Microtubules are the hollow scaffolding and railways inside your cells
Inside nearly every cell with a nucleus runs a network of tiny hollow tubes, each about 25 nanometres wide yet sometimes 50 micrometres long. These microtubules hold the cell's shape, carry cargo along their length like rail lines, and pull chromosomes apart when a cell divides.
A microtubule is assembled from pairs of two similar proteins, alpha and beta tubulin, which are about half identical in sequence and each weigh roughly 50 kilodaltons. The pairs stack end to end into strands called protofilaments, and usually 13 strands line up side by side to form the tube, leaving a hollow core known as the lumen. Because every pair points the same way, the tube has a direction: one end exposes beta tubulin and grows quickly, while the other exposes alpha tubulin and grows slowly. Whether it lengthens or shrinks depends on how many free tubulin pairs are around; above a threshold level it builds, below it falls apart, and that constant growing and shrinking can generate force.
New tubes usually sprout from organising hubs. In many animal cells the main hub is the centrosome near the cell's centre, where a third kind of tubulin, gamma tubulin, forms a ring shaped like a lock washer that serves as a template and caps the slow end. Cilia and flagella have their own base hubs, and the Golgi apparatus can seed tubes too, letting a cell build a lopsided network. Plant cells lack clear hubs and start microtubules from scattered points.
Motor proteins turn the tubes into highways. Kinesin and dynein walk along them hauling organelles, vesicles and even messenger RNA, and the same tubes form the inner skeleton of cilia and flagella. During division they make up the spindle that separates chromosomes. Their layout depends on the cell: in skin-like epithelial cells they line up from top to bottom, while in connective tissue cells they radiate outward from the centrosome and help the cell crawl.
Bacteria have cousins. Some Prosthecobacter species build similar tubes with only five protofilaments. Van Leeuwenhoek glimpsed microtubule-driven motion in 1677, though the fibres themselves were confirmed only with electron microscopes.
Source: Microtubule
