Steps of the keratin cycle. Representative images (inverse presentation), taken from time-lapse fluorescence microscopy of cultured cells producing fluorescent keratins (see Video 1), that illustrate the steps of the keratin cycle (see also Fig. 1). Images were adapted with permission from the Journal of Cell Science (Kölsch et al., 2010) and Molecular Biology of the Cell (Windoffer et al., 2004). Nucleation: Newly appearing particles that are generated in proximity to the plasma membrane (to the right) are encircled. Elongation: An elongating particle is marked by an arrow. Integration: The labeled particles (corresponding particles are color-coded in successive frames) move toward the nucleus, fuse with each other, and integrate into the peripheral keratin network. Bundling: Inward-moving filaments fuse laterally and bundle (one example is marked by an arrow). Disassembly: The demarcated filament bundle disappears gradually without filament fragmentation. Bars, 2 µm.