Centrioles are microscopic, cylindrical structures found within the cytoplasm of most animal cells. Composed primarily of tubulin protein, these organelles are integral components of the cell’s cytoskeleton, contributing to its shape and internal organization.
The Usual Number of Centrioles
A typical animal cell contains two centrioles, usually positioned perpendicular to each other. This arrangement is within the centrosome, the cell’s primary microtubule-organizing center, typically located near the nucleus.
During interphase, when a cell is not actively dividing, it maintains this pair within its single centrosome. This stable number ensures proper cellular organization and prepares the cell for future division.
What Centrioles Do and How They Multiply
Centrioles play a significant role in cell division, specifically during mitosis and meiosis. They organize spindle fibers, which are microtubule structures essential for separating chromosomes. These fibers attach to chromosomes, pulling them to opposite ends of the cell, ensuring each new daughter cell receives a complete set of genetic material.
Before a cell can divide, its centrioles must duplicate. This occurs during the S-phase of the cell cycle, mirroring DNA replication. Each existing centriole acts as a template for a new one, with a “procentriole” beginning to grow at a right angle from the side of each original centriole. This duplication results in two pairs of centrioles (four total) within the cell by the time it is ready to divide. As the cell divides, each new daughter cell inherits one of these duplicated pairs, restoring the typical number of two centrioles per cell.
When Centriole Numbers Change
While two centrioles are standard in most animal cells, variations exist. Many cell types, including most higher plant cells, fungi, and some specialized animal cells like mature neurons or red blood cells, naturally lack centrioles. These cells manage their division or function using other microtubule-organizing centers that do not involve centrioles.
In contrast, certain specialized animal cells possess numerous centrioles. For example, ciliated cells, which line various body surfaces and have hair-like projections called cilia, can produce hundreds of centrioles. In these cells, centrioles transform into structures called basal bodies, which anchor and form the base of cilia and flagella, enabling cellular movement or the movement of fluids.
Abnormal centriole numbers can also occur, particularly in disease states. Cancer cells frequently exhibit an altered number of centrioles, often having extra or unusually elongated ones. This increase in centriole number, sometimes referred to as centrosome amplification, can contribute to chromosomal instability, where chromosomes are incorrectly distributed during cell division. Such irregularities are a common feature in many cancers and can impact tumor aggressiveness.