Figure 3.

Plasticity of microtubule arrays. Microtubules are composed of tubulin subunits that can rapidly polymerize and depolymerize, and are thus said to show dynamic instability. They are here shown (a) growing rapidly from the cell center (centrosome), and shrinking more rapidly, producing a random organization. (b) A signal from the cell surface leads to (c) the selective stabilization of the growing and shrinking microtubule ends locally, and in (d) the microtubules are further stabilized by modifications that accumulate along their length. In this way, spatially random polymerization and local stabilization can lead to organized arrays of microtubules (here polarizing the cell), and allow the evolution of stable structures such as flagella and dynamic ones such as the mitotic spindle. Figure modified with permission from Figure 4-5 of Gerhart and Kirschner, Cells, Embryos, and Evolution, Oxford: Blackwell Science; © 1997.