Fig. 1 Apparent free MTs in cultured cells. (A) 3T3 fibroblast. A free MT is born just before 0 time and rapidly grows. Its minus end remains at the same position throughout the series. (B) PtK1 cell. One free MT grows across preexisting MTs (0-186 sec), and then becomes stable (442 sec). Its minus end at 442 s is slightly shifted from its initial position. A short MT is stable (0-186 sec), then slightly shortens (442). (C) Black tetra keratocyte. One free MT shortens (0 23 sec), then grows (53 sec) and disappears (123 sec). Another free MT is stable (0-53 sec), then moves laterally (123 sec). Annotated MTs are overlaid with red. Growth and shortening of long MTs can be seen in each cell. Time after the first frame shown at lower left in seconds. Scale bar, 5 µm.