Models for intermediate filament end-to-end annealing and intercalary subunit exchange. The filaments are depicted as being composed of eight tetramers per cross section, each composed of two staggered coiled-coil dimers (shown as cylinders). (a) End-to-end annealing. For diagrammatic purposes, we show the end of a red filament annealing with the end of a green filament. The annealing mechanism may involve interdigitation of the protruding dimer overhangs at the filament ends. (b) Intercalary subunit exchange. For diagrammatic purposes, we show red tetramer subunits incorporating along the length of a green filament. Both annealing and intercalary subunit exchange may occur spontaneously in cells, although it is likely that there are also accessory factors that facilitate or regulate these processes.