Centrioles are compact, cylindrical structures within a cell that help organize its landscape. Found in animal cells and some lower plant forms, these organelles are fundamental to a cell’s structural integrity and reproduction. They are located near the nucleus in a zone called the centrosome.
Centriole Structure and Organization
A centriole’s function is based on its highly organized architecture. Each centriole is a cylinder measuring approximately 250 nanometers in diameter and between 150 to 500 nanometers in length. The structure is composed of nine sets of microtubules in a “9×3” pattern. This means that nine groups of three fused microtubules form the outer wall of the cylinder, creating a shape like nine bundles of straws held in a ring.
These microtubule triplets are the building blocks, with each consisting of one complete microtubule (the A-tubule) and two partial ones (the B- and C-tubules). This intricate assembly is not found anywhere else in the cell. A cartwheel-like structure at its base stabilizes the centriole and helps organize the assembly of the triplets. This arrangement is conserved across many species, indicating its importance.
In most animal cells, centrioles exist as a pair, oriented at right angles to each other. This pair is embedded within a dense cloud of protein known as the pericentriolar material (PCM). Together, the centrioles and PCM form the centrosome, the cell’s primary microtubule-organizing center (MTOC). From this hub, the network of microtubules that forms the cell’s internal skeleton is organized. These structures are absent in higher plants and fungi, which utilize different mechanisms to organize their microtubules.
Function in Cell Division
A primary role of centrioles is their participation in cell division (mitosis and meiosis). Before a cell begins to divide, the pair of centrioles within the centrosome duplicates, resulting in two centrosomes. During prophase, the first stage of division, these two centrosomes migrate to opposite poles of the cell. This establishes the anchor points for the mitotic spindle.
The centrosomes organize the assembly of microtubules into the mitotic spindle. These microtubules extend from each pole toward the cell’s center, forming a scaffold that attaches to the chromosomes. This spindle aligns the chromosomes in the middle of the cell during metaphase. The spindle fibers then shorten, pulling the duplicated chromosomes apart to ensure each new daughter cell receives a complete set of genetic material.
This role is important for maintaining genomic stability. While some animal cells can divide after their centrioles are removed, the process is often less precise. Centrioles ensure proper spindle formation, which is related to the accurate segregation of chromosomes. Errors in this process can lead to an unequal distribution of genetic information, a common feature in cellular dysfunction.
Formation of Cilia and Flagella
Centrioles also function in creating cellular appendages called cilia and flagella. To do this, a centriole migrates to the cell’s edge and embeds in the plasma membrane. Once anchored at the cell surface, the centriole is referred to as a basal body. The basal body then serves as the foundation from which the axoneme, the microtubule core of a cilium or flagellum, grows.
Cilia and flagella are motile structures with different characteristics. Cilia are short and numerous, working in waves to move fluid across a cell’s surface, like in the respiratory tract, or to receive sensory information. Flagella are longer, whip-like structures, and cells have only one or a few. Their role is propulsion, moving the entire cell, such as a sperm cell.
The centriole’s ability to become a basal body shows its versatility. This allows the same structure to aid in both internal cell division and the external construction of appendages. Cells disassemble their cilia before division so the basal body can return to its role in the centrosome.
Impact of Centriole Abnormalities
Deviations in centriole number or structure can have significant consequences for cellular health, leading to various diseases. An incorrect number of centrioles is associated with errors in chromosome segregation due to their role in organizing the mitotic spindle. Cells with extra centrioles can form abnormal, multipolar spindles during mitosis, which can pull chromosomes in multiple directions at once. This can result in aneuploidy, where daughter cells have an incorrect number of chromosomes, a hallmark of many cancers.
Defects in the centriole’s function as a basal body are linked to a class of genetic disorders known as ciliopathies. These diseases arise from the improper formation or function of cilia and can affect numerous organs. For example, Primary Ciliary Dyskinesia (PCD) is a ciliopathy caused by defects in ciliary movement. This leads to chronic respiratory infections, sinus problems, and hearing loss because cilia cannot effectively clear mucus.
Other conditions linked to basal body dysfunction include some forms of polycystic kidney disease. In this condition, faulty cilia on kidney cells disrupt fluid flow and signaling, leading to cyst formation. Centriole defects are also connected to neurological disorders like microcephaly, a condition characterized by an abnormally small brain. This may stem from problems with the division of neural progenitor cells.