SUMMARY

The 3-dimensional structure of the fibrous cytoskeleton of 3T3 cells was examined by scanning electron microscopy of cells extracted with the non-ionic detergent Triton X-100. Detergent-extracted cells consist of the nucleus and an extensive system of fibres, the largest of which correspond to stress fibres visible by phase-contrast microscopy. The system of fibres, which is coterminous with the borders of the native cell, remains firmly adherent to the sub-stratum. The major fibres branch into smaller fibrils which appear to end by ravelling out into fine filaments that constitute a matted network in a plane very close to that of the substratum. In the nuclear region all the major fibres pass over the top of the nucleus, where they may also branch into a system of fine fibrils. Thin-section transmission electron microscopy in con-junction with heavy meromyosin treatment of extracted cells shows the fibres to be composed of native F-actin. Intermediate filaments are also present, and are prominent in the matted network, together with actin filaments. The major proteins of the residue are identified by SDS-polyacrylamide gel electrophoresis as actin, a 56000 Dalton peptide, and histones. Also present are myosin heavy chain, peptides of 225000 and 250000, and minor bands at 60000 and 94000 Daltons. The non-ionic detergent extracts 70 % of the cellular protein, including 50 % of the actin and 75 % of the myosin. The Triton-insoluble fraction of 3T3 cells appears to constitute, in addition to the nucleus, a stable cytoskeletal system, composed largely of contractile proteins and 10-nm filaments, which functions in maintenance of cell shape, in substratum adhesion, and in positioning the nucleus within the cell