Centrioles are small, cylindrical structures found within the cytoplasm of most animal cells. These organelles function as core organizing centers that direct the assembly and orientation of microtubules, the structural rods that make up the cell’s internal scaffolding. Their presence is fundamental to cellular geometry and movement, acting as architects for two dynamic processes: accurate cell division and the formation of specialized appendages.
The Centriole’s Structure and Location
A centriole consists of a hollow cylinder roughly 200 to 250 nanometers in diameter. This cylinder is built from twenty-seven microtubules, which are polymers of the protein tubulin. These microtubules are arranged in a specific pattern of nine groups, with each group containing three fused microtubules (the 9×3 arrangement). Centrioles typically exist in pairs, known as the diplosome, positioned perpendicular to one another in an L-shape. This pair is embedded within a dense cloud of protein called the pericentriolar material (PCM). The combination of the centriole pair and the surrounding PCM constitutes the centrosome, which functions as the primary microtubule-organizing center (MTOC) during interphase.
Centrioles and Cell Division: Orchestrating Mitosis
The primary function of the centrosome, and the centrioles at its core, is the organization of cell division (mitosis). Before division, the single centrosome duplicates, resulting in two organizing centers. These two centrosomes move to opposite sides of the cell nucleus, establishing the poles of the dividing cell. The pericentriolar material nucleates the growth of microtubules, which form the mitotic spindle fibers. These fibers extend across the cell, attaching to the chromosomes at their centromeres. The spindle fibers pull the duplicated chromosomes apart, ensuring each new daughter cell receives an identical and complete set of genetic material.
Beyond Division: Forming Cilia and Flagella
Centrioles also have a second function in cells that are not actively dividing: forming the base of specialized cellular appendages. One centriole leaves the centrosome and migrates to the cell membrane, transforming into a structure called the basal body. The basal body serves as the nucleation site and anchor from which cilia and flagella grow outward. Cilia are slender, hair-like projections that can be motile (e.g., sweeping mucus from the lungs) or non-motile (the primary cilium). The primary cilium acts as a sensory antenna, sensing external signals. Flagella, such as the tail of a sperm cell, are longer and provide propulsion for cellular movement. This transformation into a basal body is a prerequisite for cellular sensation and motility.
When Centrioles Malfunction: Implications for Health
Errors in centriole duplication or function can result in severe health consequences. A common defect is the overduplication of centrioles, leading to centrosome amplification. This increase causes the formation of an abnormal number of spindle poles during cell division, resulting in the missegregation of chromosomes. The resulting abnormal chromosome number, known as aneuploidy, is a characteristic of many human cancers. Furthermore, defects in the centriole’s role as a basal body cause a group of genetic disorders called ciliopathies. These disorders manifest as symptoms including polycystic kidney disease, chronic respiratory problems due to non-motile lung cilia, and various developmental syndromes.