Portion of the cytoskeleton of a 3T3 fibroblast, demembranated, depleted of soluble protein, quick frozen, deep-etched and rotary-shadowed. At higher magnification of the cytoskeleton in the accompanying micrograph, two "stress fibers" are seen to be composed of bundles of filaments that are continuous at their margins with elements of the general microtrabecular meshwork, thus strongly suggesting that the two are alternative configurations of the same structural protein. The term cytoskeleton has been used to refer to passive stiffening, or stabilizing, filamentous components of the cytoplasm, involved in the maintenance of cell shape and providing a framework of fixed origins and insertions for the motor elements of the cytoplasm.