New microtubules grow and shrink to fixed locations. A polarized MDCK cell basal cytoplast shown in Fig. 4 (A and B). Microtubules are referred to by arrow color. The red arrow shows a microtubule that arises at 1 min, apparently from the side of another microtubule. After 15 s, the minus end of the red microtubule shrinks but is stabilized at a location crossed by the orange microtubule, and it remains at this location for the duration of imaging, even after the orange microtubule depolymerizes. The red microtubule appears to have formed an end-to-end connection with another microtubule. At 3:20, the yellow microtubule grows to the location where the green microtubule begins and continues to grow at a different angle. After a few minutes, the yellow microtubule depolymerizes from its minus end, pausing at this location. The yellow arrow is carried through to show that the pink microtubule, which arises at 11 min from the side of another microtubule, grows to this same spot. There is a 3-min gap in imaging at 5 min, when the video was stopped for refocusing. Frames were collected every 5 s and are displayed at 8 frames/s. Bar, 5 µm.
