Centrioles are small, cylindrical structures found in the vast majority of eukaryotic cells, where they organize the cell’s internal machinery. They typically exist as a pair, oriented perpendicular to each other, embedded within a larger structure called the centrosome, usually near the cell’s nucleus. Centrioles maintain cellular organization and perform two major functions necessary for cell proliferation and communication.
The Unique Architecture
The defining feature of a centriole is its precise, highly conserved molecular structure, often described as a “9+0” arrangement. This refers to nine sets of triplet microtubules arranged radially in the cylinder wall, with no microtubules present in the central core. Each triplet consists of three fused microtubules, labeled A, B, and C, where the A-tubule is complete and the B- and C-tubules share segments.
Centrioles are built from tubulin and a large collection of associated proteins. In vertebrates, the structure is roughly 0.25 micrometers in diameter, giving it a barrel-like shape. The center often contains a proteinaceous hub connected to the peripheral triplets by nine radial spokes, creating a distinct cartwheel appearance.
Role in Cell Division
Centrioles are involved in cell division by organizing the centrosome, the cell’s main Microtubule-Organizing Center (MTOC). Before division, the centrosome must duplicate during the S phase of the cell cycle. As the cell enters mitosis, the two resulting centrosomes migrate to opposite poles of the nucleus.
From these poles, they direct the assembly of the mitotic spindle, a complex network of microtubules. Microtubules attach to the chromosomes, forming a path for them to be accurately separated. This process ensures that each new daughter cell receives a complete set of genetic material during chromosome segregation.
Forming Cilia and Flagella
Centrioles also serve as the foundation for two cellular appendages: cilia and flagella. When a cell exits the division cycle, the mother centriole can migrate to the cell surface and transform into a basal body. The basal body acts as a template and anchor point, initiating the growth of the microtubule shaft.
In motile structures, such as the flagellum of sperm or the cilia lining the respiratory tract, the basal body nucleates a shaft with a “9+2” arrangement: nine outer microtubule doublets surrounding two central singlet microtubules. Conversely, most cells possess a single, non-motile primary cilium, which functions as a sensory antenna. These primary cilia retain the 9+0 microtubule arrangement in their base.
Centriole Dysfunction and Disease
Failures in centriole structure or regulation are linked to several human diseases. Errors in centriole duplication, such as over-replication, lead to an abnormal number of centrosomes, a condition known as centrosome amplification. Extra centrosomes can result in the formation of multipolar mitotic spindles, which improperly distribute chromosomes.
This causes genomic instability and aneuploidy (an abnormal number of chromosomes), which is a factor in the development and progression of cancer. Defects in the centriole’s role as a basal body cause a group of disorders known as ciliopathies. These diseases arise when the cell is unable to form or maintain functional cilia. Ciliopathies affect various organ systems because cilia are widespread sensory and signaling structures. Examples include polycystic kidney disease, where fluid-filled cysts form in the kidneys, and retinal degeneration, which leads to vision loss.