Most animal cell movements are dictated by changes in the actin cortex located just beneath the plasma membrane. In order for the actin filaments of a cultured cell to be able to generate locomotion, the cell must be in close proximity to a solid surface. Contacts between the cell and the substratum are generally needed to create enough traction to drive the cell forward. Among CRE BAG 2 cells and other fibroblasts, the points of contact formed with the surface of a culture dish or imaging chamber coated with culture medium are rather strong and are known as focal adhesions.

Each focal adhesion is a site where a bundle of actin filaments, termed a stress fiber, attaches to extracellular matrix molecules via specialized anchorage proteins. The attachment sites predominantly form along the leading edge of a cell, but become increasingly distal as the cell advances. Typically, the focal adhesions are released once the cell has progressed to a certain point along its path, but as demonstrated by the cells in the video, this is not always the case. Some of the migrating CRE BAG 2 cells appear to be unable to completely release their focal adhesions and consequently lose small segments of their cellular material. The debris is particularly prominent along the right side of the field of view during the first half of the time-lapse sequence after several cells rapidly crawl out of the area.

CRE BAG 2 fibroblasts that come into contact with cell fragments often attempt to incorporate the material into their own cellular contents. Notice that the previously mentioned debris disappears quickly as a group of cells migrate over them. The fibroblasts act like a tiny cleaning crew, sweeping up all of the material they encounter.