Microtubules

In eukaryotic cells, microtubules are the thickest cytoskeletal elements in cells.

They are hollow tubes about 25 nanometers wide. They are usually made of 13 protofilaments made from repeating alpha- and beta-tubulin heterodimers.

Each microtubule has structural polarity, with a plus end and a minus end. At the plus end, where beta-tubulin is exposed, cells usually add new dimers. The minus end has alpha-tubulin, is usually less dynamic, and is often attached to a microtubule-organizing center that anchors many microtubules.

Microtubules can rapidly grow and shrink. This behavior is known as dynamic instability.

Microtubule-associated proteins bind along the sides of microtubules and help control their stability and organization.

Microtubule polarity supports directional transport, where motor proteins such as kinesin and dynein move along microtubules and carry cargo, such as vesicles, through the cytoplasm.

In eukaryotic cells such as epithelial cells and sperm cells, microtubules form the core of cilia and flagella, which have a characteristic 9+2 arrangement. These structures move fluidly across cell surfaces or propel cells through their environment.