cytoskeleton
/ SY-toh-SKEL-eh-ton /
Picture a large tent. Without poles and ropes it would collapse into a pile of fabric; with them it holds a definite shape, and you can even move it around. A living cell faces the same problem. It is mostly water wrapped in a thin, floppy membrane, yet it manages to keep a shape, resist being squashed, and move its parts around. The cytoskeleton is the internal network of protein fibers that does this job — the cell's tent poles, ropes, and roadways all at once.
More precisely, the cytoskeleton is a web of protein filaments that fills the cytoplasm of a cell. It is built from three main fiber types of different thicknesses and jobs: thin microfilaments made of actin, medium-sized intermediate filaments, and thick hollow microtubules made of tubulin. These fibers are not fixed like steel beams. Many of them are constantly being built up at one end and taken apart at the other, so the cell can rearrange its scaffold within seconds when it needs to change shape, divide, or crawl.
The cytoskeleton matters because almost everything a cell does mechanically depends on it: holding its shape, anchoring organelles, pulling chromosomes apart during division, hauling cargo from place to place, and crawling or beating tiny hairs to swim. A common misconception is that it is a passive skeleton like our bones. It is the opposite — a dynamic, energy-using system that is taken apart and rebuilt all the time, which is exactly why a cell can be both sturdy and able to move.
A white blood cell chasing a bacterium constantly rebuilds its cytoskeleton: actin filaments push out the front of the cell while the rear is pulled forward, so the cell flows toward its target.
The same scaffold that gives shape can be torn down and rebuilt to produce movement.
Despite the name, the cytoskeleton is not a fixed skeleton; its fibers are continually assembled and disassembled, and this turnover is what makes movement and division possible.