The Architecture of a Centrosome
The centrosome is a small organelle located near the nucleus in animal cells. Its structure has two primary components: a pair of barrel-shaped centrioles and a surrounding matrix of proteins known as the pericentriolar material (PCM).
Each centriole is a cylinder composed of microtubules, protein filaments that provide structural support within the cell. The walls of a centriole are constructed from nine sets of microtubule triplets, arranged in a nine-fold radial symmetry. This arrangement is referred to as a “9+0” pattern, signifying the nine outer triplets with no microtubules in the center. The two centrioles within a centrosome are positioned perpendicular to each other.
Surrounding the centrioles is the pericentriolar material, a complex assembly of proteins that appears as a dense cloud under a microscope. The PCM is responsible for anchoring microtubules that extend from the centrosome, which allows the organelle to organize the cell’s internal scaffolding.
The Centrosome’s Role in Cell Division
The most recognized function of the centrosome is orchestrating cell division during mitosis. It acts as the primary microtubule-organizing center (MTOC), ensuring genetic material is segregated accurately into two new daughter cells.
During the S phase of the cell cycle, when the cell also replicates its DNA, the centrosome duplicates. This results in two complete centrosomes, each containing a pair of centrioles. As the cell transitions into mitosis, these two centrosomes migrate to opposite ends, or poles, of the cell. This positioning establishes the two poles of the mitotic spindle, an apparatus for chromosome separation.
From each polar centrosome, microtubules grow toward the cell’s center, forming the mitotic spindle. These spindle fibers attach to the duplicated chromosomes. Once attached, the microtubules generate force to align the chromosomes at the cell’s equator.
The spindle fibers then shorten, pulling the separated sets of chromosomes toward the opposing poles. This action ensures each new daughter cell receives a complete set of genetic information.
Functions Beyond Cell Division
Even in cells that are not actively dividing, a state known as interphase, the centrosome continues to perform important organizational duties. It serves as the main hub from which a network of microtubules extends throughout the cytoplasm. This network, called the cytoskeleton, provides structural support to the cell, helps maintain its shape, and acts as a system of tracks for transporting organelles and vesicles to different locations within the cell.
The centrosome is also directly involved in the formation of cilia and flagella, which are hair-like or whip-like appendages on the surface of some cells that are involved in movement or sensing the environment. In the process of forming these structures, one of the centrioles from the centrosome, specifically the older “mother” centriole, migrates to the cell surface.
Once at the cell membrane, this mother centriole transforms into a structure called a basal body. The basal body then serves as the foundation from which the cilium or flagellum assembles. This demonstrates the versatility of the centrosome’s components, repurposing a centriole to initiate the growth of these specialized cellular extensions.
Centrosomes and Human Health
Proper regulation of centrosome number and function is important for cellular health, as defects can have significant consequences. Abnormalities in the number of centrosomes within a cell are a common feature of many types of cancer. When a cell has more than two centrosomes, it can lead to errors during cell division, resulting in an incorrect number of chromosomes in the daughter cells—a condition known as aneuploidy, which is a hallmark of genomic instability in tumors.
Mutations in the genes that code for the proteins making up the centrosome can lead to a group of genetic disorders. One such condition is microcephaly, a neurodevelopmental disorder characterized by a smaller-than-normal head and brain. This is thought to be caused by impaired cell division during brain development, stemming from faulty centrosome function.
Disruptions in the centrosome’s ability to form cilia lead to a class of diseases known as ciliopathies. Because cilia play roles in various signaling pathways and sensory functions throughout the body, their dysfunction can cause a wide range of health issues. Conditions such as polycystic kidney disease and certain forms of retinal degeneration are linked to defects in ciliary structure or function that originate with problems in the centrosome.