Centrioles are small, complex structures found within the cells of many organisms. These tiny components are fundamental to various cellular processes. They play a significant role in organizing the internal framework of the cell and facilitating movement and division. Understanding centrioles often begins with examining their unique visual characteristics, which reveal their intricate design and hint at their diverse functions.
What a Centriole Image Looks Like
When viewed under an electron microscope, a centriole presents a distinctive cylindrical shape. This barrel-like structure is approximately 200-250 nanometers (nm) in diameter and typically around 500 nm in length in vertebrate cells, though lengths can vary, for instance, in some worms they can be as short as 150 nm or as long as 2.5 micrometers in Drosophila sperm. The most striking feature visible in an image is its nine-fold radial symmetry.
Each of these nine sets consists of three microtubules, known as a microtubule triplet. These triplets are tightly bundled together, forming the wall of the cylinder. Within each triplet, one microtubule is complete, while the other two are incomplete and share portions of their walls with the adjacent microtubules, creating a strong, stable arrangement. This specific organization, often described as a “cartwheel” structure due to radial spokes extending from a central hub to the triplets, is a hallmark of centriole images.
The Centriole’s Place in the Cell
Centrioles exist in pairs within the cell, positioned perpendicularly to each other. This paired arrangement is found within a larger cellular structure known as the centrosome. The centrosome, located near the cell’s nucleus, acts as a primary microtubule-organizing center (MTOC) in animal cells. Images show these two perpendicularly oriented centrioles embedded within a cloud of dense material, called the pericentriolar material (PCM), which surrounds them.
Centrioles are primarily found in animal cells and some lower plant forms, such as mosses and ferns, but are generally absent in higher plants and fungi. In these higher plant cells, cell division and microtubule organization occur without the presence of centrioles.
Centrioles in Cellular Processes
The unique structure and specific cellular location of centrioles are directly linked to their varied functions in the cell. As components of the centrosome, centrioles play a role in organizing the cell’s internal scaffolding, particularly the microtubules. The centrosome, with its embedded centrioles, serves as a hub for initiating and organizing microtubule growth, influencing the overall shape and internal arrangement of the cell. This organization is dynamic and adapts to the cell’s needs, such as during movement or division.
A primary function of centrioles is their involvement in cell division, specifically mitosis and meiosis. During cell division, the centrosome, containing the centriole pair, duplicates, and these two centrosomes migrate to opposite ends of the cell. From these poles, microtubules assemble to form the spindle apparatus, which is essential for separating duplicated chromosomes equally into two new daughter cells. While centrioles are associated with spindle formation, research indicates that animal cells can still divide even if their centrioles are removed, although this might lead to abnormalities in spindle development and chromosome separation.
Beyond cell division, centrioles also contribute to the formation of cilia and flagella. These hair-like structures extend from the cell surface and are important for cell movement and sensing the surrounding environment. In this role, a single centriole migrates to the cell’s outer membrane and transforms into a basal body, which then acts as the foundation for the growth of cilia or flagella.