(Cell biology) A dynamic network of protein filaments that extends throughout eukaryotic cells, maintaining cellular structure and allowing cells to change shape, divide and move. There are three classes of filaments found in the cytoskeleton:

  1. Actin filaments, which are composed of (surprise!) the protein actin;
  2. Microtubules, which are formed by the protein tubulin; and
  3. Intermediate filaments, which consist of fibrous proteins such as vimentin and lamin.

The functions of each of the classes of filament are quite different. Actin filaments perform mainly structural roles: for instance, shaping the plasma membrane of animal cells (allowing them to form protuberances and invaginations on their surfaces), and are important in the generation and maintenance of cell polarity. They also form an important part of the contractile machinery of skeletal muscle. Microtubules' main job is to move stuff around the cell - for example, shifting organelles such as the endoplasmic reticulum and the Golgi apparatus into their correct alignment, or carrying cargoes of proteins in vesicles to the part of the cell where they perform their function. Intermediate filaments are basically just tough, rope-like filaments that provide mechanical stability to the cell and hold things in place.

The three types of filament are intricately connected to one another, and their functions are tightly coordinated. Defects in cytoskeletal proteins often result in severe disease; for instance, mutations in the gene for actin cause the severe muscle disease nemaline myopathy, defects of the cytoskeletal protein alpha-actinin-4 cause familial focal segmental glomerulonephritis, and mutations in the gene for beta-spectrin (a protein associated with actin filaments) result in a nasty form of hereditary hemolytic anemia.

What would life be like without the cytoskeleton? To truly appreciate something, you must imagine what life would be like without it. For a start, you would never have existed - without the cytoskeleton, your father's sperm would have been immotile; and even if they had reached your mother's egg, without the cytoskeleton the egg would not have been able to make the necessary structural rearrangements to allow your father's genetic information to enter its nucleus. Even if (by some unholy miracle) it could, the tiny fertilised cell would have been incapable of dividing into the hundred trillion cells that make up your body, or even of carrying out the normal metabolic functions that sustained its existence.

But let's say, for argument's sake, that you did reach maturity, and then someone somehow instantaneously removed every element of the cytoskeleton from every cell in your body. Immediately you would collapse, as every muscle in your body turned to useless jelly with the removal of actin filaments from the contractile machinery needed to hold you up against gravity. You would then begin to suffocate, as the muscle of your diaphragm underwent the same horrific collapse. Of course, you would be unaware of this, as the generation of thought requires the release of neurotransmitters from synapses in your brain, a process absolutely dependent on microtubular transport of vesicles rearrangements of the actin cytoskeleton. Mercifully oblivious, then, you would slowly dissolve into a puddle of viscous goo as the vital connections between your stromal cells failed, one by one, in the absence of support from intracellular actin filaments. After a few minutes you would be a partially disarticulated pile of bones and some chunks of cartilage, lying in a slowly spreading pool of blood and dissolved tissue.

The cytoskeleton is a wonderful thing. If we all spent a few moments each day appreciating the importance of our own cytoskeleton, the world would be a better place.