EXPERIMENTAL FIGURE 18-44 Mitotic spindles can form in the absence of centrosomes. Centrosome-free extracts can be isolated from frog oocytes arrested in mitosis by centrifuging eggs to separate a soluble material from the organelles and yolk. When fluorescently labeled tubulin (green) is added to extracts of the soluble material together with beads covered with DNA (red), mitotic spindles spontaneously form around the beads from randomly nucleated microtubules. See Kinoshita et al., 2002, Trends Cell Biol. 12:267–273, and Antonio et al., 2000, Cell 102:425. [Micrograph republished with permission of Nature, from Heald, R. et al., “Self-organization of microtubules into bipolar spindles around artificial chromosomes in Xenopus egg extracts,” Nature, 1996, 382:6590, pp. 420-425.]