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An intermediate filament is a rope-like protein assembly of the cytoskeleton, typically about 10 nanometres in diameter, between an actin filament and a microtubule in thickness. Different cells use different constituent proteins: keratins reinforce many epithelial cells, vimentin forms networks in many connective-tissue cells, and lamins support the cell nucleus. Their assembled subunits do not establish the uniform head-to-tail polarity characteristic of an actin filament.^[ref]

The mechanical role can be tested outside a biological cell. In 1991, P. A. Janmey and colleagues compared purified vimentin, actin and microtubule networks under deformation. The vimentin networks tolerated larger deformations before disruption, illustrating how an intermediate filament network can combine compliance with resistance to damage. This helps explain why load-bearing tissues need a network that can stretch rather than simply a rigid internal frame.^[ref]

P. A. Janmey investigated how networks of cytoskeletal proteins bear mechanical loads. A 1991 study compared purified actin, tubulin and vimentin assemblies under deformation. The networks differed in stiffness and resistance to disruption, with vimentin networks especially able to withstand large deformations.^[ref]

The comparison separated the mechanical properties of a filament network from the complexity of an intact cell. A material's response to a small deformation does not completely predict whether it survives a large one. This connects cytoskeleton composition to the distinct questions of stiffness, viscoelasticity and mechanical resilience.