Keratin filaments are packed into bundles of varying diameter, called tonofilament bundles. These are the building blocks of the cytoskeleton in epithelial cells. Each bundle is surrounded by polyribosomes and multivesicular or folded membrane complexes that form a branched, tubular network (Norlen et al. 2003). In the stratum corneum, keratin bundles are thought to pack in a cubic rod pattern (Norlen & Al-Amoudi 2004). In living cells, X-ray diffraction studies of keratin intermediate filaments indicate a filament radius of 5 nm, hexagonal geometric arrangement with an interfilament distance of 14 nm and bundle diameters of 70 nm (Hémonnot et al. 2016).
Keratin filaments can rapidly disassemble and reassemble, allowing flexibility for the cytoskeleton. Keratin building blocks accumulate at the cell periphery near focal adhesions. Polymerization is regulated by signaling molecules, e.g. heat shock proteins, 14-3-3 proteins, kinases and phosphatases (Magin et al. 2007, Kayser et al. 2003).