Microfilaments are typically smaller than the microtubules. These are thin, cylindrical, elongated structures ranging from 40-80 Å in diameter formed by joining together the molecules of actin, a protein. Hence, these are also called actin filaments as found in amoeba, yeast, skeletal and cardiac muscle cells. Contraction of muscle cells results from interactions between the actin filaments and the larger myosin filaments.
Microfilaments form an extensive network in the cytoplasm, some are associated with the plasmamembrane towards inner side and others extend in the cytoplasmic core of microvilli. These are involved in changes in cell shape during development and motility and in protoplasmic streaming in plant cells. Actin molecules are also found to be present in close association with the spindle in dividing cells. Actin-containing microfilaments are also associated with the cleavage furrow following mitosis.