Several CRE BAG 2 fibroblasts carry out the process of endocytosis, vacuoles forming along their peripheries and appearing to stream into the central regions of the cells. Though it cannot be clearly discerned in the video, the vacuoles are produced via an invagination of the plasma membrane, which pinches off to form the membrane-enclosed sac-like structures. Approximately halfway through the time-lapse sequence, a large cell that enters the upper field of view and migrates to the bottom becomes stretched very thinly across the substratum due to points of adherence it has formed with several opposing cells. Endocytosis is particularly pronounced in the cell and numerous large, round vacuoles are easily observable for a brief time.

Shortly after the vacuoles are successfully transported to cell’s center, they seem to suddenly disappear. The phenomenon is an indication of the cell’s breakdown of the vacuole membranes, which must occur in order for the cell to access the contents contained in the vesicles. Most likely the vacuoles formed by the CRE BAG 2 cells hold culture medium, which the cells digest to obtain nutrients. Without such a source of nutrients, cells would not be able to survive long in vitro.

Actively migrating fibroblasts often assume a roughly triangular form, as demonstrated by many CRE BAG 2 cells. The leading margin of a fibroblast is typically comprised of flattened lamellipodia, which are extended and contracted to pull the fibroblast along and which renders the cell very broad at its anterior end. Behind this margin, there is less surface extension activity so that the rest of the cell seems to trail along behind its front edge, tapering into a narrow tail. Commonly termed a retraction fiber or a uropod, the tail is intermittently contracted back into the main body of the cell.