Arcs: Curved microfilament bundles beneath the dorsal surface of the leading lamellae of moving chick embryo fibroblasts

Curved microfilamentous structures are frequently found in the leading lamellae of cultured fibroblasts. These structures, termed arcs, form parallel to, and about 8 μm from, the lamellar margin. Arcs move centripetally through the lamella beneath the dorsal surface at speeds of 1.5 to 3.0 μm min −1 and they disappear in front of the nucleus. The movement of arcs is related to the movement of particles transported on the cell surface. Arcs show some structural and biochemical similarities to the actomyosin stress fibres, but arcs are not obviously functional in applying force against the substratum during cell locomotion. Arcs provide clues to the organisation and regulation of the microfilament system for movement.