Fig. 1. Path photobleaching reveals persistent MT growth in the interior of a CHO cell. (A) Upper panels: low magnification images of Cy3-labeled MTs before and after bleaching a zone approximately 20x3 µm passing through the centrosomal region. The path of reduced fluorescence allows visualization of nascent centrosomal MTs. Lower panels: three MTs oriented along the direction of the path (colored yellow, red and green) can be seen to elongate persistently. Numbers in top-left corners indicate time in seconds. Images were acquired every 4 seconds. Bar, 5 µm. (B) Life history plots (length vs time) of colored MTs. Zero length represents position of the centrosome; zero time is time of bleaching. MT plus ends arrived at the plasma membrane in 60 seconds. (C) Instantaneous velocities of nascent MTs were measured as the displacement of MT ends between successive frames. The histogram shows that episodes of shortening and pause were infrequent.