Centrioles are small, cylindrical organelles found within the cells of many organisms. They are primarily composed of a protein called tubulin, which forms their microtubular structure. These structures are fundamental components for cellular organization.
Where Centrioles Reside
Centrioles are located in the cytoplasm, often near the cell’s nucleus, specifically within the centrosome. The centrosome acts as the primary microtubule-organizing center (MTOC) in most animal cells. Within the centrosome, two centrioles are positioned perpendicularly, forming a diplosome.
Each centriole is a cylinder, approximately 200-250 nanometers in diameter and 150-500 nanometers long. This cylindrical shape is formed by nine sets of microtubule triplets arranged in a ring. Surrounding these paired centrioles is a dense, amorphous material called the pericentriolar material (PCM), which provides anchoring sites for microtubules.
Cells That Contain Centrioles
Centrioles are widely present across various forms of life, commonly in most animal cells. They are also found in some lower plant forms, such as mosses and ferns, and in certain protists. However, centrioles are absent in most higher plants, including conifers and flowering plants (angiosperms), and in the majority of fungi. Despite their absence, these organisms still manage cellular processes that centrioles facilitate in other organisms. Even within animals, some specialized cells lack them or have modified centriolar structures.
How Centriole Location Dictates Function
The specific location of centrioles within the cell directly influences their primary functions, particularly in cell division and the formation of cellular appendages. During cell division, centrioles organize the mitotic spindle. As cells prepare to divide, the centrosome, containing the paired centrioles, duplicates and migrates to opposite ends of the cell.
From these poles, microtubules extend to form spindle fibers that are essential for segregating chromosomes. This positioning ensures that replicated chromosomes are accurately moved into the newly forming daughter cells. The presence of centrioles at the spindle poles helps in establishing the bipolarity of the spindle, contributing to the precise distribution of genetic material.
Centrioles also serve as basal bodies, which are structurally similar to centrioles and act as the foundation for the assembly of cilia and flagella. These motile cellular appendages, found on the surface of some cells, are involved in cell movement and sensing the environment. When functioning as basal bodies, centrioles migrate to the cell surface and become anchored, nucleating the growth of the microtubules that make up these structures. This enables the formation and proper orientation of cilia and flagella, allowing for their specific functions in cellular motility or sensing.