A centriole is a small, barrel-shaped structure found primarily in the cells of animals and some lower plants. These organelles are built from the protein tubulin, which assembles into a highly organized framework. Their precise structure allows them to carry out two main functions: organizing cell division and creating specialized external structures for movement and sensation.
Structure and Location within the Cell
The physical construction of a centriole is remarkably consistent across species, resembling a hollow cylinder. This cylinder is composed of nine sets of triplet microtubules arranged in the “9+0” configuration. Each of the nine groups is made of three fused microtubule strands, with no microtubules present in the center of the cylinder. The centriole is typically found within a larger structure called the centrosome, which acts as the cell’s main microtubule-organizing center (MTOC).
Inside the centrosome, two centrioles usually exist as a pair, oriented perpendicularly to one another. This pair is surrounded by a dense, amorphous cloud of protein known as the pericentriolar material (PCM). The PCM is responsible for nucleating the long, hollow microtubule filaments that extend throughout the cell, providing structural support and tracks for internal transport. The centrosome is often positioned near the cell’s nucleus.
Organizing the Machinery of Cell Division
The most recognized function of the centriole is orchestrating the precise mechanics of cell division, both in mitosis and meiosis. Before a cell divides, the entire centrosome, including the pair of centrioles, must duplicate during the S phase of the cell cycle. The two resulting centrosomes then separate and move to opposite sides of the cell, establishing the two poles of the dividing cell.
From these opposing poles, the centrosomes nucleate the growth of specialized microtubule filaments that form the mitotic spindle. This spindle apparatus is designed to capture and move chromosomes. The microtubules extend from the centrosome to attach directly to the replicated chromosomes. This attachment ensures that when the cell separates, the duplicated genetic material is pulled accurately toward the correct pole.
The centrioles’ role in correctly positioning the spindle poles is fundamental to the accurate segregation of chromosomes. Failure in this process can lead to daughter cells with an incorrect number of chromosomes, a condition known as aneuploidy. By defining the ends of the spindle, the centrioles ensure that the genetic material is divided evenly, maintaining genomic stability across cell generations.
Building Blocks for Cellular Antennae
Centrioles have a second, distinct function when the cell is not dividing, acting as the foundation for external hair-like appendages called cilia and flagella. In this capacity, a centriole migrates to the cell membrane and is modified into a structure known as a basal body. The basal body serves as the anchor and template from which the long, microtubule-based core of the cilium or flagellum, called the axoneme, grows outward.
This process generates two main types of cellular antennae: motile cilia and primary cilia. Motile cilia and flagella are responsible for movement, such as the rhythmic sweeping action in the respiratory tract to clear mucus or the propulsion of sperm cells by their tail. These structures feature a characteristic “9+2” microtubule arrangement in their core, which allows them to bend and move.
In contrast, primary cilia are typically non-motile and act as sensory structures, found on almost every cell in the human body. These single, slender projections detect chemical and mechanical signals from the cell’s environment, relaying information about the external world back into the cell. This sensory function is important in tissues such as the kidney, brain, and retina, influencing cell differentiation and tissue development.
Centriole Errors and Human Health
When the functions of the centriole are compromised, significant consequences can arise for human health. Errors in the regulation of centriole duplication can lead to a condition called centrosome amplification, where a cell possesses more than the normal two centrosomes. This excess often results in the formation of an abnormal, multipolar mitotic spindle, which pulls chromosomes to more than two poles.
Such faulty segregation of chromosomes creates aneuploidy, a state of genetic instability that is a common feature and driver of many types of cancer. Cancer cells often attempt to counteract this problem by “clustering” the extra centrosomes into two main groups, which restores a seemingly bipolar division but still leads to errors in chromosome number. Centrosome amplification is therefore frequently associated with increased tumor aggressiveness and metastasis.
Defects in the centriole’s role as a basal body also lead to a group of genetic disorders known as ciliopathies. These diseases result from the failure to properly form or maintain functional cilia, which impacts the signaling pathways they control. Ciliopathies affect various organ systems and include conditions such as polycystic kidney disease, Bardet-Biedl syndrome, and certain forms of retinal degeneration. The diverse health consequences stemming from centriole malfunction highlight the organelle’s fundamental importance in both cell reproduction and cellular communication.