Each type of cytoskeletal protein has a distinct distribution in cells, as illustrated in Figure 9 for intestinal epithelial cells. All three types of filaments are associated with other types of protein in the cell.
Microfilaments are present in all eukaryotic cells and are the thinnest of the filament types, having a diameter of about 6 nm (Figure 8a) and a thread-like appearance under EM. They are composed of molecules of the protein actin organised into a long helical chain, and so are also sometimes called actin filaments. Most microfilaments do not occur singly but are linked together in networks or bundles.
Networks of microfilaments are particularly prominent around the edges of cells, in a region just below the cell membrane, sometimes called the cell cortex (Figures 9a and 10). Bundles of microfilaments are found in microvilli of absorptive epithelial cells (e.g. in the intestine), and in the leading edge of moving cells, where the ability of the actin filaments to rapidly disassemble and reassemble plays a key role in cell motility.