The cytoskeleton is a network of three types of protein fibers. It provides mechanical support and allows for cell motility. The cytoskeleton can also transmit mechanical signals from the cell’s surface to the interior, and it can interact with motor proteins to produce cellular movement
Microtubules are the largest fiber, and their structures are like hollow rods which are made of columns of globular proteins called tubulins. These fibers can change length by the addition or subtraction of tubulin dimmers. The microtubules grow out from a region near the nucleus called the centrosome, a microtubule organizing center which duplicates during mitosis. Two centrioles are located in the centrosome each composed of nine sets of triplet microtubules arranged in a ring (they replicate before cell division). Microtubules provide supporting framework of the cell and serve as monorails for organelles to move along.
Microfilaments are solid rods made of actin (a globular protein) chains, and two of these chains form a helix. These fibers form a network just inside the plasma membrane. The sliding of these actin filaments and thicker myosin filaments past each other causes the contraction of muscles. Actin and myosin filaments also interact in local contractions like cleavage furrows in animal cell division and in amoeboid movement through pseudopodia.
Intermediate filaments are intermediate in size and diverse in composition. They serve as a framework especially to maintain cell shape (hold nucleus securely in place). Intermediate filaments make up the nuclear lamina which lines the nuclear envelope.