Centrioles are small, cylindrical structures found within the cytoplasm of most eukaryotic cells, particularly in animal cells. These organelles function primarily as the core components of the centrosome, which acts as the cell’s main organizing center for its internal scaffolding network. Centrioles govern processes in cellular biology, including the precise distribution of genetic material during cell division.
The Unique Architecture of the Centriole
The centriole is a non-membranous organelle built from the protein tubulin, assembled into a highly ordered, hollow cylinder. Centrioles are approximately 150 to 500 nanometers in length and about 250 nanometers in diameter, depending on the cell type.
A cross-section of a centriole reveals a signature structure known as the “9+0” pattern. This pattern consists of nine sets of triplet microtubules arranged in a radial pinwheel shape around a hollow center, with no central microtubules present. Each triplet is composed of one complete A-tubule and two partial B- and C-tubules, all linked together to form a highly stable cylinder. This intricate architecture imparts polarity to the structure, defining a proximal end and a distal end. The proximal end is where new centrioles are assembled using a cartwheel-like structure as a template, while the distal end is where proteins that help anchor the centriole to the cell membrane are found.
Centrosomes and Control of Cell Division
A functional centrosome, often called a diplosome, consists of two centrioles positioned perpendicular to each other, surrounded by an amorphous protein cloud known as the pericentriolar material (PCM). The PCM is the site where the vast majority of new microtubules are generated. These microtubules provide the cell with its internal scaffolding and transport routes.
Duplication begins during the S-phase of the cell cycle, where a new, short procentriole grows perpendicularly from the side of each existing centriole, near its proximal end. This process results in two pairs of centrioles, which remain attached until the cell prepares to divide.
Before the start of mitosis, the centrosome undergoes a process called maturation, where the PCM expands significantly and its ability to nucleate microtubules increases dramatically. The two centrosomes then separate and move to opposite sides of the nucleus, establishing the two poles of the dividing cell. From these poles, microtubules extend to form the mitotic spindle apparatus, which governs the precise movement of chromosomes. The spindle fibers attach to the replicated chromosomes and pull them apart, ensuring that each daughter cell receives a complete and identical set of genetic material.
Building Blocks for Cellular Movement and Signaling
Beyond their function in cell division, centrioles are repurposed to form the structural foundation for specialized cellular extensions. A mature centriole can migrate to the cell’s outer surface and transform into a basal body, which acts as the template for building cilia and flagella. Cilia are short, hair-like projections that are typically numerous, while flagella are longer, whip-like structures, often singular, such as a sperm tail.
The basal body seeds the growth of the internal core of these extensions, called the axoneme, by transitioning its nine microtubule triplets into nine microtubule doublets. This new arrangement, combined with two central microtubules, forms the characteristic “9+2” pattern of motile cilia and flagella. The rhythmic, coordinated beating of motile cilia is responsible for moving fluid or cells across a surface, such as the clearance of mucus in the respiratory tract.
In many cells, the centriole forms a single, non-motile extension known as the primary cilium, which lacks the two central microtubules. These primary cilia function as sensory antennae, detecting external signals and relaying information to the cell’s interior. This signaling role is crucial in various tissues, including the kidney, where the primary cilium monitors fluid flow.
When Centrioles Malfunction
The failure to correctly regulate the number of centrioles often leads to a condition called centrosome amplification, where a cell contains too many organizing centers. This numerical abnormality causes the formation of abnormal, multipolar mitotic spindles during cell division, which cannot correctly align and separate chromosomes.
The resulting unequal distribution of chromosomes produces daughter cells with an incorrect number of chromosomes, a state known as aneuploidy, which drives genomic instability. Centrosome amplification is a commonly observed characteristic in many types of human cancer.
Separately, defects in the centriole’s secondary function as a basal body lead to a class of genetic disorders known as ciliopathies. These disorders arise when the basal body is unable to properly form or maintain functional cilia and flagella. Polycystic kidney disease, for instance, is linked to primary cilia defects that impair the cell’s ability to sense and respond to fluid flow in the renal tubules. Primary ciliary dyskinesia results from structural defects in motile cilia, leading to issues like chronic respiratory infections and male infertility.