Sarcomere length fluctuations and flow in capillary endothelial cells.

Tensile force within non-muscle tissue cells is generated in actomyosin stress fibers, which are composed of contractile units called sarcomeres. The number of sarcomeres and sarcomere lengths dynamically change in the cell but the mechanisms by which these processes occur are not understood. Using live cell imaging of labeled sarcomeres, we show that sarcomere lengths continually fluctuate, with a fluctuation relaxation time of about 20 min. New sarcomeres are formed at focal adhesions and are convected into the fiber at a speed that is independent of focal adhesion size, suggesting that the speed is independent of tension. Furthermore sarcomeres were observed to disappear at specific points or "sinks" along the stress fibers. These results show that stress fibers are highly dynamic structures despite their relatively static morphology, with nascent sarcomeres forming and being incorporated into the fiber at a nearly uniform, tension-independent velocity throughout the cell. The fluctuating length of individual sarcomeres under constant tension is consistent with a model whereby sarcomere contraction/expansion speed, rather than sarcomere length, is modulated by tension.