriole duplication?
A: Key regulatory proteins include PLK4 (a kinase), STIL, SAS-6, and CPAP. These factors help initiate and guide the formation of a new centriole next to each existing one during the S phase of the cell cycle.
References
- Beisson, J.; Wright, M. (2003). “Basal body/centriole assembly and continuity”. Current Opinion in Cell Biology. 15 (1): 96–104. doi:10.1016/S0955-0674(02)00017-0
- Fishman, Emily L.; Jo, Kyoung; et al. (2018). “A novel atypical sperm centriole is functional during human fertilization”. Nature Communications. 9 (1): 2210. doi:10.1038/s41467-018-04678-8
- Lawo, Steffen; Hasegan, Monica; Gupta, Gagan D.; Pelletier, Laurence (2012). “Subdiffraction imaging of centrosomes reveals higher-order organizational features of pericentriolar material”. Nature Cell Biology. 14 (11): 1148–1158. doi:10.1038/ncb2591
- Rieder, C. L.; Faruki, S.; Khodjakov, A. (2001). “The centrosome in vertebrates: more than a microtubule-organizing center”. Trends in Cell Biology. 11 (10): 413–419. doi:10.1016/S0962-8924(01)02085-2
- Quarmby, L.M.; Parker, J.D. (2005). “Cilia and the cell cycle?”. The Journal of Cell Biology. 169 (5): 707–10. doi:10.1083/jcb.200503053