Centrioles are microscopic, cylindrical organelles found in most animal cells. Residing near the nucleus, these structures are composed primarily of a protein called tubulin and play an important part in processes that maintain cell structure and function. While a defining feature of animal cells, they are absent in higher plants and most fungi.
The Structure of a Centriole Pair
Centrioles are barrel-shaped structures, each measuring approximately 0.2 micrometers in diameter and 0.5 micrometers in length. The wall of each cylinder is composed of nine sets of short microtubules. In most animal cells, these microtubules are arranged in triplets, creating a distinctive nine-fold symmetry that is visible under an electron microscope. This organized structure provides the centriole with its shape and stability.
Centrioles are found in pairs, with the two cylinders oriented at right angles to each other. This orthogonal arrangement is a hallmark of the centrosome, the primary microtubule-organizing center (MTOC) in animal cells. The centrosome consists of the two centrioles surrounded by a dense cloud of proteins known as the pericentriolar material (PCM). This PCM, organized by the centrioles, is responsible for nucleating the microtubules that form the cell’s internal scaffolding.
Within the pair, the two centrioles are not identical; one is designated the “mother” centriole and the other, the “daughter.” The mother centriole is the older of the two and is distinguished by the presence of protein structures called distal and subdistal appendages. The distal appendages are involved in docking the centriole to the cell membrane, while the subdistal appendages serve as anchoring sites for microtubules. These structural differences underlie the distinct roles each centriole plays.
The Role in Cell Division
During cell division, such as mitosis and meiosis, the centriole pair takes on a prominent role. The centrosome orchestrates the formation of the mitotic spindle, a complex assembly of microtubules. This spindle acts as a cellular machine to segregate chromosomes accurately into two new daughter cells.
Before division begins, the centriole pair replicates, and the resulting two pairs migrate to opposite sides of the nucleus. As the nuclear membrane breaks down, these two centrosomes act as the poles of the newly forming mitotic spindle. Microtubules grow out from each centrosome, attaching to the chromosomes and ensuring their proper alignment at the cell’s equator.
The spindle fibers then shorten, an action that pulls the duplicated chromosomes apart. This process draws one complete set of genetic material to each pole of the cell. The precise positioning of the centriole pairs at the spindle poles ensures that the division of genetic material is equal, a requirement for the viability of the new cells.
The Centriole Duplication Cycle
The replication of centrioles is a controlled process synchronized with the cell’s division cycle. This duplication occurs during the S phase, the same period when the cell replicates its DNA. The process is described as semi-conservative because each original centriole serves as a template for the formation of a new one.
During the S phase, a new “procentriole” begins to form at a right angle to the base of each existing parent centriole. These procentrioles gradually elongate through the S and subsequent G2 phases of the cell cycle. By the time the cell is ready to enter mitosis, each original centriole has a complete, full-sized daughter centriole attached, resulting in two complete centriole pairs.
These two pairs remain connected and function as a single microtubule-organizing center until late in the G2 phase. Just before mitosis begins, the connection between the two pairs is severed, allowing them to separate and move to opposite poles of the cell. This duplication and separation mechanism guarantees that each daughter cell will inherit one complete centrosome.
Formation of Cilia and Flagella
Beyond their role in cell division, centrioles are also involved in the formation of cilia and flagella. These are hair-like or whip-like appendages that extend from the surface of many cell types and are involved in motility or sensing the environment. In this context, a centriole is referred to as a basal body, and the structures are interchangeable.
The process begins when a mother centriole migrates from its position near the nucleus to the cell’s periphery. It then docks with the inner surface of the plasma membrane. Once anchored, this basal body acts as a foundation, nucleating the growth of the axoneme, the microtubular skeleton that forms the core of a cilium or flagellum.
This function is distinct from the centriole’s role in mitosis and often occurs in non-dividing, differentiated cells. For example, the epithelial cells lining the human respiratory tract are covered in hundreds of cilia that work to move mucus. Similarly, the tail of a sperm cell is a flagellum that originates from a basal body, providing the motility needed for fertilization.
Consequences of Centriole Dysfunction
Errors in the number or function of centrioles can lead to various diseases. The strict regulation of centriole duplication to once per cell cycle is necessary for maintaining genomic stability. If this process is disrupted, cells with an incorrect number of centrioles can have errors in chromosome segregation during mitosis.
This failure to properly distribute chromosomes results in aneuploidy, a condition where daughter cells have an abnormal number of chromosomes. Aneuploidy is a characteristic of many cancer cells and is thought to contribute to tumor development. The presence of extra centrioles can cause the formation of multipolar spindles, which pull chromosomes in more than two directions, causing chromosomal instability.
Defects in the function of centrioles as basal bodies lead to genetic disorders known as ciliopathies. Since cilia are present on many cell types and act as signaling hubs, their malfunction can affect multiple organ systems. Diseases such as polycystic kidney disease (PKD), where cysts form in the kidneys, and certain forms of retinal degeneration and infertility are linked to defective cilia.