Pathways of Spindle Pole Formation: Different Mechanisms; Conserved Components

A central event during cell division is the transformation of an interphase network of microtubules into a bipolar spindle. For most animal cells the centrosomes, a pair of centrioles surrounded by electron-dense pericentriolar material, represent the microtubule organizing centers from which interphase microtubules are nucleated, with the microtubule minus ends at the pole and the rapidly growing, free plus ends extending away. At, or just before, the time of nuclear envelope fragmentation, the duplicated centrosomes separate from each other using microtubule-dependent motors that push against the astral microtubules nucleated by each centrosome. Microtubules penetrate the nucleus, and in a mechanism called “search and capture” (Kirschner and Mitchison, 1986) some attach to kinetochores, specialized regions that assemble onto the surface of centromeres (Fig. 1,A). As a result, most mitotic animal cells have spindles with two clearly defined spindle poles at which the microtubules (kinetochore attached,...