Life cycle of MTs

Rapid remodeling of the microtubule (MT) array is important in large motile cells because such cells are constantly in the process of changing their shape. In fibroblast-like cells, MTs form a radial array emerging from the centrosome with minus ends tethered at the centrosome and plus ends extending towards the cell periphery (reviewed by Desai and Mitchison, 1997; Keating and Borisy, 1999). Although many studies have dealt with aspects of MT dynamics, a full understanding of the MT life cycle has yet to be achieved. In particular, the first phases, namely formation at the centrosome and growth in the cell interior have not been analyzed directly because of technical limitations imposed by the high density of MTs in the centrosomal region. In previous studies, the dynamics and turnover of MTs in cultured cell lines have been analyzed primarily using fluorescently labeled tubulin. The picture that has emerged from these studies is that MTs in vivo undergo frequent transitions between phases of growth and shortening (Sammak et al., 1987; Cassimeris et al., 1988; Shelden and Wadsworth, 1993; Waterman-Storer and Salmon, 1997; Yvon and Wadsworth, 1997). However, several considerations suggest that this picture is incomplete. One basic point is that almost all observations of individual MT dynamics in vivo have been made near the cell margin. This is because the cell is thinner and the MT density is lower near the edge of the cell, thus permitting better visualization of individual MTs. Consequently, MT dynamics in the cell interior have remained essentially unexplored and the formal possibility exists that they are different from that at the cell margin. A second point is that MT dynamics in vivo show complexity not seen in vitro. In vitro, MT dynamics are characterized by transitions between well defined phases of growth and shortening (Mitchison and Kirschner, 1984a; Mitchison and Kirschner, 1984b; Horio and Hotani, 1986; Walker et al., 1988). In contrast, MTs in many cell types do not show well defined phases. In addition, MTs in vivo frequently are quiescent, neither growing nor shortening, a behavior termed pause. Growth and shortening are highly variable in terms of velocity, duration and extent but, in general, tend to be brief. For example, the mean growth length 3527