The cell dynamics of 3T3 fibroblasts can be readily observed during the high speed playback of this time-lapse sequence. When the video begins, several cells, including a binucleated fibroblast, are in the midst of crawling across the imaging chamber through the formation and retraction of surface extensions. When each broad lamellipodium is stretched out over the culture medium, sites of close adhesion to the substratum, called focal adhesions, are formed. As if they were microscopic feet, the focal adhesions remain in place as the cell advances over them, but once they move further toward the rear of the advancing cell, they usually detach from the surface. In some instances, however, focal adhesions are so strong that they remain stationary, which can cause the traction forces that drive the progression of the cell to tear bits of cytoplasm from the cell’s trailing end.

At an early point in this video, along the lower left side of the field of view, three cells can be observed in contact with one another. Similar to most fibroblasts in culture, the neighboring cells soon break apart and travel in different directions. When this occurs, several small sections of cytoplasm are left behind. For a short time, the cell debris remains untouched, but then a migrating 3T3 fibroblast passes over the material and incorporates it into its own cytoplasm.

An unusually large binucleated cell appears along the right side of the field of view. Since the expansive lamellipodia of the fibroblast are stretched so thin, endocytic vesicles formed via invagination of the membrane along the cell’s periphery can be readily seen upon close examination. The small membrane-enclosed sacs, which are especially pronounced near the end of the video, are often formed as cells feed and may contain culture medium, which provides nutrients to the fibroblasts when it is digested.