In animal cells, vacuoles are temporary cellular structures enclosed in a single membrane. They are generally formed to store or transport materials, such as food, water, or wastes. Numerous vacuoles can be observed in the BPAE cells featured in this video, and are especially pronounced in a cell located in the upper right-hand corner of the field of view early during the time-lapse sequence. Vacuoles that hold materials brought in from outside of a cell are formed via endocytosis and those that contain materials to be voided by a cell are emptied via exocytosis. The former process involves an invagination and pinching off of the plasma membrane, whereas the latter entails the fusion of a vacuole membrane with the plasma membrane.
As the BPAE cells migrate across the field of view during the high speed playback of this time-lapse sequence, oftentimes a long, thin strand of material can be seen extending from their trailing margins. The strands, which appear as if they stick to the substratum and other cells they come into contact with, are called retraction fibers. Periodically a cell’s retraction fiber contracts back into the central region of the cell. In some types of cells, this process occurs very rapidly, similar to the snapping of a piece of elastic. The retraction fibers of the BPAE cells featured in this video, however, demonstrate a much slower form of contraction that is comparable to the gradual reeling in of a fishing line.
Along the left-hand side of the field of view as the video initiates, a cell fragment can be clearly observed. The debris, which was perhaps the result of one of the cells being unable to contract its retraction fiber before it traveled too far away and the narrow strand of material snapped, undergoes various contortions and changes in shape before a migrating cell that comes across its path attempts to integrate the material into itself. Such cell fragment dynamics have been observed continuing for several hours after material is severed from a cell, suggesting that organelles are not essential for cell motility.
