Centrioles are small, cylindrical structures found within the cells of animals and certain lower plants. These organelles are located near the cell’s nucleus, within a specialized region of the cytoplasm known as the centrosome. A cell usually contains a pair of centrioles, which are positioned perpendicular to one another, forming an L-shape. They are composed of protein and lack a surrounding membrane, setting them apart from many other cellular compartments. The function of centrioles is closely linked to a cell’s ability to divide and form external appendages.
The Defining 9+0 Structure
The most striking feature of a centriole is its architecture, which resembles a hollow wooden barrel or a wagon wheel when viewed in cross-section. This structure is built from the protein tubulin, assembled into rigid tubes called microtubules. The centriole is characterized by a “9+0” arrangement, which refers to the pattern of these microtubules visible in a cross-sectional electron micrograph.
The structure consists of nine groups of microtubules, known as triplets, arranged in a ring. Each group is made up of three microtubules fused together lengthwise, like three pipes sharing a wall. These nine triplet groups form the cylinder’s wall, but the center remains empty, hence the “plus zero” in the description.
The centriole measures approximately 0.2 micrometers in diameter and usually between 0.3 to 0.7 micrometers in length. At the proximal end, where the new centriole is built, an internal structure known as the cartwheel is sometimes visible. The cartwheel consists of a central hub connected to the nine triplets by spokes. This unique, nine-fold symmetry provides mechanical stability and serves as a template for the formation of other microtubule-based structures.
Role in Cell Division
Centrioles function as organizers during the process of cell replication, or mitosis. Before a cell divides, the centriole pair duplicates itself, resulting in two centrosomes, each containing a pair of centrioles. These two centrosomes then migrate to opposite sides of the cell’s nucleus during the initial stages of mitosis.
Once positioned at the cell poles, the centrioles help to organize the formation of the mitotic spindle, a network of microtubules that spans the cell. They act as the focal points from which these spindle fibers radiate outward. The spindle fibers attach to the duplicated chromosomes and precisely align them in the cell’s center.
The organized spindle apparatus ensures that when the cell divides, the duplicated chromosomes are accurately pulled apart and delivered equally to the two new daughter cells. Maintaining the correct number of chromosomes in each generation of cells is crucial. Without this structural guidance, chromosome separation would be chaotic, leading to genetic abnormalities.
Centrioles as Basal Bodies
Beyond their function in cell division, centrioles can also transform to serve a role in building motile and non-motile appendages on the cell surface. A centriole that migrates to the cell membrane and anchors itself there is referred to as a basal body. Basal bodies act as the template and foundation for the growth of cilia and flagella, which are hair-like projections extending from the cell.
The basal body maintains the original nine-fold symmetry of the centriole, with its nine triplet microtubules. However, as the cilium or flagellum extends from the basal body, the arrangement of microtubules within the shaft changes. The resulting structure, called the axoneme, features a “9+2” pattern, consisting of nine pairs of outer microtubules and two single microtubules running down the center. This transition demonstrates the adaptability of the centriole’s core structure, allowing it to template both internal cellular organization and external motility structures.