= Intermediate filament
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>.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 16, Intermediate Filaments: structure, families and mechanical role]{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.<Viscoelastic properties of vimentin compared with other filamentous biopolymer networks by P. A. Janmey and others>[Abstract and pp. 155–160, comparison of purified networks]{ref}
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