Tiny tubes within cells that provide structural support and serve as highways for intracellular cargo transport.Microtubules are hollow cylinders inside cells, built from a protein called tubulin. They are part of the cytoskeleton, the internal scaffolding that gives a cell its shape and organizes its interior. A microtubule is roughly 25 nanometers across — far thinner than a human hair — but can stretch across much of a cell's length. They are not static pipes: they grow and shrink rapidly, assembling at one end and disassembling at the other, which lets the cell rearrange its internal structure within minutes.
Beyond support, microtubules act as tracks. Molecular motors, notably kinesin and dynein, walk along them carrying vesicles, organelles, and other cargo to specific destinations. This transport is essential in large cells such as neurons, where materials must travel from the cell body down a long axon. Microtubules also form the spindle that separates chromosomes during cell division, and they make up the core of cilia and flagella, the whip-like structures that cells use to move or to sweep fluid.
Because they are central to division and transport, microtubules are a natural target for drugs and a frequent subject in studies of cell mechanics. Their dynamic, self-organizing behavior also makes them a model system for how simple protein interactions produce large-scale order.
Tubulin proteins pair up into dimers, and dimers stack end-to-end into long chains called protofilaments. About thirteen of these chains arrange side by side in a ring, forming the hollow tube. Each dimer carries a molecule of GTP, a chemical fuel; when GTP is bound, the dimer fits readily into the growing end. After incorporation, the GTP is gradually converted to GDP, which weakens the bonds. If the cap of GTP-bound dimers at the growing tip is lost, the tube becomes unstable and peels apart — a sudden switch from growth to shrinkage known as a catastrophe.
This 'dynamic instability' lets the cell probe space and quickly rebuild its tracks. Motor proteins move along the tube by cycling through shape changes powered by ATP, stepping in a consistent direction because the tubulin dimers are arranged with a structural polarity.
Drugs that interfere with microtubule dynamics, such as paclitaxel and vinca alkaloids, are widely used in cancer chemotherapy because dividing cells depend on the spindle.
A single neuron's microtubule tracks can span distances a thousand times longer than the cell body, with cargo delivered by motor proteins that 'walk' in discrete steps.