The cytoskeleton is the flexible cellular network which helps the cells maintain their structure by elaborate linkage to cell membrane and other organelles. It is a network of three long filament systems made up of dynamic protein components. The filament system in the cytoskeleton comprises of microtubules, actin filaments, and intermediate filaments.
Components of the Cytoskeleton
Microtubules are slender, hollow, cylindrical structures having a diameter of about 25nm. Their walls are made up of a ring of 13 protofilaments. They are formed by the assembly of dimers of alpha and beta tubulin proteins. Microtubules keep on changing by adding and subtracting dimers at both ends. However, one end grows more rapidly than the other. The rapid growing end is called plus end and the other is called the minus end. The minus ends are anchored in structures called microtubule organizing structures.
Microtubules take part in a variety of functions. Most importantly they function in movement of cellular organelles, for example, movement of chromosomes during nuclear division. They are also involved in the various shape changes of the cell during periods of specialization.
Actin filaments are made up of identical actin proteins which are arranged in a long chain. They are the smallest filaments having a diameter of 6nm only. They are also called microfilaments. They are abundant in skeletal muscles where they in act with another protein called myosin to help muscles contract. In cells other than muscle cells, actin filaments are more prominent than myosin. Like microtubules, they also have plus and minus ends. Actin filaments are involved in the cytokinesis, cell movement as well as helping the muscle cells to contract.
As their name suggests, intermediate filaments are medium-sized filaments having a diameter of 10nm. Unlike the other two types, intermediate filaments are made up of a number of different proteins. They are strong and ropelike. They work together with microtubules and provide strength and support to the fragile tubulin structures. They also help to maintain cell shape and its spatial organization.