The centrosome is a small, non-membrane-bound organelle found near the center of animal cells. It functions as the primary microtubule-organizing center (MTOC), playing a part in various cellular activities. These processes include cell division, maintaining cell shape, and motility. The centrosome’s influence extends to the overall internal spatial organization of the cell.
Centrosome Composition and Structure
The centrosome is comprised of two main components: a pair of barrel-shaped centrioles and the surrounding pericentriolar material (PCM). The two centrioles are arranged perpendicular to each other and are built from a nine-fold radial arrangement of microtubules. This architecture is fundamental to their function.
Surrounding the centriole pair is the PCM, a dense, ordered matrix of proteins. This material is responsible for anchoring and nucleating microtubules, which are protein filaments that form part of the cellular skeleton. Proteins like γ-tubulin are core components permanently associated with the centrosome, while others are recruited at specific times to perform particular functions.
There is a generational difference between the two centrioles. The older “mother” centriole is fully mature and possesses structures called distal and subdistal appendages. The “daughter” centriole, which arises from centrosome duplication, is younger and lacks these appendages. This distinction has functional implications, particularly in the formation of other cellular structures.
The Centrosome’s Role in Cell Division
In preparation for a cell dividing into two, the centrosome must first duplicate itself. This duplication process occurs during the S phase of the cell cycle, in coordination with the replication of the cell’s DNA. The result is two complete centrosomes, each containing a pair of centrioles.
As the cell moves into mitosis, the two centrosomes migrate to opposite poles of the cell. From these positions, they organize microtubules into a structure known as the mitotic spindle. This spindle is a dynamic framework of protein filaments that attaches to the duplicated chromosomes.
The primary function of the mitotic spindle is to ensure the accurate segregation of chromosomes. The microtubules of the spindle pull the duplicated chromosomes apart, delivering one complete set to each of the two newly forming daughter cells. This process ensures that each new cell receives the correct genetic information.
Functions Beyond Cell Division
During interphase, the period between cell divisions, the centrosome organizes microtubules into astral arrays that radiate outwards. These networks act as a scaffold, providing structural support and maintaining cell shape and polarity. They also function as highways for the transport of vesicles and organelles within the cell.
In many cell types, the mother centriole can convert into a structure called a basal body after the cell stops dividing. This basal body then serves as the foundation for the assembly of cilia and flagella. These are hair-like or whip-like appendages on the cell surface that are involved in movement and sensory signal transduction.
The centrosome also acts as a signaling hub. Because it is not enclosed by a membrane, it can concentrate components of signaling pathways from the surrounding cytoplasm. This ability to scaffold and organize signaling molecules allows it to influence pathways involved in development and cellular stress responses.
Centrosome Abnormalities and Human Health
Errors in the number or structure of centrosomes can have significant consequences for human health. Centrosome amplification, a state of having more than two centrosomes, is a common feature of cancer cells. This surplus can lead to incorrect mitotic spindles, causing errors in chromosome segregation. This leads to aneuploidy, an abnormal number of chromosomes, which is a hallmark of cancer and drives genomic instability.
Defects in the centrosome’s function are also linked to a class of genetic disorders known as ciliopathies. Since the centrosome is directly involved in forming cilia, mutations in centrosomal genes can lead to defective cilia formation or function. These disorders can affect many organs and result in a wide range of health issues, including brain development disorders and growth failure syndromes.
Research has linked centrosome abnormalities to various conditions, including neurological disorders and certain types of infertility. Understanding how centrosomes are regulated and function may provide insights for potential therapeutic strategies for these diseases.