Frequently when 3T3 cells come into contact with other cells during their migration across a culture dish, the fibroblasts alter their course in order to move more freely. At other times, however, the fibroblasts will crawl over other cells they find in their path, as demonstrated in this video. The latter phenomenon is most common when a high volume of cells are present, which limits the number of courses the cells can follow. Several cells can be seen pulling themselves across other fibroblasts during the high speed playback of the time-lapse sequence presented here.

Initially, there is a very large cell present in the left half of the field of view, which becomes stretched very thin as the lamellipodia it extends from its surface come into contact with nearby cells. At first, it is difficult to deduce the direction that the cell will take, as it is connected to many cells from which it appears to be attempting to free itself. Eventually, however, the lamellipodia along its lower edge become very broad and take the lead, pulling the cell to the bottom of the field of view. Immediately following the movement, the trailing end of the cell, called a retraction fiber, snaps back into the body of the cell like a rubber band stretched too thin. A cell that suddenly appears along the bottom of the field of view can then be clearly observed migrating over the newly positioned fibroblast.

Many of the imaged 3T3 fibroblasts undergo cell division, further increasing cell volume. As can be observed, the cells retract their flattened lamellipodia and narrow filopodia to assume a spherical geometry before undergoing the process. As the daughter cells prepare to separate, their forms undergo many contortions, and numerous hemispherical mounds called blebs cover their central regions. The blebs, which are remnants of the former rounded cell shape, disappear as the new cells settle on the substratum and extend out lamellipodia and small, pointy filopodia from their surfaces.

The majority of the 3T3 cells in this video exhibit a single, large nucleus. Near the end of sequence, however, a few binucleated cells appear and begin migrating across the field of view. The nuclei in the binucleated cells are notably smaller than many of the nuclei that can be observed in the mononucleated cells.