Fig. 4 Representative life history plots of free MTs. Upper curve in each plot,- plus end; lower curve,--minus end. The reference point for each curve was the position of the minus end at zero time. Plus ends express different behavior: (A) dynamic instability; (B) growth; (C) growth and shortening that brings the MT to complete depolymerization; (D) stability with minor oscillations. Minus ends are usually stable (A-D), but in some cases they slowly depolymerize (E). (F) shows an example of a MT that polymerizes and then moves along its axis with plus end leading and minus end trailing. All plots shown are from PtK1 cells. MT origin (birth) and complete depolymerization (death) are indicated by arrows.