Centrioles are small cylindrical structures found in many animal cells and other eukaryotic organisms. They are best known for helping organize microtubules, especially during cell division, but they also contribute to the formation of structures such as cilia and flagella.
A typical animal cell contains a pair of centrioles arranged at roughly right angles to each other. Together with surrounding proteins, this pair forms the centrosome, a major microtubule-organizing center. Understanding how centrioles are built and how they behave helps explain how cells maintain their internal organization, move materials, and divide accurately.
What is a centriole?
A centriole is a short, barrel-shaped structure made primarily of microtubules. It is usually about 0.2 micrometers in diameter and around 0.3–0.5 micrometers long, although its dimensions can vary.
Centrioles occur in pairs in many animal cells. The two centrioles differ according to their age and role: the older mother centriole has specialized features that the younger daughter centriole initially lacks.
Centrioles are not surrounded by a membrane. Instead, they are composed of a precisely organized arrangement of protein filaments, surrounded by other proteins that help them perform their cellular functions.
Their most important role is not to act as miniature structures that directly pull chromosomes apart. Rather, centrioles help organize the larger microtubule-based machinery that performs this and other tasks.
The structure of a centriole
The defining feature of a centriole is its arrangement of nine microtubule triplets around a central axis.
Each of the nine repeating units contains three closely associated microtubules, conventionally called A, B, and C tubules. The A tubule is a complete microtubule, while the B and C tubules share portions of their walls with the neighboring tubule.
This produces a characteristic 9-fold symmetry: nine triplet units form a cylinder around the center of the centriole.
The triplets are connected and stabilized by various proteins. At the proximal end, the centriole has a distinctive internal organization often described as a cartwheel. The cartwheel helps establish the centriole’s ninefold symmetry during its formation.
Although a centriole is tiny, its architecture is remarkably ordered. Its structure is also closely related to that of basal bodies, which anchor cilia and flagella.
Centrioles and the centrosome
A centriole should not be confused with a centrosome.
The centrosome is a larger cellular structure consisting of a pair of centrioles embedded in a protein-rich material called pericentriolar material, or PCM. The PCM contains proteins that nucleate and organize microtubules.
In many animal cells, the centrosome serves as the cell’s main microtubule-organizing center (MTOC).
Microtubules are dynamic protein filaments that form part of the cytoskeleton. They help maintain cell shape, position organelles, transport materials within the cell, and organize chromosomes during cell division.
The centrioles provide an important structural framework for the centrosome, while the surrounding pericentriolar material performs much of the direct microtubule-nucleating activity.
How centrioles help organize microtubules
Microtubules are built from a protein called tubulin. They continually assemble and disassemble, allowing the cell to remodel its internal structure as conditions change.
The centrosome can act as a site where new microtubules are initiated. Proteins in the pericentriolar material, particularly gamma-tubulin-containing complexes, help start microtubule formation.
Centrioles therefore play an organizational role rather than simply serving as structures that produce microtubules themselves.
This organization is especially important in animal cells because microtubules extend outward from centrosomes and establish an organized network throughout the cytoplasm.
What happens to centrioles during cell division?
Centrioles have an important role in preparing cells for division.
Before a cell divides, its centriole pair duplicates. Each original centriole gives rise to a new daughter centriole, producing two centriole pairs. The duplication process is tightly regulated so that, under normal circumstances, only one new centriole forms next to each existing centriole during a cell cycle.
As the cell progresses toward mitosis, the two centrosomes move apart. They become the two major poles of the developing mitotic spindle, a microtubule-based structure that organizes and separates duplicated chromosomes.
The spindle helps ensure that each daughter cell receives the appropriate set of chromosomes.
Centrioles are therefore closely associated with centrosome duplication and spindle organization. However, the relationship is more nuanced than the common description that “centrioles pull chromosomes apart.” The microtubules and associated spindle proteins carry out the mechanical work of chromosome segregation, while centrosomes help organize the spindle.
Some animal cells can form functional spindle structures without centrioles under particular experimental or developmental circumstances. This shows that centrioles are important organizers but are not universally indispensable for every aspect of spindle formation.
Centriole duplication is carefully controlled
Centriole duplication resembles DNA replication in one important respect: both are normally limited to once per cell cycle.
During centriole duplication, a new procentriole begins to form beside each preexisting centriole. The new structure initially grows perpendicular to its parent. As it matures, it develops the characteristic architecture and specialized features of a centriole.
The process involves several conserved proteins that control the initiation and assembly of the new centriole. Tight regulation is essential. Excessive centriole formation can produce abnormal numbers of centrosomes and interfere with accurate chromosome segregation.
After duplication, the newly formed centrioles do not immediately become fully mature copies of their parents. Centriole maturation occurs over subsequent stages of the cell cycle and can include the acquisition of specialized structures and proteins.
The mother centriole has specialized features
The two centrioles in a centrosome are not identical.
The older mother centriole develops structures called distal appendages and, in many cells, subdistal appendages. These specialized protein structures give the mother centriole capabilities that the younger daughter centriole does not initially possess.
Distal appendages are particularly important when the mother centriole becomes a basal body, the structure that anchors a cilium to the cell surface.
The difference between mother and daughter centrioles is therefore important for understanding why centriole age matters. A centriole’s functions change as it matures.
Centrioles and cilia
One of the most important functions of mature centrioles is their ability to become basal bodies.
A cilium is a slender, membrane-covered projection extending from the surface of a cell. Many cilia contain an internal structure made from microtubules and associated proteins. In motile cilia, coordinated microtubule movements allow the cilium to beat and move fluid across the cell surface.
Before a cilium forms, a centriole can migrate toward the cell membrane and become a basal body. The basal body anchors the cilium and helps organize its internal microtubule structure.
This connection explains why centriole biology is closely linked to cilia biology. Problems affecting centriole formation or function can consequently interfere with the development or operation of cilia.
Centrioles are closely related to basal bodies
Centrioles and basal bodies have essentially the same basic nine-triplet microtubule architecture, but they occupy different cellular contexts.
A centriole is typically associated with the centrosome or exists as part of the centriole cycle. A basal body is positioned at the base of a cilium or flagellum and serves as an anchoring and organizing structure.
The same physical structure can effectively change roles depending on where it is located and what cellular structures are being assembled around it.
Do all cells have centrioles?
No.
Centrioles are characteristic of animal cells and are also found in many other eukaryotic organisms, including numerous unicellular species. But they are absent from several major groups of organisms.
Most higher plants, for example, do not contain typical centrioles in their vegetative cells. Yet plant cells still organize microtubules and form mitotic spindles. They use other microtubule-organizing mechanisms instead.
Centrioles are also absent from some animal cell types. Mature human red blood cells, for example, lack the typical cellular structures found in nucleated cells because they lose their nucleus and most organelles during maturation.
Thus, centrioles are important components of particular cellular systems, not universal features of every eukaryotic cell.
Why centriole structure matters
The precise architecture of centrioles allows them to perform several related functions.
Their ninefold organization provides a stable framework for centrosome development and for the formation of basal bodies. Their duplication and maturation are coordinated with the cell cycle. Their association with pericentriolar material allows centrosomes to organize microtubules. And their conversion into basal bodies connects centriole biology with cilium formation.
These roles make centrioles important at several levels of cell biology: cell division, cytoskeletal organization, intracellular organization, and cilium formation.
What happens when centriole function goes wrong?
Because centrioles participate in cell division and cilium formation, abnormal centriole number or structure can have significant consequences.
Errors in centriole duplication can lead to abnormal centrosome numbers. Multiple centrosomes can disrupt the normal organization of the mitotic spindle and increase the likelihood of chromosome-segregation errors.
Abnormalities in centriole formation or maturation can also interfere with cilia. Since cilia have important roles in processes ranging from movement of fluid across epithelial surfaces to cellular signaling and development, defects in the structures that build them can affect many biological systems.
Centriole abnormalities have therefore attracted considerable attention in the study of developmental disorders and cancer biology. Importantly, the presence of abnormal centrioles is not by itself a diagnosis or a simple explanation for disease; centriole biology is part of a much larger network of cellular processes.
The key idea to remember
A centriole is a small, non-membrane-bound cylinder built from nine microtubule triplets. Two centrioles, together with surrounding pericentriolar material, form a centrosome that organizes microtubules in many animal cells.
Centrioles duplicate once during a normal cell cycle, help establish the centrosomes that organize the mitotic spindle, and can mature into basal bodies that anchor cilia. Their highly ordered structure is therefore directly connected to their ability to organize larger cellular structures.
Rather than thinking of centrioles simply as “parts of the centrosome,” it is more accurate to view them as specialized microtubule-based organizers whose functions change as they duplicate, mature, and move between cellular contexts.
