Those who view the high speed playback of this time-lapse sequence are sure to experience suspense at the cellular level. One cannot help but wonder if the extremely long retraction fiber trailing behind the 3T3 cell attempting to progress out of the upper field of view is going to snap back into the central portion of the cell or is going to suddenly break, sacrificing bits of the cell’s cytoplasm, perhaps forever. Tension builds as the contractile tail stretches thinner and thinner. Other cells migrate over the narrow strand and appear as if they may hinder the return of the retraction fiber to the cell body from whence it came. Only when the rest of the fibroblast is almost completely out of view does the focal adhesion between the fiber and the substratum finally release, allowing the slender extension to suddenly retract and the cell to finish its migration.

Though the previously mentioned fibroblast at last successfully draws in its trailing end, a small rounded mound of material near the bottom of the field of view serves as a visual indication of what could have happened. The small mass is a fragment of a cell that must have been lost by one of the 3T3 cells present in the imaging chamber before the beginning of the time-lapse sequence. Such debris typically results from the inability of a cell to release portions of its membrane-enclosed cytoplasm from the substratum in a timely manner, that is, before the rest of the cell travels too far away to maintain the connection. The focal adhesions found at the sites of connection represent locations where bundled actin filaments called stress fibers are anchored via specialized transmembrane proteins.

Sometimes cell debris is quickly reintegrated into the cell that originally lost the material. The fate of the cell debris in the opening sequence of this video, however, does not enjoy this fate. Instead, a cell rapidly approaches from the lower left-hand corner of the field of view and appears to add the fragment to its own cytoplasm. The debris is first discovered by the migrating cell’s narrow, searching filopodia, but then seems to be grabbed up from the left in a flurry of surface extensions. However, the outline of the debris can still be observed, and since it loosens and releases from the cell before it exits the upperside of the field of view, the material apparently never was truly integrated into the fibroblast. The surface extensions of other 3T3 cells in the area attempt to subsume the fragment once it is abandoned by the exiting cell.