Actively migrating LLC-MK2 cells seem to contemplate joining a relatively stable colony of epithelial cells. As they locomote, they are generally broadest at their leading margins, where lamellipodia are most prominent. Their posterior edges exhibit a tapered form and a thin strand of material, commonly called a retraction fiber, can usually be seen trailing behind the rest of the migrating cells. Near the end of the time-lapse sequence, the retraction fibers of several epithelial cells overlap the main colony, which appears as if it has become entwined in the sticky strands of a spider’s web.

Vacuole formation can be observed in several of the cells. The sudden appearance of the small, round bodies along the peripheries of the cells is due to endocytosis. Though the individual steps cannot be discerned in the video, the ingestion process involves an invagination of the plasma membrane, which is then pinched off to trap extracellular material in a membrane-enclosed sac. The newly formed vacuoles can then be transported to other portions of the cell. The vacuoles in the LLC-MK2 cells, which likely contain culture medium, can be seen streaming toward central cytoplasmic regions, from where they seem to magically disappear. The vanishing of the vacuoles is an indication that the bodies have been broken down in order that the cells may access and utilize their contents.

In addition to vacuoles, several other structures can be identified in the epithelial cells. Each cell possesses a large, round nucleoli-containing nucleus and countless mitochondria. Imaged under a light microscope, the mitochondria look like small granules. Their constant movement is a testament to the dynamic nature of the power-generating organelles.