Numerous albino Swiss mouse embryo 3T3 fibroblasts traverse a microscope imaging chamber with crawling-like motions. The motile cells migrate via cycles of extension and contraction manifested as the formation and recession of lamellipodia and other membrane-bound cytoplasmic extensions. In culture, fibroblasts are contact inhibited, which can be readily observed when the cells collide with one another. Following a brief period of connection, the cells alter their dynamics and attempt to migrate in different directions. The high cell volume in the chamber imaged to produce this time-lapse sequence, however, renders it difficult for the cells to freely migrate in any direction since they soon bump into other cells.

In the center of the field of view, some of the 3T3 cells appear to be pulled in several directions at once. Due to limited space, the fibroblasts are necessarily in contact with multiple cells at any given moment. Even as they extend lamellipodia into the least occupied area and attempt to migrate along that path, temporary connections with neighboring cells are formed and are not easily broken. Cytoplasmic strands can be seen stretching thinner and thinner between the cells as they slowly progress further apart.

Large, round nuclei surrounded by an abundance of mitochondria, which have the appearance of tiny granules, can be clearly observed in the 3T3 cells. Note that most cytoplasmic organelles are generally absent from the peripheral region of the cell directly beneath the plasma membrane, known as the cell cortex. An area of high actin concentration, the cortex is chiefly responsible for the pronounced flexibility in shape of animal cells, including their ability to form lamellipodia, filopodia, and other surface extensions.

Most animal cells also form a temporary contractile ring composed of actin during cell division, which functions in the partitioning of large quantities of cytoplasm. Cell division is demonstrated multiple times in this video, first in the upper right-hand corner of the field of view and later along the left-hand side. Parental 3T3 fibroblast cells characteristically retract all surface extensions before undergoing the mitotic process.