Epithelial cells experience significant contact inhibition to migration. When two epithelial cells come into contact with one another, the ruffling of their overlapping surface extensions generally halts and the cells adhere to one another. Unlike fibroblasts, which experience a much milder form of contact inhibition, epithelial cells do not usually separate and travel in new directions once they become stuck together. Consequently, A-549 cells and other epithelial lines typically form small colonies in culture, which over time can develop into sheet-like structures reminiscent of the epithelial sheets they form in vivo. Several of the A-549 epithelial cells in this time-lapse sequence are organized into a colony, and accordingly do no exhibit ruffling lamellipodia along the borders they share with neighboring cells.

Though colonies of epithelial cells are capable of locomoting as a group, the colony in this video remains relatively stable. The long narrow cell that appears in the beginning of the sequence and a large, unbound cell that features a flattened skirt of lamellipodia around its perimeter are much more active, however. The rapid extension and retraction of lamellipodia called ruffling can be seen along the leading margin of the narrow cell, while its tapered back end trails behind the cell’s anterior. Ruffling occurs along the front edge of the larger cell as well, but can also be seen at several other points along the cell’s periphery.

When the previously mentioned tapered cell exits along the bottom of the field of view, several small pieces of cell debris are left behind. The fragments are severed from the A-549 cell because the focal adhesions between the cell and the substratum are not successfully released. The rest of the cell advances so far ahead from the points of junction that the tension becomes too great for the thin strands of cytoplasm connecting the adhesions to the cell body. The large cell locomoting nearby soon sweeps the debris up in its ruffling lamellipodia, presumably to be integrated into its own cellular contents.