microtubule
/ MY-kroh-TOOB-yool /
Imagine a drinking straw, but built by stacking many tiny LEGO bricks into a hollow tube. Microtubules are the cell's straws: stiff, hollow cylinders that are the thickest of the three cytoskeletal fibers. Because they are hollow and rigid, they make excellent support beams and, just as importantly, excellent rails for carrying cargo across a large cell.
Each microtubule is built from a protein called tubulin, which comes as a pair (alpha-tubulin and beta-tubulin joined together). These pairs stack end to end into long rows called protofilaments, and thirteen protofilaments lie side by side to form the wall of the tube. Like microfilaments, microtubules have two distinct ends: a fast-growing plus end and a slower minus end usually anchored near the cell's center. They are famously unstable, often growing and then suddenly shrinking back in a behavior called dynamic instability, which lets the cell quickly explore and reorganize its interior.
Microtubules do several big jobs. They radiate from the centrosome to act as the cell's internal highways, along which motor proteins haul vesicles and organelles. During cell division they reorganize into the mitotic spindle that separates the chromosomes. And in cilia and flagella, a special ring of microtubules called the axoneme bends to make these structures beat. Because dividing cancer cells depend on the spindle, several anticancer drugs work by jamming microtubule assembly.
In a long nerve cell, vesicles full of neurotransmitter are carried from the cell body all the way down the axon to the tip by motor proteins walking along microtubule tracks.
Microtubules act as long-distance railways inside the cell.
Dynamic instability looks wasteful — microtubules keep building and collapsing — but this trial-and-error growth is exactly how the spindle quickly finds and captures chromosomes during division.