Behaviour and structure of the leading lamella in moving fibroblasts. I. Occurrence and centripetal movement of arc-shaped microfilament bundles beneath the dorsal cell surface.
Arc-shaped bundles of microfilaments are frequently found beneath the dorsal surface of the leading lamella of chick embryo fibroblasts. These structures, called arcs, form parallel to, and 2-10 micron from the leading edge of the lamella and they then move centripetally through the lamella and disappear in front of the cell nucleus. Arcs move centripetally at a mean speed of 1.33 (+/- 0.08 S.E.) micron min-1 relative to the substratum. Arcs are specifically associated with cells that are actively protrusive. They are common in fanshaped fibroblasts and in those cells that are newly spreading on a substratum. Arcs are absent from fibroblasts that have only small lamellae or that are polygonal. The correlation between are formation and protrusive activity is also clearly shown by the locomotory behaviour of fan-shaped cells. When protrusion of the lamella is increased by tail detachment, fibroblasts often develop numerous arcs, which chase each other backwards through the lamella. Conversely, arc formation ceases during contact inhibition of locomotion. In cells with large convex-edged lamellae there is a dorsal submembraneous sheath of microfilaments. All of the filaments comprising the sheath are oriented parallel to the margin of the lamella. Arcs are regions of the sheath where the microfilaments are more densely packed and hence visible by phase-contrast microscopy. There is no obvious relationship between the dorsally situated arcs and microfilament sheath, and the stress fibres that are associated with the ventral cell surface. The similarities between the movement and behaviour of arcs and the centripetal transport of particles on the surface of the lamella suggest a role for the microfilament sheath in the movement of particles.