RK13 cells crawl over surfaces through the coordinated use of surface protrusions. As demonstrated in the video, the epithelial cells repeatedly extend and contract broad, flattened lamellipodia, which seem to roll in and out over the surface of the imaging chamber like waves. The characteristic movement of the structures, which intermittently rear up on the upper surface of a cell and sweep rearward, is known as ruffling. Lamellipodial ruffling is a visual manifestation of dynamic changes in the cortical actin filaments that underlie the plasma membrane.

Actin is polymerized at the periphery of a surface extension, resulting in a deformation of the cell. In more rearward locations, the actin cortex contracts through the fragmentation and depolymerization of actin filaments. Myosin molecules are involved in cellular contraction. The elastic properties provided to the cell by myosin are demonstrated when the retraction fiber or uropod that trails behind the leading margin of a migrating cell rounds up, detaches from the substratum, and appears to snap back into the main cellular process.

The actin filaments that comprise the prominent lamellipodia along the edges of the featured RK13 cells are organized into large branching nets. In the much smaller and narrower protrusions called filopodia that can also sometimes be observed extending and receding from the peripheries of the cells, filaments of actin are loosely bundled together by fimbrin and other cross-linking proteins. When either type of protrusion is extended over the substratum, they can form attachments with the surface that are utilized as anchorage points as the cell contracts. Traction enables the bulk of a cell to travel over the sites, which are usually released as they become increasingly distal form the leading edge.