Cells carry an internal scaffold that builds and dismantles itself constantly
Early biologists dismissed the cytoskeleton as dull jelly holding organelles in place. It turned out to be a restless network of protein cables and tubes that shapes the cell, hauls cargo, pulls chromosomes apart and powers every muscle contraction you make. Even bacteria, it emerged in 1992, have one.
Three kinds of filament do the work in complex cells. Microfilaments, built from actin, the most abundant protein in the cell, are about 7 nanometres across. Microtubules, made of tubulin, are much wider at 25 nanometres. Intermediate filaments fall in between at 8 to 12 nanometres and are assembled from different proteins depending on the cell type; in neurons they are called neurofilaments. All three can grow or fall apart quickly as the cell's needs change.
The jobs are remarkably varied. The network gives a cell its shape and resists deformation, and by anchoring to neighbours and surrounding tissue it holds whole tissues together. It can contract, letting cells crawl. It helps swallow material from outside, separate chromosomes and pinch a dividing cell in two, and it forms flagella and cilia. Motor proteins walk along its tracks carrying vesicles and organelles.
Muscle offers the most visible example. A nerve signal releases calcium inside a muscle cell. Normally the protein tropomyosin blocks myosin from grabbing actin, but troponin senses the calcium and lifts that block. Myosin motors then tug on parallel actin filaments, and when many cells do this together, the whole muscle shortens.
Nikolai Koltsov proposed in 1903 that a network of tubules determines cell shape, and the French embryologist Paul Wintrebert introduced the word cytoskeleton in 1931. Faults in the scaffold appear in several neurodegenerative diseases: in Alzheimer's, the tau proteins that normally stabilise microtubules malfunction, and cytoskeletal defects also feature in Parkinson's, Huntington's and ALS. More speculatively, Stuart Hameroff and Roger Penrose have suggested microtubule vibrations in neurons play a part in consciousness.
Source: Cytoskeleton
