A centrosome is a small organelle that serves as the main microtubule organizing center in animal cells. It acts like a command hub for the cell’s internal skeleton, generating the protein filaments (called microtubules) that give a cell its shape, move its cargo, and pull chromosomes apart during division. Every animal cell that’s preparing to divide contains two centrosomes, one at each pole, forming the anchor points for the machinery that splits the cell in half.
Structure of a Centrosome
A centrosome has two core components: a pair of barrel-shaped structures called centrioles, and a surrounding cloud of proteins called the pericentriolar material (PCM). The centrioles sit at the center and are arranged in a distinctive pattern of nine sets of microtubule triplets radiating outward, giving them a pinwheel-like cross section. The two centrioles in each centrosome are oriented perpendicular to each other.
The PCM is where the real action happens. It forms a porous scaffold made largely of proteins with coiled-coil shapes, meaning they twist around each other like rope strands. This design allows them to link together into a sturdy but flexible mesh. Embedded throughout this scaffold is a ring-shaped protein complex built from a special form of tubulin called gamma-tubulin. These rings are the launchpads for new microtubules. At the concentrations of tubulin found inside a cell, microtubules almost never form on their own. Instead, the gamma-tubulin rings act as templates that mimic the end of a microtubule, giving new filaments a surface to grow from.
How Centrosomes Drive Cell Division
The centrosome’s most important job is building the mitotic spindle, the structure that separates chromosomes when a cell divides. As a cell enters mitosis, its two centrosomes move to opposite sides of the cell and dramatically ramp up their microtubule production. They recruit more gamma-tubulin complexes and additional scaffolding proteins, and key enzymes chemically activate these components through phosphorylation. The result is a roughly tenfold increase in microtubule turnover compared to a resting cell, which lets the centrosome rapidly send out filaments in all directions.
These microtubules extend outward in a process called “search and capture.” Each filament grows, shortens, and regrows in random directions until it connects with a kinetochore, a specialized docking site on each chromosome. Once a microtubule latches onto a kinetochore, it stabilizes. Bundles of these stabilized filaments, called K-fibers, physically pull sister chromosomes to opposite poles of the cell. If this process goes wrong and chromosomes are distributed unevenly, the daughter cells end up with the wrong number of chromosomes, a condition called aneuploidy.
The Centrosome Duplication Cycle
Centrosomes replicate once per cell cycle, and this process is tightly synchronized with DNA replication. During the G1/S transition (the point when a cell commits to copying its DNA), each existing centriole begins growing a new daughter centriole perpendicular to itself. This daughter centriole, called a procentriole, elongates throughout the G2 phase until it reaches roughly the same size as its parent. By the time mitosis begins, the cell has two complete centrosomes ready to anchor opposite ends of the spindle.
The enzyme that coordinates this timing is the same one that triggers DNA replication: a complex of Cdk2 paired with cyclin E. Blocking Cdk2 activity stops centrosome duplication. Conversely, when DNA replication is artificially stalled, centrosomes can keep duplicating on their own, sometimes producing extra copies. The cell uses several safeguards to prevent this over-duplication, including proteins originally known for their roles in DNA replication control. When these safeguards fail, the consequences for the cell can be serious.
Centrosomes and Cancer
One of the most clinically significant things about centrosomes is what happens when cells accumulate too many of them. In studies of invasive breast tumors, roughly 80% of tumors contain cells with amplified centrosomes, meaning extra centrioles, excess pericentriolar material, or both. About 60 to 80% of these same tumors are aneuploid.
The connection is straightforward: extra centrosomes create extra spindle poles. Instead of a clean two-pole spindle that divides chromosomes evenly, a cell with three or four centrosomes can form a multipolar spindle that distributes chromosomes chaotically. Research on breast tumors has shown a direct linear correlation between centrosome size, centrosome number, and the degree of chromosomal instability. Tumors with the most unstable chromosome counts had significantly larger and more numerous centrosomes than tumors with relatively stable (though still abnormal) chromosome counts.
This relationship appears to be an early event in tumor development rather than a late consequence. Centrosome amplification has been found in ductal carcinoma in situ (DCIS), a pre-invasive stage of breast cancer, and in experimental systems centrosome defects appear before nuclear changes associated with aneuploidy. The current understanding is that centrosome amplification may initiate chromosomal instability, which then fuels further tumor evolution.
The Centrosome’s Role in Building Cilia
When a cell exits the division cycle and enters a resting or differentiated state, centrosomes take on an entirely different job. The older of the two centrioles (the “mother”) migrates from its position near the nucleus to the cell surface, docks with the plasma membrane, and serves as the foundation for a primary cilium. This is a single, non-moving antenna that protrudes from the cell surface and detects signals from the surrounding environment, including chemical gradients, mechanical forces, and light in some cell types.
This dual identity means the centrosome effectively switches between two architectures depending on what the cell needs. In dividing cells, it sits near the nucleus and organizes microtubules. In quiescent cells, it relocates to the membrane and organizes a cilium. The two states are generally mutually exclusive: most differentiated, non-dividing cells assemble primary cilia, while actively dividing cells typically do not. When a quiescent cell re-enters the division cycle, the cilium is reabsorbed and the centrosome resumes its role as a microtubule organizing center.
This is distinct from motile cilia, the waving hair-like structures found on cells lining the airways or reproductive tract. Motile cilia are generated by basal bodies, which can be produced in bulk without a centrosome. Primary cilia, by contrast, can only be formed by centrosomes.
Centrosome Defects and Genetic Disorders
Because centrosomes are so central to cell division, mutations in the genes encoding centrosomal proteins cause a cluster of related developmental disorders. The best studied is autosomal recessive primary microcephaly (MCPH), characterized by a significantly smaller brain due to insufficient neuron production during embryonic development. All seven genes implicated in MCPH encode centrosomal proteins.
The developing brain is especially vulnerable because its neural progenitor cells divide at an extraordinarily high rate and rely on precise spindle orientation to balance the production of new progenitors against differentiated neurons. When centrosomal proteins are disrupted, these progenitors experience spindle defects, abnormal centriole duplication, and impaired cell cycle control. In mouse models, loss of one key centrosomal protein (Cdk5rap2) causes not only small brain size but also infertility, anemia from blood cell precursor defects, increased aneuploidy, and heightened sensitivity to radiation. Related syndromes involving centrosomal gene mutations also cause dwarfism, reflecting the centrosome’s importance in cell proliferation throughout the body.
Cells That Divide Without Centrosomes
Despite the centrosome’s central role, it is not strictly required for cell division. Higher plants lack centrosomes entirely and build their spindles using gamma-tubulin complexes distributed along existing microtubules rather than concentrated at a single organizing center. When the gamma-tubulin complex is knocked out in plant cells, the spindle collapses and division fails, confirming that the nucleation machinery matters even when the centrosome structure does not.
Vertebrate cells also have a backup system. In landmark experiments, researchers destroyed both centrosomes in mammalian cells during prophase and found that the cells still formed functional bipolar spindles. Even destroying just one centrosome produced a bipolar spindle with one centrosomal pole and one acentrosomal pole. In these cases, chromosomes themselves promote microtubule assembly and motor proteins sort the filaments into a spindle-shaped array. This centrosome-independent pathway operates during normal division as well but is masked by the dominant centrosomal pathway. It likely explains why certain cell types, including egg cells undergoing meiosis, divide successfully without centrosomes as a matter of course.