FILAMENT ARRANGEMENTS IN NEGATIVELY STAINED CULTURED-CELLS - ORGANIZATION OF ACTIN

Abstract

Treatment of spread, cultured cells with Triton X-100 followed by negative staining reveals the organization of the unextracted intracellular filamentous elements actin, microtubules and the 100 .ANG. filaments. The organization of the actin-like filaments of human skin fibroblasts and mouse 3T3 cells is described. The cytoplasmic stress fibers were composed of bundles of colinear actin-like filaments. In addition to these large stress fibers much smaller bundles of thin filaments and randomly oriented thin filaments were also observed. A thick bundle of thin filaments, 0.2-0.5 .mu.m in diameter delimited the concave cell edges most prominent in well-spread stationary cells. The leading edge and ruffled border of human skin fibroblasts appeared as a broad web, or meshwork of diagonally oriented thin filaments interconnecting radiating, linear bundles of thin filaments about 0.1 .mu.m in diameter. These bundles corresponding to the microspikes described earlier ranged from about 1.5-6.0 .mu.m in length and were separated by 1-3 .mu.m laterally. The leading edge of 3T3 cells showed a similar organization but with fewer radiating thin filament bundles. Both the filaments in in the bundles and in the meshwork formed arrowhead complexes with smooth muscle myosin subfragment-1 which were unipolar and directed towards the main body of the cell. The findings are discussed in relation to the mechanism of non-muscle cell motility.