Centrioles are barrel-shaped cellular structures built primarily from the protein tubulin, organized into a cylindrical pattern. These microscopic components are typically found in pairs, positioned at right angles to one another, near the cell’s nucleus. In cells that possess them, centrioles reside within a larger, amorphous mass of protein known as the pericentriolar material, which together forms the centrosome. This entire assembly functions as a central hub for organizing the internal scaffolding of the cell and plays a significant role in several processes necessary for life.
Primary Functions of Centrioles
The primary role of centrioles involves organizing the cell’s internal skeleton, specifically the network of microtubules. During cell division, the centrosome, containing the centrioles, functions as the main microtubule-organizing center (MTOC) in many organisms. As the cell prepares to divide, the centrosome duplicates and the two resulting bodies move to opposite sides of the cell nucleus.
From these opposing poles, the mitotic spindle is nucleated and assembled. These spindle fibers attach to the chromosomes, ensuring that the genetic material is accurately separated and distributed evenly between the two nascent daughter cells during mitosis or meiosis.
Beyond cell division, centrioles also have a direct role in cellular movement and sensory function. A centriole can migrate to the cell’s surface and transform into a structure called a basal body. The basal body then anchors and organizes the growth of cilia or flagella. These structures are used for locomotion, such as propelling a single-celled organism through water, or for moving fluid over a tissue surface, like the cilia lining the human respiratory tract.
Life Forms That Possess Centrioles
Centrioles are standard components in the Kingdom Animalia, where they are found in nearly every cell type, from sponges to mammals. The presence of a centrosome containing a pair of centrioles is the defining MTOC structure for all animal cells that undergo division.
Many single-celled eukaryotes, classified under the Kingdom Protista, also possess centrioles. They utilize centrioles to form the basal bodies necessary for their flagella or cilia. The motile forms of many algae, such as Chlamydomonas, rely on these structures to organize the flagella that allow them to swim toward light sources.
Centrioles are not universally present in plants, but they are retained in several ancestral groups. These lower plants, including bryophytes and pteridophytes, produce motile sperm cells that require flagella for movement through water. Centrioles are present in the cells that develop these motile gametes, forming the basal bodies necessary to construct the flagella.
Ancient Seed Plants
Even some ancient seed plants, such as cycads and Ginkgo biloba, still produce flagellated sperm. This necessitates the presence of centrioles in those specific reproductive cells.
Organisms That Do Not Utilize Centrioles
The absence of centrioles is common in several major life forms that have evolved alternative strategies for cellular organization. The prokaryotes, including all bacteria and archaea, lack these structures. Since prokaryotic cells lack a nucleus and complex membrane-bound organelles, they do not possess the microtubule-based cytoskeleton or the complex division machinery that centrioles help organize.
Higher plants, such as flowering plants (angiosperms) and cone-bearing plants (gymnosperms), have lost centrioles. These plants still undergo mitosis but have evolved a decentralized system for spindle formation. Instead of a single, centralized centrosome, they use numerous dispersed microtubule organizing centers (MTOCs) scattered throughout the cytoplasm and around the nuclear envelope.
In these higher plant cells, the mitotic spindle is constructed through a process known as acentrosomal division, where microtubules are nucleated from the surface of the nucleus itself. These MTOCs organize the spindle fibers to separate chromosomes without the need for a centriole structure to define the poles. This adaptation is linked to the fact that these plants no longer produce motile cells and do not require the basal bodies that centrioles form.
Most species within the Kingdom Fungi also do not possess centrioles, relying on a distinct mechanism for spindle assembly. Instead of a centrosome, fungi utilize a specialized organelle called the spindle pole body (SPB). The SPB is a dense, plaque-like structure embedded within the nuclear envelope in many fungal species, including common yeasts.
Unlike the animal centrosome, the fungal SPB remains intact within the nuclear envelope throughout mitosis. The SPB nucleates the microtubules that form the mitotic spindle. The nuclear envelope itself often remains intact during division in many fungi, meaning the SPB is directly responsible for organizing the spindle from within or across the nuclear boundary, a process different from the open mitosis seen in animal cells.