Any of numerous microscopic protein fibres, typically 7–9 nm in diameter, that form one of the main components of the cytoskeleton of eukaryotic cells. Each microfilament consists of two helically twisted strands, each with distinct ‘plus’ and ‘minus’ ends, and comprising a chain of globular subunits of the protein actin. They can shorten or extend by the removal or addition of subunits and are linked by cross-linking proteins to form three-dimensional networks. Bundles of microfilaments often occur just beneath the cell surface—cortical microfilaments—typically oriented parallel to the long axis of the cell, and some are anchored to the plasma membrane. With the aid of the motor protein myosin the microfilaments can slide relative to each other, causing contractile movements, as in muscle cells, or other changes in cell shape, such as those occurring in amoeboid movement. They are also involved in the transport of materials within the cell, and in the peripheral flow of cytoplasm and cell organelles known as cytoplasmic streaming. Hence microfilaments play a crucial role in the growth of cell extensions, for example the pollen tube that develops from a germinating pollen grain. Compare microtubule. See also intermediate filament.